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Organic long-persistent luminescence stimulated by visible light in p-type systems based on organic photoredox catalyst dopants | Nature Materials
Abstract . Organic long-persistent-luminescent (OLPL) materials demonstrating hour-long photoluminescence have practical advantages in applications owing to their flexible design and easy processability. However, the energy absorbed in these materials is typically stored in an intermediate charge-separated state that is unstable when exposed to oxygen, thus preventing persistent luminescence in air unless oxygen penetration is suppressed through crystallization. Moreover, OLPL materials usually require ultraviolet excitation. Here we overcome such limitations and demonstrate amorphous OLPL systems that can be excited by radiation up to 600?nm and exhibit persistent luminescence in air. By adding cationic photoredox catalysts as electron-accepting dopants in a neutral electron-donor host, stable charge-separated states are generated by hole diffusion in these blends. Furthermore, the addition of hole-trapping molecules extends the photoluminescence lifetime. By using a p-type host less reactive to oxygen and tuning the donor–acceptor energy gap, our amorphous blends exhibit persistent luminescence stimulated by visible light even in air, expanding the applicability of OLPL materials. You have full access to this article via your institution. Download PDF Download PDF Main . Long-persistent luminescence (LPL) is a phenomenon in which emission continues for a long period after photoexcitation 1 . LPL emitters are used as glow-in-the-dark paints for clock faces and emergency lights. High-efficiency LPL materials are composed of metal-oxide microcrystals and small amounts of rare-earth ions that act as charge-trapping and emission sites 2 , 3 . In these inorganic LPL materials, holes or electrons generated by the photoexcitation of the metal-oxide crystal are accumulated in dopants that act as charge-trapping sites. Gradual charge recombination followed by thermal detrapping produces hour-long emissions 4 , 5 , 6 . Inorganic LPL materials are insoluble in any solvent, and require grinding into microparticles before dispersion into solvents or polymers for practical applications. Most inorganic LPL systems require ultraviolet to blue excitation light below 450?nm due to the limited metal-oxide absorption bands 7 , 8 , 9 . We have reported LPL emissions from mixtures of organic molecules 10 . This organic LPL (OLPL) system can be fabricated from a solution process 11 and the fabricated films can be transparent and flexible 12 . LPL emissions can be tuned from greenish-blue to red with the addition of fluorescent materials 13 . In contrast to conventional organic room-temperature phosphorescent (RTP) materials 14 , which store their energy in triplet excited states and exhibit radiative transition from the triplet excited states to the singlet ground states 15 , OLPL systems accumulate energy into charge-separated states similar to inorganic LPL materials. LPL and RTP can be identified from their emission decay profiles 16 , 17 , 18 . Current OLPL systems require inert gas conditions to exhibit LPL because LPL is quenched completely in air. These OLPL systems, which consist of an electron-donating and electron-accepting material, store absorbed energy into geminate pairs of air-unstable donor radical cations and acceptor radical anions. A donor–acceptor mixed crystal has been reported to exhibit OLPL in air due to oxygen diffusion being suppressed by crystallization 19 , but crystalline systems cannot take advantage of OLPL material properties, such as flexibility and processability. Air-stable amorphous OLPL systems will enable applications that are unique to organic materials, such as solution process coating and flexible polymer films. For practical applications of OLPL systems, the air stability needs to be improved without crystallization. Organic field-effect transistors use radical cations and anions as the active charge-transport species; however, many air-stable organic transistors have been reported 20 . Radical anions (n-type organic semiconductors) are more unstable than radical cations (p-type organic semiconductors) in air. The reported OLPL system can be considered as an n-type OLPL system due to the low donor concentration where only electrons can diffuse through acceptor molecules (Fig. 1a ). The unstable radical anions can be ascribed to the high probability of a reaction with oxygen. A p-type OLPL system, in which radical cations diffuse (Fig. 1a ), is expected to be more stable to oxygen. Air-stable organic transistors have been achieved by making the lowest unoccupied molecular orbital (LUMO) level deeper than the reduction potential of oxygen (?3.5?eV) 21 , 22 . To improve the air stability of the OLPL systems, a p-type OLPL system with deep highest occupied molecular orbital (HOMO) and LUMO levels is required. Fig. 1: Emission mechanism of a p-type OLPL system. a , Schematic diagrams showing charge-separated states in n-type (left) and p-type (right) OLPL systems. In n-type systems, electrons diffuse through the host acceptor molecules until they recombine radiatively with a radical donor cation. In p-type systems, hole diffusion occurs in a donor host until charges recombine with a radical acceptor anion. b , A mixture of neutral donors and acceptors forms radical ion pairs with Coulombic interaction as the charge-separated (CS) state by photoinduced charge transfer (CT) from the ground state, whereas cationic acceptors or anionic donors form neutral radicals. The weaker Coulombic interaction is expected to reduce the probability of charge recombination (CR). c , Chemical structures of electron acceptors, donors and hole-trapping molecules. d , HOMO and LUMO levels of materials and reduction potential of oxygen. HOMO and LUMO levels were obtained from electrochemical (EC) measurements. When the redox potential was out of the range of electrochemical measurement, the HOMO and LUMO levels were estimated from the optical gap (OG) that was calculated from the absorption edge. e , Schematic diagram of a p-type OLPL with a hole-trapping mechanism. Full size image To satisfy these requirements, we focused on cationic electron acceptors, which are known as a type of organic photoredox catalyst 23 . These organic photoredox catalysts are ideal electron donors or acceptors because of their high reduction or oxidation potential in the excited state and their ability to form a stable one-electron oxidized or reduced state. Many organic photoredox catalysts have a large enough energy gap to exhibit luminescence in the visible region. Furthermore, a mixture of neutral donors and acceptors forms radical ion pairs (D δ+ –A δ? ) with Coulombic interaction by photoinduced charge separation, whereas cationic acceptors or anionic donors form neutral radicals rather than radical anions or radical cations (D δ+ – (A + ) δ? , (D ? ) δ+ –A δ? ) (Fig. 1b ). Even if there are counter-ions, the formation of neutral radicals is expected to reduce the Coulombic interaction in the charge-separated state 19 , 24 , 25 . Further stabilization of the charge-separated states is required despite the reduced Coulombic interaction. Similar to inorganic LPL systems, the charge-separated state is expected to be stabilized by a special separation of the ion pairs by the introduction of carrier-trapping materials 26 , 27 . In this article we report p-type OLPL systems made from cationic electron acceptors as a dopant and neutral electron donors as a host. These systems exhibit LPL in air. The addition of hole-trapping materials improved the performance sevenfold under nitrogen gas without changing the emission spectrum. In addition, the p-type OLPL system can be excited by a wavelength from 300 to 600?nm. By tuning the energy gap between the electron donor and acceptor, near-infrared (NIR) LPL emission is achieved. The development of such visible-light-active p-type OLPL systems that work in air will contribute to various future practical applications of OLPL systems. Cationic photoredox catalysts 2,4,6-triphenylpyrylium tetrafluoroborate (TPP + ) and 2,4,6-tris(methoxyphenyl)pyrylium tetrafluoroborate (MeOTPP + ) were used as electron acceptors 28 , 29 , 30 and semiconducting host molecules 3,3′-di(9 H -carbazol-9-yl)biphenyl ( m CBP) 31 and 1,3,5-tris(1-phenyl-1 H -benzimidazol-2-yl)benzene (TPBi) 32 were used as electron donors (Fig. 1c ). 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TCTA) 33 , 4,4-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl(α-NPD) 34 and 2,8-bis(diphenylphosphoryl)dibenzo[ b , d ]thiophene (PPT) 35 were used as hole-trapping materials. The analytical results of these materials by ultraviolet–visible absorption, fluorescence, and phosphorescence spectra are shown in Supplementary Fig. 1 . Energy levels of the lowest singlet excited state ( 1 LE) and triplet excited states ( 3 LE) were estimated from the onset of fluorescence and the phosphorescence spectra, respectively. HOMO and LUMO levels were obtained from the first redox peaks of the cyclic voltammograms or differential pulse voltammograms (Supplementary Fig. 2 ). When the redox potential was out of range of electrochemical measurement, the HOMO and LUMO levels were estimated from the optical gap calculated from the absorption edge (Fig. 1d and Supplementary Table 1 ). LPL films with a 1:99 molar ratio of an acceptor:donor system were fabricated by conventional melt casting 11 . The obtained films were considered to be amorphous, based on the X-ray diffraction analysis (Supplementary Fig. 3 ). Steady-state photoluminescence (PL) and LPL spectra, emission decay profiles and steady-state PL quantum yields ( Φ PL ) were obtained under nitrogen gas (Supplementary Fig. 4 ). Since the emission intensity of LPL is much weaker than that of steady-state PL under photoexcitation, the presented Φ PL do not contain emissions from LPL 36 . The LPL duration was defined as the time until the emission intensity dropped below 1?pW after 300?s of excitation. The triplet charge-transfer excited state ( 3 CT) level was assumed from the singlet charge-transfer excited state ( 1 CT) level that was obtained from the onset of the PL spectrum as most LPL systems have a small energy gap between the 1 CT and 3 CT (ref. 37 ). When the films were excited by 365?nm light, the TPP + /TPBi, TPP + / m CBP and MeOTPP + / m CBP films exhibited a long LPL emission in which the decay profiles followed a power-law decay (Fig. 2a ) 38 , 39 . Because of the strong electron-accepting properties of TPP + , the mixed film exhibited an appreciable CT absorption over 500?nm (Supplementary Fig. 1 ). This power-law emission decay indicates the generation of intermediate charge-separated states and successive gradual bulk charge recombination, which led to LPL. The emission spectra of these films were attributed to the CT excited states between the donors and acceptors 16 , 17 . Fig. 2: Photoluminescence properties of OLPL systems under nitrogen gas. a , Emission decay profiles (excitation: 365?nm, 1?mW, 300?s, 300?K, under nitrogen) of TPP + /TPBi, TPP + /mCBP, MeOTPP + /TPBi and MeOTPP + /mCBP films on a log–log plot. b , Steady-state PL spectra and emission spectra 10?s after photoexcitation (LPL) (excitation: 365?nm, 1?mW, 300?s, 300?K, under nitrogen). The LPL spectrum of the TPP + /mCBP was not measurable because of the weak emission intensity excited at 365?nm. c , Excitation-wavelength dependence of emission decay profiles of TPP + /TPBi film (excitation: 300?mW, 300?s, 300?K, under nitrogen). Source data Full size image The TPP + /TPBi, TPP + / m CBP and MeOTPP + / m CBP films form 1 CT excited states (Fig. 2b ) and the 1 CT energy levels are lower than those of the locally excited states of donors and acceptors ( 3 LE D and 3 LE A ). Therefore, the LPL that was caused by the recombination of accumulated charges occurs from the CT states. Because the Φ PL of the CT excited states is low, the non-radiative deactivation pathway from the CT excited state to the ground state is also available. The TPP + /TPBi film exhibits a CT emission at 603?nm with a shoulder peak at ? 555?nm. The shoulder peak decreased with an increase in TPP + concentration because of the self-absorption of TPP radicals at 500–600?nm (Supplementary Fig. 5a ) 17 , 40 , 41 and disappeared at the higher TPP + concentration due to the strong self-absorption of the TPP radical (Supplementary Fig. 5b ). The TPP + /TPBi film showed the longest LPL duration of 1,830?s. The LPL duration decreased with an increase in TPP + concentration because the charge recombination probability increased at the higher TPP + concentration (Supplementary Fig. 5c ). The TPP + / m CBP film exhibited a broad NIR emission at 731?nm from the smaller energy gap between the TPP + LUMO and m CBP HOMO. Because of the low Φ PL of the CT emission and low NIR sensitivity of the photodiode for detection, the LPL duration was 33?s. The MeOTPP + / m CBP films exhibited a CT emission at 624?nm with an LPL duration of 610?s. In contrast, the MeOTPP + /TPBi film did not form a CT excited state although the HOMO and LUMO levels were appropriate. The TPBi may not act as a donor because of the closed HOMO levels between them. Instead, this film exhibited the fluorescence and RTP of the MeOTPP + (Supplementary Fig. 6 ). These results indicate that the formation of the lowest 1 CT state is important for efficient LPL emission in p-type OLPL systems. As a blended film of donor and acceptor molecules has donor, acceptor and charge-transfer absorption bands, OLPL systems can be excited by various wavelengths, offering a major advantage over inorganic LPL systems, which are limited mostly to ultraviolet to blue excitation wavelengths. The excitation spectra of the TPP + /TPBi and MeOTPP + / m CBP films indicate that these films can be excited by wavelengths from 300 to 600?nm (Supplementary Fig. 7 ). To confirm the excitation wavelength dependence of the LPL emission, these films were excited by 365, 400, 455, 500, 550 and 600?nm light-emitting diodes (LEDs). The LPL emission was observed at all excitation wavelengths, although the LPL duration decreased, which correlates with the absorption intensity (Supplementary Figs. 2c and 6c ). Unlike the emission lifetime from normal excited states, such as conventional fluorescence and phosphorescence, the duration of the LPL depends on the number of stored charges and varies with the excitation wavelength. When the hot exciton mechanism is considered as the charge separation process, the charge separation efficiency also depends on the excitation wavelength 42 , 43 . The 600?nm photoexcitation and NIR LPL emission, which corresponds to the biological window, is expected to be suitable for bioimaging 44 . The LPL performance improved sevenfold with addition of hole-trapping material to the p-type OLPL system. Previously, we reported that the LPL duration could be enhanced by doping electron-trapping materials into an n-type OLPL system 13 . The dopants can receive electrons from the acceptor molecules because the LUMO of the dopant is lower than that of the acceptor. By tuning the appropriate LUMO level, the trapped electrons remained for more than one week 45 . In the p-type OLPL system, TCTA was doped into the TPP + /TPBi system as the HOMO level of the TCTA (?5.3?eV) was shallower than that of the TPBi (?6.0 eV) (Fig. 1d and Supplementary Fig. 2 ). The PL and LPL spectra of the TPP + /TPBi/TCTA film with a 1:99:1 molar ratio (Fig. 3a ) was nearly identical to that of the TPP + /TPBi film (Fig. 2b ), except that the LPL duration time was significantly improved almost sevenfold, to 14,600?s (Fig. 3b ). The TPBi/TCTA (99:1) film also exhibited no LPL because of the closed LUMO levels between them, although it formed a CT excited state (Supplementary Fig. 8 ). This result indicates that the TCTA can act as a hole trap (Fig. 3c ). Fig. 3: Photoluminescence properties of TPP + /TPBi/TCTA film under nitrogen gas. a , PL and LPL (delay, 10?s) spectra of TPP + /TPBi/TCTA film (excitation: 365?nm, 1?mW, 300?s, 300?K, under nitrogen). b , Emission decay profiles (excitation: 365?nm, 1?mW, 300?s, 300?K, under nitrogen) of TPP + /TPBi, TPP + ?/TPBi/PPT, TPP + /TPBi/mCBP, TPP + /TPBi/TCTA and TPP + /TPBi/α-NPD films. c , Emission mechanism of TPP + /TPBi/TCTA film. The TCTA molecule accepts holes from TPBi and forms radical cations as the HOMO level of the TCTA is shallower than that of the TPBi. Thermal detrapping from TCTA regenerates radical cations of TPBi and recombines with TPP radical. d , Top: delta absorption spectra of TPP + /TPBi, TPP + /TPBi/TCTA and TPBi/TCTA films before and after photoexcitation (300?K, under nitrogen); bottom: absorption spectra of TCTA in DCM that contains 0.1?M TBAPF 6 with and without electrical oxidation (300?K, under nitrogen). Absorption data at 1650–1700?nm were omitted because of the quartz substrate absorption. Source data Full size image To confirm the hole trapping by TCTA, the absorption spectra of TPP + /TPBi, TPP + /TPBi/TCTA and TPBi/TCTA films were obtained 45 . After photoexcitation, a clear broad absorption above 800?nm was observed in the TPP + /TPBi/TCTA film (Fig. 3d ). This peak corresponds to the radical cation of TCTA in dichloromethane (DCM) under electrochemical oxidation, although a wavelength shift can be observed because of the polarization effect in the solid film (Fig. 3d ). In contrast, the TPBi/TCTA film exhibited no absorption of the TCTA radical cation after photoexcitation (Fig. 3d ). Therefore, the formation of the TCTA radical cations in the TPP + /TPBi/TCTA film indicates a hole diffusion from a TPBi radical cation to a TCTA molecule. The hole trapping was confirmed by the temperature dependence of the LPL duration 46 . Because hole detrapping is endothermic, the LPL intensity increased by raising the temperature in the TPP + /TPBi/TCTA film (Supplementary Fig. 9b ). In contrast, the LPL duration of the TPP + /TPBi film decreased gradually with an increase in temperature because the non-radiative process was enhanced at high temperatures (Supplementary Fig. 9c ). Thus, detrapping in the TPP + /TPBi/TCTA film for the LPL intensity overwhelmed the non-radiative process at the measured temperatures. To confirm the generality of the hole-trapping mechanism, several trap molecules with different HOMO levels (PPT, m CBP and α-NPD) were investigated (Supplementary Fig. 10 ). While the LPL emission spectra were the same for all systems, the emission duration varied significantly. These differences mainly depend on the HOMO levels, suggesting the LPL emission originates from the hole diffusion mechanism. Since the HOMOs of PPT (?5.9?eV) and m CBP (?5.7?eV) are comparable to that of TPBi (?6.0?eV), they do not act as effective traps because the holes can easily detrap at room temperature. However, the HOMOs of both TCTA (?5.3?eV) and α-NPD (?5.2?eV) are located at much shallower levels compared with that of TPBi, and therefore provide effective hole traps. The generation of α-NPD radical cation was also confirmed by excited-state absorption spectra (Supplementary Fig. 10e ). These results suggest the large HOMO gap (<0.5?eV) is required for efferent hole traps at room temperature. Optical analysis of TPP + /TPBi, MeOTPP + / m CBP and TPP + /TPBi/TCTA films was carried out in air to confirm the LPL emission of the p-type OLPL systems because the LUMO levels of the TPP + (?4.0?eV) and MeOTPP + (?3.8?eV) are lower than the reduction potential of oxygen (?3.5?eV). Although the reported n-type OLPL system of m -MTDATA/PPT showed no LPL emission in air (Supplementary Fig. 9a ), all p-type OLPL films exhibited LPL in air (Fig. 4a,b and Supplementary Fig. 11b ). However, the observed LPL durations of all films in air were shorter than in nitrogen because the energy transfer from the triplet excited state of the emitters to the molecular oxygen with a triplet ground state (triplet quenching) cannot be prevented 47 . An irreversible reaction between TPP radical and oxygen is known 48 ; however, this reaction is considered to be slow in the solid state because the LPL intensity recovers considerably with evacuation (Supplementary Fig. 12 ). On the other hand, because of the irreversible reaction with oxygen, the emission intensity does not recover completely. The charge recombination in LPL emission generates 1 CT and 3 CT excited states 49 , and the 3 CT excited states are quenched by oxygen. In contrast, the emission spectra did not change because of the lack of 3 LE contribution (Fig. 4c,d ). In addition to oxygen, water also can deactivate the radicals. To confirm the LPL quenching by water, the crushed TPP + /TPBi/TCTA sample was dispersed into water and excited at 365?nm (Supplementary Fig. 13 and Supplementary Video 1 ). As a result, LPL was also observed in water. Due to the slow diffusion of water into the solid samples, the effect of quenching by water is very small. Fig. 4: Optical properties of OLPL systems in air. a , b , LPL duration under nitrogen and in air of TPP + ?/TPBi ( a ) and TPP + ?/TPBi/TCTA ( b ) films excited at 365?nm. The optical properties were obtained under nitrogen, and then after exposure to air for 24?h. The sample was evacuated again, and the second measurements were repeated. c , d , PL and LPL (delay, 10?s) spectra in vacuum and in air of TPP + ?/TPBi ( c ) and TPP + /TPBi/TCTA ( d ) films excited at 365?nm. Source data Full size image The LPL duration of the TPP + /TPBi/TCTA in air was extended to 1,685?s, which is almost the same as the TPP + /TPBi film in nitrogen (Supplementary Video 2 ). These results indicate that the p-type OLPL system with deep HOMO levels can emit LPL in air but cannot prevent triplet quenching by oxygen. Future developments of low-oxygen-diffusion hosts or advanced encapsulation techniques to prevent oxygen are required to obtain efficient LPL emissions in air. We demonstrated p-type OLPL systems based on the cationic organic photoredox catalyst TPP + and MeOTPP + as acceptors. These systems can be excited by a wavelength from the ultraviolet range to 600?nm and exhibit a yellow-to-NIR LPL emission. The p-type OLPL with deeper HOMO and LUMO levels can prevent reaction with oxygen and exhibited LPL in air. The hole-trapping dopant enhanced the LPL duration strongly without changing the emission spectrum. The tunable absorption wavelength from 300 to 600?nm of OLPL systems provides a major advantage over inorganic materials, and the absorptions and emissions that were fitted to the biological window are expected to have future bioimaging applications. For efficient charge separation, the oxidized state of the donor and the reduced state of the acceptor must be stable. For the hole-trapping mechanism, the electron donor, acceptor and trap molecules need to have a cascading HOMO and LUMO relationship. The hole-trapping material requires an energy gap of about 0.5?eV shallower than the acceptor’s HOMO level for efficient hole trapping. For efficient LPL emission, the CT excited state needs to be lower than that of the 3 LE. For the melt-cast process, the donor host needs to form good amorphous films. The p-type OLPLs can prevent reaction with oxygen in the excited state, making it possible to produce LPL films by a simple solution processing. However, because triplet quenching by oxygen cannot be prevented perfectly in amorphous films, low-oxygen-diffusion hosts or oxygen barrier materials are required for future practical applications. Future development of p-type OLPL made by polymers will enable printable and flexible film OLPL systems. Methods . Materials . TPP + , MeOTPP + and m -MTDATA were from Merck. TPBi, m CBP and α-NPD were from TCI Chemical. PPT was synthesized according to the literature 35 . Film fabrication . In a nitrogen-filled glovebox, mixtures of electron donors and acceptors were placed on a template glass substrate (surface area, 100?mm 2 ; depth, 0.5?mm) and heated to 280?°C for 10?s. After melting, the substrate was cooled rapidly to room temperature. Optical and electrical measurements . A DCM solution of each material (10 ?5 ?M) was used to measure absorption, fluorescence, phosphorescence and absolute PL quantum yields ( Φ PL ). The absorption spectra were measured using a ultraviolet–visible–NIR spectrophotometer (Lambda 950, Perkin Elmer). The PL spectra at room temperature and the phosphorescence spectra at 77?K were measured using spectrofluorometers (FP-8600, JASCO; PMA-12, Hamamatsu Photonics). The Φ PL were measured using an integrating sphere with a PL measurement unit (Quantaurus-QY, C11347-01, Hamamatsu Photonics) in a glovebox. The sample was excited by a 365?nm LED (0.1?mW) for 60?s. Emission spectra were obtained every 20?ms integration time during photoexcitation (60?s) and after photoexcitation (60?s). PL lifetime was measured using a streak camera system (C14832-110, Hamamatsu Photonics). Samples were photoexcited by a 343?nm femtosecond (pulse width, 190?fs, 1?kHz) laser (Pharos and HERO, Light Conversion). The phosphorescence lifetime of TPP + and MeOTPP + was obtained by time-resolved emission spectra at 77?K measured by spectrometers (PMA-12, Hamamatsu Photonics). Cyclic voltammetry and differential pulse voltammetry were carried out using an electrochemical analyser (Model 610, BAS). Measurements were performed in dried and oxygen-free dimethylformamide or tetrahydrofuran using 0.1?M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as a supporting electrolyte. A platinum wire was used as a counterelectrode, with glassy carbon as a working electrode, and Ag/Ag + as a reference electrode. Redox potentials were referenced against ferrocene/ferrocenium (Fc/Fc + ). Corresponding HOMO and LUMO energies were calculated from first reduction or oxidation peaks using an absolute value of ?4.8?eV to vacuum for the Fc/Fc + redox potential 50 . The absorption spectra of the TPP + , TCTA and α-NPD radical species were measured by the in situ ultraviolet–visible–NIR spectroelectrochemical technique in a UV-3600 Plus (Shimadzu). TPP + , TCTA and α-NPD radical cations were generated by electrical oxidation on a platinum mesh electrode surface in a DCM solution that contained 0.1?M TBAPF 6 (BAE, 013510 SEC-C, thin-layer quartz glass spectroelectrochemical cell kit) using an electrochemical analyser (BAS, Model 610E). The absorption spectra of TCTA and α-NPD radical cations were obtained 30?s after photoexcitation by a 365?nm LED. X-ray diffraction analysis was performed with a D8 Discover (Bruker). LPL measurements . The LPL spectra and decay profiles were obtained using a measurement system in a glovebox 13 . The fabricated films were placed in a dark box and excited by LEDs of various wavelengths with bandpass filters (Thorlabs; band width, ±5?nm), an excitation power of 1?mW?cm ?2 and an excitation duration of 300?s. The PL and LPL spectra were recorded using a multichannel spectrometer (PMA-12, Hamamatsu Photonics). Emission decay profiles were obtained without wavelength sensitivity calibration using a silicon photomultiplier (MPPC module, C13366-1350GA, Hamamatsu Photonics). The temperature dependence and air stability were measured in a cryostat (PS-HT-200, Nagase Techno-Engineering) connected to a turbo molecular pump (HiPace, Pfeiffer Vacuum) and excited for 60?s. The LPL properties were measured under a vacuum. Samples were kept under air (humidity, 30–40%) in the dark for one week and the optical properties were measured in air. Data availability . Source data are provided with this paper. Additional information is available from the authors on request. References . 1. Xu, J. & Tanabe, S. Persistent luminescence instead of phosphorescence: history, mechanism, and perspective. J. Lumin. 205 , 581–620 (2019). CAS ? Google Scholar ? 2. Li, Y., Gecevicius, M. & Qiu, J. Long persistent phosphors—from fundamentals to applications. Chem. Soc. Rev. 45 , 2090–2136 (2016). CAS ? Google Scholar ? 3. 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Google Scholar ? 50. Liu, Y., Liu, M. S. & Jen, A. K.-Y. Synthesis and characterization of a novel and highly efficient light-emitting polymer. Acta Polym. 50 , 105–108 (1999). CAS ? Google Scholar ? Download references Acknowledgements . This work was supported by the Japan Science and Technology Agency (JST) FOREST project (grant number JPMJFR201H); JST ERATO, Adachi Molecular Exciton Engineering Project (grant number JPMJER1305); JSPS KAKENHI (grant numbers JP18H02049 and JP21H02020); JSPS Core-to-core project; the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), the OIST Proof of Concept (POC) Programme; and the Kyushu University Platform of Inter/Transdisciplinary Energy Research, young researcher/doctor student support programme. We thank K. Tokumaru for helpful discussions. We thank K. Kusuhara and N. Nakamura for their assistance with the preparation of TPBi, m CBP and TCTA. Author information . Affiliations . Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan Kazuya Jinnai?&?Chihaya Adachi JST, ERATO Adachi Molecular Exciton Engineering Project, Fukuoka, Japan Kazuya Jinnai,?Ryota Kabe,?Zesen Lin?&?Chihaya Adachi Organic Optoelectronics Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan Ryota Kabe?&?Zesen Lin International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Fukuoka, Japan Chihaya Adachi Authors Kazuya Jinnai View author publications You can also search for this author in PubMed ? Google Scholar Ryota Kabe View author publications You can also search for this author in PubMed ? Google Scholar Zesen Lin View author publications You can also search for this author in PubMed ? Google Scholar Chihaya Adachi View author publications You can also search for this author in PubMed ? Google Scholar Contributions . K.J. and R.K. designed this project. K.J. and R.K. carried out all the experiments. K.J and Z.L. performed the electrochemical and in situ spectroelectrochemical measurements. K.J. and R.K. analysed all data. R.K and C.A. supervised the project. All authors contributed to writing the paper and critically commented on the project. Corresponding authors . Correspondence to Ryota Kabe or Chihaya Adachi . Ethics declarations . Competing interests . The authors declare no competing interests. Additional information . Peer review information Nature Materials thanks Hugo Bronstein and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Supplementary information . Supplementary Information . Supplementary Figs. 1–14 and Table 1. Supplementary Video 1 . A melt casted TPP + /TPBi/TCTA sample was crashed and dispersed into water without nitrogen bubbling in NMR tube. Sample was photoexcited at 365 nm at room temperature. Supplementary Video 2 . LPL emission of TPP + /TPBi in air (top, left), TPP + /TPBi/TCTA in air (top, right), TPP + /TPBi in N 2 (bottom, left) and TPP + /TPBi/TCTA in N 2 (bottom, right). Source data . Source Data Fig. 2 . This contains data plotted in Figs. 2a, 2b (both panels) and 2c. Source Data Fig. 3 . This contains data plotted in Figs. 3a, 3b and 3d (both panels). Source Data Fig. 4 . This contains data plotted in Figs. 4a, 4b, 4c and 4d. Rights and permissions . Reprints and Permissions About this article . Cite this article . Jinnai, K., Kabe, R., Lin, Z. et al. Organic long-persistent luminescence stimulated by visible light in p-type systems based on organic photoredox catalyst dopants. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01150-9 Download citation Received : 16 February 2021 Accepted : 05 October 2021 Published : 29 November 2021 DOI : https://doi.org/10.1038/s41563-021-01150-9 Share this article . Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative .
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