Recent progress in heavy atom-free organic compounds showing unexpected intersystem crossing (ISC) ability.

In this review, recent progress in heavy atom-free triplet photosensitizers was summarized. The general approaches include attaining S1/Tn states sharing similar energy levels or proper molecular geometry to satisfy the angular momentum reservation in intersystem crossing (ISC). ISC via the higher singlet excited state (Sn, n > 1) → Tm (m > 1), which is a rarely reported phenomenon, was also discussed. The ISC of some Bodipy dimers was proposed to be via the 'doubly excited state', but recent studies show that the ISC mechanism of these Bodipy dimers is charge separation/recombination. These new findings in the study of triplet photosensitizers are useful for photovoltaics, photodynamic therapy and photocatalysis, as well as in fundamental photochemistry studies.