Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.


Introduction
Chromophores have increasingly become functional organic materials (Müller, 2007). As a consequence of their photophysical properties, such as fluorescence and aggregationinduced emission (AIE) (Hong et al., 2009;Hong et al., 2011;Hu et al., 2014;Mei et al., 2015;Mazumdar et al., 2016), and their electrochemical characteristics they find broad application in organic light-emitting diodes (OLEDs) (Müllen and Scherf, 2006;Park et al., 2011;Kalyani and Dhoble, 2012;Li, 2015), dye-sensitized solar cells (DSSCs) (Grätzel, 2001;Mishra et al., 2009), organic photovoltaics (OPVs) (Yam, 2010;Su et al., 2012;Ameri et al., 2013), organic field effect transistors (OFETs) (Kymissis, 2008;Torsi et al., 2013) and in bio-or environmental analytics Nilsson et al., 2002;Wagenknecht, 2008;Kim and Park, 2009;Cairo et al., 2010), as well as in medicinal and pharmaceutical applications (Dua et al., 2011;Arora et al., 2012;Saini et al., 2013;Kumar and Kumar Jain, 2016;Taylor et al., 2016;Asif, 2017;Taek Han et al., 2017;Petronijevic et al., 2018;Soleymani and Chegeni, 2019). The ongoing quest for novel, efficient syntheses of new functional chromophores with well-defined features and thoroughly fine-tuned properties remains a challenge. Research on diversity-oriented syntheses concepts for functional chromophores has become highly relevant over the past one and a half decades (Müller and D'Souza, 2008;Briehn and Bäuerle, 2002;Zhu et al., 2014;D'Souza and Müller, 2007). In particular, multicomponent reactions (MCRs) (Weber et al., 1999;Bienaymé et al., 2000 Dömling and Ugi, 2000;Zhu and Bienaymé, 2006;Müller, 2016;Riva et al., 2016) have been established as a promising tool for the synthesis of functional π-electron systems (Levi and Müller, 2016a;Levi and Müller, 2016b;de Moliner et al., 2017; in recent years. By definition MCRs proceed in a one-pot fashion, where more than two starting components react to form a product containing most of the deployed atoms. In sensu stricto, the process is not only conducted in a single reaction vessel, but also without changing the solvent, filtration or any other workup operations. Indeed, MCR represent a reactivity-based concept (Müller, 2014), where reactivities of functional groups can be employed in three different ways. In a domino MCRs, all compounds are present in the vessel from the beginning of the process. In a sequential MCR, reactants are added in a well-defined order while the reaction conditions maintained constant. Finally, MCRs taking advantage of defined order of transformation of the reactants, yet, with variably adjusting the reaction conditions from step to step are considered as consecutive processes. All MCR have in common high levels of functional and structural diversity with a minimum of purification operations at the end of the one-pot processes.
MCRs are generally well suited to generate acyclic or cyclic scaffolds that either bear chromophores as functional substituents (scaffold approach) or can be considered de novo formed chromophores (chromogenic approach) (Scheme 1) (Levi and Müller, 2016b). While the former approach allows placing multiple chromophores in close proximity with defined configuration and conformation by the constituted scaffold, also in an electronically non-conjugative fashion, the latter approach is chromogenic and forms the chromophore scaffold of interest with broad structural diversity in a one-pot fashion.
Both approaches are advantageous for advanced chromophore research since large chromophore libraries are rapidly and convergently generated. This allows extensive exploration of electronic properties by electrochemical and photophysical investigation in combination with advanced computational chemistry. Ultimately, the evolving structure-property relationships lead to a more comprehensive understanding and to a rational experimentally founded design of functional chromophores, which are highly requested for high-tech applications.
Guided by the synthetic chromogenic concept of MCR we have structured this review, which updates our previous overview on MCR approaches to functional chromophores (Levi and Müller, 2016b), according to the chromophore classes as defined by their acyclic or (hetero)cyclic central structural elements. Starting from merocyanines, we span the arc from five-to six-membered heterocycles or (hetero)arene and azo chromophores to complexes, discussing predominantly chromophore approaches and some scaffold approaches for illustrating and highlighting in a flashlight fashion most recent developments.

Merocyanines
Merocyanines are donor-acceptor polyenes (Mishra et al., 2000;Kulinich and Ishchenko, 2009a;Kulinich and Ishchenko, 2009b;Hamer, 2009). Due to the polymethine chain and the resulting conjugated π-system, they can be classified as polymethine dyes. According to the classical definition, the class of merocyanines comprises the streptocyanines and their analogues, in which both the nitrogen atom and the carbonyl group form part of a heterocyclic system. In general, merocyanines exhibit tunable electronic properties (Hamer, 1964;Mishra et al., 2000;Kulinich and Ishchenko, 2009b;Arjona-Esteban et al., 2015;Lenze et al., 2015). By varying the terminal groups, the substituents, and the polymethine chain length, functional dyes can be prepared with applications in optoelectronic and nonlinear optical materials, optical information carriers, solar concentrators, electroluminescent devices (Kronenberg et al., 2008;Bürckstümmer et al., 2010;Würthner and Meerholz, 2010), and as fluorescent probes and markers (Valinsky et al., 1978;Williamson et al., 1983;Nandi et al., 2014). Due to their polar nature, merocyanines are generally characterized by absorption and/or emission solvatochromism (Brooker et al., 1951;Reichardt and Welton, 2010). Predominantly, merocyanine dyes are prepared via aldol condensation. Only a few multicomponent synthesis routes for the preparation of these dyes are reported (Wurthner, 1999;Wurthner et al., 1999). One of the first MCRs for the synthesis of merocyanine dyes gave access to dye 1 and 2 through a formylation and condensation sequence of thiazoles or indolines with hydroxypyridones and a formylation reagent generated in situ from DMF in acetic anhydride in almost quantitative yields in the sense of the chromophore concept. Mechanistically, the DMF in acetic anhydride first reacts with hydroxypyridones and the subsequently formed intermediate reacts with an electron-rich methylene base or heterocycle. In addition, benzothiazole and benzoxazole dyes 3 and 4, respectively, were obtained in remarkable yields (39%-42%) from the in situ generated methylene bases (Supplementary Material S1).
It should be noted that all alkyl-substituted derivatives are characterized by a single extremely narrow absorption band in the visible (λ max,abs = 495-533 nm). All of the obtained indoline 2, benzothiazole 3 and benzoxazole dyes 4 display red or orange solid-state luminescence. Only a few of the thiazole dyes 1 fluoresce in solid state. Thiazole dyes 1 absorb light in a range of 528-555 nm. Because of their electronic peculiarity that contributions of the nonpolar and the zwitterionic resonance structures are almost identical both in the electronic ground and excited state, these dyes find application in color copy (Gregory, 2012).
A variety of indolone-based merocyanines with a push-pull character on the basis of insertion-alkynylation Michael addition sequence can be synthesized (Muschelknautz et al., 2014;D'Souza et al., 2010;Muschelknautz et al., 2013). Recently, even a white light emitter generated by aggregation-induced double emission (AIDE) in combination with partial energy transfer is postulated (Denißen et al., 2020). Indolone-based merocyanines, with a Boc group, also exhibit interesting photophysical properties such as aggregationinduced emission (AIE) or crystallization-induced emission enhancement (CIEE) (Denissen et al., 2017a).
The Boc-substituted 3-arylallylidene indolones 5 can be synthesized via a consecutive four-component approach following the chromophore concept. The synthesis via Heck condensation sequence is not stereoselective regarding the allylidene double bond adjacent to the indolone, which exhibits photoisomerization in daylight (Scheme 2A) (Wilbert and Müller, 2022).
Notably, the respective diastereomers of compound 5 share very similar photophysical properties. In general, most of the derivatives display exclusively solid-state emission. Among them, compounds 5a and 5b possess the highest fluorescence quantum yields reaching 18%. Furthermore, the oxindole-based merocyanines 5c and 5d are also luminescent in solution and exhibit positive emission solvatochromism. Additionally, 5c exhibits aggregation-induced emission enhancement (AIEE) (Supplementary Material S2).
Using a Heck-Knoevenagel sequence, 7-donor-substituted phenothiazinyl merocyanines 6 are prepared with yields up to 54%. The optimal reaction conditions of the three-component reaction involving heteroaryl halides, acrolein and ethyl cyanoacetate were developed by statistical design of experiments and Bayesian optimization (Scheme 2B) (Stephan et al., 2022).
The phenothiazine-based merocyanines 6 are red-orange dyes that reveal longest absorption bands in a range from 466 to 491 nm. Using the Hammett plot, a correlation of the red shift of the absorption band with increasing donor strength can be proven, which indicates an essential charge transfer character. The fluorescence of the chromophores is located in a range from 730 to 780 nm with Stokes shifts of 6,500-8,200 cm −1 . The carboxylic acids obtained by saponification of merocyanine esters can be implemented in DSSC and achieve solar cell performance with relative efficiencies up to 93% compared to standard ruthenium dye N3.
Further comparably small merocyanines with outstanding photophysical properties such as AIE behavior and tunable solidstate emission are proposed by chromophores with aroyl-S,N-ketene acetal building blocks . The previous reported systems were modified by synthesis of bichromophores. For this purpose, blue emitters were introduced after condensing aroyl chlorides and a N-(p-bromobenzyl) 2-methyl-benzothiazolium salt by a Suzuki coupling of pinacolboronic esters. This threecomponent one-pot procedure enabled an extraordinarily large substance library of N-benzyl aroyl-S,N-keteneacetal bichromophores 7 (Scheme 2C) .
The synthesized fluorophores show a rainbow-like tuning of the solid-state emission color. Depending on the substitution pattern, a color impression from blue to orange-red can be achieved (Supplementary Material S3).
The variation of both chromophores allows different communication pathways between them. This can result in full or partial energy transfer processes leading to dual emission, or aggregation-induced switching of fluorescence. This selective excitation of either the blue emitter or the aroyl S,N-ketene acetal provides specific emission behavior in solution or in aggregates, which can be exploited for application as a pH sensor or as a detector of water content in alcoholic beverages.
The bichromophore systems could be further developed by preparing asymmetrically bridged aroyl-S,N-ketene acetals and aroyl-S,N-ketene acetal multichromophores 8 via multiple Suzuki one-pot sequences starting from electron donating p-bromo substituted benzylaroyl-S,N-ketene acetals. Phenyl and more complex dye systems were accepted as linker systems within the three-component Suzuki reaction. In order to synthesize the multichromophore systems, triborylated benzene and triphenylamine as well as tetraborylated tetraphenylethene were used as linkers (Supplementary Material S4) (Biesen et al., 2022).
Similar to previous synthesized bichromophores, the bridged compounds 8 exhibit AIE and ET properties. The emission behavior in solution and in the solid state as well as starting point for the formation of aggregates can be determined by the employed linker molecule. The interaction of the aroyl S,N-ketene acetals and the linker influences photophysical properties such as emission color and intensity, fluorescence quantum yield and lifetime. Furthermore, due to the different combination of the linker and aroyl S,N-ketene acetal, partial and complete energy transfer processes as well as different emission behavior upon aggregation (AIE, AIEE and ACQ) is observed. Interestingly, the emission properties of bichromophores 7 and multichromophores 8 can be enhanced by encapsulation in polystyrene particles.
The UV/Vis spectra of merocyanine 9-13 exhibit the longest wavelength absorption maxima in a broad spectral range of λ max,abs = 367-580 nm with molar extinction coefficients ε between 21,000 and 52,000 M −1 cm −1, which accounts to a dominant charge transfer transition from the donor to the acceptor part. The emission maxima can be detected in a range of 412-668 nm with Stokes shifts ranging from 1,200 to 8,000 cm −1 (0.147-0.990 eV). The energy of the hypothetical E 0-0 transition determined from the intersection of the absorption and emission bands represents the optical band gap and lies between 2.083 and 3.197 eV. A correlation analyses by plotting the optical band gaps E 0-0 against the first oxidation potentials E 1/2 of redox active systems of consanguineous series furnishes linear correlations and, by extension, two parameter correlations (oxidation potential and emission maximum) with the optical band gaps. Thus, given this planar correlation, for a number of merocyanines optical band gaps can be predicted based on the first oxidation potentials and emission maxima.
Consecutive three-component alkynylation addition sequences can be performed to synthesize cyclohexene-embedded merocyanines 14 and 15. A stronger nucleophilic amine moiety like pyrrolidine can be employed to synthesize cyanines 16 in excellent yields via a pseudo four-component synthesis by carbonyl condensation of the heterocyclic amine (Scheme 3B) (Papadopoulos et al., 2022).
Merocyanines 14 and 15 and cyanines 16 are obtained as yellow to orange solids. The prepared merocyanine derivatives are found to be non-luminescent in solution and in solid state, whereas the cyanines exhibit luminescence with low quantum yield of 1%. The absorption maxima of 16 are located at 443 and 459 nm with absorption coefficients over 110·10 3 M −1 cm −1 for the two derivates. The absorption maxima of the merocyanines 14/15 and component 16 are located at 365-390 nm and 442 nm, respectively. Due to the extension of the system, the red shift of the absorption band can be explained.
3 Five-membered heterocycles 3.1 Oxygen heterocycles Furans are found in many natural products (Keay and Dibble, 1996) and as a result of their biological activity, they are often structural moieties in pharmaceuticals (Iyer and Gopalachari, 1973;Dikshit et al., 1974;Durani et al., 1989;Dikshit et al., 1991;Nieves-Neira et al., 1999;Mortensen et al., 2001;Chène, 2003;Issaeva et al., 2004;Thuita et al., 2008;Wenzler et al., 2009;Barrett and Croft, 2012). The Molisch test is a colometric detection method that can be used to detect carbohydrates that have been converted into furfuraldehydes by thermal acid degradation. This detection is carried out via reaction with α-naphthol and results in a violet coloration (Molisch, 1886). But aside from their pharmacological importance, they can also be considered as photonic chromophores (Liu and Luh, 2002). In general, many furan heterocycles exhibit strong fluorescence. Due to their excellent semiconducting properties, they are found in organic solar cells (Zheng and Huo, 2021). In addition, furan possesses strong non-linear optical properties which makes it useful in a number of non-linear optical applications (Kamada et al., 2000;Gündüzalp et al., 2016). Classic synthetic routes for the production of substituted furans rely either on cyclocondensation of dicarbonyl compounds or on furan ring substitution (Zeni and Larock, 2004;Brown, 2005;Duc, 2019). The preparation of 2,5-disubstituted furans is commonly carried out via a Paal-Knorr reaction in the sense of a cyclocondensation of 1,4-diketones (Amarnath and Amarnath, 1995), yet a consecutive multicomponent approach to symmetrically 2,5-disubstituted furans via a Sonogashira-Glaser addition-cyclization sequence is known (Klukas et al., 2014).
Butenolides are unsaturated lactones consisting of a dihydrofuran-2-one group. They are important structural building blocks in natural products (Seitz and Reiser, 2005;Roethle and Trauner, 2008;Kitson et al., 2009;Igarashi et al., 2011) and often possess high biological activity such as antibiotic, anticancer and antitumor (Shiomi et al., 2005;Wu et al., 2005;Fei et al., 2007;Liu et al., 2008;Isaka et al., 2009;Wang et al., 2009;Kitani et al., 2011;Uchida et al., 2011;Chen et al., 2012;Csuk et al., 2012). The coupling of a chromium methoxycarbene complex with a ketone or an imide lithium enolate followed by a propargylic organomagnesium reagent lead to the synthesis of novel hydroxy-substituted bicyclic [4.3.0]-galkylidene-2-butenolides 17 and 18 (Scheme 4A) (Suero et al., 2012). Upon modulating substituents on the starting materials, high diversity and complexity of the bicycles can be realized. For instance, alkyl, aryl, heteroaryl, alkenyl and alkynyl groups are well tolerated in the bicyclic compound 17 as a group R 2 resulting from the ketone component. The synthesis of compound 18 proceeds enantioselectively due to the inserted chiral imide enolate auxiliary.
Chromophores 19 exhibit a weak emission in the range of around 485 and 515 nm. However, the fluorescence intensity increases selectively in the presence of Al(III) ions due to CHEF (chelation-enhanced fluorescence). Complexation ensues between the Al(III) ions and presumably the two syn-O-atoms of the functional alcohol groups, which can be experimentally verified by 1 H NMR as well as via calculations (Sun et al., 2012;Jo et al., 2016;Naskar et al., 2018).
Several multicomponent syntheses of pyrroles have been reported (Balme, 2004;Estevez et al., 2010;Estévez et al., 2014). For instance, tetraaryl-1,4-dihydropyrrolo[3,2-b] pyrroles 20 are formed via Mannich reaction of benzaldehydes, anilines and βdiketone derivatives in the sense of a chromophore approach (Scheme 4C) (Martins et al., 2018). The in situ generated Schiff base reacts with the enol form of butane-2,3-dione. Then, the cyclic enamine intermediate adds to another Schiff base. The pyrrolopyrrole is finally formed cyclization and subsequent oxidation (Janiga et al., 2013). These compounds can be classified as heteropentalenes which consist of two fused heterocyclic fivemembered rings (Gribble and Joule, 2009) with 10 π-electrons and, thus, elevating them for further usage in electronic devices (Biswas et al., 2016). Chromophores 20 are promising candidates as sensitizing dyes in optoelectronic applications. For the MCR synthesis of compounds 20, niobium pentachloride is used as a Lewis acid catalyst. The reaction proceeds via two competing mechanisms, which can be controlled by the reactivity of the selected benzaldehyde and aniline derivatives.
Most components absorb between 294 and 382 nm and emit around 420 nm. As said, the photophysical properties suggest that these materials can be used as sensitizing dyes in optoelectronic devices. The modulation of the optical properties can be particularly controlled by the substituent R 1 benzaldehyde derivative. For the electron-withdrawing substituent NO 2 , the absorption bands redshifts to the region between 500 and 600 nm. In contrast, the electron-releasing p-methyl substituent CH 3 causes a blueshift of the absorption bands.
The emission maxima of selected GBBR dyes 21 and 23 are found in a narrow range from 447 to 460 nm. By the substituent diversity on the GBB cores, the emission can be tuned. Thus, a redshift in absorption and emission wavelengths occurs by expanding the π-electron conjugation with conjugated (hetero)aryl groups and their modification. For instance, the orange bispyridinium salt 24 is prepared by alkylation of the pyridine ring of yellow chromophore 21a (Supplementary Material S5). While compound 21a shows no specific interaction with quadruplex DNA structures, the bispyridinium salt 24 reveals significant strong interaction and can be potentially applied as selective binder for the further development of new anticancer drugs.
The synthesized imidazole chromophores 25 are blue fluorescent and show excited state proton transfer (ESIPT). In the solid state as well as in solution, intense fluorescence is exhibited due to the four phenyl rings on the imidazole core limiting the intermolecular interactions between neighboring molecules.
The representatives with donor-acceptor substitution pattern strongly fluoresce in solution (Supplementary Material S6). The absorption spectrum of 26a, which contains the biaryl substituent in 3-position, is hypsochromically shifted (λ max,abs = 294 nm). Compound 26a exhibits the highest molecular absorption coefficient (ε = 66,000 M −1 cm −1 ) and Stokes shift (Δ] = 10,400 cm −1 ) in this series of biaryl-substituted isoxazoles. However, the quantum yield of 26a (Φ F = 0.17) is considerably lower compared to 5-biaryl-substituted isoxazoles 27, which range from 0.62 to 0.86. Emission maxima of compounds 26a-c are detected at 376, 411, and 554 nm, respectively. The bathochromic shift of 27b is caused by the electron-withdrawing cyano substituent in comparison to the trifluoromethyl substituent of 26a. The strong bathochromic shift of compound 27c results from the strong electron-donating dimethylamino donor.
Another interesting catalytic approach for the generation of isoxazol-5(4H)-one is catalysis by enzymes. Polyethylene imine (PEI) derivatives are synthetic enzymes with good catalytic activities for various reactions (Vasylyev et al., 2006;Abu-Reziq et al., 2008;Avenier and Hollfelder, 2009). The benefit of this catalyst system is that it can be recycled up to 15 times without any noticeable loss of catalyst activity. Mechanistically, the condensation reaction of hydroxylamine and ethyl acetoacetate is followed by cyclization. The resulting isoxazolones undergo a Knoevenagel-type condensation with aromatic aldehydes to form isoxazol-5(4H)-ones 28 in good to excellent yields (Scheme 6B) (Oliveira et al., 2021).
The shown derivatives 28 fluoresce in solution and they also display positive solvatochromicity. Thus, small Stokes shifts are observed in non-polar solvents and medium to large Stokes shifts in polar solvents. In general, the chromophores have large molar extinction coefficients and low fluorescence quantum yields. In particular, the fluorescent derivative 28d represents a promising candidate as a probe for bioimaging due to its ability to selectively stain early endosomes in living cells.
Another push-pull chromophore, where the isoxazole scaffold acts as the electron acceptor and the aromatic ring as the donor, is 4-(arylmethylene)-5-oxo-4,5-dihydroisoxazole-3-carboxylic acid 29. The reaction proceeds via a two-step one-pot reaction starting from aromatic aldehydes, diethyl acetylenedicarboxylate and hydroxylamine-O-sulfonic acid (Scheme 7A) (Tasior et al., 2021). The reaction is initiated by a Michael addition of hydroxylamine to diethyl acetylenedicarboxylate. The formed enamine then reacts with aromatic aldehydes and the isoxazole derivatives are obtained by elimination of the sulfur trioxide and subsequent transesterification.
Frontiers in Chemistry frontiersin.org 10 Another five-membered ring containing a nitrogen and a sulfur atom is thiazole. The thiazole core is found in naturally occurring compounds such as vitamin B 1 , penicillins and dolastatin analogues. The latter are cytostatic drugs (Maderna et al., 2014), which can possibly be used in the treatment of cancer (Bhat et al., 2009;Halasi et al., 2010;Anuradha et al., 2019;Sharma et al., 2020). In general, thiazoles show biological activity and are therefore of great interest for medicinal chemistry (Kashyap et al., 2012; Mahmoodi and SCHEME 7 (A) Synthesis of isoxazol-5-one 29 via a two-step one-pot reaction and the three obtained fluorophores (Tasior et al., 2021). (B) One-step synthesis of 5-amino-4-carboxamidothiazoles 30 based on the chromophore approach and the synthesized chromophores (Tong et al., 2017). Ghanbari Pirbasti, 2016; Hussein and Turan, 2018). Furthermore, organic semiconductors based on thiazole for organic electronics are known (Lin et al., 2012a;Lin et al., 2012b). The synthesis of thiazoles proceeds via MCR, for example via the Hantzsch reaction or via cyclization/oxidation of a corresponding peptide precursor in a biomimetic approach (Hantzsch, 1881;Evans et al., 1979;Boden and Pattenden, 1994;Videnov et al., 1996). Thiazoles can also be prepared via condensation of thiocarboxylic acids with isocyanides (Scheme 7B I) (Tong et al., 2017). 5-Amino-4-carboxamidothiazoles 30 obtained by the triflate-catalyzed reaction display interesting fluorescence properties as potential ESIPT chromophores. Alternatively, thiazole-based chromophores 30 are also produced by reacting a suspension of carboxylic acid with Lawesson's reagent in boiling toluene followed by addition of an isocyanide and Y(OTf) 3 in a one-pot fashion (Scheme 7B II).
5-Amino-4-carboxamidothiazoles 30 (λ max,abs = 333-460 nm) possess high extinction coefficients (ε > 10 4 M −1 cm −1 ) and fluoresce with violet, blue, green to yellow color depending on the substitution pattern (λ max,em = 393-558 nm). Upon UV excitation, compound 30a emits in the green region, 30b in the in violet to blue region and 30c as well as 30d in the yellow region. An aryl substituent in 2position with electron-withdrawing and electron-donating antiauxochromes and auxochromes can significantly increase the quantum yield. The quantum yield can also be increased by expanding the conjugated system via group R 1 . The insertion of a multiple bond between the thiazole ring and further arenes in substituent R 1 causes a bathochromic shift. In particular, nitrophenyl substituted thiazoles can be excited with visible light and fluoresce with large Stokes shifts, high quantum yields and pronounced solvatochromism. These dyes have potential for the double ESIPT process.
Indoles are desirable due to their widespread medical use (Kang et al., 2009;Kochanowska-Karamyan and Hamann, 2010;Kaushik et al., 2013;Álvarez et al., 2013;Zhang et al., 2015a;Zhang et al., 2015b;Parisi et al., 2015;Hu et al., 2017) and their occurrence in many natural products (Lindquist et al., 1991;Pettit et al., 2002;Cruz-Monserrate et al., 2003;Simon and Petrášek, 2011). For instance, indole constitutes the core of many alkaloids, some hormones and also dyes. Among indole dyes indigo is the most prominent representative (Amat et al., 2011). Considering indoles photophysically, the heterocycle displays fluorescence and strong phosphorescence (Bünau and Birks, 1970). The absorption and emission maxima of indole derivatives in aqueous solution lie typically at 270 and 355 nm, respectively (Bridges and Williams, 1968). Similar absorption and emission values are found for indolylmalonamides 31. In addition, when irradiated with long-wave UV light (λ exc = 366 nm) chromophores 31 with high molar absorptivity fluoresce and exhibit large Stokes shifts. The synthesis of indolylmalonamides 31 are performed via a lanthanum (III)catalyzed three-component amidation reaction of coumarin-3carboxylates, indoles and amines in the sense of the chromophore approach (Scheme 8A) (Jennings et al., 2016). After Lewis acidcatalyzed Michael addition indolylmalonamides 31 are obtained by Lewis acid-catalyzed amidation.
The UV/Vis spectra of 2-alkynyl-4-(7-azaindol-3-yl) pyrimidines 32 show absorption maxima at λ max,abs = 293-296 nm with molar extinction coefficients ranging between 23,100 and 43,900 L mol −1 cm −1 for aliphatic substituents and 48,000 and 77,000 L mol −1 cm −1 for aromatic substituents, respectively. Electron-donating substituents R 2 redshift the absorption maxima to longer wavelengths. Most of the compound fluoresce with emission maxima at about 447 nm with high Stokes shifts (Δ] = 10,300-11,900 cm -1 ). It should be noted that this method provides an efficient access to alkynyl meriolins, a new biological active class of potential apoptosis inducers.
Amine-appended spiro[indoline-3,4′-pyridines] 33, for example, can be applied as ON-OFF chemosensors for Cu(II) ions via a fluorescence response. These chemosensors show high selectivity and have already been applied for imaging Cu(II) ions in human hepatocellular liver carcinoma cells. Spiro[indoline-3,4'pyridine] 33 are prepared via a one-pot four-component reaction involving dialkyl but-2-ynedioate, primary amines, isatin, and malononitrile (Scheme 9A) . The environmental benign reaction is catalyzed by iodine and carried out in aqueous ethanol solution. The proposed mechanism suggests activation of the C-3 carbon of isatin by iodine. The isatin-iodine complex reacts with malononitrile via Knoevenagel condensation. In the presence of the iodine catalyst, the nucleophilic attack of the primary amine occurs. Addition of dialkyl but-2-ynedioate initiates the intramolecular Michael addition. Subsequently, chromophores 32 are formed by proton tautomerization.
The chromophores 33 display strong emission in DMSO in a range of 458-493 nm with large Stokes shifts (Δṽ = 5,335-11,641 cm −1 ). In the presence of Cu(II) ions the fluorescence decays and the color change can be observed by the naked eye. Chemosensor 33d with the highest quantum yield (0.95) has successfully been used for in vitro fluorescence cell imaging of Cu(II) ions in human hepatocellular liver carcinoma cells.
Further spiro indoles 34 can also be generated under sustainable conditions. The three-component reaction of 1,3-indanediones, isatins, and aromatic amines is catalyzed either by PEG-OSO 3 H or by [NMP]H 2 PO 4 to produce spiro[diindenopyridine-indoline] triones 34 under conventional heating and ultrasonication (Scheme 9B) (Sindhu et al., 2015). The PEG-OSO 3 H is a polymeric acidsurfactant based catalyst that can be recycled and reused without significant loss of activity. Implementing of acidic ionic liquid [NMP]H 2 PO 4 as a catalyst, solvents are not required as it also functions as the medium. The condensation reaction proceeds with a variety of aromatic amines and different isatins with excellent yields (88%-95%) following the chromophore approach.
All dyes 34 are deep red with broad absorption bands with maxima between 264 and 274 nm and they show strong fluorescence in methanol in a range of 282-596 nm with large Stokes shifts (Δṽ = 11,000-13,000 cm −1 ).
All dyes 35 and 36 (λ max,abs = 239-287 nm) exhibit fluorescence in methanol in a range from 366 to 417 nm with large Stokes shifts (Δṽ = 8,800-25,000 cm −1 ). In addition, compound 35a is the strongest antibacterial agent against Staphylococcus aureus and Bacillus subtilis in this series.
Hybridizing several biologically active moieties in a single molecule via MCR also applies to chromophore 37, a scaffold consisting of oxindole, pyrrole, and pyrazole (Jamwal et al., 2013;Karrouchi et al., 2018). Oxindole bearing pyrrolo[2,3-c]pyrazole 37 is prepared by an acid-promoted sequential three-component reaction between benzoylacetonitriles, phenylhydrazine, and 3phenacylideneoxindoles in the sense of the chromophore approach (Scheme 10B) (Nazeri et al., 2020). In the first step, phenylhydrazine reacts with benzoylacetonitrile via cyclocondensation. This is followed by Michael addition with 3phenacylideneoxindoles and concluded by cyclocondensation to give the desired products.
Absorption maxima of the violet fluorescent compounds 37 can be detected at around 220 and 355 nm. Electron-donating groups on the aromatic ring of the pyrrolo[2,3-c]pyrazoles display a slight redshift causing a decrease of the absorption and photoluminescence intensity.
Isoindoles are likewise important scaffolds of natural products and pharmaceuticals (Speck and Magauer, 2013;Kaur Bhatia, 2017;Csende and Porkoláb, 2018). The 3-substituted isoindolinone derivatives 38 and 39 have potential as cell sensors or drug carriers. The reaction proceeds as a Lewis acid-catalyzed, solidphase MCR between chiral β-keto lactam, an aldehyde, an isocyanide, and a dienophile mediated by microwave energy. The The 3-substituted isoindolinone derivatives 38 and 39 display significant fluorescence with large Stokes shifts (Δṽ = 3,900 nm).
Frontiers in Chemistry frontiersin.org 14 (ε = 5,300 L mol −1 cm −1 ) and an emission maximum of 542 nm (Φ F = 0.3). Low cytotoxicity, water solubility, and rapid cell penetration of dyes 39a and 39b make them promising candidates as molecular probes for cell sensing and cellpenetrating transport agents.
Intensive blue and greenish luminesce can by observed for several dyes 40 in solution at low concentration (Supplementary Material S8).
The UV/Vis spectra of the 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones 40 display two distinct absorption maxima in a range from 259 to 274 nm (ε ≈ 55,000 L mol −1 cm −1 ) and 359-379 nm (ε ≈ 3,500 L mol −1 cm −1 ). Electron-donating substituents R 1 shift the absorption and emission maxima to longer wavelengths, accompanied by a significant increase in luminescence as seen for dye 40b (Supplementary Material S8). Suppression of luminescence can also be caused by free rotation of the aryl substituent on the imide. Therefore, 4-aryl-1H-benzo[f] isoindole-1,3(2H)-diones 40 also possess AIE, where the intramolecular motion is suppressed upon aggregation. For instance, the fluorescence quantum yield of compound 40a thereby increases more than eightfold in the solid state compared to emission in solution.
Another approach to access chromophores with an isoindole core proceeds via sequential MCR of the Knoevenagel adduct of ninhydrin and malononitrile, isocyanide, amine, and elemental selenium. The iodide-catalyzed process very likely involves a radical ring enlargement and furnishes luminescent benzooxazino-isoindole derivates 41 under mild reaction condition according to the chromophore concept (Scheme 11B) (Sedighian  (Sedighian et al., 2021).
Frontiers in Chemistry frontiersin.org 16 of dyes 41 with hydrogen bond accepting solvents, lowering the ground state and increasing the excited state energies of the chromophores, resulting in a hypsochromic shift (Zakerhamidi and Sorkhabi, 2015).
Both pyridoindolizines exhibit strong green fluorescence with maxima in a range from 448 to 490 nm and with quantum yields of up to 0.81.
An interesting sustainable approach is the three-step one-pot synthesis of α-acyloxy carboxamides 44 by Passerini reaction in sodium phosphate buffer solution as a medium. The three steps of the sequence starting from salicylic alcohol derivatives include a Trametes versicolor laccase catalyzed aerobical oxidation, aldol condensation and 6-O-glucose ester promoted Passerini MCR. By modification of alcohols, aldehydes, or isocyanides, α-acyloxy carboxamides containing a coumarin scaffold 44 are obtained in variable yields (Scheme 12A) (Paprocki et al., 2020). The synthesis can also be started with the third step employing coumarin-3carboxylic acids as starting materials in the terminal Passerini reaction.
Another approach to produce coumarin-based dyes proceeds via condensation of 3-acetyl coumarin with acetyl chloride, an aromatic aldehyde, and acetonitrile to obtain blue light emitting fluorescent dyes named 'Beta Fluors' 45 (Supplementary Material S10) (Soumya et al., 2014). Phenyl boronic acid acts as a green catalyst for this reaction.
All chromophores 45 absorb with longest wavelength maxima between exhibit 303-346 nm and emit blue light from λ max,em = 382-436 nm with large Stokes shifts.
The absorption and emission spectra of the prepared CPC 46 are located in the range of 451-460 nm and 532-549 nm, respectively. Apart from the large Stokes shift, the molecule exhibits positive solvatochromic behavior. The peak at 490-510 nm in the solvatochromic absorption spectrum can be attributed to intramolecular charge transfer (ICT) from the donor (coumarin building block) to the acceptor (pyridone building block). Due to its special photophysical and electrochemical properties, this could be utilized as a fluorescent labeling agent not only for the visualization of latent fingerprints on various surfaces, but also as a detection of nitrite (NO 2 ) via cyclic voltammetry (CV) and chronoamperometry (CA).
The synthesized chromophores 47 exhibit blue fluorescent properties, which can be slightly affected by the electronic properties of the aryl residue on the dihydropyrimidinone moiety, although there is no direct conjugation with the coumarin core. Based on the internal charge transfer process, the 3,4-dihydropyrimidin-2(1H)-one/thione, are promising candidates for novel chemical and biological probes in addition as useful pH indicators. As a selective Hg 2+ chemosensor, chromenone-pyrazoles derivatives 48a and 48b can be applied, which are formed via a solvent-free one-pot sequence of salicylaldehyde derivative, 4-hydroxy-6-methyl-2H-pyran-2-one and hydrazine formed using SrFe 12 O 19 as a catalyst. Through an one-pot multicomponent reaction using SrFe 12 O 19 chromenone-pyrazole derivatives can be synthesized (Supplementary Material S12) (Ziarani et al., 2022).
The fluorescence properties of the prepared chromenone-pyrazole derivatives 48 are strongly determined by the residue on the pyrazole group. Thus, only the prepared non-aromatic introduced residues reveal fluorescence properties. The fluorescence emission of 48a and 48b in ethanol are at λ ex = 348 nm and λ ex = 300 nm, respectively.
Most HTC derivatives 49 display bright fluorescence in chloroform (λ max,em = 409-511 nm) with large Stokes shifts. Electron-donating groups in para/meta positions of the aromatic ring stemming from aldehydes/ketones result in bathochromic shifts. Furthermore, the electrochemical properties indicate that these compounds can be used as hole transporting materials.

Frontiers in Chemistry frontiersin.org
Fluorescence of compounds 50 is detected in the visible blue to green region. The chromophores 50 exhibit a bathochromic shift for both emission and absorption maxima with increasing solvent polarity. In non-protic solvents, the emission maxima is redshifted due to an intermolecular charge transfer process (ICT).
Chromenopyridinones derivatives are also bioactive (Fayed et al., 2021) and find application in the treatment of bronchial asthma (Ukawa et al., 1985) and they show anticancer activities (Bizarro et al., 2018). Furthermore, these heterocycles can be applied for the construction of chemosensors (Kozhevnikov et al., 2003). The one-pot three-component condensation of 4hydroxycoumarins with ammonium acetate and 3formylchromones gives highly substituted chromenopyridinone derivatives 51 in an environmentally benign solid-state melt reaction catalyzed by L-proline (Supplementary Material S14) (Paul and Lee, 2016). Mechanistically, the process can be rationalized by L-proline catalyzed Knoevenagel condensation of 4-hydroxycoumarin and 3-formylchromones via an iminium ion intermediate, followed by cyclocondensation with NH 4 OAc. The product 50 is thus obtained by a ring opening-ring closing sequence. The process follows the chromophore concept and produces the dyes in high yields (75%-93%).
Dye 52a strongly emits in the range of 450-550 nm and the highest fluorescence intensity was detected in MeOH and the lowest in non-polar solvents. The methyl group in the 6-position of the coumarin ring accounts for the strong emission of component 52a. In addition, electron-donating components as R 1 and R 2 groups are instrumental for ICT.
All dye 53-55 show blue to green fluorescence with broad maxima upon excitation at 290 nm. Furthermore, the structures of the derivates 53a and 54a exhibit interesting, exchangeable, intramolecular H-bonding, which open an access to new phenol containing pharmacophores.
The absorption spectra of dyes 56 and 57 are characterized by two to three distinct maxima, with the longest wavelength absorptions in a range from 414 to 422 nm. The emission maxima of the chromophores 56 and 57 can be detected at wavelengths ranging from 462 to 498 nm and 450-468 nm, respectively. Most dyes 56 give fluorescence quantum yields ranging from less than 0.01 up to 0.10, as also the chromphores 57 reveal quantum yields of around or less than 0.1. The compounds 56a and 57a have shown to be suitable for the detection of Fe(III) and Cu(II) ions and can thus be used as chemical sensors.
An approach to increase sustainability of common reactions is the use of solvent-free syntheses to reduce organic solvents. As mentioned before, the multicomponent synthesis of fluorescent 4H-chromene derivatives 58 can be mediated by amberlite IRA-400 Cl resin, which functions both as the solvent and the reusable catalyst (Supplementary Material S17) (Harichandran et al., 2017). 2-Hydroxybenzaldehydes, 1,3-diketones, and nucleophiles (Nu) react involving by Knoevenagel condensation and subsequent Michael addition in excellent yields.
The UV/Vis spectra of the 4H-chromen dyes 58 display absorption maxima ranging from 252 to 394 nm and emission maxima in a range from 405 to 462 nm, blue fluorescence. Chromophore 58b has been found to be the best fluorophore in the series. Compounds 58a and 58b are the only derivatives containing an electron donating amino group at the R 1 position. These chromophores exhibit smaller Stokes shifts and higher fluorescence quantum yield (Δ] ≈ 6,400 cm −1 , Φ F = 0.03-0.09) than the other congeners.
Depending on the selection of the donor and acceptor components, the photophysical properties of the 8-donor-5acceptor-substituted psoralens 59 and 60 can be fine-tuned. The optical properties of the donor-acceptor psoralens are characterized by large Stokes shifts, partially high fluorescence quantum yield in solution, and strong solid-state fluorescence. Of particular interest are observed solvatochromism, acidochromism, and aggregationinduced emission (Supplementary Material S18). In addition, the pronounced charge transfer character of the longest wavelength absorption band is confirmed experimentally and computationally.
Fluoresceins can also be categorized as xanthene dyes. These are characterized by interesting photophysical properties such as high molar absorptivity, high fluorescence quantum yields, and high photostability. Typically, fluoresceins absorb in the range from 400 to 700 nm (Sjöback et al., 1995;Song et al., 1998). Depending on the pH value, fluorescein is present as an anion, cation, or in its neutral form, which influences the photophysical properties (Martin and Lindqvist, 1975;Diehl and Markuszewski, 1989). This pH dependency can be exploited for the application as an indicator . The UV/Vis spectra of the fluorescein derivatives 62 were measured in NaOH so that the fluorescein derivatives are present in the dianionic form (pH > 8) (Sacoman Torquato da . The dianionic form shows longest wavelength absorption and emission maxima and highest fluorescence quantum yields due to its conjugated system. The absorption (λ max,abs = 476-512 nm) and emission (λ max,em = 515-525 nm) of dyes 62 lie in the typical range of fluorescein and high fluorescence quantum yields are realized (Φ F = 0.60-0.93). Derivatives containing halogens show a bathochromic shift in emission and absorption behavior as well as a decrease in fluorescence intensity. The fluorescein dye derivatives can be formed by a pseudo threecomponent reaction of phenol and phthalic anhydride derivatives in the sense of the chromophore approach (Scheme 14B). This MCR can be catalyzed by the electrophilic NbCl 5 . The reaction proceeds in two steps via twofold Friedel-Crafts reaction and subsequent carbonyl addition. In addition, the interesting photophysical of fluoresceins 62 make them suitable for application in DSSC.

Nitrogen heterocycles
Pyridine (azine) is found in many bioactive natural products, such as vitamin B 6 (Hill et al., 1996) or nicotinamide adenine dinucleotide, which is a pivotal coenzyme in the metabolism (Noctor et al., 2006). Furthermore, pyridine is an important synthetic building block for pharmaceuticals (Chang et al., 2005;Reddy et al., 2006;Altaf et al., 2015;Nagham Mahmood Aljamali, 2021). Pyridine is an excellent moiety for the assembly of fluorescent compounds due to its strong electron withdrawing properties, good rigid structure, and strong coordination ability (Xie et al., 2021). Numerous chemosensors with a pyridine core are reported (Hirano et al., 2000;Jiang and Guo, 2004;Machado et al., 2014;Ma et al., 2017). For instance, a fluorescent chemosensors can be used for the detection of Fe(III) and Hg(II) ions based on an ON-OFF mechanism.
The coordination of Fe(III) ions and 2-aminopyridine-based compound 63 results in an increase of the absorption intensity of both bands λ max,abs = 246 and 335 nm. In contrast, Hg(II) ions lead to a decrease in absorption intensity with a slight bathochromic shift. In the presence of Fe(III) and Hg(II) ions, the fluorescence spectrum shows quenching of emission intensity by 81% and 55%, respectively. Quenching is very likely caused by the paramagnetic nature of the Fe(III) ions and by the heavy metal ion effect for Hg(II). Due to the small spectral shift upon emission decrease, photo-induced electron transfer (PET) is a plausible mechanism. It is noteworthy that selective quenching occurs only for Fe(III) and Hg(II) ions, which makes the chromophore a suitable chemosensor for these two metal ions. Iron is involved in many biological processes and its selective detection can be a valuable tool for biological studies (Sigel and Sigel, 1998;Que et al., 2008).
In general, the absorption band of imidazo[1,2-a]pyridine is located near 280 nm and an efficient fluorescence band is observed at 370 nm (Stasyuk et al., 2012). The emission properties of the dyes 64 are mainly affected by the amidine and aldehyde building blocks. Thus, the products starting from methyl 2-aminoisonicotinate, methyl 2-aminoisonicotinate, pyrimidin-2-amine, and pyrazin-2amine fluoresce intensively in solution. All the mentioned imidazo [1,2-a]pyridines exhibit intrinsic fluorescence with a broad range of colors and they are also promising for use in chemosensors. The fluorescence mainly depends on the electron-donating effect and the degree of conjugation of aldehyde moiety (Supplementary Material S20).
The GBB synthesis of imidazo[1,2-a]pyridines 65 proceeds under mild conditions and short reaction times giving good yields after simple non-aqueous workup (Supplementary Material S21) (Khan et al., 2012a;Khan et al., 2012b). In this variant of the three-component condensation, bromodimethylsulfonium bromide (BDMS) serves as the catalyst. Thereby, even sterically demanding amidines can be transformed with good yields. The released water from condensation reacts with BDMS to liberate HBr which protonates the imine. Mechanistically, the key step of the formation of imidazo[1,2-a]pyridines 65 is a [4 + 1] cycloaddition followed an aromatizing 1,3-H shift.
Absorption maxima of the fluorescent compounds 65 can be detected at around 250 and around 335 nm. A chlorine substituent at C 2 -position of the imidazo[1,2-a]pyridine results in a slight redshift of the absorption maxima. A 2,4-methoxy substituent on the aromatic ring shifts the emission maximum to a longer wavelength (λ max,em = 470 nm).
The obtained imidazo[1,2-a]pyridine derivatives 66 display a blue color under the UV lamp. Based on the aldehyde component, the fluorescence properties can be modified.
The pseudo three-component reaction of mucobromic acid and two molecules of a series of 2-substituted benzimidazoles provides benzo [4,5]imidazo[1,2-a]pyridine derivates 67 in moderate to good yields. Mechanistically, a nucleophilic substitution of benzimidazoles with mucobromic acid occurs in the presence of the base potassium carbonate. After decarboxylation, Michael addition of another benzimidazole is performed. With the aid of the base, a dehydrohalogenation reaction proceeds and the desired product is obtained by cyclization (Supplementary Material S23) (Yang et al., 2022). Via Michael addition, other NH-containing heterocyclic nucleophiles can also be introduced to the system. The introduced heterocyclic rings in C 1 position of benzo[4,5] imidazo[1,2-a]pyridine 67 lead to the cancellation of the coplanarity of the whole molecule and hinder the strong intermolecular π-π interaction and the tight π-π stacking between the neighboring molecules. These electrostatic interactions are able to constrain intramolecular motion in the solid state, causing an enhancement of fluorescence in the aggregation state. 67a could be used to detect picric acid, a nitroaromatic explosive.
Pyrazole[3,4-b]thieno[2,3-e]pyridines 68 contain pyrazoles and thienyl moieties fused to a central pyridine core and can be accessed by Hantzsch dihydropyridine synthesis followed by a solvent-free oxidative aromatization with H 2 O 2 (Scheme 15B) (Yao et al., 2014). The aromatic substituent of the aldehyde can be electronwithdrawing or electron-releasing and it influences the emission behavior. The dyes 68 feature a blue to green fluorescent donor-πconjugated acceptor system with emission maxima in a range from 430 to 505 nm.
The dyes fluoresce in a range of 434-464 nm with quantum yields of 0.10-0.17. The fluorescence is increased by electrondonating groups (69c) due to the conjugation of the system, Frontiers in Chemistry frontiersin.org where electron-withdrawing groups (69d) diminish the fluorescence.
Highly substituted pyridines can be formed via potassium hydroxide catalyzed pseudo four-component reaction of aldehydes, malononitrile and nucleophiles, such as thiols, alcohols or amines in the sense of the chromophore concept (Scheme 15C) (Khan et al., 2012b). Mechanistically, one molecule of malononitrile reacts via a Knoevenagel condensation with aldehyde and the other malononitrile undergoes a Michael addition with the previously formed electrophile followed by a concomitant nucleophilic addition to the cyano-substituent of the adduct. The functionalized 2-amino pyridines 70 are formed via cyclization and oxidative aromatization.
The UV/Vis spectra of 2-amino-3,5-dicarbonitrile-6-thio-pyridines 70 display absorption maxima in a range from 329 to 356 nm and emission maxima in a range from 386 to 451 nm. The highest quantum yield in comparison to other thio-pyridine derivatives (Φ F = 0.00-0.05) is detected for compound 70a (Φ F = 0.18), which possesses a strongly electron-donating group at the C 4 aryl substituent of the pyridine ring. The amino and oxo pyridine derivatives exhibit quantum yields of 0.27-0.57 and 0.10 to 0.14, respectively. The chromophores are potential candidates as new fluorescent probes or luminescence materials. Furthermore, it was reported that when sterically hindered aldehydes are used not the expected thio-pyridine derivatives but the corresponding 1,4-dihydropyridines (DHPs) were obtained.
1,4-Dihydropyridines 71 and 72 are formed via catalyst-free MCR of amine hydrochloride salts or ammonium chloride, aldehydes, and acetals (Scheme 16A) (Sueki et al., 2014). Ammonium chloride and 3,3-diethoxypropionate activated by protons react to give an imine intermediate that tautomerizes to the enamine. This enamine intermediate forms with an aldehyde upon elimination of ethanol an α,β-unsaturated imine. Subsequently, the enamine intermediate reacts with the α,βunsaturated imine via Michael anellation to form tetrahydropyridine. Upon elimination of ammonia 1,4-DHP is generated. Based on the two approaches A and B, a wide range of various 3,4,5-trisubstituted 1,4-DHPs with ethoxycarbonyl groups at the 3-and 5-positions 71 can be formed (Scheme 16A I). Further modification can be achieved by changing the ethoxycarbonylgroups to other electron-withdrawing groups (EWGs) and the corresponding 3,4,5-substituted 1,4-DHPs 71 are obtained in moderate to good yields in the sense of the chromophore concept (Scheme 16A II).
The MCR products 71 and 72 exhibit fluorescence in the range from 403 to 542 nm and quantum yields up to 0.94. 1,4-DHPs with extended π-conjugation exhibit emission maxima at longer wavelengths. Photophysical properties such as the fluorescence wavelength and the quantum yield can be varied by modifying the substituents of the 1,4-DHPs.
All chromophores 73 display three distinct absorption maxima in the UV/Vis spectra, the first around λ max,abs = 230 nm, the second around 255 nm, and the third around 365 nm. Also, the 1,2,3triazole-linked pentasubstituted 1,4-DHPs show strong fluorescence in solution with emission maxima between 439 and 451 nm. It is worth noting that the substituent on the phenyl ring has only a minor influence on the fluorescence properties. Moreover, the compounds 73 are proven to possess antibacterial, antifungal and antioxidant properties.
The In/SiO 2 catalyzed Hantzsch reaction of various substituted aryl aldehydes provides access to blue-green fluorescent 1,4-DHPs 74 (Scheme 16B) (Affeldt et al., 2012) The UV/Vis spectra of 74 exhibit the absorption maxima at around 350 nm. The emission maxima can be detected in a range of 402-516 nm with large Stokes shifts ranging from 4,200 to 11,000 cm −1 . In particular, for derivatives 74a the largest Stokes shift is measured, due to intramolecular charge transfer mechanism in the excited state from the dimethylaniline to the dihydropyridine chromophores.
The three synthesized 1,4-DHPS 75, 76 and 77 display absorption in the UV region and emission in the blue-green region.
The photophysical study reveals that the hydroxyphenylbenzoxazole and DHP fluorophores in the hydroxyphenylbenzoxazole-DHP structure behave independently after excitation. In addition, ESIPT emission can be observed.
The  -Sirjani et al., 2018). The reaction is catalyzed by both polyoxometalate anions and cations. The cations activate the carbonyl groups in aromatic aldehydes and dimedones due to their Lewis acidic nature and the anions abstract the α-proton of the dimedone furnishing 1,8-dioxodecahydroacridines 78 in excellent yields based on the chromophore concept (Scheme 16C).
The emission spectra of 78c recorded in different solvents and at different temperatures exhibit a maximum at around 520 nm under all conditions. Based on a pseudo five-component reaction of cyclopentanone, two molecules of aromatic aldehyde, N-pyridinium substituted ortho-hydroxyaryl methyl ketone and ammonium acetate 2-   (Batalin et al., 2021). For the aromatic aldehyde either benzaldehyde or up to 3 methoxy groups with various substitution patterns on the aromatic ring were furnished, which influenced the yields and the amount of pyridium salt added. The reaction proceeds via a modified Kröhnke reaction (Yan et al., 2007), in which the first step is an aldol condensation between the cyclopentanone and the aromatic aldehyde in the presence of ammonium acetate. The resulting E-cross conjugated dienone undergoes a Michael reaction with the pyridime salt and forms the intermediate illustrated in the Supplementary Material S27, which after elimination of the pyridinium cation forms the unsaturated 1,5-diketone. In the final step, the nucleophilic addition of ammonia leads to the compounds 79 or 80.
The derivatives exhibit interesting fluorescence properties such as excited-state intramolecular proton transfer (ESIPT) in solution. Excitation maxima are measured at 374 nm and emission maxima at 434 nm under neutral conditions. In acidic conditions, excitation maxima and emission maxima for compounds 79 and 80 are found at 332-476 nm and 484-606 nm, respectively. An increase in acid concentration results in either an increase in fluorescence intensity or quenching of fluorescence in the case of newly formed fluorophores in solution, according to the specific derivative. In the crystalline state, double emission due to ESIPT was observed with fluorescence maxima of the enolimine tautomers ranging from 414 to 426 nm. The fluorescence maxima of the keto amine tautomers are within the region of 594-624 nm with high Stokes shifts.
Pyrimidine derivatives display biological and pharmacological characteristics, including antitumor (Atwal et al., 1989;El-Subbagh et al., 2000), antibacterial (Cieplik et al., 2011;Selvam et al., 2015) and anticancer (Joshi et al., 2016;Mangal and Jangid, 2016) properties. Pyrimidine nucleosides as well as alkaloids and antibiotics have been isolated from natural sources (Lagoja, 2005). Access to different pyrimidine derivatives is possible for example via Biginelli MCR of aldehydes, urea and methylene active compounds under dielectric heating (Dabiri et al., 2007;Tu et al., 2009). In addition, some chromophores with a pyrimidine core show interesting photophysical properties. C 6 unsubstituted tetrahydropyrimidines 81 exhibit aggregationinduced emission enhancement (AIEE) and size-independent emission (SIE) characteristics . They emit blue or green fluorescence in aggregates (λ max,em = 399-491 nm) with fluorescence quantum yields of up to 0.93. For derivatives with aromatic substituents in the R 2 and R 4 position, a strong emission in aggregates can be observed compared to substituents with alkyl groups. The derivative 81a shows SIE characteristics since emission maximum of suspension particles, powder, film and crystals are identical at and appear at 434 and 484 nm, respectively. The synthesis is performed under mild conditions and catalyzed by the organocatalyst urea. The five-component reaction allows the construction of a substance library in which all starting materials except aldehyde are varied giving yields between 21% and 63% in the sense of the chromophore approach (Scheme 17A). The reaction contains four elementary steps starting with an aminovinylation, followed by aza-ene-type reaction, nucleophilic addition, and cyclization.
The 3,4-dihydropyrimidin-2-(1H)-ones derivative 82 display well-defined color regions such as red, orange and yellow with good intensity. The UV-visible absorption spectra in DMF exhibit absorption maxima in the region from 399 to 493 nm.
The synthesized dihydropyrimidines derivatives 83 exhibit fluorescence in solution with large Stokes shifts, due to a proton transfer process. Moreover, fluorophores reveal double fluorescence emission. The emission at short wavelengths is attributed to the excited enol forms and at longer wavelengths to the tautomeric species, which can be associated with the intramolecular excited state proton transfer process (ESIPT). Further studies confirm a cytotoxic activity of the compounds and their potential application as fluorescence probes.
Pyrimidines can also be employed as metal ion sensors. For example, an acetonitrile solution of 2-arylthieno[2,3-d]pyrimidyl-4amine 84d changes color from colorless to yellow at higher concentrations of Pd(II) ions, which is visible to the naked eye. The absorption spectrum reveals a decrease of the three bands of the dye (λ max,abs = 221, 244 and 332 nm) on expense of a new band with an isosbestic point at 362 nm in presence Pd(II) ions. The synthesis of 2-arylthieno[2,3-d]pyrimidin-4-amines 84 proceeds via a modified Gewald reaction (Gewald et al., 1966) with four components, where the additional component exploits the reactivity of the two neighboring functional groups of the classical three-component product. The starting materials are various α-methylene bearing ketones, malononitrile, aryl or heteroarylnitrile derivatives and elemental sulfur, which form functionalized products in good to excellent yields in the sense of the chromophore approach (Scheme 17C) (Abaee et al., 2017).
The furo[2,3-d]pyrimidines-2,4[1H,3H]-diones 85 display dual channel sensing of Cu(II) ions in solution and in the membrane phase. The electroanalytic study exhibits an ion selective electrode response toward Cu(II) ion in membrane phase. In the presence of Cu(II) ions, the absorption maximum decreases at the longest wavelength of the four absorption maxima (λ max,abs = 210, 271, 325 and 375 nm) and results in a decolorization of the initially yellow solution. The emission spectra are not affected by Cu(II) ions. The proposed complex based on the 1 H NMR study shows that the furan oxygen atom and the alkylamino NH group are directly involved in the coordination of Cu(II) ions (Supplementary Frontiers in Chemistry frontiersin.org Material S30). The coordination can be cleaved by sequestering agents such as EDTA. The chemical sensor is recoverable and efficiently reused several times.
All pyrazolo[1,5-a]pyrimidines display intense fluorescence. The compounds 86 show absorption maxima between 267 and 296 nm and emission maxima between 304 and 332 nm. The amino group at the C 7 -position of compound 87 enhances the optical properties (λ max,abs = 336-360 nm, λ max,em = 393-414 nm) in comparison to the aryl group of compounds 86. If dimedone is replaced by benzoylacetonitriles in the synthesis, pyrazolo[1,5-a] quinazolines can be prepared through cyclocondensation in refluxing toluene.
The tetracycles 88 and 89 show strong fluorescence in EtOH upon irradiation with UV light (λ exc = 360 nm), also with high fluorescence quantum yields. Moreover, emissions in the visible, allow applications as fluorescent probes, OLEDs, or luminescent materials. In fact, naphthyridine derivatives have already been used as luminescent materials for molecular recognition due to their planar rigid structure (Peng et al., 2005;Lu et al., 2006).
Nitroquinolines 90 can be produced by reacting p-nitroaniline, benzaldehydes and phenyl acetylene in presence of the Lewis acid niobium pentachloride under mild conditions (Supplementary Material S32) (dos Santos et al., 2017).
The optical properties of the compounds 90 can be altered by substituents on the benzaldehyde. The two absorption bands appear in a range of 250-280 nm and 325-393 nm, ascribed to the π-π*transition and n-π*-transition. Electron-donating substituents cause a bathochromic shift in the absorption spectra. The nitroquinolines can be reduced with hydrazine monohydrate in the presence of 10% Pd/C to give aminoquinolines, which exhibit high quantum yields up to 0.83.
If ketones are used instead of aldehydes in the three-component reaction in the sense of the scaffold approach, access to substance libraries of highly conjugated, fluorescent spirofluorenonaphthoquinolines 91 are formed (Scheme 18B) (Meerakrishna et al., 2016). Using aryl alkynes with electrondonating groups in the copper-catalyzed reaction of ketones, alkynes, and amines (KA 2 coupling) lead to higher yields due to the increased nucleophilicity of the copper acetylide. Mechanistically, the reaction proceeds by Cu(I)-catalyzed nucleophilic addition of phenylacetylide to 9-fluorenone to give a propargyl alcohol, which reacts with aminoanthraquinone forming an amino allene that undergoes intramolecular arylation and subsequent aromatization. Structurally different spirofluorenophenalenoquinoline derivatives 92 are obtained by using 1-aminopyrene as the amino component in an analogous reaction.
The absorption spectra of chromophores 91 are generally characterized by an absorption maximum between 537 and 614 nm, whereas dye 92 shows two absorption maxima 391 and 412 nm. The compounds 91 exhibit orange-red fluorescence, while pyrenospirofluoreno-naphthoquinoline 92 shows emission in the deep blue region with the highest Stokes shifts (Δ] = 3,738 cm −1 ). The emissions of 91 are accompanied with low quantum yields presumably caused by the ketone bridge (Usta et al., 2009;Jacques et al., 2014).
All isatin-based spiro compounds 93 show similar absorption and emission spectra. A red shift of the emission maxima can be observed by increasing the solvent polarity and fluorescence quantum yields and lifetimes are also affected by the polarity of the solvent. The solvatochromism is characteristic for quinolines (Mataga and Tsuno, 1957;Atkinson and Speakman, 1971). Furthermore, the isatin-based spiro compounds 93 can be used as an ON-OFF switch chemosensor for Cu(II) ions.
A three-component reaction of 4-hydroxycoumarin, aldehydes and primary amines produces a huge substance library of quinoline chromophores 94 (Scheme 19A) (Ataee-Kachouei et al., 2019). The synthesis of chromeno[4,3-b]quinolin-6-ones and their symmetrical and unsymmetrical dyes 95-98 is catalyzed by naturally occurring halloysite nanotube (HNT) with the general formula of (Al 2 (OH) 4 Si 2 O 5 ·2 H 2 O) (Rawtani and Agrawal, 2012). The solvent-free conditions, excellent yields, short reaction times and the low-cost, environmentally friendly and reusable catalyst characterize this reaction procedure as a green approach. The fluorophores 94-98 fluoresce blue and green and emission maxima can be detected in a range from 400 to 535 nm.
The absorption maxima of the hexahydroquinolin-5-ones 101 can be detected at around 365 nm and emission at around 450 nm. The compounds 101 exhibit high Stokes shifts ranging from 4,800 to 5,800 cm −1 and moderate quantum yields up to 0.28. In addition, some of the synthesized hexahydroquinolines show anticancer activities.
The intramolecular charge transfer band in the absorption spectrum and emission solvatochromicity of the donor-acceptor quinoline dyes 102 account for polar excited states. The quinolinebased chromophores 102a and 102b achieve quantum yields of up to 0.40 and 0.59, respectively.
The UV/Vis spectra of indol-3-yl substituted dyes 103 show similar absorption and emission maxima at around 354 and 415 nm, respectively. The highest fluorescence quantum yields (Φ F = 0.42-0.70) are obtained in polar and protic solvents. In addition, the compounds 103 undergo reversible fluorescence quenching under acidic conditions. These optical properties result from the localized electron density of the frontier orbitals on the isoquinolinamine moiety and the equal energy gaps of the associated frontier molecular orbitals.
The electron properties of the 3,10-diaryl 10H-phenothiazines derivatives 104 can be fine-tuned by varying electronic substituents on the phenothiazine that extend the π-electron conjugation and the fundamental conformational change from intra-to extraconfiguration. Using an elaborated structure-property relationship 3D diagram based on the correlation between the first oxidation potential (E 0/+1 0 ) and the Hammett parameters σ + p (R 1 ) and σ p (R 2 ), a prediction or tailoring of the oxidation potential is possible. The modification of the electronic substituents allows to tune the emission color over the entire spectral range from blue to red.
Placing the heterocycle 1,3,5-triazine instead of the aryl radical in 3position, also a considerable color spectrum can be covered by modulating the acceptor strength of the triazine moiety in the solid state under excitation under UV light (λ exc = 365 nm). The triazine building blocks have immense importance due to their extraordinary biological activities, especially in chemical medicine (Verma et al., 2020). Synthetically, after brominelithium exchange of the starting component 3-bromo-N-anisyl-phenothiazine with BuLi and introduction of the trichloro-1,3,5-triazine core by subsequent double nucleophilic substitution, the phenothiazine-triazine chromophores 105 can be obtained (Scheme 20B) (Kloeters et al., 2022).
In general, compounds 105 show high fluorescence quantum yields in solution and in the solid state. By tuning the substitution pattern on the triazine, photophysical properties such as thermally activated delayed fluorescence (TADF) and white light emission can occure. Thus, solvatochromism studies of electron-deficient substituted triazine chromophores reveal strong charge transfer character and a small singlet-triplet energy gap, hence these derivatives in particular 105a identify as TADF candidates. While the electron rich triazine dyes show a reversible shift of the spectral emission upon protonation. Furthermore, a white light emission can be observed for derivatives 105d (Supplementary Material S38).
The absorption spectra of compounds 106 exhibit four absorption bands. Three appear at shorter wavelengths, which can be assigned to the phenothiazinyl units, and the longest wavelength maximum can be attributed to the central 2,5-di (hetero)aryl-substituted thiophene moiety. The molar decadic extinction coefficient increases with the number of phenothiazinyl units. The thienyl-bridged oligophenothiazines 106 emit in a wavelength range from 506 to 521 nm with large Stokes shifts between 4,800 and 5,600 cm −1 , Frontiers in Chemistry frontiersin.org which are characteristic for oligophenothiazines (Sailer et al., 2008). Fluorescence quantum yields of chromophores 106 range from 0.15 to 0.18. A cathodic shift of oxidation potentials is observed for the series with increasing number of phenothiazinyl electrophore units. A consistently reversible oxidation range can be demonstrated for compound 106c. Molecular modelling reveals lowest energy conformers that exhibit a sigmoidal and helical structure. TD-DFT calculations and even semiempirical ZINDO (Zerner's intermediate neglect of differential overlap) calculations confirm the trends of the absorption bands with the longest wavelengths. Thus, the charge transfer can be largely assigned between the electrophore moieties from the neighboring phenothiazinyl moieties to the central thienyl unit. Diacceptor substituted phenothiazine 107 can be accessed via a lithium formylation-Knoevenagel condensation (LiForK) sequence . The consecutive pseudo SCHEME 20 (A) Sequentially Pd-catalyzed arylation-amination consecutive three-component synthesis of 3,10-diaryl 10H-phenothiazines 104 (TRZ = 2,4diphenyl-1,3,5-triazine) (Mayer et al., 2020;Mayer and Müller, 2021). (B) One-pot sequence to synthesize disubstituted phenothiazine-triazine dyes 105 (Kloeters et al., 2022). (C) Pseudo five-component Sonogashira-Glaser cyclization synthesis of thienyl-bridged oligophenothiazines 106 (Urselmann et al., 2016). (D) One-pot LiForK synthesis of a 3,7-diacceptor substituted phenothiazine 107 .
The interactions between the substituents in the dithieno [1,4]thiazines (108 and 109) is stronger than in the corresponding phenothiazine 107 since dithieno [1,4]thiazines are generally higher polarizable. Dithieno [1,4]thiazines 108 and 109 are characterized by cathodically shifted oxidation potentials and red-shifted, more intense absorption bands compared to the corresponding phenothiazines 107. The analysis of the structure-property relationships points out that the photophysical and electrochemical properties as well as the electronic structure are significantly determined by the thiophene anellation mode of the products 108 and 109. Thus, strong acceptors in syn-syn-dithieno[1,4]thiazines 108 and 110a possess a rather folded structure as well as weak fluorescence (Φ F = 0.01). In contrast, equally substituted anti-anti isomers 109 and 111a show an almost planar ground state geometry and very intense near-infrared fluorescence (Φ F = 0.52). In principle these red light or NIR emitters can be promising for potential application in biomedical imaging (Hong et al., 2017) or OLEDdevices (Qu et al., 2006).

(Hetero)Arene
Exciplexes (excited complexes) as well as excimers (excited dimers) are emitting charge transfer complexes, which are formed by excitation of one of the constituent chromophores, which collides with a second chromophore that is in the electronic ground state (Balzani, 2001;Balzani and Venturi, 2003). The Ugi four-component reaction provides an access to unimolecular exciplex emitting dyads 112 consisting of an N,Ndimethylaniline moiety as a donor and anthracene, naphthalene or pyrene as acceptor chromophores in the sense of the scaffold approach (Scheme 21A) (Ochs et al., 2019).
The chromophores of the donor-acceptor dyads 112 are electronically decoupled in the ground state and electronically coupled in the excited state. This results in exciplex emissions due to the polar nature of the excited state. The formation of the exciplex depends on partial electron transfer with matching redox potentials and on spatial proximity of the donor and acceptor units. Furthermore, the N,N-dimethylaniline-acceptor chromophores 112 are capable of photoinduced intramolecular electron transfer (PIET) and exhibit emission solvatochromism with red-shifted emission upon increasing solvent polarity (Supplementary Material S40). Based on TD-DFT calculations, the qualitative assignment of the experimental absorption and emission bands is possible.
Structurally diverse benzo [b]phospholes 114 are synthesized via facile regiocontrolled one-pot sequential coupling of an arylzinc reagent, an alkyne, dichlorophenylphosphane (or phosphorus trichloride and a Grignard reagent), and an oxidant (hydrogen peroxide or sulfur) (Scheme 21B) (Wu et al., 2014). For this MCR two common approaches with similar initiation steps can be applied. Cobaltcatalyzed migratory arylzincation forms the intermediate that reacts after transmetalation to an organic copper species with PhPCl 2 followed by oxidation with hydrogen peroxide or sulfur powder. Alternatively, the formation of the benzo[b]phosphole oxides or benzo[b]phosphole sulfides 114 proceeds by reaction of the copper species with PCl 3 , followed by addition of Grignard reagents and final oxidation.
Most benzo[b]phosphole derivatives 114, especially benzo [b]phosphole oxides, are fluorescent in solution. Electrondonating amino groups as substituents in the R 1 position led to a significant redshift. In general, the longest wavelength absorption maxima appear between 317 and 394 nm. The emission maxima are located between 385 and 484 nm with fluorescence quantum yields of up to 0.93. In general, benzo [b] phospholes possess interesting optoelectronic properties and find application in organic electronic devices (Tsuji et al., 2009;Tsuji et al., 2010).

Azo chromophores
Azo dyes have a long history among synthetic dyes and are used in a wide range of applications, including in cosmetic, textile and paper industries (Şener et al., 2006;Benkhaya et al., 2017;Benkhaya et al., 2020). Azo dyes are characterized by one or more azo bridges as integral chromophore. The common preparation method of azo dyes is the diazotization of an aromatic primary amine followed by coupling with one or more electron-rich π-nucleophiles (Gürses et al., 2016). Due to the great importance and application of this dye class, several green syntheses of this class of compounds have been explored (Safari and Zarnegar, 2015;Nikpassand and Pirdelzendeh, 2016;Nikpassand et al., 2018;Nikpassand, 2020). For instance, the microwave assisted threecomponent reaction of arylazopyrazoles, benzaldehydes and dimedone proceed via Mannich cyclization and condensation to form pyrazoloquinazolinone azo dyes 115 (Supplementary Material S42) (Elgemeie et al., 2015). The absorption maxima of azo dyes 115 appear Frontiers in Chemistry frontiersin.org between 380 and 498 nm. In the presence of a nitro aryl substituent on the azo part, a bathochromic shift of the absorption maxima can be observed. Azo pyrimido[4,5-b]quinoline derivatives 116 are prepared via an unsymmetrical Hantzsch synthesis with dimedone or 1,3cyclohexadione, azo aldehydes, and 6-amino-1,3-dimethyluracil in the presence of choline chloride/oxalic acid (ChCl/Oxa) as a green solvent and recyclable substance catalyst (Scheme 22A) (Gholami et al., 2020). The UV/Vis spectra exhibit two absorption bands. The longest wavelength absorptions are found between 352 and 362 nm.
Recently, a multicomponent synthesis of two boron Schiff bases 118a and 118b through the condensation reaction of 2hydroxynaphthaldehyde with the corresponding amines and in Frontiers in Chemistry frontiersin.org situ generated diphenylborinic acid has been reported (Scheme 23A) (Corona-Lopez et al., 2021). The longest wavelength absorption maxima can be detected between 396 and 404 nm. The emission maxima lie between 474 and 525 nm with large Stokes shifts, but low quantum yields. In addition, boron complexes 118a and 118b can be classified as semiconductors based on the determined optical band gaps E g between 2.57 and 2.78 eV for the n-π* electronic transitions of the substituent groups.
Further tetracoordinated boron atoms bearing Schiff bases 119 can be synthesized via a three-component condensation reaction starting from damnacanthal as a biogenic component in a remarkably rapid manner (Supplementary Material S43) (Garcia-Lopez et al., 2022).
The organoboron esters 119 are fluorescent and exhibit emission maxima at around 400 nm. Notably, the oscillator strength of the electronic transition can be influenced by the position of the substituent. Thus, the derivative with the nitro group substituent in 4-position displays a larger extinction coefficient. Similar to the organoboron complexes 118, the chromophores 119 also show low quantum yields around 0.01 and can also be classified as semiconductors due to the optical band gaps (E g ≈ 252 eV). As a result of the damnacanthal utilized as the starting material, 119 possess biological activity and in particular 119b showed cytotoxicity activity against MDA-MB-231 breast cancer cells.
A fast and efficient approach to chromophores applicable also in biological fields such as boron hydrazone dyes 120, can be achieved by microwave-assisted domino multicomponent condensation reaction of diverse aryl aldehydes, benzoylhydrazide, or 4nitrobenzoylhidrazine, and diphenyl boronic acid (Supplementary Material S44) (Molina-Paredes et al., 2019).
The UV/Vis spectra of boron complexes 120 exhibit either one or two absorption maxima and longest wavelength absorption bands can be detected in a range from 368 to 448 nm with molar extinction coefficients ε between 13,000 and 55,000 M −1 cm −1 . The emission maxima appear between 420 and 520 nm with moderate Stokes shifts (Δ] = 2,980-3,400 cm −1 ) for most complexes, except for dye 120c (Δ] = 7,330 cm −1 ). Although the organoboron dyes have low fluorescence quantum yields, they can be used to stain silk fibroin. Therefore, they can be employed in the development of scaffolds for tissue engineering due to their confirmed nontoxicity.
The emission and absorption of the fluorescent BODIPY dyes 123 lie in the typical range of BODIPY fluorophores and exhibit quantum yields of up to 0.61. The dyes 121 can be utilized as a fluorescent probe for in vivo imaging of phagocytosing macrophages.
Moreover, by varying the functional group of the BODIPY dye further substance libraries of BODIPY complexes can be obtained by MCR. The Passerini reaction of formyl-containing BODIPY derivatives with benzoic acid and t-butyl isocyanide yields highly substituted BODIPY dyes 125 and 126 (Supplementary Material S45) (Ramirez-Ornelas et al., 2016). The formyl-containing BODIPY complex are prepared via Liebeskind−Srogl crosscoupling or Vilsmeier reaction starting from Biellmann BODIPYs (Goud et al., 2006).
The photophysical properties of the BODIPY dyes are not affected by the ligations to the para position of the 8-phenyl group or to 2-position directly on the BODIPY core, but rather are influenced by the free motion of the 8-aryl. For example, boronic complexes containing an unhindered 8-phenyl (125a and 126a) show a low fluorescence response, due to free rotational motion of the ring. In contrast, high quantum yields (Φ F = 0.82) are detected for aryls with methylation at the ortho positions (126b), as mesityl groups prevent the aryl from rotating freely due to a higher rotational barrier. In addition, these complexes can be used to stain blood cells with very intense and stable signals at a very low exposure time.
Luminescence in metal complexes typically results from metalto-ligand charge transfer in the excited state (Fredericks et al., 1979;Striplin and Crosby, 1994). Therefore, their photophysical properties are characterized by high Stokes shifts and long luminescence lifetimes, which are especially essential in the bioanalytical field (Ma et al., 2014;Albada and Metzler-Nolte, 2016). For instance, organotin compounds derived from Schiff base complexes can be applied as analytical luminescent chemosensors for the identification of metals (Vinayak and Nayek, 2019). In addition, since these complexes are capable of staining silk fibroin, they can potentially be used as scaffolds for tissue engineering (Lara-Ceron et al., 2017). Organotin complexes with n-butyl (127) and phenyl residues (128) can be prepared by microwave-assisted three-component condensation reaction of 2hydroxy-1-naphthaldehyde, L-amino acids, and diorganotin oxides (Scheme 24A). Intrinsically fluorescent amino acids, such as tryptophan, tyrosine, and phenylalanine can be thereby implemented.
The UV/Vis and fluorescence spectra of compounds 127 and 128 display similar maxima. Two absorption maxima are observed at 420 nm, which can be assigned to n-π* transitions of the carboxylate and imine groups, and at 335 nm, which can be assigned to the π-π* intraligand charge transfer within the naphthyl segment. The absorption maximum of the π-π* transition also differs from the free ligands (λ max,abs = 303 nm). The bathochromic shift of the bands for the tin complexes can be attributed to azomethine-N→Sn coordination. The emission can be detected at a maximum of 465 nm fluorescence quantum yields ranging from 0.08 to 0.21.
By replacing the amine with just one primary amine group by a component with three reactive moieties, such as benzene-1,3,5-Frontiers in Chemistry frontiersin.org tricarbohydrazide, organotin compounds with a C 3 -symmetric Schiff base 129 with excellent yields can be generated (Scheme 24B) (Canton-Diaz et al., 2021). The photophysical properties of the two organotin complexes with three central tin atoms are similar to those with one central tin atom in which [N-(2-oxido-1naphthaldehyde)-4-hydroxybenzyhydrazidate] was employed (Jiménez-Pérez et al., 2015b). However, the extinction coefficient of 129a exhibits a larger value (ε = 68,400 M −1 cm −1 ) and thus a larger oscillator strength. Theoretical calculations attribute the S 0 →S 1 excitation to natural transition orbitals. The tin atoms do not interact electronically, as both organotin complexes show quantum yields in chloroform of approximately 0.52, with lifetimes of about 3 ns. Both complexes show green emission in solution.
Sn(IV)-porphyrins have an affinity for oxygen donor ligands (Arnold and Blok, 2004). Therefore, these complexes are suitable for the construction of axially coordination bound multiporphyrin arrays (Redman et al., 2001;Prodi et al., 2002;Scandola et al., 2006;Shetti et al., 2012). In a one-step procedure meso-pyridyl Sn(IV)-porphyrin, meso-hydroxyphenyl-21,23-dithiaporphyrin, and Ru(II)-porphyrin react to give Sn(IV)-porphyrin-based oligomers 130 with yields ranging from 60% to 70% (Supplementary Material S46) (Dvivedi et al., 2014). Here, the Ru(II)-porphyrins are coordinated as peripheral ligands to the meso-pyridyl group(s) of the Sn(IV)-porphyrin. The similar overlapping absorption bands of all three constituent monomers with slight shifts in their absorption maxima and the weak interaction of the porphyrin units in the oligomers observable in the electrochemical study indicate that the various porphyrin units of compound 130 act as decoupled supramolecular arrays. However, the steady state fluorescence study shows emission quenching in presence of the Ru(II)porphyrin units, presumably caused by strong spin-orbit coupling. SCHEME 25 (A) One-pot catalyst-free Ugi 4CR for the synthesis carboxamide-modified-metallophthalocyanines 131 and 132 (Afshari et al., 2019). (B) One-pot synthesis of fluorophore decorated macrocyclic peptides 133 (Rotstein et al., 2011).

Peptoidic dyes
Phthalocyanines are structurally related to porphyrins. This macrocyclic chromophore can be employed in Ugi 4CR in a scaffold approach to access sidechain modified metallophthalocyanines 131 and 132 (Scheme 25A) (Afshari et al., 2019). The electrochemical and optical properties as well as the solubility of phthalocyanines can be adjusted by rational tuning of the metal center as well as modification and functionalization of substrates. In the case of dye 131, where cobalt, copper, iron, nickel, and zinc ions are implemented as metal centers, the phthalocyanine acts as the carboxylic acid component in the Ugi reaction. For the synthesis of the cobalt complex 132, tetra-amino cobalt (II) phthalocyanine acts as the starting material, underling the substrate diversity of the synthetic route.
The phthalocyanines containing carboxamide moieties 131 and 132 are characterized by high stability, diminished aggregation and increased of monomerization, resulting in good solubilty of the metallophthalocyanines in common solvents, especially in water.
Peptides perform a wide range of physiological functions in the human organism and possess biochemical properties that are valuable for drug development (Sánchez and Vázquez, 2017). Therefore, synthetic peptides have found numerous applications in the cellular arena and tagging them with fluorophores makes them suitable fluorescent imaging agents and activity-based probes (Blum et al., 2005;Loving and Imperiali, 2009;Sainlos et al., 2009;Baumes et al., 2010;Kwan et al., 2011). However, peptides have limited stability against proteolysis in vivo (Henninot et al., 2018;Lee et al., 2019). Cyclic peptides possess higher in vivo proteolytic stability and better cellular permeability (Matsuzaki et al., 1997;Gudmundsson et al., 1999;Fletcher et al., 2008). A possible synthetic route to fluorophore decorated macrocyclic peptides 133 is offered by the multicomponent reaction of peptides, aziridine aldehydes and isocyanides bearing a solvatochromic fluorophore (Scheme 25B) (Rotstein et al., 2011). The fluorescent peptide macrocycles 133 are fluorescent and their maximum emission incorporated di-and tripeptides lies in the range from 495 to 500 nm.

Conclusion and outlook
MCRs are a valuable tool for synthesizing functional organic chromophores with unique photophysical and electrochemical features. Two strategies, the scaffold and the chromophore concepts, have to provide the targeted structures, where chromophores are either ligated to a scaffold (which also can constitute a new chromophore) or where the chromophore, mostly linear and cyclic conjugated systems, is formed in a chromogenic fashion. Besides aiming for new chromophores, arrays of established chromophores, or providing systems for establishing systematic structure property relationships, in recent years diversity-oriented syntheses relying on green approaches involving environmentally friendly solvents, catalysts and purification processes have become increasingly important and intellectually challenging in the field of MCRs. Tailormade functional π-systems accessed by MCR not only provide fluorophores with AIE effects for extensive application possibilities as luminescent materials in optoelectronics (OLED, OFET, DSSC, OPV), but also reach by analytics applications beyond to life science and biomedical engineering. Furthermore, newly explored effects, such as AIE (aggregation-induced emission) or TADF (thermally activated delayed fluorescence), demand suitable tunable chromophores. Therefore, existing and still undiscovered MCRs will also provide synthetic solutions for tackling new actual and future scientific problems in chromophore research, which has become an evergreen in chemical sciences.

Author contributions
LB wrote the first draft of the manuscript and was involved in reviewing and editing. TJJM conceptualized the topic, was involved in reviewing and editing and was responsible for funding acquisition.