Abstract
The suppression of thermally driven triplet deactivation is crucial for efficient persistent room-temperature phosphorescence (pRTP). However, the mechanism by which triplet deactivation occurs in metal-free molecular solids at room temperature (RT) remains unclear. Herein, we report a large pRTP intensity change in a molecular guest that depended on the reversible amorphous–crystal phase change in the molecular host, and we confirm the large contribution made by the rigidity of the host in suppressing intermolecular triplet quenching in the guest. (S)-(−)-2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl ((S)-BINAP) was doped as a guest into a highly purified (S)-bis(diphenylphosphino)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl ((S)-H8-BINAP) host. It was possible to reversibly form the amorphous and crystalline states of the solid by cooling to RT from various temperatures. The RTP yield (Φ p) originating from the (S)-BINAP was 6.7% in the crystalline state of the (S)-H8-BINAP host, whereas it decreased to 0.31% in the amorphous state. Arrhenius plots showing the rate of nonradiative deactivation from the lowest triplet excited state (T1) of the amorphous and crystalline solids indicated that the large difference in Φ p between the crystalline and amorphous states was mostly due to the discrepancy in the magnitude of quenching of intermolecular triplet energy transfer from the (S)-BINAP guest to the (S)-H8-BINAP host. Controlled analyses of the T1 energy of the guest and host, and of the reorganization energy of the intermolecular triplet energy transfer from the guest to the host, confirmed that the large difference in intermolecular triplet quenching was due to the discrepancy in the magnitude of the diffusion constant of the (S)-H8-BINAP host between its amorphous and crystalline states. Quantification of both the T1 energy and the diffusion constant of molecules used in solid materials is crucial for a meaningful discussion of the intermolecular triplet deactivation of various metal-free solid materials.
Highlights
Room-temperature phosphorescence (RTP) with an emission lifetime of more than 100 ms—i.e., persistent RTP— from metal-free molecular solids occurs after ceasing exposure to fluorescence-independent excitation light (Clapp. 1939; Zhang et al, 2007; Hirata et al, 2013)
The molten materials were placed on a quartz substrate on a hotplate at 250°C which is higher temperature than a melting point of 208°C of (S)-H8-BINAP host (Supplementary Figure S3), and the substrate was quenched to room temperature (RT) to prepare an amorphous 5 wt% (S)-BINAP-doped (S)-H8-BINAP film
Triplet quenching caused by endothermic triplet–triplet energy transfer has been reported for metal-free and/or heavy atom-free chromophores with small kp and knr(RT) values, even when the T1 energy of the host is much larger than that of the guest, and the solid materials are under high vacuum (Hirata et al, 2013; Totani et al, 2013)
Summary
Room-temperature phosphorescence (RTP) with an emission lifetime of more than 100 ms—i.e., persistent RTP (pRTP)— from metal-free molecular solids occurs after ceasing exposure to fluorescence-independent excitation light (Clapp. 1939; Zhang et al, 2007; Hirata et al, 2013). Room-temperature phosphorescence (RTP) with an emission lifetime of more than 100 ms—i.e., persistent RTP (pRTP)— from metal-free molecular solids occurs after ceasing exposure to fluorescence-independent excitation light Because autofluorescenceindependent pRTP can be detected using small-scale and lowcost photo detectors, chemicals and materials with pRTP characteristics are crucial for state-of-the-art security, sensing, and bioimaging applications (Deng et al, 2013; Zhang et al, 2014; Fateminia et al, 2017; Zhen et al, 2017; Louis et al, 2019; Zhou and Yan, 2019). Because pRTP from metal-free chromophores is a slow process (Hirata, 2017), the suppression of thermo-driven triplet deactivation is necessary for efficient pRTP. The mechanism by which the triplet deactivation of metal-free molecular solids occurs at RT remains unclear
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