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 (T ₁ ) 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 T ₁ to the ground state (S ₀ ) 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 T ₁ -S ₀ transition energy and local excited T ₁ -S ₀ transition characteristics is intrinsically less than 10 ⁰ s ^-1 even when vibrations freely occur. This information will assist researchers to appropriately design materials without limitations regarding intermolecular interactions to control T ₁ lifetime at room temperature and facilitate triplet energy harvesting.