Room-Temperature Phosphorescence from Metal-Free Organic Materials in Solution: Origin and Molecular Design

Metal-free organic materials with room-temperature phosphorescence (RTP) is hardly achieved in solution owing to the ambiguous underlying mechanism. By combining thermal vibration correlation function rate theory and a polarizable continuum model (PCM) coupled with the Tamm–Dancoff approximation method, concentrating on β-hydroxyvinylimine boron compounds C-BF2 and S-BF2, we showed that the increased intersystem crossing (kisc) and radiative decay rates (kp) are responsible for the strong RTP of S-BF2 in solution. From C-BF2 to S-BF2, the T2 state is increasingly dominated by the n → π* transition, largely enhancing the kisc of S1 → T2 (up to 3 orders of magnitude) and kp of T1 → S0. Impressively, the nonradiative decay rate of T1 → S0 is slightly increased by suppressing the out-of-plane twisting motions. This mechanism is also tenable for several designed RTP molecules through further experimental demonstration, which will pave a new way to design organic materials with single-molecule phosphorescence for applying to organic light-emitting diodes.

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