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Persistent room-temperature phosphorescence (pRTP) under ambient conditions is attracting attention due to its strong potential for state of art bioimaging and security applications because materials with pRTP can be utilized for high contrast emission imaging independent of autofluorescence using small-scale and low-cost photo-detectors. To extract efficient pRTP, the control of the radiative rate from the lowest triplet excited state (T1) (kp), the nonradiative rate of intramolecular vibrational relaxation at room temperature (RT) from T1 (knr(RT)), and the triplet quenching rate at RT caused by interactions with the ambient surroundings (kq(RT)) is crucial. In the last six years, knr(RT)+kq(RT) have been suppressed in a variety of heavy atom-free molecular materials under ambient conditions, which has allowed electrons in T1 to partly access the pRTP pathway with small kp. However, a key strategy to suppress knr(RT) and kq(RT) as well as increase kp for the efficient persistent RTP is not found yet.
Here, we investigate kp, knr(RT), and kq(RT) of heavy atom-free molecular materials showing pRTP. Cooperative analysis using microscope and quantum chemical calculation indicated that no appearance of pRTP of many heavy atom-free molecules are mostly caused by kq(RT) while minimization of the intermolecular triplet energy transfer using appropriate design of materials could largely decreased kq(RT). For knr(RT), analysis using vibrational spin–orbit coupling at RT indicates that knr(RT) of many heavy atom-free molecules is intrinsically small and approaches the small kp. To increase kp independent of knr(RT), the control of electronic structure of the high order excited state is highly important. This is first overall analysis of kp, knr(RT), and kq(RT) of heavy atom-free molecules, which will be important for realizing efficient pRTP from a variety of heavy atom-free molecular materials. In symposium, intrinsic characteristics of pRTP that general long persistent emitters could not reach are also explained.
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