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-BF ₂ and S-BF ₂ , we showed that the increased intersystem crossing (k _isc ) and radiative decay rates (k _p ) are responsible for the strong RTP of S-BF ₂ in solution. From C-BF ₂ to S-BF ₂ , the T ₂ state is increasingly dominated by the n →π∗ 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.