Vibrational Radiationless Transition from Triplet States of Chromophores at Room Temperature

The radiationless transition rate based on intramolecular vibrations from the lowest excited triplet state (T₁) at room temperature [k_nr(RT)] is crucial for triplet energy harvesting in optoelectronics and photonics applications. Although a decrease of k_nr(RT) of chromophores with strong intermolecular interactions is often proposed, scientific evidence for this has not been reported. Here we report a method to predict k_nr(RT). We optically estimated k_nr(RT) of various molecularly dispersed chromophores with a variety of transition characteristics from T₁ to the ground state (S₀) under appropriate inert liquid or solid host conditions. Spin-orbit coupling (SOC) without considering molecular vibrations was not correlated with the estimated k_nr(RT). However, the estimated k_nr(RT) was strongly correlated with a multiplication of SOC considering vibrations freely allowed at room temperature and the Franck-Condon factor. This correlation revealed that k_nr(RT) of many heavy-atom-free chromophores with a visible T₁-S₀ transition energy and local excited T₁-S₀ transition characteristics is intrinsically less than 10⁰ s^-1 even when vibrations freely occur. This information will assist researchers to appropriately design materials without limitations regarding intermolecular interactions to control T₁ lifetime at room temperature and facilitate triplet energy harvesting.