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Nat. Commun., 2021, 12, 1364 Thermochromic aggregation-induced dual phosphorescence via temperature-dependent sp3-linked donor-acceptor electronic coupling T. Wang, Z. Hu, X. Nie, L. Huang, M. Hui, X. Sun, and G. Zhang October 19, 2021, Journal club B4 K. Kawaguchi Introduction The authors present an RTP design strategy of molecular solids by combining the concept of AIE and the donor–sp3 linker– acceptor dyad molecular motif and demonstrate that such a design yields an unexpected phenomenon of thermochromic phosphorescence (TCP). The authors also show that the temperature-dependent emission is largely dictated by how strongly the two largely locally excited triplet emitting states (i.e., donor triplet 3LED and acceptor triplet 3LEA) associate. UV-vis absorption Mostly LE state Fig. 1a Normalized absorption spectra (left) of TPA1-5 in optically dilute THF solutions and corresponding hole (blue)– electron (green) distributions (right) calculated based on the TD-DFT method. Supplementary Table 1. Transition information of 4 lowest-lying excited states (maximum peak) in THF TD-DFT calculation DFT functional corrected for the molecule with long-distance charge transfer. ・ωB97X ・CAM-B3LYP DFT basis function Triple-zeta valence + polarization Solvent effect Supplementary Figure 3. UV-vis absorption spectra of (a) TPA1, (b) TPA2, (c) TPA3, (d) TPA4, (e) TPA5, and (f) TPA6 in various optically dilute solvents. Steady-state emission Fig. 1b Steady-state emission spectra of TPA1-5 in a bicomponent solution mixture showing the AIE process with different THF/water ratios (0–95%, v/v) and related AIE photos under a hand-held UV lamp (water fraction: 0 and 95%, TPA concentration: 2.0×10−3 mol/L). AIE process With increasing water fraction, a typical AIE process can be observed due to the restriction of intramolecular motions and possibly newly emerged, lowest emissive states as aggregates. Afterglow emission of TPA1 FESEM images Always form polycrystal Nano crystal Large particle size Nano crystal Large particle size Nano crystal Supplementary Figure 5. Scanning electron microscopy images of TPA1-6 Single-crystal XRD analysis The donor and acceptor moieties of TPAs adopt a highly twisted conformation due to the separation by a sp3 linker (O or CH2), resulting in a twist angle within a range of ~110– 120°. As expected, the combined effect of a propeller-shaped TPA donor and an additional twist exerted by the sp3 linker is apparent: it makes π–π stacking essentially nonexistent, while suppressing various molecular motions as aggregates, which is a typical AIE mechanism. TPA1 TPA3 TPA4 TPA5 UV absorption and excitation spectra Emission spectra and photos of the aggregates Broad shoulder peak Second emissive triplet excited state (Not T2 state) Phosphorescence Fig. 4a Normalized emission spectra of TPA1-5 in air at room temperature (excitation: 365 nm for TPA1, TPA2, and TPA4; 430 nm for TPA3; and 450 nm for TPA5) and corresponding photos showing TPA1-5 excited by 365nm UV light. Decay profiles Fluorescence Phosphorescence AIE-RTP Fig. 4b Time-resolved decay profiles of TPA1-5 in air at room temperature (right: fluorescence; left: RTP). Time-resolved emission spectra ofTPA1 The shoulder peak intensity decays faster vs. that of the main RTP peak at room temperature Fig. 4c Time-resolved emission spectra of TPA1. d Normalized emission spectra of TPA1 at 77 K. Supplementary Figure 7. Variation of the emission intensity ratio of TPA1 between 485 and 525 nm at a different delay time at room temperature. Time-resolved emission spectra of TPA1 At 77 K, the steady-state and delayed emissions show a tremendous increase in the shoulder band, which perhaps indicates a second emissive triplet state. This temperature-dependent phosphorescence decay kinetics suggests that the second emissive triplet excited state is feeding the lowest T1 at room temperature, whereas the communication is cut off at 77 K. Emission in PMMA film Visually, a color change from green to sky blue in the afterglow could be noted. T1L T1 H The temperature dependency can therefore be interpreted as: (1) a higher temperature produces hotter excitons that may prefer ISC favorable for relaxation to the T1L site and vice versa; (2) communications among these emitting states at local minima by thermal motions (e.g., vibrations) may be cut off at frigid temperatures, so that more distinct separation in spectrum could be revealed. Fig. 4e Normalized phosphorescence emission spectra of TPA1 dissolved in PMMA film (excitation: 375 nm; concentration: 3%, w/w) at different temperatures. f Schematic illustration of the ternary emission process; process 1: fluorescence; process 2: intersystem crossing at 77 and 298 K; process 3: favored intersystem crossing at 298 K; process 4 and 6: phosphorescence; and process 5: thermally activated conformation transformation. Time-resolved emission spectra of TPA2 r.t. 77 K Indicating a higher energy barrier between the two emitting states when the CH2 linker replaces O at room temperature. Emission spectra in PMMA films Stronger high-energy shoulder peak TPA4 TPA2 TPA3 TPA5 Excitation spectra in PMMA films Emission spectra and photos of the aggregates Due to the introduction of the carbonyl group in TPA3–5, the fluorescence emission almost totally turns into RTP because of a promoted ISC process, and the fluorescence emission can only be measured before 480 nm Fig. 4a Normalized emission spectra of TPA1-5 in air at room temperature (excitation: 365 nm for TPA1, TPA2, and TPA4; 430 nm for TPA3; and 450 nm for TPA5) and corresponding photos showing TPA1-5 excited by 365nm UV light. Decay profiles In the aggregated states, all three samples (TPA3–5) exhibit a broad RTP emission dominated by T1L and much shorter RTP lifetimes due to strong electronic coupling between the TPA-localized 3π– π* and the ketone-localized 3n–π* state. Fluorescence Phosphorescence Fig. 4b Time-resolved decay profiles of TPA1-5 in air at room temperature (right: fluorescence; left: RTP). Time-resolved emission spectra of TPA3 Time-resolved emission spectra of TPA4 Time-resolved emission spectra of TPA5 Lifetime and quantum yield data Properties of binary mixture The results not only suggest that the two RTP bands do originate from the donor and acceptor, respectively, but also point to the importance of the sp3 chemical linker: cutting off communications at low temperature but not high temperature, something completely different from blending. Computational investigation Geometry optimization & Frequency calculation UM06-2X-D3/6-31G(d) Single-point calculation PWPB95-D3(BJ)/def2-QZVPP Vertical emission TD-M06-2X/TZVP Range-separation method LC-ωPBE Higher gap Computational investigation SOC: ωB97X-D3/TZVP Geometry optimization CAM-B3LYP/6-31G(d) Geometry optimization UCAMB3LYP/6-31G(d) Emission energy TD-LC-ωPBE*/TZVP Emission energy TD-LC-ωPBE*/TZVP Application explorations The rationale is that the electron-withdrawing pyridyl group is smaller than cyanobenzene, which should yield an even more separated T1H and T1L energy gap to make the visual phosphorescence color change more dramatic and spectroscopically more resolvable. The lone pair electron in the pyridine moiety is likely to make molecular stacking even more difficult in the solid state, so that no PMMA matrix is needed for the TPA6-based phosphorescence sensing module. Fig. 7a Chemical structure of TPA6. Application explorations Fig. 7b Normalized steady-state and RTP emission (Δt = 3 ms) spectra of TPA6 in air at room temperature; inset: photo showing TPA6 solid excited by 365-nm UV light. c Relative steady-state emission spectra of TPA6 at different temperatures. d Relationship between the emission intensity ratio of 455 and 425 nm (I455/I425) and temperature. Application explorations Fig. 7e Normalized temperature-dependent phosphorescence emission spectra of TPA6. f Image showing delayed emission (>50 ms) color change at different temperatures. Synthesis Synthesis Synthesis (C-N coupling) Synthesis of 4-methoxy-N, Ndiphenylaniline (TPA-OMe). Diphenylamine (1 equiv., 0.95 g), 4iodoanisole (1.2 equiv., 1.34 g), 2,2'dipyridyl (0.02 equiv., 0.018 g), CuI (0.02 equiv., 0.021 g) and potassium tert-butylate (t-BuOK, 1.5 equiv., 0.94 g) were added into a round-bottom flask containing 20 mL of toluene. The reaction was heated to reflux under N2 for 4 h. After the reaction finished and cooled down to room temperature, the reaction solvent was wash by deionized water twice (30 mL×2). Then the organic layer was collected, and the solvent was removed by the rotary evaporator in vacuum. The obtained crude product was further purified by column chromatography, giving the white solid (95%, 1.42 g). Synthesis (Demethylation) Synthesis of 4-(diphenylamino)phenol (TPA-OH). To a solution of TPA-OMe (1 equiv., 1.4 g) in dichloromethane (45 mL) was slowly added BBr3 (1.5 equiv., 1.91 g) at 0C. The mixture was allowed to be stirred at 0C for 0.5 h under the protection of N2, and then the system temperature was heated to 25 C. After being stirred at 25C for 10 h, a saturated NH4Cl aqueous solution (60 mL) was added into the reaction system to quench the reaction. Then the resulting mixture was extracted with dichloromethane three times (60 mL×3), and the organic layer was collected. The dichloromethane was removed by the rotary evaporator under reduced pressure, and the obtained crude product was purified by column chromatography, affording TPA-OH as the off-white solid (85%, 1.13 g). Synthesis (Ullmann ether synthesis) Synthesis of 4-(4-(diphenylamino)phenoxy)benzonitrile (TPA1). TPA-OH (1 equiv., 0.32 g), 4- bromobenzonitrile (1.5 equiv., 0.33 g), caesium carbonate (Cs2CO3, 2 equiv., 0.80 g) were added into the round-bottom flask containing 4 mL of DMF. The reaction mixture was heated to reflux and allowed to react overnight. After the reaction finished, 20 mL of deionized water was added to quench the reaction. And then mixture was extracted with dichloromethane three times (20 mL×3). The organic layer was collected and removed under reduced pressure. The obtained crude product was purified by column chromatography, giving the white crystal (92%, 0.39 g). Synthesis (Suzuki-Miyaura coupling) Synthesis of 4-(4(diphenylamino)benzyl)benzonitrile (TPA2). 150 mL of degassed THF and 15 mL of Na2CO3 aqueous solution (2 mol/L) were added into a round-bottom flask and protected by N2. Then 4-cyanobenzyl bromide (1 equiv., 0.8 g), 4(diphenylamino)phenylboronic acid (1.2 equiv., 1.54 g), Pd(PPh3)4 (0.03 equiv., 0.15 g) were added to the flask. Then the reaction system was heated to reflux for 24 h. After the reaction system cooled down to room temperature, the solvent was removed under reduced pressure. Then the collected solid was re-dissolved in dichloromethane (100 mL) and washed with deionized water (100 mL) twice. The organic layer was collected and removed by the rotary evaporator in vacuum. The crude product was further purified by column chromatography to afford TPA2 as the white solid (65%, 0.95 g).