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 beta-hydroxyvinylimine boron compounds C-BF2 and S-BF2, we showed that the increased intersystem crossing (k(isc)) and radiative decay rates (k(p)) are responsible for the strong RTP of S-BF2 in solution. From C-BF2 to S-BF2, the T-2 state is increasingly dominated by the n -> pi* transition, largely enhancing the k(isc) of S-1 -> T-2 (up to 3 orders of magnitude) and k(p) of T-1 -> S-0. Impressively, the nonradiative decay rate of T-1 -> S-0 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.