Exploration of violet-to-blue thermally activated delayed fluorescence emitters based on “CH/N” and “H/CN” substitutions at diphenylsulphone acceptor. A DFT study

Abstract

The violet-to-blue thermally activated delayed fluorescence (TADF) emitters were created employing several substituents based on 5,5-dimethyl-5,10-dihydropyrido [2,3-b][1,8] naphthyridine-diphenylsulphone (DMDHPN-DPS) called 1a via “CH/N” and “H/CN” substitutions at the diphenylsulphone acceptor (DPS) moiety. The parent compound 1a was selected from our former work after extensive research employing “CH/N” substitution on Dimethyl-acridine (DMAC) donor moiety. There is a little overlap amid the highest occupied molecular orbitals (HOMOs) and lowest un-occupied molecular orbitals (LUMOs) due to the distribution of HOMOs and LUMOs primarily on the DMDHPN donor and the DPS acceptor moieties, respectively. It resulted in a narrower energy gap (∆E_ST) between the lowest singlet (S₁) and triplet (T₁) excited state. In nearly all derivatives, the steric hindrance results in a larger torsional angle (85°–98°) between the plane of the DMDHPN and the DPS moieties. The predicted ΔE_ST values of the compounds with “H/CN” substitution were lower than those of the comparable “CH/N” substituents, demonstrating the superiority of the reversible inter-system crossing (RISC) from the T₁ → S₁ state. All derivatives have emission wavelengths (λ_em) in the range of 357–449 nm. The LUMO → HOMO transition energies in the S₁ states are lowered by the presence of –CN groups or –N = atoms at the ortho or meta sites of a DPS acceptor unit, causing the λ_em values to red-shift. Furthermore, the λ_em showed a greater red-shift as there were more–CN groups or –N = atoms. Three of the derivatives named 1b, 1g, and 1h, emit violet (394 nm, 399 nm, and 398 nm, respectively), while two others, 1f and 1i, emit blue shade (449 nm each) with reasonable emission intensity peak demonstrating that these derivatives are effective violet-to-blue TADF nominees. The lower ΔE_ST value for derivative 1i (0.01 eV) with λ_em values of 449 nm make this molecule the finest choice for blue TADF emitter amongst all the studied derivatives. We believe our research might lead to the development of more proficient blue TADF-OLEDs in the future.

1 Introduction

Owing to their potential use as solid-state lightening sources and high-resolution flat panel display, organic light-emitting diodes (OLEDs) having multi-layer structures got a lot of interest since Tang and his colleagues initially described them (Tang and VanSlyke, 1987; Adachi et al., 2001; Zhang et al., 2014a; Chan et al., 2018; Liu et al., 2018; Wada et al., 2020). Because traditional fluorescent material can only harvest singlet excitons, OLEDs can only attain an internal quantum efficiency (IQE) of 25% (Shuai et al., 2000; Mehes et al., 2012; Sun et al., 2015). In contrast, noble heavy-metal-based phosphorescent materials may harvest both singlets as well as triplet excitons and hence can achieve an IQE of ≈ 100% through effective spin-orbit coupling interaction (SOC) (Adachi et al., 2001; Deaton et al., 2010; Zhang et al., 2012a; Xu et al., 2021). Noble metal-containing phosphorescent materials are, however, rather expensive and unreliable for blue emission. Therefore, a novel approach for obtaining high photoluminescence is required (Finkenzeller, 2008; Chi and Chou, 2010; Zhang et al., 2014a; Yu et al., 2022).

Adachi’s group, recently, has purported a novel triplet-harvesting mechanism named thermally activated delayed fluorescence (TADF) for highly proficient OLEDs as a workable alternative method with greater singlet yield (Endo et al., 2009; Mehes et al., 2012; Uoyama et al., 2012; Youn Lee et al., 2012; Li et al., 2020). For proficient TADF emitters, a narrower energy gap (∆E_ST) amid the lowest singlet excited state (S₁) and lowest triplet excited state (T₁) is essential for reversible inter-system crossing (RISC) from the T₁ → S₁ state (Berberan-Santos and Garcia, 1996; Wada et al., 2020). The ∆E_ST is associated with exchange energy (j) between the HOMOs and LUMOs in a molecule (Lu et al., 2015a). A tiny spatial overlap (ρ) amid HOMO-LUMO is believed to be an essential component (Lu et al., 2015b) for obtaining a low ∆E_ST value, and this is realized by joining the donor (D) and the acceptor (A) fragments in structures via steric hindrance, for instance, bulk, spirojunction, or homoconjugation (Kawasumi et al., 2015; Lu et al., 2016; Liu et al., 2020; Zhang et al., 2022).

Diphenylsulphone (DPS) unit is a versatile fragment with favorable features for TADF materials owing to a twist angle in the center and a high electron-accepting capacity (Huang et al., 2014; Tao et al., 2014; Bryden and Zysman-Colman, 2021). The sulfonyl group has an electron-withdrawing characteristic because of significant electronegativity of the oxygen atom in the group. It can also prevent compounds from undergoing π-conjugation because of its tetrahedral structure. High-performance TADF-OLEDs have been recognized by several sulfone-based compounds. As a result, in recent years, DPS has emerged as the most popular electron-accepting moiety for TADF emitters (Ye et al., 2013; Wu et al., 2014; Li et al., 2015; Liu et al., 2015). Several DPS-containing compounds with TADF properties have previously been described by Adachi’s group (Zhang et al., 2012b; Ye et al., 2013; Wu et al., 2014; Li et al., 2015; Liu et al., 2015; Cui et al., 2020). Dimethylacridine-Diphenylsulphone (DMAC-DPS), among all the previously described DPS-based emitters, has been revealed to be a powerful blue TADF emitter with emission wavelength (λ_em) of 460 nm in toluene and ∆E_ST (CT) value of 0.02 eV (Zhang et al., 2014a; Luo et al., 2016). In order to adjust the emission color, our group has published a chain of derivatives using H/R substitution on the D and A units (R = CH₃ and CN) and “CH/N” substitution on the D-fragment (Wang et al., 2017a). It was discovered that modifying the D and A fragments with push-pull substituents is a useful technique for adjusting the λ_em, reducing the ∆E_ST, and enhancing the optical characteristics of designed molecules (Fan et al., 2016a; Lin et al., 2017a; Wang et al., 2017b; Yang et al., 2020; Jiao et al., 2021; Zhang et al., 2022). The “CH/N” as well as “H/CN” substitution on the acceptor unit, which results in bathochromically-shifted λ_em values and decreases ∆E_ST values, is a successful strategy, according to our analysis (Sun et al., 2008a; Sun et al., 2008b; Li et al., 2012).

In this contribution, we use the parent molecule DMDHPN-DPS (1a) as a starting point from our aforementioned report (Wang et al., 2017a). Then, by “CH/N” as well as “H/CN” substitution at meta and ortho sites of the DPS A-fragment, we were able to change the emission color to fashion blue TADF emitters. Figure 1 illustrates the structures of the parent molecule and all the symmetric substituted molecules. By computing ∆E_ST and λ_em values with the optimal Hartree-Fock percentage (OHF%) method in the exchange-correlation of the time-dependent density functional (TD-DFT) theory, we were able to analyse the TADF of these designed molecules.

FIGURE 1

2 Computational methodology

At first, the Gaussian-09 program was utilized to perform the ground state geometry optimization (S₀) for all of the derivatives employing B3LYP/6-31G(d) method. In addition, vibrational frequency analysis was accomplished to validate the local minima, and they turned up no imaginary frequencies (Irfan et al., 2014; Gao et al., 2017a; Gao et al., 2017b; Liu et al., 2022). Following that, the charge-transfer index (q), which is a measure of electron density re-distribution within a molecule, was determined from D to A using the HOMO and LUMO distribution. Subsequently, to examine the orbital composition using the Multiwfn tool, the optimal HF% (OHF%) was calculated using the relationship OHF = 42q. Based on S₀ geometry, the vertical absorption energies for singlet E_VA (S₁) as well as triplet E_VA (T₁) were computed using different functionals including M06-HF, M06-2X, BMK, MPW1B95, PBE0, and B3LYP having different HF% of 100%, 54%, 42%, 31%, 25%, and 20%, respectively, with 6-31G(d) basis set. The HF percentage (HF%) of various functionals is given in Supplementary Figure S1. Afterward, the best-fit straight line of the double log plots of E_VA (S₁, T₁) against HF% was used to calculate the vertical excitation energy E_VA (S₁, OHF) as seen in Figure 2 and S1. Eventually, the ∆E_ST and zero-zero transition energies (E_0-0) were predicted using the following proven formulae of Adachi et al. (Huang et al., 2013; Zhang et al., 2014b; Tian et al., 2016; Cui et al., 2020).

FIGURE 2

Here, ∆E_stokes (energy loss during Stokes-shift) is about 0.09 eV, and ∆E_V (difference in vibrational energy levels between the zero-zero transitions and the vertical transitions) is around 0.15 eV for the conjugated compounds. E_VA (T₁, B3LYP) depicts the vertical excitation energy for the triplet (T₁) state calculated via B3LYP/6-31G(d) method. The C is the correction factor whose value for BMK, M06-2X, and, M06-HF functionals is about 1.10, 1.18, and 1.30, respectively. The S₁ state geometries were subsequently optimized using functionals with an HF% near the OHF (%). The MPW1B95 functional was selected for S₁ state optimization for all the studied derivatives as the OHF was determined to be closest to 31% as shown in Table 1. Then, using the TD-MPW1B95/6-31G(d) method with a polarizable continuum model (PCM) in the toluene medium, the absorption (λ_ab) and emission (λ_em) wavelengths were determined based on optimized S₀ and S₁ state geometries respectively. The Gaussian-09 software is used for all calculations (Fan et al., 2016b; Cai et al., 2016; Liu et al., 2020). Software such as Multiwfn, PyMOlyze, Origin, Gaussview, and Gausssum were used for postprocessing the findings.

3 Results and discussion

3.1 Optimized geometries at S₀ and S₁ states

Generally speaking, the photophysical characteristics of molecules with conjugated systems depend greatly on the dihedral angle and bond length. Stronger absorption, more effective emission, and improved fluorescence characteristics are frequently the results of optimal conjugation and planarity. Supplementary Table S1 shows the optimized geometrical parameters at S₀ and S₁ state of the parent and designed derivatives at B3LYP/6-31G(d) coupled with TD-MPW1B95/6-31G(d) level, employing DFT and TD-DFT, respectively. It is observed that the “CH/N” derivatives possess smaller C–N = bond lengths than the original C-C bond lengths at the pyrimidine/pyridine ring of the A-fragment because of the greater electronegativity of the nitrogen atom which draws the electron density towards itself and shortens the C–N = bond. In contrast, the C–C bond lengths in “H/CN” derivatives are longer than the prior C–C bond lengths because the –CN group will increase the electron-withdrawing strength of the A-moiety and elongates the C–C bond length inside the ring.

Bond lengths mostly alter on the substituent atom and nearby atom when compared to the original molecule 1a. For instance, C₄–N₁₄ and C₄–N₆ bond lengths of 1b, where C₁₄ and C₆ have been replaced by –N = atom, have decreased by 0.064 and 0.069 Å, respectively, in the S₀ state and 0.071 and 0.070 Å in the S₁ optimized state. Also, the lengths of the neighboring bonds C₁₂–N₁₄ and N₆–C₈ have lessened by 0.054 Å and 0.051 Å, respectively, in the S₀ state and 0.072 Å in the S₁ state. Conversely, for 1i, where –CN group has been used to replace the H-atom at C₈ and C₁₂, and the C₈–C₆, C₁₀–C₈, C₁₂–C₁₀, and C₁₄–C₁₂ bond lengths have risen by 0.065, 0.011, 0.010 and, 0.066 Å in S₀ state and 0.076, 0.004, 0.005, and 0.076 Å in S₁ state, respectively. The bond length values for C₁₂–C₁₄ and C₆–C₈ are decreased by 0.019 Å in S₀ to S₁ transition while increased by 0.003 Å for C₈–C₁₀ and C₁₀–C₁₂. The geometrical parameters compared in the S₀ state and S₁ states for the parent and designed molecules showed a bond lengths alteration of up to 0.038 Å in C–C and C–N = bonds and up to 0.067 Å in C–S bonds. For the C–S bonds, the bond lengths alteration in the S₀ state and S₁ state are more pronounced. All of the proposed compounds have shorter C–S bonds in the S₁ (0.045–0.067 Å) than they do in the S₀. But their neighboring N–C and C–C bond lengths increase in the range of (0.002–0.023 Å) in S₁ than those in the S₀ state with only a few exceptions. Hence, it is obvious that the “CH/N” derivatives possess smaller C–N = bond lengths while “H/CN” derivatives possess larger C–C bond lengths. In both types of derivatives, “CH/N” and “H/CN”, the conjugation is increased compared with parent molecule 1a, hence there is a red-shift in wavelength.

We know that the greater the value of the torsional/dihedral angle (β) between D and A fragments, the smaller will be the HOMO-LUMO overlap and the lower will be the value of △E_ST. It can be seen from Supplementary Table S1 that all the derivatives have large dihedral angle values (∼85°–98°) between the D and A fragments in the S₀ state except 1b (70.2°) due to the greater steric hindrance and 1f (115.8°) due to the smaller steric hindrance. The larger β values for almost all the derivatives are optimistic for blocking the electrical interaction between D and A units (Zhang et al., 2014b; Lin et al., 2017b).

3.2 Frontier molecular orbitals

It is acknowledged that the frontier molecular orbitals (FMOs) offer crucial details on the nature of the TADF. The HOMO-LUMO electron density graphs at the S₀ state are displayed in Figure 3. Using the below relation in the Multiwfn program, the overlap between HOMO and LUMO (ρ) was calculated using.

FIGURE 3

As is obvious from Figure 3, the HOMOs are particularly localized on the DMDHPN donor fragment while the LUMOs are largely distributed on the DPS acceptor moiety. Additionally, we found that the ρ values are ranging from 0.160 to 0.256, showing an ease of charge transfer between HOMO and LUMO, and a low electron exchange energy (j), which makes the △E_ST very modest except 1f (0.325) and 1b (0.464). Because the LUMO is mainly found on the A-unit of substituted derivatives, more substantial changes can be found in LUMO energy levels than HOMO energy levels, as seen in Figure 4. The energetic gaps (ΔE_H-L) between HOMO and LUMO are in the range of 3.84–4.69 eV in the S₁ state and 3.40–4.45 eV in the S₀ state.

FIGURE 4

When –N = atom or –CN groups are added to A-fragments, the values of ΔE_H-L are lower than they would be for the parent molecule because the enhanced electron-accepting capacity of the DPS acceptors lowers the LUMO energy level, as shown in Supplementary Table S2. However, the LUMO energy levels are reduced more effectively by the –CN group than by the –N = atom, which reduces the ΔE_H-L.

Additionally, the shift in ∆E_H–L values is more apparent as the greater number of –CN groups or –N = atoms there are. Given that the λ_ab and λ_em values are connected to the ∆E_H–L values, (Li et al., 2012), the import of –CN groups or of –N = atoms to the A-fragments may alter the absorption and emission spectra, as illustrated in Supplementary Tables S4, S5. As a result, it is expected that the newly created derivatives would exhibit red-shifted λ_em when compared to the parent molecule 1a.

3.3 Singlet-triplet energy gap

Using the Multiwfn program, the E_0-0 (S₁), E_0-0 (³CT), as well as E_0-0 (³LE) are attained based on the computed CT amount (q) (Table 1). At first, double log plots of E_VA (S₁, T₁) versus HF% for the compounds under investigation were plotted (Figure 2). Then the E_0-0 (S₁), E_0-0 (³CT), as well as E_0-0 (³LE) of these designed derivatives were computed using Eqs 1–3 as shown in Table 3. The calculated findings for the substituted molecules show that the addition of –CN groups or of –N = atoms on the A-moiety improves its capacity to pull electrons, which eventually leads to a decrease in the ∆E_ST value. Additionally, it is clear that as there are more –CN groups or of –N = atoms, the ∆E_ST values decrease, particularly at meta-position. It is also obvious that the “CH/N” derivatives have lower ΔE_ST values than the original molecule but are larger than the “H/CN” derivatives. When the lowest T₁ state is taken into account, the computed E_0-0 (³CT), as well as E_0-0 (³LE), appear to have the lowest energy levels. The ¹CT–³CT splitting is denoted by ∆E_ST (CT), while the energy difference amid the lowest T₁ and the S₁ state is denoted by ΔE_S1-T1 (Huang et al., 2013). All the newly designed molecules are symmetric substitution derivatives and have lower ΔE_ST values as compared with a parent molecule. According to Table 2, for 1i, the T₁ state is ³CT in nature, which is ideal for an effectual RISC, whereas, for others, T₁ states are ³LE in nature. The calculated ∆E_ST (CT) value using the OHF method for 1i is 0.01 eV resulting from the small ρ value. The 1i has the smallest ∆E_ST value compared with all the other substituted derivatives because 1) it is a “H/CN” substituted derivative which is more effective than “CH/N” substituted derivatives 2) two H-atoms have been replaced with two CN-groups which will further enhance the electron-withdrawing strength of the A-fragment 3) it is meta-substituted and is at a greater distance from sulphonyl group. Even though some other designed molecules, including the selected violet-blue to blue derivates like 1b, 1f, 1g, and 1h, possess a lower ³LE state, the effective TADF is made feasible via reversible internal conversion from ³LE to ³CT state followed by a subsequent RISC from ³CT to ¹CT state. So, by enhancing the electron-withdrawing capacity of A, it is beneficial to raise the ³LE state and lower the ∆E_ST (Hussain et al., 2019; Hassan et al., 2022; Hussain et al., 2023).

3.4 Photophysical properties

Figure 5 displays the emission and absorption spectra of the parent molecule and all derivatives. Table 3 lists the estimated emission and absorption wavelengths (λ_em and λ_ab) in the toluene medium at the TD-MPW1B95/6-31G(d) theory level. All the investigation molecules have λ_em and λ_ab values corresponding to the values of ΔE_H-L. The designed derivatives show that the λ_em and λ_ab values range from 357 to 449 nm and 321–418 nm, respectively. The λ_em bands instigate from S₁ → S₀ transitions which is mainly the transference of the electrons from LUMO → HOMO, and the electronic shifts are π* → π type. The addition of –CN group or –N = atom to DPS acceptor moiety has resulted in the bathochromically-shifted values for λ_em and λ_ab. Furthermore, by increasing the numbers of –CN groups or –N = atoms, the λ_em, and λ_ab values display a more significant red-shift. Particularly, the derivatives 1f and 1i, display a more noticeable red-shift in λ_em of about 97 nm each. Additionally, the meta-positioned derivatives show more red-shift in λ_em and λ_ab values than ortho-positions with the same number of –CN groups or of –N = atoms. Actually, at meta-position, the –CN group or –N = atom is at a larger distance from the sulphonyl group and experiences less steric hindrance compared to the ortho position which experiences more steric repulsion. It will increase the stability and charge transfer characters of the substituted derivatives which results in lowering the ΔE_ST and increasing the λ_em value. It is evident from the calculated results that the three molecules named 1b, 1g, and 1h are showing violet-blue emission (394, 399, and 398 nm) and two of them named 1f and 1i are showing blue emission of 449 nm with reasonable emission intensity peak demonstrating that the studied compounds are effective violet-blue to blue TADF materials. We can see that both 1f and 1i have the same value of emission wavelength (λ_em) of 449 nm lying in the pure blue region but 1i is regarded as a comparatively better TADF candidate as it has a lower ΔE_ST 0.01 eV value as compared to 1f (0.26 eV). The constructed molecules demonstrated that the sequence of λ_ab, as well as λ_em, is coherent with the propensity of the electron-withdrawing strength of A-fragment.

FIGURE 5

The energy (or wavelength) differential between a molecule’s absorption (λ_ab) and emission (λ_em) maxima is called as the Stokes-shift (∆υ). It represents the energy loss occurring during the S₁ → S₀ transition. In general, a smaller Stokes-shift is advantageous since it decreases the spectral overlap between excitation and emission signals. Since self-absorption is reduced by a narrow Stokes-shift, more of the absorbed energy is transformed into light output, enhancing OLEDs’ total energy efficiency. Every constructed molecule, except 1b and 1f, exhibits a smaller stokes-shift than the original molecule as shown in Table 3. It is also obvious that the “H/CN” substituents exhibit a relatively smaller value compared with “CH/N” derivatives hence more effective in reducing the energy loss during the S₁ → S₀ transition.

4 Conclusion

In conclusion, we have projected the photophysical as well as electronic characteristics for a variety of freshly created substituents to improve the efficacies for blue TADF emitters based on the parent system DMDHPN-DPS. The HOMOs are primarily concentrated on the DMDHPN-donor moiety, whereas the LUMOs are located at the DPS-acceptor unit. The λ_em of all the designed derivatives ranges from 357 to 449 nm. The import of –CN groups or of –N = atoms on DPS moiety lowers the transition energy from LUMO to HOMO, causing a red-shift in λ_em. Additionally, the red-shift in λ_em values becomes more significant by increasing the quantity of –CN groups or of –N = atoms. The derivatives 1f and 1i display a more evident red-shift of 97 nm. Additionally, meta-position substituents exhibit greater red-shifted λ_em values than ortho-position substituents with the same number of –CN groups or of –N = atoms. Actually, at meta-position, the –CN group or –N = atom is at a larger distance from the sulphonyl group and experiences less steric hindrance compared to the ortho position which experiences more steric repulsion. It will increase the stability and charge transfer characters of the substituted derivatives which result in lowering the ΔE_ST and increasing the λ_em value. The small ρ values for the designed molecules due to the greater β values of 85°–98° contribute to the realization of the charge transfer state and small ΔE_ST. The estimated findings showed that, out of all the analyzed molecules, three violet-blue emitters (394–399 nm) and two blue emitters (449 nm) with reasonable emission intensity and smaller ΔE_ST are favorable to be effective violet-blue to blue TADF emitters with 1i being the best TADF candidate as it has smallest ΔE_ST values of 0.01 eV. We believe that in the future, understanding and building effective violet-blue to blue TADF-based OLEDs will be made easier with the aid of our theoretical designs.

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