Open AccessCCS ChemistryRESEARCH ARTICLES6 Oct 2022Highly Emissive Luminogens in Both Solution and Aggregate States Pengbo Han, Guiquan Zhang, Jia Wang, Yejun Yao, Yanping Qiu, He Xu, Anjun Qin and Ben Zhong Tang Pengbo Han State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Guiquan Zhang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Jia Wang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Yejun Yao State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Yanping Qiu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , He Xu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Anjun Qin *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Aggregation-Induced Emission, AIE Institute, South China University of Technology, Guangzhou 510640 School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen 518172 Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202272 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic light-emitting materials have attracted considerable attention because of their promising applications in diverse areas. Most fluorophores emit brightly in either dilute solutions or aggregate states; the former generally suffer from aggregation-caused quenching problem, and the latter encounter intensity loss at low concentrations. Herein, we propose a new strategy to overcome these dilemmas by balancing the planar and distorted structures of terphenyl-based luminogens and obtain three luminogens, 2PB-AC, 2Me2PB-AC, and 2T2PB-AC, with bright emission in both solution and aggregate states. Among them, 2PB-AC shows absolute photoluminescence quantum yields (ФPL) higher than 90% in both tetrahydrofuran solution (90.2%) and aggregate states (92.7% for powder and 95.3% for crystal). Thus, 2PB-AC could be an efficient probe to realize dual-channel explosive detection in both solution and aggregate states. Moreover, it could be used to image live-cell lipid droplets at a wide range of concentrations. In addition, benefiting from its thermodynamically favorable intersystem crossing process, 2Me2PB-AC could be doped in polymethyl methacrylate matrix to provide efficient room-temperature phosphorescence. Thus, this work provides a feasible strategy for the design of luminogens with highly efficient emission in both solution and aggregate states, greatly facilitating and broadening their practical applications. Download figure Download PowerPoint Introduction Organic luminescent materials have been widely applied in organic light-emitting diodes,1–3 bioimaging agents,4,5 sensors, memory devices,6–8 and so on. Fluorophores with simple molecular structures, facile property tunability, and thermal stability are desirable for the construction of high-performance devices.9–12 However, traditional fluorophores generally suffer from an aggregation-caused quenching (ACQ) effect, where they emit intensely in dilute solution while exhibiting weak or even no emission in aggregate states.13–15 In contrast, luminogens with aggregation-induced emission (AIEgens) are weakly or nonemissive in dilute solution but emit brightly in aggregate states.16–20 To broaden the practical applications, it is meaningful but challenging to construct luminogens that emit strongly in both solution and aggregate states. Several studies have been reported to this end. For example, Stang et al.21 prepared two tetragonal prismatic metallacages by using self-assembly chemistry, from which intense emission in both molecular and aggregate states were observed. Yang et al.22 developed a self-assembled pure organic stack that produced bright fluorescence in solution and aggregate states, which in turn overcame the putative disadvantages of heavy metal complexes. However, specific stereogeometries are required for these self-assembly stacks. In addition to self-assembly chemistry, molecular structural engineering was used for the construction of highly emissive molecules in solution and aggregate states. For instance, Tang et al.23 reported two triphenylamine (TPA) derivatives that exhibit intense emission with photoluminescence quantum yields (ФPL) of 79.4% in dimethylformamide (DMF) solution and 54.8% in a crystallinestate because of their conjugation-induced rigidity. Zhao et al.24 prepared a fluorophore by combining a planar diphenyldiacetylene core with a distorted cyanostilbene unit, which showed a ФPL of 98.2% in tetrahydrofuran (THF) solution, but the value decreased to 60.7% in the solid state. Zhu et al. prepared a dual-state balanced fluorophore with thermally activated delayed fluorescence via a rigid- and flexible-alternating design approach. However, the ФPL values in solution and aggregate states were both less than 35.3%.25 Therefore, it is quite challenging to realize highly emissive fluorophores with ФPL values higher than 90% in both solution and aggregate states, and a simple and more feasible molecular design strategy is highly desired. Herein, a simple and feasible molecular design strategy was proposed for the preparation of luminogens with efficient emission in both solution and aggregate states. Since substantial rigidity is necessary to limit intramolecular motion,26,27 a planar terphenyl moiety was selected as the core to produce high emission in solution states of the resultant molecules (Figure 1a).28 Moreover, because molecules with intense emission in the solid state require considerable twisting structures to prevent the formation of excimers, a TPA moiety was used to suppress possible intermolecular π–π stacking.29–31 In addition, a cyano (CN) group was attached to the terphenyl core to construct a donor–acceptor structure and to form intermolecular C–H···N hydrogen bonds with adjacent molecules,32,33 which could in turn improve solid-state emission because of the efficient restriction of intramolecular rotation. Figure 1 | (a) Molecular engineering toward highly emissive luminogens in both solution and aggregate states. (b) Construction of an efficient RTP system using 2MePB-AC as a guest and PMMA as a host and its application in advanced encryption. (c) Schematic illustration of detection of explosive and (d) imaging of lipid droplet using 2PB-AC as the probes. Download figure Download PowerPoint Following this design strategy, three terphenyl derivatives, 2PB-AC, 2Me2PB-AC, and 2T2PB-AC, with the same donor (D) and acceptor (A) units but different side groups, were designed and synthesized. Unlike traditional ACQ fluorophores and AIEgens, they showed strong emission in their powder, crystalline, and solution states, and the ФPL values reached as high as 95.7%. Because of its thermodynamically favorable intersystem-crossing (ISC) process, 2MePB-AC doped in polymethyl methacrylate (PMMA) matrix furnished efficient room-temperature phosphorescence (RTP), and this guest–host system was applied in advanced encryption (Figure 1b). Due to its high emission with the ФPL greater than 90% in both solution and aggregate states, 2PB-AC in both states was used as a probe to sensitively detect explosive, providing a dual-channel explosive detection strategy (Figure 1c). Furthermore, 2PB-AC also served as a bio-probe to efficiently image lipid droplets at a broad range of concentrations (Figure 1d). Thus, this work presents a concise yet more precise strategy for the construction of highly emissive luminogens in both solution and aggregate states, which could further facilitate and broaden their practical applications. Experimental Methods Materials PMMA (molecular weight = 350,000 by Gel Permeation Chromatography [GPC]) was purchased from Sigma-Aldrich (Shanghai, China). Picric acid (PA) was purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy (polyethylene glycol)-2000 (DSPE-PEG2000) was purchased from Shanghai Ponsure Biotech (Shanghai, China). BODIPY and Nile Red were purchased from Thermo Fisher Scientific (Shanghai, China). CCK-8 Cell Proliferation and Cytotoxicity Assay Kit were obtained from Shanghai Beyotime Biotechnology (Shanghai, China). MCF-7 (human breast cancer cells) cells were obtained from the cell culture center of the Institute of Basic Medical Sciences, Chinese Academy of Medical Science (Beijing, China). 2PB-AC, 2Me2PB-AC, and 2T2PB-AC were synthesized according to the routes shown in Supporting Information Figure S1 and further purified by vacuum sublimation before use. Structural characterization 1H and 13C NMR spectra were measured on a Bruker AV 500 spectrometer (BRUKER, Massachusetts, USA) in deuterated dichloromethane (DCM) or chloroform. High resolution mass spectra were recorded on a GCT premier CAB048 mass spectrometer (Waters, Massachusetts, USA) operating in matrix-assisted laser desorption ionization time-of-flight mode. The crystal structure of 2PB-AC was grown successfully from DCM/n-hexane by a slow solvent evaporation, and the single crystal X-ray diffraction analysis was conducted on a Gemini A Ultra diffractometer at 170 K. Photophysical property measurement UV–vis absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer (Shimadzu, Kyoto, Japan). Photoluminescence (PL) spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer (HORIBA, Kyoto, Japan). Absolute fluorescence quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer (Hamamatsu, Naka-ku Hamamatsu, Japan) C11347 Quantaurus_QY. Fluorescence lifetimes were determined with a Hamamatsu C11367-11 Quantaurus-Tau time-resolved spectrometer (Hamamatsu, Naka-ku Hamamatsu City, Japan). Transient PL decay spectra were conducted on a FL980 (Edinburgh Instruments, Edinburgh, United Kingdom) where the lifetimes in 50 μs time range were measured with a Time-Correlated Single Photon Counting laser and the lifetimes in ms and s time range were measured with a micro-second flash lamp and Xeon lamp, respectively. Thermal property measurement Thermogravimetric analysis (TGA) analysis was carried out on a thermal analysis TGA Q5000 and differential scanning calorimetry (DSC) analysis was performed on a DSC Q1000 under dry nitrogen at a heating rate of 10 °C min−1. Computational detail The ground states (S0) of 2PB-AC, 2Me2PB-AC, 2T2PB-AC were optimized using density functional theory (DFT) and the first singlet excited state (S1) of monomers was calculated using time-dependent DFT (TDDFT) as implemented in the Gaussian 16 software. The optimization and frequency calculations were performed employing B3LYP exchange functional with 6-311G(d,p) basis set. The spin–orbital coupling (SOC) matrix between S1 and T1, T2, T3 states were calculated by the Quantum Chemistry Program Package. Preparation of doped PMMA films 2Me2PB-AC (2 mg) was mixed with PMMA (200 mg) in 2 mL toluene solution. This solution was then heated at 90 °C for 20 min followed by sonication for 10 min to completely dissolve all the components. Then, 0.1 mL of the solution was drop-casted on clean glass substrate. Finally, the drop-casted thin films were dried at 90 °C for 2 h prior to the photophysical studies. Synthesis of nanoparticles DSPE-PEG2000 (4.2 mg) and 2PB-AC (2.1 mg) were dissolved in THF (1 mL). The solution was quickly injected into 10 mL of deionized water and stirred in a fume hood for 24 h. For use in cytotoxicity testing, the sample was further filtered through a membrane filter with a diameter of 220 nm. Cell culture MCF-7 cells were cultured in Dulbecco’s modified Eagle’s medium (Gibco) containing 10% fetal bovine serum and 1% penicillin-streptomycin. All cells were cultured at 37 °C under 5% CO2 atmosphere. The cells were ensured to be in good condition before the experiment. Cell viability assay MCF-7 cells were inoculated into 96-well cell culture plates at a density of 1 × 104 cells per well and incubated for 24 h. Cytotoxicity assay Cells were incubated with different concentrations of 2PB-AC nanoparticles (0, 0.75, 1.5, 3, 6, 12, 24, 48, and 96 μM) in fresh medium for another 24 h. Then the medium was removed and washed three times with 1× PBS (phosphate-buffered saline) solution. The CCK-8 assay was performed according to the production protocol. Confocal laser scanning microscopy for cells imaging To evaluate the imaging performance of 2PB-AC, MCF-7 cells were treated with different concentrations of 2PB-AC NPs (1, 5, and 10 μM) at 37 °C for 2 h. Then the cells were observed by the confocal laser scanning microscope with an excitation wavelength of 405 nm. To study the subcellular distribution of 2PB-AC NPs, MCF-7 cells were first stained with 2PB-AC NPs (5 μM) for 2 h. Then, the cells were washed three times with 1× PBS solution and stained with BODIPY or Nile Red (3 μM for each) for 30 min. Then confocal laser scanning microscopy (CLSM) imaging was performed. Photostability of 2PB-AC NPs MCF-7 cells were stained with 2PB-AC NPs for 2 h according to above procedures. Subsequently, the 2PB-AC NPs-labeled cells were continuously irradiated with 405 nm laser from CLSM with the laser power set to 10% of its full output. The fluorescence intensity of the cells was measured every 1 s. As a comparison, BODIPY 493/503-stained MCF-7 cells were also continuously irradiated by 488 nm laser. Results and Discussion Synthesis and thermal properties of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC The synthetic routes to 2PB-AC, 2Me2PB-AC, and 2T2PB-AC are shown in Supporting Information Figure S1. They were obtained in excellent yields (85–94%), and their chemical structures were fully characterized by 1H NMR, 13C NMR, high-resolution mass spectrometry, and single-crystal X-ray diffraction. Their original NMR spectra were given as appendix in Supporting Information Figures A1–A12. They also exhibited excellent thermal stabilities. The temperatures for 5% weight loss (Td) of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC were as high as 360, 339, and 373 °C, and their melting temperatures (Tm) were 194, 185, and 220 °C, respectively ( Supporting Information Figure S2), which could meet the requirements for general applications. Photophysical properties of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC After spectroscopically confirming the structures of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC, their photophysical properties in THF solutions were first investigated. The UV–vis spectra showed absorption bands located at 300–400 nm (Figure 2a), which confirmed existing intramolecular charge transfer (ICT) processes because of their D-π-A structures.34 Notably, the absorption peaks of 2MePB-AC and 2T2PB-AC were slightly blue-shifted compared with that of 2PB-AC, suggesting that the π-conjugation of the former compounds becomes weaker than the latter by the introduction of substituents. Upon excitation, 2PB-AC, 2MePB-AC, and 2T2PB-AC emitted intensely in the blue and sky-blue regions with peaks at 475, 460, and 483 nm, with ФPL values of 90.2%, 80.1%, and 67.8%, respectively (Figure 2a). In comparison with that of 2PB-AC, the PL peak of 2Me2PB-AC was blue-shifted by 15 nm, whereas a redshift of 8 nm was observed for the PL peak of 2T2PB-AC, which might be due to weaker π-conjugation and a stronger ICT process, respectively. It is worth noting that these compounds exhibited large Stokes shifts (Δλ) of 118–179 nm, which will greatly avoid self-absorption during their applications. Moreover, 2PB-AC, 2MePB-AC, and 2T2PB-AC showed distinct solvatochromism while maintaining bright emission in different solvents (Figure 2b and Supporting Information Figure S3). For instance, 2PB-AC produced intense emissions with a ФPL value higher than 83% in different solvents. Interestingly, the ФPL gradually increased along with the redshift of emission peaks, which was not seen for fluorophores with twisted intramolecular charge transfer (TICT) feature.35–37 These different photophysical behaviors imply that these compounds emit from planar intramolecular charge transfer (PICT) excited states instead of TICT ones.38 Figure 2 | (a) UV–vis and PL spectra of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC in THF solutions. (b) PL peak and absolute quantum yields of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC in solutions; λex, 330 nm; concentration, 10 μM.(c) PL spectra and (d) lifetime decay profiles for emission peaks at 468, 415, and 437 nm for 2PB-AC, 2Me2PB-AC, and 2T2PB-AC powders, respectively; λex = 330 nm. Download figure Download PowerPoint To further understand the intense emission of these compounds in dilute solutions, a computational study was performed by using TDDFT. The optimized ground- and excited-state conformations, as well as the lowest unoccupied molecular orbitals (LUMOs) and highest occupied molecular orbitals (HOMOs) of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC, are presented in Supporting Information Figure S4. Their HOMOs are mainly localized on the TPA moieties, with a slight delocalization onto the adjacent phenyl rings, while their LUMOs are mainly concentrated on the terphenyl and CN groups ( Supporting Information Figure S4a). The spatial separation of the HOMOs and LUMOs of these compounds could result in effective ICT processes, which is also consistent with the distinct solvatochromism effects of these compounds. Their molecular structures exhibited more planar conformations in the excited state than in the ground state, as evidenced by their reduced torsion angles ( Supporting Information Figure S4b). Their planar excited-state conformations can effectively suppress the molecular motion and strengthen radiative decay, resulting in bright emission in dilute solutions. Meanwhile, the photophysical properties of the powders of 2PB-AC, 2MePB-AC, and 2T2PB-AC were also studied. As shown in Figure 2c, they emitted in the violet to blue region, with peaks at 468, 415, and 437 nm, respectively. In contrast with their isolated molecules in THF solutions, the emission peaks of the 2PB-AC, 2MePB-AC, and 2T2PB-AC powders were considerably blue-shifted, implying shorter conjugation lengths in the solid states than in solutions and/or the attenuation of the ICT process.23 Notably, the ФPL values for 2PB-AC, 2Me2PB-AC, and 2T2PB-AC powders were as high as 92.7%, 68.1%, and 53.2%, respectively. These results unambiguously demonstrate that these compounds emit brightly in both solution and aggregate states. In particular, both the ФPL values of 2PB-AC in the solution and aggregate states are higher than 90%. Moreover, as depicted in Figure 2d, the PL decay curves of 2PB-AC, 2MePB-AC, and 2T2PB-AC present single-exponential decay processes with short exciton lifetimes of 2.65, 1.77, and 1.22 ns, respectively, suggesting that they emit fluorescence. To further confirm these processes and to experimentally determine their lowest excited singlet (S1) and triplet (T1) states, the fluorescent and phosphorescent spectra of the 2PB-AC, 2MePB-AC, and 2T2PB-AC powders were measured at 77 K ( Supporting Information Figure S5), from which large experimental singlet-triplet energy gaps (ΔEST) of 0.51, 0.6, and 0.63 eV, respectively, were found, which rules out the possibility of thermally activated delayed fluorescence. In addition, the PL decay spectra for the emission bands near the T1 peaks of the 2PB-AC, 2Me2PB-AC, and 2T2PB-AC powders were investigated. The corresponding PL lifetimes were 7.67, 3.63, and 1.81 ns, respectively ( Supporting Information Figure S6 and Table S1), further confirming that they are all fluorescent compounds. To further decipher the emission mechanism in aggregate states, a single crystal of 2PB-AC (CCDC 2095218) was grown successfully in a DCM/n-hexane mixture by slow solvent evaporation, and its crystal structure was analyzed by X-ray diffraction crystallography. The results show that the 2PB-AC in the crystalline state adopted a planar conformation with angles in the range of 5–46° ( Supporting Information Figure S7a). No close π–π stacking was found, but multiple intermolecular C–H···π interactions with distances of 3.052–3.808 Å were observed ( Supporting Information Figure S7b). Moreover, strong intermolecular C–H···N hydrogen bonding interactions with a distance of 2.645 Å were also observed between molecules. These multiple and strong intermolecular interactions can effectively rigidify molecular conformations and reduce nonradiative energy dissipation in aggregate states.35,39 Therefore, 2PB-AC in the crystalline state exhibited bright blue emission with a ФPL as high as 95.3% ( Supporting Information Figure S8). In addition, as shown in Supporting Information Figure S9, the PL decay spectrum of the 2PB-AC crystal exhibited single-exponential decay with short exciton lifetimes (4.25 ns), further confirming that the 2PB-AC crystal emits fluorescence. To better understand the emission behaviors of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC, their singlet and triplet energy levels and SOC were studied. As shown in Figure 3, the energy gap between S1 and T1 states of 2PB-AC is 0.46 eV, while that between S1 and T2 is as low as 0.06 eV. Furthermore, the SOC matrix element value <S1|Hso|T2> is only 0.05. These results might rule out the possibility of an ISC process causing the intense emission of 2PB-AC. However, an obvious ISC process was observed for 2Me2PB-AC because large SOC matrix element values for the close-lying S1-T2 and S1-T3 were deduced. In addition, the main triplet states were located below the S1 states, as indicated by the thermodynamically favorable ISC process.40 Furthermore, compositional analysis of the related excited states was performed to verify the validity of the ISC process ( Supporting Information Figure S10). The results show that the holes and particles of the S1 state were distributed on the TPA moiety and CN group, respectively, exhibiting an evident charge-transfer (CT) state. In contrast, for T2 and T3, the holes and particles were centered on the TPA moiety or CN group, which are indicative of a typical local-excited state. These results correspond to the allowed El-Sayed’s rule of 2Me2PB-AC for S1-T2 and S1-T3. Therefore, the ФPL value of 2Me2PB-AC is lower than that of 2PB-AC. Furthermore, the introduction of sulfur atoms facilitates the ISC processes for the close-lying S1-T2 and S1-T3 with the allowed El-Sayed’s rule and large SOC matrix element values, which further reduced the ФPL of 2T2PB-AC compared with that of 2Me2PB-AC. Therefore, the attachment of side chains to the terphenyl core can regulate ISC processes and fine-tune the emission of its derivatives. Figure 3 | Relative excitation energies, the SOC matrix element values and their corresponding hole and electron wavefunctions of 2PB-AC, 2Me2PB-AC, and 2T2PB-AC (hole and electron wavefunctions are shown below and above in the schematic). Download figure Download PowerPoint Photoactivated phosphorescence of [email protected] films Owing to their thermodynamically favorable ISC processes and in consideration of effective restriction of the nonradiative transitions of guest molecules by the host matrix, we tried to dope 2MePB-AC and 2T2PB-AC into PMMA matrices at very low concentrations to realize RTP (Figure 4a).41–45 Interestingly, [email protected] showed notable RTP, whereas no obvious RTP was found for [email protected] This phenomenon might be explained by the lower T1 energy level of 2T2PB-AC as compared with 2Me2PB-AC, which leads to a high nonradiative transition decay. This reason was further confirmed by the fact that only 2Me2PB-AC exhibited strong phosphorescence in THF solution at 77 K ( Supporting Information Figure S11). Notably, after continuous photoirradiation with a handheld 365 UV lamp (3 W) for 40 s, the phosphorescence of [email protected] lasted for more than 5 s at room temperature (Figure 4b and Supporting Information Video S1). As illustrated in Figure 4c, 2Me2PB-AC (1 wt %)@PMMA exhibited strong violet-blue fluorescence at 414 nm with a ФPL value of 73.9%, and no obvious phosphorescence was found in its delayed emission spectrum (Figure 4d, delay time = 0.1 ms), because the T1 excitons of 2Me2PB-AC were quenched by the surrounding oxygen molecules.46–49 However, the density of oxygen molecules at the excited spots decreases with longer excitation times, and as a result, [email protected] exhibited strong phosphorescence at 517 nm with phosphorescence quantum yields (ФP) of 19.7% (Figure 4d and Supporting Information Figure S12). To further exclude the influence of the molecule itself on the emission behavior in the [email protected] system before and after photoirradiation, the phosphorescence behavior of 2Me2PB-AC was studied. The lifetimes of 2Me2PB-AC powder with emission peaks at 414 and 517 nm exhibited almost no change before and after photoirradiation for 40 s under ambient conditions ( Supporting Information Figure S13), suggesting that the photoactivation phosphorescence of [email protected] is not because of the change of the molecule itself, such as crystal movement.50 Furthermore, to confirm the oxygen quenching effect, [email protected] was excited in vacuum and air, independently, which showed that a strong green phosphorescence of [email protected] was visible by the naked eye under 365 UV light irradiation only in vacuum ( Supporting Information Figure S14). These results suggest that the photoactivated phosphorescence of [email protected] is caused by the consumption of residual O2 in the PMMA matrix. Moreover, the lifetimes of the emission at 517 nm were 2.39 ns and 331.97 ms before and after photoirradiation, respectively, under ambient conditions (Figure 4e), which is consistent with our observations. Figure 4 | (a) Guest–host strategy for development of efficient flexible RTP materials from nonemissive host (PMMA) and fluorescent guest (2Me2PB-AC). (b) Photophysical behaviors of [email protected] film taken at different time intervals before and after