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 ( 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 T₂ state is increasingly dominated by the n → π* transition, largely enhancing the k_isc of S₁ → T₂ (up to 3 orders of magnitude) and k_p 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.