Electrostatic Interaction-Induced Room-Temperature Phosphorescence in Pure Organic Molecules from QM/MM Calculations

Room temperature phosphorescence (RTP) from pure organic material is rare due to the low phosphorescence quantum efficiency. That is why the recent discovery of crystallization induced RTP for several organic molecules aroused strong interests. Through a combined quantum and molecular mechanics CASPT2/AMBER scheme taking terephthalic acid (TPA) as example, we found that electrostatic interaction not only can induce an enhanced radiative decay T₁ → S₀ through the dipole-allowed S₁ intermediate state, but also can hinder the nonradiative decay process upon crystallization. From gas phase to crystal, the nature of S₁ state is converted to ¹(π,π∗) from ¹(n,π∗) character, enhancing transition dipole moment and serving as an efficient intermediate radiative pathway for T₁ → S₀ transition, and eventually leading to a boosted RTP. The intermolecular packing also blocks the nonradiative decay channel of the high-frequency C=O stretching vibration with large vibronic coupling, rather than the conventional low-frequency aromatic rotation in crystal. This mechanism also holds for other organic compounds that contain both ketones and aromatic rings.