The radiationless transition rate based on intra-molecular 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.