TANG Zhong Academy team \”Advanced Materials\”: 1 + 1> 2, quaternary ammonium phosphonium triphenyl making traps, length up to 7 hours the organic luminescent material

Long emission (LPL) material having a storage and slow release properties of the excited state energy, has shown great potential in the life sciences, biotechnology and medical fields PV. Currently, the most successful LPL material is a transition metal and rare earth materials, its luminous length ranging from a few minutes to a few hours, some systems can last for days or even weeks. However, the high cost of inorganic LPL, demanding manufacturing process, greatly limits its applications. Organic LPL (OLPL) Material easy synthesis and processing, easy modification function, attracted much attention in the. In order to prolong the light emission time OLPL, some researchers use a triplet excited by a heteroatom, a carbonyl functional group, and between the heavy atom effect reinforcing polymer system significantly improves the efficiency of the cross at the triplet state at room temperature. In addition to an excited triplet state, the triplet protection to extend the emission lifetime is also important, through the use of a crystal, a metal – organic frameworks and aggregation means H, research in this area has been made significant progress. Nevertheless, in the light-emitting time OLPL still difficult compared with LPL. The results presented in long in order to improve the light-emitting OLPL time, Hong Kong University of Science and Technology TANG Zhong academicians Task Force Inspired by LPL long-lasting light-emitting mechanism, the use of quaternary ammonium triphenyl phosphate TPP-3C2B an organic electron traps, prepared a Long luminescent material OLPL , these quaternary ammonium phosphonium photoinduced charge transfer both strong electron acceptor, but also as a protective trap, in order to stabilize and protect the excited free radicals, enabling the system to slow recombination, has a long-lasting light emission characteristics, length up to 7 hours light, much higher than the current level OLPL. This study presented a simple way to long-acting organic phosphors prepared, opens the door to the development of new materials OLPL. 2, quaternary ammonium phosphonium triphenyl making traps, length up to 7 hours the organic luminescent material \”/>

Long light emission mechanism of LPL and OLPL

[123 ] 2, quaternary ammonium phosphonium triphenyl making traps, length up to 7 hours the organic luminescent material \”/> Figure 1Inorganic and organic light emitting long-acting mechanism. FIG. (A) long-lasting luminescent inorganic material may be achieved by an electron or hole trap mechanism, in the electron trap, after excitation (Ex), the excited electrons reach the electron passes through the conduction band trap; in the hole trap, filling holes in the valence band electrons propagate through, in both cases, the relaxation is blocked, either because the excited electrons have moved away, either holes are filled, thermal interference can recover electrons or holes to generate afterglow emission (Em); (b) long-term organic light-emitting mechanism, wherein the cationic organic quaternary phosphonium core as a trap, an aromatic amine as an electron donor, firstly 1) between the donor and acceptor molecules photoinduced charge transfer (CT), and 2) charge separation (CS), before the final 3) charge recombination (CR), a plurality of CS can occur, thereby forming OLPL.
Researchers believe that the reason why the characteristics of LPL having a long-term use because of emission of the trap effect, traps may be previously stored energy, followed by a slow release of thermal energy interference out. Inspired by long-term light emission mechanism of LPL, OLPL researchers designed a system, the system consists of two parts: a transmitter and a trap, the advantages of this system with the LPL OLPL ingenious combination of performance together.

Research OLPL luminescent properties

2, triphenyl quaternary ammonium phosphonium making traps, organic luminescent material length up to seven hours \”/> FIG 2 . (a) TPP-3CB, TPP-4CB, TPP-3C2B and TPP-4C2B crystalline powder chemical structure at 298 K when, (b) PL spectra and (c) time-resolved PL decay (in nm when 480); λex = 310nm;. (d) at 254 nm UV excitation, TPP-3CB, TPP-4CB, TPP-3C2B and TPP-4C2B pictures and phosphorescence quantum yield (ΦP)
to verify the above vision is correct, you first need to find can be used as a strong electron acceptor ion nucleus, the researchers found that organic quaternary ammonium bromide salt TPP-3CB, TPP-4CB, TPP-3C2B and TPP-4C2B can achieve this functionality. the four seed crystals are distorted tetrahedral geometry (108.7° -110.8 °), the distance between phosphorus and bromine ions 4.50-4.75Å, four kinds of salts is not π-π interaction or bromide anion -π interaction, and there is only CH ·· π CH ··· Br interaction. These interactions and electrostatic interactions cosuppression crystal molecular movement, enhancing the efficiency of the phosphor of OLPL. Four kinds of crystal maximum emission wavelength was 480 nm, surprisingly, their excitation spectrum when the emission maximum 480 nm excitation showed a band at 310 nm, which is due to the quaternary phosphonium bromide counterion core caused by photoinduced charge transfer, time-resolved PL decay curves show that at room temperature (298 K) at 480 nm excitation, TPP-3CB, the average life TPP-4CB, TPP3C2B and TPP-4C2B crystals was 157,200,159,164 ms. Under 254 nm excitation handheld UV lamp, the researcher can observe from the crystals to a strong blue-green light.

QM / MM simulation OLPL emission process

2, triphenyl quaternary ammonium phosphonium making traps, length up to 7 hours the organic luminescent material \”/> 3. FIG excited with QM / MM calculated singlet and triplet levels in TD-DFT. (a, b) TPP-3CB for obtaining the crystal structure 43 is surrounded by molecules of S1 (a) and T1 (b) the optimal geometry of the excited state; (c) calculated from the optimized structure of molecular orbital T1; (d) in a TPP-3CB electron starts from the ground into the first charge transfer excited (red) a radical of ( RP), the second charge transfer (blue) into the T1 state; Jablonski diagram after (e) show modified after absorption (Abs) is formed of a singlet radical (1RP), the spin-orbit charge transfer (the SOCT) and super hyperfine coupling (HFC) can help 1RP and conversion between a triplet radical pair (3RP), before the phosphor is observed, the RP to the relaxed state T1.
researchers at TPP-3CB crystals objects, method (QM / MM) molecular mechanics and quantum mechanics to simulate the combination of the crystal photophysical process, optimizing the excited singlet state (S1) and triplet(T1). In S1 geometry, TPP-3CB S1 and T1 of the molecular orbitals exhibit complete charge transfer from the anion to the phosphonium cation bromo, providing excitation radical pair. Initially, the transition from S0 to S1 track distance larger, resulting from the S1 to the T1 state exchange energy is small, the energy gap less than 0.1 eV, which is conducive to cross between systems. Such that the bromine anion and spin-orbit coupling constant conversion of up to 327.39 cm-1, which further enhances the conversion between the S1 and T1. Thus, a complete charge transfer and strong bromo SOC is phosphorus complex from S1 to T1 spin-orbit charge transfer (the SOCT) process provides a solid foundation. In addition, effects on excited free radicals are also vulnerable to the hyperfine coupling (HFC), which provides a further channel for the inter-system crossing; radical due to the different positions, the local magnetic field around there are differences, resulting spin mixture, wherein rephasing and spin flip transition is conducive to cross between systems. Thus, upon excitation, the molecule first be excited singlet radical pair (1RP), and HFC SOCT but due not only become possible cross 1RP excited triplet radical pairs in inter-system (3RP), but also 3RP the system can easily cross into the 1RP.
2, quaternary ammonium phosphonium triphenyl making traps, length up to 7 hours the organic luminescent material \”/> Fig 4. TPP-3C2B:. Photophysical properties and chemical structure of the DMA (a) in excitation at 365 nm after stopping, at different times (0-7 h) measured TPP-3C2B: DMA emission spectra of crystals; (b) TPP-3C2B: DMA crystals UV excitation at 365 nm and photo subsequent afterglow; ( c) for a period of 7 hours PL intensity at 500nm afterglow kinetics measurements; (d) left: TPP-3C2B chemical structure, receptor labeled a, N, N- dimethylaniline (DMA) labeled D donor right: by charge transfer, and finally followed by a CS charge recombination light emission to achieve a long-lasting free radicals
researchers believe that using a strong electron-accepting molecules, excited by the introduction of charge separation (. CS) to \”capture\” the charge carriers, is expected to further extend OLPL light-emitting time. they DMA and TPP-3C2BIn dichloromethane (DCM) was mixed, and then crystal growth, doped to the DMA in the TPP-3C2B found that these crystals at 500 show a strong emission of light at 365 nm excitation. Since DMA and TPP-3C2B no absorption band at 365 nm, indicating that TPP-3C2B: DMA crystal induced excitation light having a charge transfer characteristics. The researchers also performed TPP-3C2B: DMA transient absorption spectroscopy and found to form a new band at 475 nm at. In summary, after UV excitation 365 nm 60 s, OLPL produced herein had a long life of charge-separated state, continuous emission up to 7 hours. TPP-3C2B: DMA crystal photogenerated radical may be protected from the effects of atmospheric oxygen, to produce a long-lasting luminescence phenomenon. These crystals also have good stability after placing in the dark for 45 days did not change the emission time. Simply DMA / TPP-3C2B (10: 1 molar ratio) with DCM / EA (1: 1 v / v) mixture, can adjust the light emission time OLPL in the range of 1 to 7 h. As compared with TPP-3C2B, crystals grown in the light emission time 8-36 molar equivalents of both the DMA 7 h.

Summary Inspired by LPL in the trap mechanism, Hong Kong University of Science and Technology academician Zhong Tang this group proposed a three-phenyl quaternary ammonium phosphate OLPL, when to TPP-3C2B organic trap, light emission time of the material up to 7 h, and the DMA / TPP-3C2B with DCM / EA were mixed, can adjust the light emission time OLPL in the range of 1 to 7 h, stored 45 days in a dark environment, materials luminescent properties did not change. The organic trap TPP-3C2B: DMA exhibit long-lasting effect of the light emitting important factor, effectively protect the system against oxygen quenching, is also very important for extending the light emission time. Original link: https: //onlinelibrary.wiley.com/doi/10.1002/adma.202001026

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