Molecular physics of persistent room temperature phosphorescence and long-lived triplet excitons

Persistent room temperature phosphorescence (pRTP) is important to high-resolution imaging independent of autofluorescence and the scattering of excitation light for security and imaging applications. Although efficient and bright pRTP is crucial to imaging applications, photophysical processes from the triple states of heavy-atom-free chromophores have been explained by making many assumptions that are potentially based on incorrect photophysical explanations. This often confuses researchers in their efforts to control and enhance the pRTP characteristics. This paper introduces recent advances in our understanding of photophysical processes from the lowest triplet excited state of heavy-atom-free chromophores based on statistical evidence from experimental and theoretical viewpoints. After the introduction of two photophysical processes showing persistent RT emissions and the characteristics of the persistent emissions, physical parameters relating to pRTP and appropriate techniques for measuring the parameters are explained. For molecularly dispersed heavy-metal-free chromophores in a solid state, recent understandings of the physical parameters verified by correlations from optically estimated and theoretical viewpoints are summarized. Using the photophysical insights obtained for the dispersed chromophores, uncertainties regarding the photophysical processes of aggregated chromophores are discussed. After highlighting recently developed materials showing efficient pRTP, the potential advantages of pRTP over previous persistent emissions are discussed considering recent demonstrations of persistent emitters. This review quantitatively summarizes the relationship between the molecular backbone and physical parameters of pRTP characteristics and guides the reader in their efforts to appropriately design materials with efficient pRTP and control long-lived triplet excitons for promising applications.