Long term luminescence (LPL) materials have the ability to store and slowly release excited state energy, which has shown great potential in life science, biomedicine, photovoltaic and other fields. At present, the most successful LPL materials are transition metal and rare earth metal materials. Their luminescent time varies from a few minutes to a few hours. Some systems can even last for days or weeks. However, the high cost of inorganic LPL and the strict requirements of the preparation process greatly limit its application. Organic LPL (olpl) materials are easy to be synthesized and processed, and easy to be modified. In order to prolong the luminescent time of olpl, some researchers have used the excited triplet state to significantly improve the efficiency of triplet state at room temperature through the cross of heteroatom, carbonyl functional group, heavy atom effect and polymer enhanced system. In addition to the excited triplet, it is also very important to protect the triplet to prolong its luminescence life. By using crystal, metal organic framework and H-aggregation, great progress has been made in this field. However, it is difficult to compare olpl with LPL in the light-emitting time. Results in order to improve the luminescent time of olpl, the research group of academician Tang benzhong of Hong Kong University of science and technology, inspired by the long-term luminescent mechanism of LPL, prepared a kind of long-term luminescent material of olpl with tpp-3c2b as organic electron trap. These quaternary ammonium phosphors are not only the strong electron acceptor of photoinduced charge transfer, but also the protective trap, so as to stabilize and protect the excitation The free radicals of the system can be slowly recombined and have long-term luminescent properties. The luminescent time is up to 7 hours, which is far higher than the existing olpl level. In this study, a simple method for the preparation of organic long-term phosphors is proposed, which opens a door for the development of new olpl materials. Long term luminescent mechanism of LPL and olpl 1. Schematic diagram of inorganic and organic long term luminescent mechanism. (a) The long-term luminescence of inorganic materials can be realized by electron or hole trap mechanism. In the electron trap, after excitation (Ex), the excited electrons pass through the conduction band to the electron receiving trap; in the hole trap, the electrons propagate through the valence band to fill the holes, in both cases, the relaxation is blocked, either because the excited electrons have migrated away, or It is the hole that is filled up, and the thermal interference will restore the electron or hole, resulting in afterglow emission (EM); (b) the organic long-term luminescent mechanism, in which the cation quaternary phosphorus nucleus is the organic trap, and the aromatic amine is the electron donor. First, 1) the photoinduced charge transfer (CT) occurs between the donor and the receptor molecules, then 2) the charge separation (CS), and finally 3) the charge recombination (CR) Previously, multiple CS may occur to form an olpl. The researchers believe that the reason why LPL has long-term luminescence is that the trap can store energy in advance, and then release energy slowly through thermal interference. Inspired by the long-term luminescent mechanism of LPL, researchers designed an olpl system, which consists of two parts: emitter and trap. This system combines the advantages of olpl with the performance of LPL. Study on the luminescent properties of olpl Fig. 2. (a) chemical structure of tpp-3c B, tpp-4cb, tpp-3c2b and tpp-4c2b crystal powders at 298 K, (b) PL spectrum and (c) time-resolved PL attenuation (at 480 nm); λ ex = 310nm; (d) photo of tpp-3cb, tpp-4cb, tpp-3c2b and tpp-4c2b and their phosphorescence quantum yield (Φ P). In order to verify the above assumption, it is necessary to find the ion nucleus that can be used as strong electron acceptor. The researchers found that the organic quaternary ammonium bromide tpp-3cb, tpp-4cb, tpp-3c2b and tpp-4c2b can achieve this function. The four crystals have distorted tetrahedral geometry (108.7 ° - 110.8 °), the distance between phosphorus and bromine ions is 4.50 – 4.75?, none of the four salts has π - π interaction or bromine anion - π interaction, only C-H ·· π and C-H ·· br interaction. These interactions and electrostatic interactions together inhibit the molecular movement in the crystal and enhance the phosphorescence efficiency of olpl. The maximum emission wavelength of the four crystals is 480 nm. Surprisingly, the excitation spectrum of the four crystals at 480 nm shows that there is a maximum excitation band at 310 nm, which is caused by the photoinduced charge transfer from the bromide counterion to the Quaternary phosphorus nucleus. The time-resolved PL decay curve shows that at room temperature (298 K) 480 The average lifetimes of tpp-3cb, tpp-4cb, tpp3c2b and tpp-4c2b are 157, 200, 159, 164 MS under nm excitation. Under the excitation of 254 nm hand-held UV lamp, researchers can observe strong blue-green light from the crystal. QM / mm simulated olpl luminescence process figure 3. Calculate singlet and triplet excited states with QM / mm at TD-DFT level. (a, b) tpp-3cb crystal structure is used to obtain the optimal geometry of S1 (a) and T1 (b) excited states surrounded by 43 molecules; (c) molecular orbital calculated from the optimized T1 structure; (d) electrons in tpp-3cb start from the ground state, enter the free radical pair (RP) in the first charge transfer excitation (red), and enter the T1 state in the second charge transfer (blue); and (E) repair The modified Jablonski diagram shows that the single line free radical pair (1rp) formed after absorption (ABS), spin orbit charge transfer (SOCT) and hyperfine coupling (HFC) can help the transition between 1rp and triplet free radical pair (3rp). Before phosphorescence is observed, RP relaxes to T1 state. Taking tpp-3cb crystal as the object, the researchers used QM / mm to simulate the photophysical process in the crystal, and optimized the excited singlet (S1) and triplet (T1). In the S1 geometry, the S1 and T1 molecular orbitals of tpp-3cb exhibit complete charge transfer from bromine anion to phosphorus cation, providing excited free radical pairs. At first, the orbit distance of transition from S0 to S1 is large, which results in the exchange energy from S1 to T1 is very small, and the energy gap is less than 0.1eV, which is conducive to the cross between systems. Moreover, the spin orbit coupling constant is 327.39 cm-1, which further enhances the conversion between S1 and T1. Therefore, the complete charge transfer characteristics and strong SOC provide a solid foundation for the spin orbit charge transfer (SOCT) process of p-br complex from S1 to T1. In addition, excited state radicals are also susceptible to hyperfine coupling (HFC), which provides another channel for the cross between systems; due to the different positions of radicals, the local magnetic field around them is also different, which leads to spin mixing, in which the re phase and spin flip transition are conducive to the cross between systems. In this way, the molecule initially becomes the excited singlet radical pair (1rp), but because of the action of SOCT and HFC, not only 1rp is easy to cross into the excited triplet radical pair (3rp), but also 3rp can easily cross into 1rp. Figure 4. Tpp-3c2b: chemical structure and photophysical properties of DMA. (a) After stopping excitation at 365 nm, the emission spectra of tpp-3c2b: DMA crystal measured at different time (0-7 h); (b) tpp-3c2b: DMA crystal at 365 nm Photos of the afterglow excited by UV at nm and the subsequent afterglow; (c) afterglow dynamics measurement of the PL intensity at 500nm within 7 hours; (d) left: chemical structure of tpp-3c2b, labeled as a receptor, N, N-dimethylaniline (DMA) labeled as D donor, right: free radical pair of long-term luminescence is realized by charge transfer, followed by CS and finally charge recombination. The researchers believe that the strong electron acceptor and excited state charge separation (CS) are used to "capture" the charge carrier, which is expected to further extend the luminescence time of olpl. They mixed DMA with dichloromethane (DCM) solution of tpp-3c2b, and then grew the crystals. They doped DMA into tpp-3c2b, and found that these crystals showed strong emission light at 500 under 365 nm excitation. Because both DMA and tpp-3c2b have no absorption band at 365 nm, it shows that the excitation of tpp-3c2b: DMA crystal has photoinduced charge transfer characteristics. The transient absorption spectrum of tpp-3c2b: DMA was also analyzed, and a new band was found at 475 nm. All in all, the olpl prepared in this paper has a long-life state of charge separation, lasting for 7 hours. Tpp-3c2b: DMA crystal can protect the photogenerated free radicals from the influence of oxygen in the atmosphere, thus producing a long-term luminescence phenomenon. These crystals also have good stability. After 45 days in dark environment, their luminescent time has not changed. By simply mixing DMA / tpp-3c2b (10:1 mole ratio) with DCM / EA (1:1v / V), the luminous time of olpl can be adjusted in the range of 1 to 7 hours. Compared with tpp-3c2b, the photoluminescence time of the crystals grown in 8-36 mole equivalent DMA is 7 h. In summary, inspired by the trap mechanism in LPL, academician Tang benzhong of Hong Kong University of science and technology proposed a triphenylquaternium phosphorous olpl. When tpp-3c2b was used as organic trap, the luminescent time of the material was up to 7 h, and the mixture of DMA / tpp-3c2b and DCM / EA could be from 1 to 7 The luminescent time of olpl was adjusted by h-range. After 45 days storage in dark environment, the luminescent properties of the material did not change. Organic trap is an important factor for tpp-3c2b: DMA to show long-term luminescent effect. It is very important to protect the system from oxygen quenching and to prolong the luminescent time. Original link: https://onlinelibrary.wiley.com/doi/10.1002/adma.202001026 advanced polymer science established "photoelectric" and other communication groups, added small editors as friends (micro signal: polymer Xiang, please note: name unit Title Research direction), invited to join the group.
Source: polymer science frontier
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