A Simple Approach to Achieve Organic Radicals with Unusual Solid-State Emission and Persistent Stability

Open AccessCCS ChemistryCOMMUNICATION6 Jun 2022A Simple Approach to Achieve Organic Radicals with Unusual Solid-State Emission and Persistent Stability Xueqian Zhao†, Junyi Gong†, Parvej Alam, Chao Ma, Yanpei Wang, Jing Guo, Zebin Zeng, Zikai He, Herman H. Y. Sung, Ian D. Williams, Kam Sing Wong, Sijie Chen, Jacky W. Y. Lam, Zheng Zhao and Ben Zhong Tang Xueqian Zhao† Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Junyi Gong† Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Parvej Alam Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Chao Ma Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Yanpei Wang State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan Province 410082 , Jing Guo State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan Province 410082 , Zebin Zeng State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan Province 410082 , Zikai He School of Science, Harbin Institute of Technology, Shenzhen, HIT Campus of University Town, Shenzhen, Guangdong Province 518055 , Herman H. Y. Sung Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Ian D. Williams Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Kam Sing Wong Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Sijie Chen Ming Wai Lau Centre for Reparative Medicine, Karolinska Institutet, Hong Kong 999077 , Jacky W. Y. Lam Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 , Zheng Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Science and Engineering, Chinese University of Hong Kong, Shenzhen, Guangdong Province 518172 and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 School of Science and Engineering, Chinese University of Hong Kong, Shenzhen, Guangdong Province 518172 HKUST-Shenzhen Research Institute, South Area Hi-Tech Park, Nanshan, Shenzhen, Guangdong Province 518057 Center for Aggregation-Induced Emission, SCUT-HKUST Joint Research Institute, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong Province 510640 AIE Institute, Guangzhou Development District, Huangpu, Guangzhou, Guangdong Province 510530 https://doi.org/10.31635/ccschem.021.202101192 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Stable organic radicals are promising materials for information storage, molecular magnetism, electronic devices, and biological probes. Many organic radicals have been prepared, but most are non- or weakly emissive and degrade easily upon photoexcitation. It remains challenging to produce stable and efficient luminescent radicals because of the absence of general guidelines for their synthesis. Herein, we present a photoactivation approach to generate a stable luminescent radical from tris(4-chlorophenyl)phosphine (TCPP) with red emission in the crystal state. The mechanistic study suggests that the molecular symmetry breaking in the crystal causes changes of molecular conformation, redox properties, and molecular packing that facilitates radical generation and stabilization. This design strategy demonstrates a straightforward approach to develop stable organic luminescent radicals that will open new doors to photoinduced luminescent radical materials. Download figure Download PowerPoint Introduction Stable luminescent radicals or doublet emitters are next-generation luminophores in which the emission originates from the radiative decay of the unusual lowest doublet excited state (D1) to the doublet ground state (D0).1–3 Their potential applications in, for example, organic light-emitting diodes (OLEDs),4 light sources,5 and chemical sensors are explored.6–8 Recently, it has been demonstrated that radical emitters can easily achieve the upper limit (100%) internal quantum efficiency of OLEDs due to their unusual doublet nature.9,10 Thus, the potential of luminescent radicals for practical applications have attracted considerable attention in the last decade and have brought this research field back to life. Thanks to the extensive efforts of scientists, several luminescent radicals, such as tris-2,4,6-trichlorophenylmethyl,11 perchlorotriphenylmethyl,12 and biphenylmethyl,7,13–15 have been successfully developed as luminous cores by exploiting their steric and delocalization effects. In addition, strategies such as chemical modification,16–18 physical doping,19 and even supramolecular assembly20–23 have been utilized to improve their stability and luminescence quantum yield (QY). In spite of this gratifying progress, stable radical species showing strong light emission are still relatively rare and mainly limited to triarylmethyl radicals.8,24–27 On the other hand, similar to many conventional luminophores, luminescent radicals exhibit low luminescence quantum efficiency at high solution concentrations or in the solid state due to the aggregation-caused quenching effect,28 and such a problem must be solved because thin films or aggregates of luminescent materials are widely utilized in practical applications. However, the research on luminescent radicals is still in the preliminary stage. Besides, most of the stable luminescent radicals are generated by time-consuming chemical synthesis. As a result, the development of new methods to generate luminescent radicals is challenging but also exciting and meaningful. In this work, we demonstrate that the photoactivation of tris(4-chlorophenyl)phosphine (TCPP) crystals can generate stable radicals with unusual red emission at ambient conditions. Figure 1a shows the schematic diagram of the photoinduced radical generation process. X-ray crystallographic analysis combined with density functional theory (DFT) calculations reveal that the symmetry breaking in TCPP crystals upon UV irradiation results in charge separation, which serves as a driving force for the generation of stable solid-state emissive radicals. The TCPP radicals exhibit persistent stability and show an emission half-life of over one week even when stored at ambient conditions. Interestingly, the red radical emission is quenched only by destroying the crystal packing or irradiating it with strong visible light, suggesting that crystal packing acts as a protective cage to stabilize the formed radicals. After recrystallization, the ground sample can regenerate stable radicals again by UV exposure. Taking advantage of the reversible feature of radical generation, a photocontrollable anticounterfeiting film was demonstrated. Although there have been some achievements in generating stable luminescent radicals by wet chemistry reactions, new strategies for the design and synthesis of stable and efficient radicals in the absence of any external additives are also highly desirable. Thus, the current work may open up a new avenue of achieving stable and photoresponsive solid-state luminescent radicals. Results and Discussion TCPP sample was purchased from a commercial source and purified by multiple column chromatography followed by recrystallization three times in dichloromethane and hexane mixture before use. The structure of TCPP was confirmed by NMR spectroscopies and high-resolution mass spectrometry (HRMS) as well as single-crystal X-ray crystallography ( Supporting Information Figures S1–S4). After recrystallization, the final products appeared as colorless both in solution and the solid state and were almost nonemissive ( Supporting Information Figure S5). The UV–vis spectrum of TCPP crystals exhibited an absorption band centered at around 250 nm, which was consistent with its colorless appearance (Figure 1b). However, upon 365-nm UV irradiation for a few seconds, the colorless crystals gradually turned orange. Two new absorption peaks that appeared at 496 and 520 nm were suggestive of the generation of new species with narrow band gaps. As most of the triphenylphosphine derivatives are electron-rich, they lose electrons readily during the oxidation process to generate several oxidized products such as radicals, oxides, or homocoupling products. These oxidized products often show redder absorption than their neutral counterparts.29–31 To verify the origin of the new photogenerated species, high-performance liquid chromatography (HPLC) and 31P NMR analysis were carried out. Results show that the signals at a retention time of 2.3 min and a chemical shift of 27.2 are in good agreement with those of TCPP oxide ( Supporting Information Figure S6). However, a mixture of TCPP oxide and pure TCPP was found to be colorless rather than orange. Thus, it is speculated that the orange color originates from TCPP radicals because TCPP oxide can be generated from the radicals. To verify this hypothesis, electron paramagnetic resonance (EPR) spectroscopy was carried out which indicated that the i-TCPP (irradiated TCPP) crystals exhibit a strong radical signal with g value of 2.0035. On the other hand, the pristine TCPP crystals were radical silent (Figure 1c). We speculated that the crystal structure can only protect the radicals generated inside the core but not on the surface. Hence, only a trace amount of TCPP oxide will be formed by the reaction of TCPP radicals with ambient oxygen. Therefore, these results indicate that the photoirradiation of TCPP crystals can generate radicals inside the crystal core. The three-dimensional image reconstruction of crystalline i-TCPP by confocal microscopy confirmed the existence of luminescent radicals inside the crystal core ( Supporting Information Figure S7). Interestingly, i-TCPP showed bright red emission at 620 nm with a QY of 4.6%, which was in great contrast with the nonemissive characteristic (QY < 0.1%) of TCPP in toluene solution or in the crystal state (Figure 1d). This typical behavior of i-TCPP is similar to that of luminogens with aggregation-induced emission,32–34 which exhibit strong emission in the aggregate or solid state but weak or no emission in solution ( Supporting Information Figure S8). The photoluminescence (PL) emission lifetime of i-TCPP crystals was measured to be 3.9 ns, suggestive of the fluorescent nature of the light emission (Figure 1e). To elucidate the origin of the red emission, a further detailed investigation of the photophysical property was conducted. Because impurity even in trace amounts may exert great influence on the luminescent property,35,36 the purity of TCPP was confirmed by HPLC and elemental analysis (EA). The difference between the obtained EA value and the theoretical value is <0.3% which suggests the high purity of the present molecule ( Supporting Information Figure S6a). Different batches of TCPP samples were also purchased from different commercial sources and purified through the same procedure. Despite the fact that they exhibit varied luminescence properties, all the purified samples showed identical optical properties with no or weak emission, demonstrative of excellent data reproducibility and reliability ( Supporting Information Figure S9). It is now clear that pure TCPP crystals generate red-emitting stable radicals readily under UV photoirradiation. As both radicals and TCPP oxide are formed after photoirradiation and the TCPP oxide is nonemissive (QY < 0.1%), the red emission must be due to the radical formation. Notably, the red emission of i-TCPP crystals along with its EPR signal disappears upon grinding ( Supporting Information Figure S10). This observation supports the significance of the crystalline state where the crystal lattice acts as a protective cage to protect the TCPP radicals from water and atmospheric oxygen. Figure 1 | (a) Schematic diagram of the photoinduced radical generation process. (b) UV–vis spectra, (c) EPR spectra, and (d) PL spectra of TCPP crystals before and after UV light exposure for seconds at room temperature, λex = 365 nm. The insets in (b) and (d) show the photographs of color and emission changes, QY = quantum yield. (e) Time-resolved PL decay curves of i-TCPP measured at λmax = 620 nm at room temperature, λex = 400 nm. Download figure Download PowerPoint To understand the optical properties and the generation mechanism of TCPP radicals, we analyzed TCPP's single-crystal structure and molecular stacking. Single-crystal structure analysis (Figure 2a and Supporting Information Table S1) indicates that TCPP adopts a D2h symmetry rather than the commonly observed C3 symmetry for most of its analogs like triphenylphosphine ( Supporting Information Table S2). The dihedral angles of the phenyl rings are not all identical. In particular, the value (87.6°) of one pair (R2–R2′) was much larger than the others (66.4° and 50.8°). Another structural characteristic is that one phenyl ring (R1) is almost parallel with the lone pair of the phosphorus atom. However, the optimized molecular structure of TCPP in the gas phase exhibited a C3 conformation, and the dihedral angles of the phenyl rings were all equal. This indicates that the symmetry breaking leads to the D2h conformation of the TCPP molecule in the crystal state, which was first noticed and described as an "abnormal" conformation by Bin Shawkataly et al. in 1996.37 Although TCPP adopts no donor–acceptor structure, DFT calculations based on its D2h point group show distinct charge separation with the highest occupied molecular orbital (HOMO) located mainly on the phosphorous atom and the phenyl ring that is parallel with the plane. On the other hand, the lowest unoccupied molecular orbital (LUMO) was located mainly on the two phenyl rings vertical to the plane. No HOMO and LUMO separation of TCPP was observed in the gas phase (Figure 2b). It is worth noting that molecules with charge separation are promising for generating radicals through the photoinduced electron transfer (PET) process. However, the radicals generated are often transient, sensitive, and quickly react with water and oxygen which lowers their stability at ambient conditions. On the contrary, the TCPP radicals show excellent stability at ambient conditions, which may be correlated with their asymmetric conformations and crystal structures. Further crystal analysis shows that the TCPP molecules adopt an alternate intermolecular donor–acceptor arrangement with the closest distance between phenyl rings of about 3.29 Å, which is beneficial for the exciton separation and stabilization through the electron hopping mechanism (Figure 2c). In consideration of the considerable stability of the radicals within i-TCPP crystals and the molecular arrangement caused by this unique symmetry breaking, we propose that the red emission of i-TCPP crystals is due to TCPP radicals generated through the PET process (Figure 2d).22,38–40 Figure 2 | (a) Molecular conformations of neutral TCPP obtained by DFT using BLYP/def2-SVP in the gas state (top) and crystal state (bottom) with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity. (b) Electron cloud distributions of TCPP in the gas and crystal states calculated by TD-DFT M062X/def2-TZVP, ORCA 4.1 program. (c) Crystal packing diagrams of TCPP. (d) Proposed mechanism of the photoinduced single-electron transfer process. Download figure Download PowerPoint To confirm the rationality of our hypothesis, a theoretical calculation was carried out based on the high-precision double-hybrid functional PWPB95 with a def2-TZVP basis set. The free energy of the TCPP dimer and the TCPP radicals (cations and anions) in the ground state as well as that of the dimer in the excited state was calculated (Figure 3a). Results show that both radical cations and anions exhibit a much lower free energy than the dimer in both ground state and excited state, suggesting that the formation of radicals is thermodynamically favorable. Interestingly, the optimized molecular configuration of the TCPP radical anion is almost identical to that of the TCPP crystal, and both belong to a point group of D2h (Figure 3b and Supporting Information Table S3). It is worth mentioning that despite the very crowded stacked structure, no interaction, such as a CH···π interaction, exists in the packing structure. Therefore, the molecular configuration of TCPP in the crystal state is responsible for the formation and stabilization of radicals.41,42 The spin distribution of TCPP radicals in the crystal structure was also calculated (Figure 3c). For the TCPP radical cation, the unpaired electron is mainly distributed on the phosphorus atom and the benzene ring, parallel to the principal plane. For the TCPP radical anion, the unpaired electron is distributed on benzene rings that are vertical to the plane. Such distributions suggest that the radicals formed are heterogenous instead of homogenous, which is consistent with the complex hyperfine splitting pattern of the EPR spectrum. The absorption and emission spectra of TCPP radicals are also simulated by time-dependent (TD)-DFT (Figures 3d and 3e). Interestingly, the superimposed absorption spectra of TCPP radical cation and anion show two peaks at 457 and 482 nm, which correlate well with the experimental data and suggest the reliability of the calculation results. Furthermore, in-depth analysis indicates that the two absorption peaks at 457 and 482 nm arise from the HOMO–9→singly occupied molecular orbital (SOMO) transition of the radical cation and the SOMO→LUMO+7 transition of the radical anion, respectively (Figure 3d). The detailed energy level diagrams and wave functions of the frontier molecular orbitals of the TCPP radical anion and cation are shown in Supporting Information Figure S11. The emission spectra of the TCPP radical cation and anion are also calculated using TD-DFT calculations based on optimized structures in the first excited state by M062X/def2-TZVP with two emission bands centered at around 549 and 667 nm and oscillator strengths of up to 0.021 (SOMO, LUMO+7) and 0.105 (HOMO-4, SOMO), respectively ( Supporting Information Table S4 and Figure S12). Therefore, the red emission of the i-TCPP crystal is mainly contributed by its radical cations, which is also confirmed by the excitation spectra ( Supporting Information Figure S13). Figure 3 | (a) Schematic diagram showing the free energy profile of the process obtained by DFT using PWPB95/def2-TZVP. (b) Molecular structures of TCPP radical cation and anion in the gas phase obtained by DFT using the BLYP/def2-SVP. (c) The spin density distribution of TCPP radical anion and cation with single point (SP) calculation based on structures in the gas phase by DFT using the M062X/def2-TZVP. (d–e) Absorption and emission spectra of TCPP radical anion and cation simulated on TD-DFT calculation using PBE0/def2-TZVP with structures in the gas phase. Download figure Download PowerPoint The bright red emission of the i-TCPP crystal enables it to serve as a potential optical crystalline material and also a reporter to evaluate the stability of radicals through a visualization method by measuring the emission change with time. As shown in Figure 4a, i-TCPP crystals were placed at ambient conditions without any protection, and their emission signals were measured by a PL machine at different intervals of time. The radicals and their red emission can survive for more than one week in the crystal state at ambient conditions. Notably, re-photoirradiation of the solid sample by UV light can regenerate the radicals. This emission is an "on–off" process that can be repeated many times and is technically feasible by alternate irradiation with UV and visible light (Figure 4b). Figure 4 | Plots of relative PL intensity at 620 nm under (a) ambient condition and (b) under UV (365 nm) or visible light (520 nm) irradiation. Download figure Download PowerPoint Interestingly, a TCPP-loaded filter paper shows pressure responsiveness upon UV irradiation which makes it possible to utilize as a smart responsive material for applications such as photopatterning or anticounterfeiting (Figures 5a and 5b). By immersing a filter paper in a dichloromethane solution of TCPP, a TCPP-loaded filter paper was prepared. By stamping the freshly soaked paper with a seal of a Chinese character "唐" of Tang and then irradiating the paper with UV light, the patterned area exhibited a red emission while the rest of the paper only showed autofluorescence. The red emission could be erased upon exposure to a green lamp (520 nm). Since the radicals are only stable in the crystal state, we speculate that exerting pressure can induce the formation of microcrystals in the filter paper. Figure 5 | (a) Preparation of an anticounterfeiting paper with a Chinese character of "唐" (Tang). (b) The fluorescence images were acquired upon exposure with a UV lamp (254 nm) and erased upon exposure with a green lamp (using a 520-nm infrared semiconductor laser). Scale bar: 0.5 cm. Download figure Download PowerPoint Conclusion The photoinduced generation of radicals in TCPP crystals with bright red emission has been reported. A mechanistic study reveals that radicals are generated within the TCPP crystals, and the red emission is mainly ascribed to the radical cation. Theoretical calculation integrated with crystal structure analysis indicates that the unique symmetry breaking results in molecular conformation and photoredox property change of TCPP crystals, facilitating the generation of radicals through PET. The TCPP crystal lattice protects the radicals very well from quenching by water and oxygen to endow the TCPP radicals with considerable stability at ambient conditions. The present results not only demonstrate the possibility of in-situ generation of stable radicals with the assistance of light irradiation but also open a new window to producing photoresponsive stable luminescent radicals. Further potential applications of this material are being explored. Footnote CCDC 2080878 contains the supplementary crystallographic data for TCPP.37 These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (see www.ccdc.cam.ac.uk). Supporting Information Supporting Information is available and includes materials and methods, characterizations, DFT calculations, crystallographic data, photophysical data, and imaging data. Conflicts of Interest There are no conflicts to declare. Funding Information This project was financially supported by the National Natural Science Foundation of China (grant no. 21788102), the Natural Science Foundation of Guangdong Province (grant nos. 2019B121205002 and 2019B030301003), the Research Grants Council of Hong Kong (grant nos. 16305618, 16305518, C6014-20W, C6009-17G, and AoE/P-02/12), the National Key Research and Development Program (grant no. 2018YFE0190200), the Innovation and Technology Commission (grant no. ITC-CNERC14SC01), and the Science and Technology Plan of Shenzhen (grant nos. JCYJ20180306174910791, JCYJ20170818113530705, JCYJ20170818113538482, and JCYJ20160229205601482). Preprint Acknowledgment Research presented in this article was posted on a preprint server prior to publication in CCS Chemistry. 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 4Issue 6Page: 1912-1920Supporting Information Copyright & Permissions© 2021 Chinese Chemical SocietyKeywordsphotoinduced electron transfersymmetry breakinganticounterfeitingluminescent radicalstris(4-chlorophenyl)phosphineAcknowledgmentsWe gratefully acknowledge the contributions of Lili Du and PHILLIPS, David Lee at the University of Hong Kong (HKU), Zhijiao Tang at Shenzhen University, Congwu Ge and Xike Gao at the Shanghai Institute of Organic Chemistry (SIOC) Chinese Academy of Sciences (CAS), Qiang Wei at the Ningbo Institute of Industrial Technology (CNITECH), Chinese Academy of Sciences (CAS), Xing Feng at the Guangdong University of Technology, and Ruoyao Zhang at the Hong Kong University of Science and Technology for the experimental measurements. Downloaded 3,224 times PDF DownloadLoading ...