Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Dual-Responsive Thermally Activated Delayed Fluorescence of Spiropyran Derivatives Liangwei Ma, Guanghui Wang, Bingbing Ding and Xiang Ma Liangwei Ma Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237 Google Scholar More articles by this author , Guanghui Wang Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237 Google Scholar More articles by this author , Bingbing Ding Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237 Google Scholar More articles by this author and Xiang Ma *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100992 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail For the first time, a series of photochromic dyes SP1-3 with dual-responsive thermally activated delayed fluorescence (TADF) characteristics were reported in this research. Benefitting from the reversible light-responsive property of SP1-3, the eigen TADF of these compounds could be regulated between 588 and 678 nm or turned on/off by different wavelengths of light irradiation in a polyvinylpyrrolidone (PVP) matrix. The emission lifetime of the spiropyran derivative was prolonged to millisecond level for the first time. Moreover, the delayed fluorescence of SP1 and SP3 could also be manipulated by mechanical force because of the mechanochromistic properties of SP1 and SP3. This research is a significant breakthrough for photochromic materials and may broaden its scope. Download figure Download PowerPoint Introduction Spiropyran (SP), one of the most important photochromic dyes, has attracted overwhelming attention due to its potential applications in various fields like anticounterfeiting, sensors, biological imaging, information encryption, super-resolution imaging, and so forth.1–6 Along with the isomerization of SP molecules from SP form to merocyanine (MC) form, a new peak at the long wavelength region was aroused both in the absorption and emission spectra.7 Significant variations of emission intensity make it easy to precisely detect the luminescence signal. Taking advantage of this characteristic, SP derivatives have already been utilized in cell and super-resolution imaging.1,5,8,9 Most current works on luminous photochromic dyes are focused on emission wavelength or intensity. However, lifetime, which was detected to be on the nanosecond level for SP derivatives due to the nature of its fluorescence,10–14 is rarely studied. If the luminescence lifetime of photochromic dyes could be prolonged from the nanosecond to millisecond or even second level, the performance could be dramatically improved in many applications. For example, taking advantage of the time dimension of the emission, the signal-to-noise ratio of bioimaging, or the security of the encrypted information could be enhanced dramatically.15–20 Although the phosphorescence of SP derivatives was investigated in previous work, it could only be detected at 77 K, which restricted their applications.21 It is still a challenge to construct long-life luminous photochromic material at room temperature. Thermally activated delayed fluorescence (TADF) is a kind of long-lived emission due to its triplet state-involved nature. It can utilize the triplet excitons reverse intersystem crossing (RISC) process from the lowest triplet excited state (T1) to the lowest singlet excited state (S1) then decay to the electronic ground state (S0).22–24 By narrowing the energy gap between singlet and triplet excited states (ΔEST), 100% internal quantum efficiency could be achieved in organic light-emitting diodes (OLEDs) for TADF emitters by harvesting both singlet and triplet excitons simultaneously.25 Other than electronic devices, TADF materials have been applied in time-resolved fluorescence imaging and oxygen sensing by utilizing emission lifetime and oxygen sensitivity, respectively.15,16,26–28 More recently, stimuli-responsive TADF and room-temperature phosphorescence (RTP), which is another kind of long-lived emission,29 materials have also been reported.30–37 The delayed emission of these compounds could be switched “ON” or “OFF” by changing the molecular conformation, packing mode, or the surrounding of the crystals with mechanical force, chemical species, or light stimulation.38 However, it is difficult to apply these crystalline materials in various fields because the stimuli-responsive properties are dependent on changes of conformation, packing mode, or surrounding. Combining photochromism with TADF or RTP is a unique way to manipulate the delayed emission because of the responsiveness of photochromic materials. Recently, our group has copolymerized SP and phosphors with acrylamide.39 The obtained materials showed tunable luminescence via a tunable energy transfer process between SP and phosphors. However, the construction of photochromic dyes with eigen delayed emission, which would broaden the scope of photochromic materials, is still a challenge.35 Herein, we report a series of photochromic SP derivatives ( SP1-3, Scheme 1a). After doped into a PVP matrix, these dyes exhibit light and mechanical force dual-responsive TDAF with a millisecond lifetime, which is the longest lifetime for SP derivatives to date. Benefitting from the stimuli-responsive properties of SP, the eigen TADF of [email protected], [email protected], and [email protected] could be switched between different wavelengths or switched “ON” upon light or mechanical force stimulation. Also, the TADF can recover to its initial state after further irradiation of visible light. Scheme 1 | (a) Top: Chemical structure of SP1-3. Bottom: Simulation structure of SP1, and single-crystal X-ray diffraction structures of SP2 and SP3. (b) Reversible isomerization of SP between SP and MC form under UV and visible light irradiation. Download figure Download PowerPoint Experimental Methods Detailed synthetic procedures of compounds SP1, SP2, and SP3 are listed in the Supporting Information ( Supporting Information Figure S15). Their structures are characterized by 1H NMR, 13C NMR, high-resolution electrospray ionization (ESI) mass spectroscopy, and single crystal X-ray diffraction ( Supporting Information Figures S16–S35, Table S3). OLEDs were fabricated with device configuration of ITO/PEDOT:PSS(30n m)/PVK:20% emitter(30 nm) /TPBI(40 nm)/LiF(1 nm)/Al ( Supporting Information Figure S14). Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed with the Gaussian 09 (Revision E.01) software package. Ground-state (S0) geometries were optimized with the B3LYP and 6-31G* basis. The excitation energies in the singlet and triplet states were obtained using the TDDFT method based on the optimized S0 molecular structure. Natural transition orbitals (NTOs) were generalized using Muitiwfn software.40–42 SP1 was chosen as a model molecule to analyze the photophysical properties of these compounds. Results and Discussion An ethanol solution of SP1 exhibited three distinct absorption peaks around 273, 296, and 330 nm (Figure 1a), which were assigned to the absorption of SP form.31 Upon 365 nm light irradiation, all absorption peaks slightly decreased, and a distinct absorption peak around 550 nm gradually formed, which was attributed to the absorption of the MC form.43,44 The photostationary state (PSS) was reached after 120 s of sustained UV light irradiation. Identifiable with the absorption spectra change, the color of the ethanol solution changed from faint yellow to amaranthine upon UV light irradiation. In addition, the absorption spectra and color could gradually recover to its initial state after visible light irradiation. Moreover, an isosbestic point could be observed at 356 nm with variable absorption spectra. Altogether, these phenomena indicate the occurrence of the photoisomerization process in ethanol solution. Figure 1 | UV–vis absorption (a) and emission (b) spectra changes of SP1 in ethanol upon irradiation of 365 nm light depending on different times. Inset: Photograph of the solution under daylight (a) and UV light (b) upon alternating 365 nm and visible light irradiation; Concentration: 2.5 × 10−5 M; λex = 560 nm. Fluorescence (c) and delayed emission (d, delayed time = 0.1 ms, gate time = 2.0 ms) spectra change of [email protected] upon irradiation of 365 nm light depending on different times. Lifetime decay spectra of emission bands of [email protected] at 588 nm (e) and 678 nm (f) at different temperatures; λex = 520 nm. Download figure Download PowerPoint In its initial state, almost no emission was recorded for the ethanol solution of SP1 (Figure 1b). After irradiation of UV light, the emission intensity around 643 nm ( Supporting Information Figure S1, τ = 1.40 ns), which should be attributed to the emission of MC form, gradually increased, and the intensity kept increasing until the PSS was attained. In line with the absorption spectra, the emission spectra could recover to its initial state after further irradiation with visible light. Based on the fatigue resistance ( Supporting Information Figure S2), the fluorescence “ON” and “OFF” switch could be repeated multiple times. Similar photochromic and fluorescence switching phenomena could also be observed for SP2 and SP3 in ethanol solution ( Supporting Information Figures S3 and S4). It is worth noting that the absorption spectra of the SP form of SP2 and SP3 extended to 600 nm, which might result from the intramolecular charge transfer transition from N atom to the carbonyl group. The photochromic process of SP1-3 did not occur in solid state, which might result from the tight molecular packing.7,43,45,46 To provide a relatively permissive environment for SP molecules, these molecules were doped into a PVP matrix and named [email protected], [email protected], and [email protected], respectively, by dissolving SP (20 mg) and PVP (2.0 g) in ethanol (20 mL) and evaporating the solvent in a vacuum. Powder X-ray diffraction (PXRD) spectra confirmed the amorphous state of all obtained materials ( Supporting Information Figure S5). As shown in Supporting Information Figure S6a, [email protected] exhibited three broad absorption peaks around 300, 339, and 424 nm in the initial state. The broad peak around 572 nm should be assigned to the residual MC form in the sample. Similar to the solution, the peak absorbance around 424 and 572 nm were enhanced after irradiation of 365 nm light, accompanied by the color conversion from yellow to amaranthine. The absorption spectra and color of the powder were also restored to its initial state after further irradiation with visible light. These phenomena indicated the occurrence of photochromism of SP1 in the PVP matrix. [email protected] exhibited a relatively strong fluorescence emission around 572 nm (τ = 2.96 ns, Supporting Information Figure S7) and a weak emission around 617 nm (Figure 1c) in the initial state (ΦPL = 20.9%, Supporting Information Table S1).47,48 Along with the decrease of emission intensity around 572 nm, the intensity of the peak around 617 nm increased after sustaining irradiation of the 365 nm light. It is interesting that the maximum emission wavelength gradually shifted from 622 to 653 nm upon UV light irradiation (τ = 3.80 ns, Supporting Information Figure S7). Besides, the emission spectra could also recover to its initial state when further irradiated with visible light. Similar phenomena on absorption and fluorescence spectra could also be observed for [email protected] and [email protected] ( Supporting Information Figures S8a and S8d). To explain the redshift, the excitation spectra of SP1 in ethanol and PVP matrix were measured at different wavelengths ( Supporting Information Figure S9). In ethanol solution, as the detection wavelength increased from 600 to 750 nm, the maximum of the excitation spectra varied from 551 to 571 nm. Previous works indicated that this phenomenon resulted from the presence of trans-trans-cis (TTC) and trans-trans-trans (TTT) isomers of MC form in ethanol solution (Scheme 1b), due to the cis (C) or trans (T) configuration of the methine bridge connecting the indole and benzene parts. Moreover, the TTT isomer was more stable than the TTC isomer and the emission wavelength was longer, too,49–57 which showed comparable properties when doped into the PVP matrix. As illustrated in Supporting Information Figure S9b, the excitation spectra of [email protected] at 560 nm was identified with 572 nm, except some difference in intensity, since both emission peaks around 560 and 572 nm originated from the SP form. When the detection wavelength was set at 620, 656, and 700 nm, the maxima of the excitation spectra were measured at 590, 599, and 607 nm, respectively. This data could be interpreted as the existence of TTC and TTT isomers of the MC form in the PVP matrix. We proposed that, in the initial state, [email protected] was composed of SP form (mainly) and TTC isomers (partially), which responded for the emission peak around 572 and 620 nm, respectively. Upon UV light irradiation, the SP form rapidly isomerized to the MC form, which was composed of the TTT and TTC forms, and the generation of the TTT form should be slower than the TTC form. Thereby, an intermediate emission around 630 nm, originating from the TTC form, could be observed during the photochromism process. The TTC form was then further converted to the TTT form under UV light irradiation. Therefore, the emission was gradually redshifted from 630 to 653 nm. [email protected] showed a broad delayed emission peak around 588 nm (Figures 1d and 1e, τ = 0.41 ms), which approached the emission wavelength of the fluorescence of SP1, in the initial state at room temperature. The lifetime gradually decreased with decreasing temperature (Figure 1e). Consequently, the delayed emission was assigned to TADF of SP1.25,58–61 After irradiation with 365 nm light for about 1 s, the peak at 588 nm dramatically decreased and a new peak around 630 nm appeared, which could be assigned to the TADF of the TTC isomer of the MC form because its fluorescence was located at 622 nm, and the lifetime decreased with decreasing temperature (Figure 1f). Further irradiation with UV light caused the emission intensity at 588 nm to continually decrease because of the isomerization of SP form, while the emission peak around 630 nm shifted to 678 nm (Figures 1d and 1e, τ = 0.79 ms), and was assigned to the TADF of the TTT isomer of MC form. The explanation of the redshift of TADF is the same as the fluorescence since TADF is also the radiative transition from the S1 state. The delayed emission excitation spectra of [email protected] at different wavelengths could also confirm this conclusion ( Supporting Information Figure S7c). Different from [email protected], almost no delayed fluorescence was observed for [email protected] or [email protected] in the initial state, due to the large energy difference between the S1 and T1 states (>0.5 eV) and the nonfluorescence of SP2 and SP3. With sustained UV light irradiation, the delayed emission around 648 and 649 nm ( Supporting Information Figures S8b and S8e) was gradually enhanced for [email protected] and [email protected], respectively. The delayed emission could also be assigned to the TADFs of MC2 and MC3 because the lifetime of the delayed emission decreased with decreasing temperature ( Supporting Information Figure S10) and the prompt emissions of MC2 and MC3 in the PVP matrix were about 637 and 643 nm. Further visible light irradiation of these powders showed that TADF emission could be switched from 678 to 588 nm for [email protected] or switched off for [email protected] and [email protected]. Benefitting from the satisfactory fatigue resistance of SP1-3 ( Supporting Information Figure S11), this light-induced TADF switch could be repeated many times. DFT and TDDFT calculations were carried out on the B3LYP/6-31G* level to investigate the inherent mechanism of the TADF.62 As shown in Figure 2, the highest occupied NTO (HONTO) and lowest unoccupied NTO (LUNTO) of SP1 were orthogonal and almost not overlapped. Although the spin-orbit coupling (SOC) matrix element value between S1 and T1 state was not satisfactory (0.086 cm−1, Supporting Information Table S2), which was calculated by a simplified method by selecting the geometry of the excited state,33,63 the small energy difference (0.03 eV) between the S1 and T1 states could support the TADF mechanism.22,25,64 However, a much larger energy difference between S1 and T1 (ΔEST = 1.08 eV) was calculated for MC1, which could cause the T1 → S1 up-conversion to become problematic. The large energy difference should result from the planar donor–acceptor molecule configuration of MC1, which led to a larger overlap between the corresponding transition orbitals. However, it should be noticed that the T3 state was the closest triplet state below the S1 state, the S1, T1, and T3 states were primarily characterized by its locally-excited nature, and the T2 state transition was primarily the charge-transfer characteristic transition. Besides, the energy gap between S1 and T3 (ΔEST = 0.08 eV) and S1 and T2 (ΔEST = 0.12 eV) were quite small. Even more important, the energy difference between T1 and T2 was almost 1 eV. In general, excitons tend to relax to the ground state from the lowest singlet or the lowest triplet state. However, if the energy gap between excited state was too large, like azulene, excitons could radiatively decay from the high excited state, like S2, instead of the lowest excited state to the ground state because the large energy gap would slow the internal conversion (IC) rate.65,66 Analogously, once the T2 → T1 transition rate is slowed by the large energy gap, the RISC rate from a high excited triplet state to S1 state might compete with the IC rate of T2 → T1. The large energy difference between the T2 and T1 states and the charge-transfer transition nature of the T2 state made it possible to generate the TADF from the T2 state.67–69 Interestingly, the energy difference between the T2 and T3 states was calculated to be 0.04 eV, indicating the possibility of a thermally activated reverse IC (RIC). Also, the SOC matrix element value between S1 and T3 states was very large (ca. 19.29 cm−1, Supporting Information Table S2). Therefore, the TADF could be generated from the RISC process of the T3 state, either before IC or after thermally activated RIC from T2.22,64 Both RISCs from the T3 and T2 states were the possible paths to generating the TADF, but it was difficult to determine the dominant path.62,70,71 Similar situations could also be observed for MC2 and MC3 ( Supporting Information Figure S12 and Table S2). Figure 2 | Top: Energy diagram of the first 10 singlets and triplet excited states of SP1 and MC1 from TDDFT calculations. Bottom: NTOs of SP1 and MC1 corresponding to the S1, T1, T2, and T3 states at their respective optimized geometries. Download figure Download PowerPoint [email protected] and [email protected] also show the mechanical responsive properties when pressed under 20 MPa pressure. As shown in Figure 3a, the absorption peak of [email protected] around 555 nm was gradually enhanced under pressure, accompanied by a color conversion from yellow to amaranthine. With further visible light irradiation after the stationary state was reached, the absorption spectra and color could recover to their initial state. These phenomena indicated the occurrence of mechanochromism.72–75 In line with the absorption spectra change, the fluorescence emission of [email protected] around 575 nm gradually decreased and a new emission peak around 625 nm, which should be attributed to the emission of TTC isomer of MC form, increased under pressure (Figure 3b). It is quite interesting that no such red-shifting phenomenon in the photochromism process was observed during the mechanochromism process. The delayed emission was also identified with the behavior of fluorescence spectra, which only exhibited a TADF emission peak around 633 nm during the mechanochromism process (Figure 3c). When irradiating with UV light after the stationary state was reached, the maximum delayed emission wavelength would shift to about 670 nm, which identified with the delayed emission of TTT isomers (Figure 3d). Besides, the fluorescence and TADF emission could recovery to the initial state after irradiation of visible light. A reasonable explanation is that although the SP form molecule could isomerize to the MC form under high pressure, the TTC isomer could not be converted to the TTT isomer under mechanical force stimulation. Under high pressure, the SP form molecule converted to TTC isomer resulted in variations of absorption, prompt emission, and delayed emission spectra change. Further irradiation with UV light after the stationary state was reached, the TTC isomer converted to its TTT configuration which induced the emission spectra red shift. Similar phenomena could also be observed for [email protected] ( Supporting Information Figure S13). Figure 3 | Absorption (a), prompt emission (b), and delayed emission (c) spectra change of [email protected] under 20 MPa pressure. (d) Normalized delayed emission of [email protected] before (black line) and after (orange line) pressurizing, and the delayed emission after irradiation of UV light when the stationary state was reached under pressure; λex = 520 nm. Download figure Download PowerPoint Conclusions A series of eigen TADF characterized photochromic dyes SP1-3 are presented in this study. The lifetime of the delayed emission was prolonged to the millisecond level, which is the longest lifetime for SP derivatives to date. Benefitting from the stimuli-responsive properties of SP, both the eigen fluorescence and TADF wavelength or intensity of [email protected] could be reversibly switched by light or mechanical force. We believe this research will contribute to an expanded scope of photochromic materials and provide a new path to apply photochromic materials in various fields like cell imaging, super-resolution imaging, and so forth. Supporting Information Supporting Information is available and includes additional experimental details on the synthesis of compounds, Table S1–S3, and Figures S1–S35. Conflict of Interest There is no conflict of interest to report. Funding Information We gratefully acknowledge the financial support from the National Natural Science Foundation of China (nos. 21788102, 22020102006, 21722603, and 21871083), project support by the Shanghai Municipal Science and Technology Major Project (grant no. 2018SHZDZX03), Program of Shanghai Academic/Technology Research Leader (no. 20XD1421300), “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (no. 19SG26), the Innovation Program of Shanghai Municipal Education Commission (no. 2017 01-07-00-02-E00010), and the Fundamental Research Funds for the Central Universities. Acknowledgments The authors gratefully thank Prof. Dongge Ma (South China University of Technology) for their kind assistance on OLEDs.
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