Thermally activated delayed fluorescence and high-contrast mechanochromism of anthrone-based donor–acceptor systems

The development of materials that emit in the deep-red to near-infrared region of the spectrum has attracted significant attention due to their potential as optical sensing and imaging reagents in biology. Herein, we report the synthesis and optoelectronic characterization of four anthraquinone-based emitters, T-tBuCz-AQ, T-MeOCz-AQ, C-tBuCz-AQ, and C-MeOCz-AQ, and two pyrazoloanthrone-based emitters, tBuCz-PA and DMAC-PA. Depending on the donor, these compounds emit in the spectral range between 640 and 750 nm in the neat film, while the emission of the 10 wt% doped films in poly(methyl methacrylate) (PMMA) is blue-shifted between 600 and 700 nm and has low photoluminescence quantum yields between 2.6% and 6.6%. Of these compounds, T-tBuCz-AQ, T-MeOCz-AQ, and C-tBuCz-AQ exhibited thermally activated delayed fluorescence (TADF) in 10 wt% doped films in PMMA, while the crystals of T-tBuCz-AQ also showed TADF. Compound tBuCz-PA showed a high-contrast and reversible photoluminescence (PL) response upon mechanical grinding and hexane fuming.


Experimental Section
General Synthetic Procedures.Reagents and solvents were obtained from commercial sources and used as received.Air-sensitive reactions were performed under a nitrogen atmosphere using Schlenk techniques, no special precautions were taken to exclude air or moisture during work-up and crystallization.Anhydrous toluene was obtained from a MBraun SPS5 solvent purification system.Flash column chromatography was carried out using silica gel (Silia-P from Silicycle, 60 Å, 40-63 µm).Analytical thin-layer-chromatography (TLC) was performed with silica plates with aluminum backings (250 µm with F-254 indicator).TLC visualization was accomplished by 254/365 nm UV lamp.HPLC analysis was conducted on a Shimadzu LC-40 HPLC system.HPLC analysis was conducted on a Shimadzu Prominence Modular HPLC system.HPLC traces were performed using an ACE Excel 2 C18 analytical column.GCMS analysis was conducted using a Shimadzu QP2010SE GC-MS equipped with a Shimadzu SH-Rtx-1 column (30 m × 0.25 mm).
1 H and 13 C spectra were recorded on a Bruker Advance spectrometer (400 MHz for 1 H, 125 MHz for 13 C).The following abbreviations have been used for multiplicity assignments: "s" for singlet, "d" for doublet, "t" for triplet and "m" for multiplet.Theoretical Calculations.All ground-state optimizations have been carried out at the Density Functional Theory (DFT) level with Gaussian16 (Frisch et al., 2016) using the PBE0 functional (Adamo and Barone, 1999) and the 6-31G(d,p) basis set (Petersson et al., 1991).Excited-state calculations have been performed at Time-Dependent DFT (TD-DFT) within the Tamm-Dancoff approximation (TDA) (Hirata and Head-Gordon, 1999) using the same functional and basis set as for ground state geometry optimization.Spin-orbit coupling matrix elements (ξ) were calculated based on the optimized singlet excited state geometry.Molecular orbitals were visualized using GaussView 6.0 (Dennington et al., 2016).Calculations were automated using an in-house designed software package, Silico, which uses a number of 3 rd party libraries and programs, including extraction and processing of results: cclib (Allouche, 2011), generations of 3D images (Humphrey et al., 1996): VMD & Tachyon (Edward et al., 1998).
Electrochemistry measurements.Cyclic Voltammetry (CV) analysis was performed on an Electrochemical Analyzer potentiostat model 620E from CH Instruments at a sweep rate of 100 mV/s.Differential pulse voltammetry (DPV) was conducted with an increment potential of 0.004 V and pulse amplitude, width, and period of 50 mV, 0.05, and 0.5 s, respectively.Samples were prepared as DCM solutions, which were degassed by sparging with MeCN-saturated argon gas for 5 minutes prior to measurements.All measurements were performed using 0.1 M DCM solution of tetra-n-butylammonium hexafluorophosphate ([ n Bu4N]PF6]).An Ag/Ag + electrode was used as the reference electrode while a platinum electrode and a platinum wire were used as the working electrode and counter electrode, respectively.The redox potentials are reported relative to a saturated calomel electrode (SCE) with a ferrocenium/ferrocene (Fc/Fc + ) redox couple as the internal standard (0.46 V vs SCE) (Pavlishchuk and Addison, 2000).

Photophysical measurements.
Optically dilute solutions of concentrations on the order of 10 -5 or 10 -6 M were prepared in spectroscopic or HPLC grade solvents for absorption and emission analysis.Absorption spectra were recorded at room temperature on a Shimadzu UV-2600 double beam spectrophotometer with a 1 cm quartz cuvette.Molar absorptivity determination was verified by linear regression analysis of values obtained from at least four independent solutions at varying concentrations with absorbance ranging from 0.078 to 0.144 for T-tBuCz-AQ; 0.086 to 0.154 for T-MeOCz-AQ; 0.075 to 0.130 for C-tBuCz-AQ; 0.078 to 0.143 for C-MeOCz-AQ; 0.012 to 0.061 for tBuCz-PA and 0.002 to 0.010 for tBuCz-DMAC.
To prepare the 10 wt% doped films of emitters in a PMMA matrix, 90% w/w (90 mg) of host was dissolved in 1 mL of solvent and to this, 10% w/w (10 mg) of emitter was added.Thin films were then spin-coated on a quartz substrate using a spin speed of 1500 rpm for 60 s.Absolute photoluminescence quantum yields (ΦPLs) were determined using an integrating sphere that is equipped with an FS5 spectrometer.The ΦPLs were measured in air and N2 environment by purging the integrating sphere with N2 gas flow for 2 min.Steady-state PL spectra were measured using a xenon lamp as the source.Time-gated PL spectra (delayed emission/phosphorescence) were measured using a pulsed microsecond flash lamp (μF1) by the multi-channel scaling (MCS) mode in FS5.The time-gated PL spectra for the samples were collected between 1-9 ms (λexc = 450 nm).
Temperature-dependent (100 to 298 K) measurements were performed using an Oxford Instruments OPTISTAT DN-V cryostat controlled by an Oxford Instruments Mercury iTC temperature controller connected to the FS5 spectrometer.Samples were allowed to equilibrate at each temperature before measurements were conducted.
The singlet-triplet energy splitting (∆EST) in 2-MeTHF was estimated from the onset of prompt fluorescence spectra and phosphorescence emission at 77 K. Prompt fluorescence spectra (1-100 ns) were generated using the time-resolve PL technique using a 375 nm picosecond pulsed laser diode.Prompt fluorescence lifetimes were measured using a picosecond pulsed diode laser (375 nm).Phosphorescence lifetimes were measured using a pulsed xenon microsecond flash lamp.
Fitting of time-resolved luminescence measurements: Time-resolved PL measurements were fitted to a sum of exponentials decay model, with chi-squared (χ 2 ) values between 1 and 2, using the EI FLS980 software.Each component of the decay is assigned a weight, (wi), which is the contribution of the emission from each component to the total emission.The average lifetime was then calculated using the following: • Two exponential decay model: where A1 and A2 are the preexponential-factors of each component.
• Three exponential decay model: where A1, A2 and A3 are the preexponential-factors of each component.
After cooling, the mixture was passed through a Celite pad and concentrated in vacuo.The combined organic layer was dried with anhydrous sodium sulfate and concentrated in vacuo.The resulting mixture was purified by silica gel column chromatography to yield the desired compound.

Rf
1 H and 13 C NMR spectra referenced residual solvent peaks with respect to TMS (δ = 0 ppm).Melting points were measured using open-ended capillaries on an Electrothermal 1101D Mel-Temp apparatus and are uncorrected.High-resolution mass spectrometry (HRMS) was performed at the University of Edinburgh.Elemental analyses were performed by the School of Geosciences at the University of Edinburgh.Single-crystal XRD structures (CCDC: 2271716-2271719) of the target compounds have been deposited.
kp and kd are the rate constants of prompt fluorescence and delayed fluorescence, respectively, kISC = intersystem crossing rate constant, kRISC = reverse intersystem crossing rate constant, ΦP and Φd are the prompt fluorescence and delayed PL quantum yields.Rate constants are calculated based on the methodology described by Tsuchiya et al.(Tsuchiya et al., 2021).

Figure S30 .Figure S31 .
Figure S30.Repeated switching of the λPL upon mechanical pressure and hexane fuming of tBuCz-PA.

Table S2 .
Excited state energies of S1, T1 and T2 and spin-orbit coupling constants calculated at optimized S1 geometry using PBE0/6-31G(d,p) level of theory.Absorption spectra of tBuCz, MeOCz and PA in toluene.