Thermochromic Aggregation-Induced Dual Phosphorescence via Temperature-Dependent sp 3 -Linked Donor-Acceptor Electronic Coupling
Aggregation-induced emission (AIE) has proven to be a viable strategy to achieve highly efficient RTP in bulk by restricting molecular motions. Here we show that by utilizing triphenylamine (TPA) as an electronic donor which connects to an acceptor via an sp 3 linker, six TPA-based AIE-active RTP luminophores were obtained. Distinct dual phosphorescence bands emitting from largely localized donor and acceptor triplet emitting states, respectively, could be recorded at lowered temperatures; at room temperature, only a merged RTP band is present.Theoretical investigations reveal that the two temperature-dependent phosphorescence bands both originate from local/global minima from the lowest triplet excited state (T1). The reported molecular construct serves as an "intermediary case" between a fully conjugated donor-acceptor
<!>AIE characterization.<!>Computational investigation on the origin of dual phosphorescence.<!>Discussion<!>Methods<!>Corresponding Author
<p>system and a donor/acceptor binary mix, which may provide important clues on the design and control of high-freedom molecular systems with complex excited-state dynamics. uppressing nonradiative decays (e.g., intramolecular motions) and promoting the intersystem crossing rate (ISC) are crucial to realize organic room-temperature phosphorescence (RTP) intense enough for practical applications 1,2 . Up till now, several RTP design principles have been proposed, such as introducing bulky hindrance groups 3,4 and spiro linkers 5 , constructing aggregates [6][7][8] , polymers 9,10 and self-assemblies [11][12][13] , host-guest [14][15][16] and bonding [17][18][19] interactions, as well as tuning charge-transfer (CT) states 20,21 . Recently, we put forward a new RTP design principle based on an sp 3 -linker connected donor-acceptor dyad, where an angled intramolecular CT state is believed to enhance spin-orbit coupling due to electron circular motion during transition 22,23 . For example, a proton-activated "off-on" RTP molecular probe could be developed based on this principle 22 . However, most RTP emitters still require stringent environmental factors such as low temperature, solid matrix assistance, and/or oxygen exclusion, which can be rather cumbersome for practical applications. Since 2001 24 , aggregationinduced emission (AIE) luminophores have been a research hotspot because of their enormous potentials in solid-state display [25][26][27] , bio-probes [28][29][30] , circularly polarized luminescence 31,32 , and "on-off" sensors 33 , with an advantage being "emitting in their own microenvironments". Therefore, a universal design method for the development of RTP materials may be built upon less environmental-sensitive AIE core structures with RTP's increasing use in chemical sensing 34,35 , data storage 36,37 , bioimaging 38,39 and OLEDs 40,41 . Herein, we present an RTP design strategy of molecular solids by combining the concept of AIE and the donor-sp 3 linker-acceptor dyad molecular motif and demonstrate that such a design yields an unexpected phenomenon of thermochromic phosphorescence (TCP). We also show that the temperature-dependent emission S 3 is largely dictated by how strongly the two largely locally excited triplet emitting states (i.e., donor triplet 3 LED and acceptor triplet 3 LEA) associate (Figure 1a). The TPA moiety was selected as an AIE activator as well as an electron donor with the acceptor varying in chemical structures. Shown in Figure 1b, five TPA-functionalized AIE-active RTP molecules were synthesized via simple Suzuki-Miyaura or Ullmann-type coupling reactions and the AIE properties of TPA1-5 were investigated.</p><!><p>In solution, absorption spectra conducted in THF (Figure 2a) are consistent with calculations by the time-dependent density functional theory (TD-DFT) method with the optimally tuning range-separated functional (LC-ωPBE*) and the TZVP basis set 42,43 . According to the theoretical results (Supplementary Figure 1 and Supplementary Table 1), the reddest absorption band (>280 nm) of TPA1-2 is mainly contributed by the second and the third singlet excited states (S2 and S3), while that of TPA3-5 is dominated by S3 and the fourth singlet excited state (S4). The hole-electron distribution analysis performed by Multiwfn 3.7 (dev) program 44 indicates that lower electronic transitions of all TPAs are mainly contributed by local excitations (Figure 2a, right), and the lowest singlet excited state (S1) is a dark state with a strong CT character, transitions to localized excited states (e.g., S2 and S3, Supplementary Figure 2). Therefore, it can be inferred that TPAs are non-emissive due to a combination of excited-state energy dissipation via intramolecular rotation (propeller-shaped TPA moiety) and a lowest forbidden CT pathway.</p><p>With increasing water fraction, a typical AIE process (Figure 2b) can be observed due to restriction of intramolecular motions and possibly newly emerged, lowest emissive states as aggregates. It has to be noted that the observed emission maximum shift mainly results from the change of mixture solvent polarity 45 . Moreover, all TPA aggregates in THF/water (5/95, v/v) contain longlived emissions, and the afterglow can even be observed for certain (e.g., TPA1) aggregates (Supplementary Figure 3). The scanning electron microscopy (SEM) images (Supplementary Figure 4) show that some of these aggregates are in fact ordered and form nanocrystals (TPA1, TPA2 and TPA5); the increased particle sizes of TPA3 and TPA4 may originate from reduced intermolecular interactions given that they exhibit high molecular complexity.</p><p>To assist further understanding of their AIE behaviours, single-crystal measurements of TPA1 and TPA3-5 were conducted to investigate the origin of the emission, except for TPA2 which always generates polycrystals. As shown in Figure 3, the donor and acceptor moieties of TPAs adopt a highly twisted conformation due to the separation by an sp 3 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 sp 3 linker is apparent: it makes - stacking essentially non-existent while suppressing various molecular motions as aggregates, which is a typical AIE mechanism.   Photophysical properties in the aggregation state. These collected aggregates were then investigated by both steady-state and delayed luminescence spectroscopy. The optimal excitation wavelengths were assessed from solid-state UV-vis absorption and excitation spectra (Supplementary Figure 5). For TPA1 and TPA2, the steady-state emission exhibits a main AIEfluorescence band (em = 403 and 437 nm, respectively, Figure 4a) with nanosecond lifetimes (Figure 4b, left). A low-energy band over 500 nm with a lifetime of >300 ms (Figure 4b, right) is presumably due to the AIE-RTP. When the delayed spectrum of TPA1 was examined, we observed a high energy broad shoulder (~485 nm) that is >1600 cm -1 above from lowest triplet excited state emission (T1). To shed light on the origin of this shoulder peak, time-resolved emission spectra were collected (Figure 4c), where the shoulder peak intensity decays faster vs.</p><p>that of the main RTP peak at room temperature (Supplementary Figure 6 and 7a). At 77 K, the steady-state and delayed emissions show a tremendous increase in the shoulder band (Figure 4d), which perhaps indicates a second emissive triplet state 2 . For delayed emissions, the relative intensity ratio between the shoulder band and the main peak maintains constant (Supplementary Figure 8). 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.</p><p>8</p><p>To exclude the influence of ground-state aggregation and trace impurity contamination (which can be significant in molecular solids), the phosphorescence spectra (t = 1 ms) of single-crystal purity TPA1 dissolved in PMMA were also recorded at various temperatures, where a consistently blue-shifted trend could be noted with decreasing temperature (Figure 4e), further indicative of such a second triplet state being separated from the lowest T1. Visually, a colour change from green to sky blue in the afterglow could be noted. A schematic illustration for explaining this ternary emission (fluorescence and dual phosphoresce) is presented in Figure 4f. For highest twisted molecules with many vibrational and rotational freedoms like these TPA derivatives, we anticipate a very rough potential energy surface (PES) in the T1 state: more than one local minima can therefore become the emitting states. It is reasonable to assume that a molecular geometry further away from the equilibrium position gives off emission at a longer wavelength, which is designated as T1 L ; the other emitting state closer to equilibrium position is denoted as T1 H (L/H stands for low/high). The temperature dependency can therefore be interpreted as: 1) a higher temperature produces hotter excitons that may prefer intersystem crossing (ISC) favourable for relaxation to the T1 L 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.</p><p>For TPA2 aggregates, although little to no T1 H emission band is presented at room temperature (Figure 4a and Supplementary Figure 9), a new peak shows up in the low-temperature phosphorescence spectrum (LTP, 77 K, Supplementary Figure 10) at 455 nm in addition to the main band (= 525 nm), indicating a higher energy barrier between the two emitting states when the CH2 linker replaces O at room temperature. Similarly, when TPA2 was molecularly dissolved in the PMMA film, a much stronger high energy shoulder peak (= 440 nm) gradually emerged (Figure 5a) compared to the aggregated state. The experimental data above appear to indicate that two T1 states communicate at room temperature both intermolecularly and intramolecularly, since the aggregates tend to give a much weaker T1 H phosphorescence band irrespectively of the temperature, compared to discrete molecules in PMMA. At this stage, we speculate that the T1 L state is a TPA-localized triplet state with T1 H being an acceptor (cyanobenzene) centric triplet one. To verify the hypothesis, two important experiments were conducted: 1) TPA3 to TPA5 with an aromatic ketone acceptor were prepared. It is wellknown that benzophenone exhibits a prominent 3 n-* state 46,47 with a characteristic vibrational progression in the phosphorescence spectrum separated by ~1300-1600 cm -1 . Indeed, such spectroscopic patterns from the T1 H state characteristic of benzophenone 3 n-* were unambiguously obtained (Figure 5b and 5c) when the temperature is reduced to or below 200 K for the two samples dissolved in PMMA films (particularly for the CH2-linked TPA4 where no oxygen conjugation interferes with the vibration), further revealing that phosphorescence emissions consist of two emissive triplet states from benzophenone and TPA subunits, respectively.</p><p>It has to be noted that below 150 K, the T1 L state appears largely suppressed, again suggesting preferential ISC relaxations at different temperatures (Figure 4f).</p><p>For TPA5, the 4-oxygen substituted acetophenone acceptor is expected to exhibit a mixed 3 -* and 3 n-* band for T1 H in PMMA at 77 K (em= 455 nm) (Figure 5d). Temperature-dependent excitation spectra (Supplementary Figure 11) of TPA1-5 dissolved in PMMA films also suggest the existence of two different emissive triplet states. In the aggregated states, all three samples (TPA3-5) exhibit a broad RTP emission dominated by T1 L (Figure 4a) and much shorter RTP lifetimes (Figure 4b, Supplementary Table 2) due to strong electronic coupling between the TPAlocalized 3 -* and the ketone-localized 3 n-* state; such coupling is also present, albeit weak, at 77 K based on the phosphorescence spectra (Supplementary Figure 12-14). Notably, 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 (Figure 4b, left). A binary molecular mixture between TPA and the benzophenone precursor at 1:1 ratio was used to repeat the same experiment (Supplementary Figure 15), where it was found that the 1:1 physical mixture generates RTP bands belonging to the two molecules irrespective of the temperature. 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 sp 3 chemical linker: cutting off communications at low temperature but not high temperature, something completely different from blending.</p><!><p>To further back the proposed T1 H -T1 L dual phosphorescence states model, we first selected TPA1 as example to carry out a series of theoretical calculations via TD-DFT at the LC-ωPBE*/TZVP level in the gas state, which mimics the state of monomers in PMMA. The calculated energy diagram of the vertical excitation and spin-orbit coupling constants reveals multiple ISC channels from S1 to a higher triplet state during RTP generation (Supplementary Figure 16), pointing to the possibility of preferential ISC channels depending on temperature. Moreover, the vertical emission calculations demonstrate that 1) the S1 state is almost degenerate with the acceptor T2 state, which boosts the ISC process, and 2) the energy gap between T1 and T2 states is relatively large (Supplementary Figure 17). However, according to the energy gap law for internal conversion 48 : log(kIC) ≈ 12-2, where kIC (s -1 ) is the internal conversion rate and  (μm -1 ) is the wavenumber difference between T1 and T2, the calculated kIC for TPA1 is ~3.95×10 -10 s -1 , which indicates that triplet excitons can only emit as photons from T1.</p><p>To shed light on the origin of dual phosphorescence, we further conducted relative Gibbs free energies and the three global/local-minima geometries (T1-1 to T1-3) of T1-state TPA1 (Figure 6).</p><p>As Figure 6 shows, there are different TPA1 conformations showing different T1 states, which exhibit T1 emissions contributed by donor and acceptor. Due to the existence of energy barriers (TS1 and TS2), the conformation transformation should be decided by transition energy, which can be compensated at room temperature while inhibited at low temperature, such as 77 K. T1 H and T1 L are not completely localized on donor and acceptor of TPA1 perhaps because cyanobenzene is a type of extremely weak emitter, which is consistent with spectroscopic evidence 12 that TPA1 T1 L emission is always dominant. Therefore, we further conducted the calculations of TPA4, where benzophenone is a better emitter than cyanobenzene. As anticipated, two T1 species are localized on TPA and benzophenone subunits, perfectly corresponding with the experimental results (Figure 5c and S14). Therefore, we can conclude that the dual phosphorescence mainly originates from T1 states of TPA derivatives in specific conformations, lining up with the dualphosphorescence model in Figure 4f. geometries, the optimal geometries with dihedrals of highlighted atoms, and main molecular orbital transition contributions for T1 emissions (>10%) (H= HOMO, L=LUMO).</p><p>Application explorations. Finally, to showcase the application value of the AIE thermochromic dual phosphorescence molecules, we synthesized TPA6 with a TPA donor and a pyridine acceptor (Figure 7a). The rationale is that the electron-withdrawing pyridyl group is smaller than cyanobenzene which should yield an even more separated T1 H and T1 L energy gap to make the visual phosphorescence colour change more dramatic and spectroscopically more resolvable.</p><p>Meanwhile, the lone pair electron in the pyridine moiety is likely to make molecular stacking even  13 more difficult in the solid state, so that no PMMA matrix is needed for the TPA6-based phosphorescence sensing module. Figure 7b shows that the TPA6 solid exhibits a fluorescence emission at 425 nm (= 0.7 ns). Meanwhile, an obvious afterglow can be observed by the naked eye and an RTP emission band can be collected at 550 nm with a lifetime of 83 ms in air at room temperature. As the temperature decreases, a new phosphorescence band (T1 H ) appears with an emission maximum at 455 nm in the steady-state emission spectrum (Figure 7c), which can be used as a ratiometric phosphorescence/fluorescence (P/F) sensing scheme (Figure 7d) demonstrated in many previous reports. 49,50 However, here we show for the first time that a delayed spectrum can further become sensitive to an external stimulus and responds to temperature change dramatically (Figure 7e and 7f), which could be potentially used as the thermometer for indicating extremely low-temperature environment.</p><!><p>In summary, by introducing a TPA-based AIE-gen into the donor-sp 3 linker-acceptor structure, several ternary emissive AIE-active RTP molecules with prompt fluorescence and dual phosphorescence were obtained. The phenomenon is explained by the presence of a TPA-localized T1 L state, which couples to a higher acceptor T1 H state at room temperature but cuts the electronic coupling off at lowered temperatures. Therefore, a universal principle for designing a dualphosphorescence thermochromic material can be deduced and demonstrated. The current strategy benefits design for single-component, dual-phosphorescence-based molecular probes. More importantly, we expect many more structurally different molecular can be used in this strategy with a possibility of generating more than two phosphorescence bands to obtain kinetically more complex molecular systems.</p><!><p>Synthesis. TPA1-6 were synthesized by Suzuki-Miyaura and Ullmann-type coupling reactions.</p><p>Further detailed information on synthetic procedures is provided as Supporting Information.</p><p>Measurements. 1</p><!><p>X. Sun: xiang.sun@nyu.edu G. Zhang: gzhang@ustc.edu.cn</p>