The El-Sayed’s rule analogy enables long-lived room temperature phosphorescence in twisted biphenyls

Carbonyl groups in organic phosphors can significantly benefit the process of intersystem crossing (ISC) but may lead to rapid photodegradation. Recently, twisted molecules consisting of an aromatic donor and acceptor free of carbonyls have been used to enhance ISC without unstable photochemistry. The strategy is thought to be analogous to the El-Sayed rule, where electrons go through circular motions and thus boost spin-orbit coupling (SOC) during photoexcitation. However, such a design has not been systematically explored. Here, by progressively twisting the two benzene planes of a series of carbonyl-free biphenyl compounds to various degrees from methyl substitution, we show that, both experimentally and theoretically, this "El-Sayed rule analogy" is different from the well-established El-Sayed rule: the SOC is more effective between higher-lying excited states and can be efficient for ISC from singlet to triplet excited states, which leads to an "ultralong" phosphorescent state due to ineffective ISC from triplet to ground states.