Intersystem Crossing Mechanisms in the Room Temperature Phosphorescence of Crystalline Organic Compounds

Reports on the room temperature phosphorescence of metal-free organic crystals have been surging in the past few years. Together with interests in the rare phenomenon, these compounds have attracted attention for such potential applications as bio-imaging probes, oxygen sensors, and organic light-emitting diodes. For common organic compounds, phosphorescence is the emission from a triplet excited state, which is usually produced from a singlet excited state through intersystem crossing, a forbidden spin-flip of an electron. The mechanism of the forbidden process is the key to understanding such rare phenomenon and designing new phosphorescence materials. In this account, we make commentaries on the main intersystem crossing mechanisms proposed to date of the room temperature phosphorescence of heavy-atom-free, crystalline organic compounds, focusing on our own findings.