Vibronic Coupling of Competing Internal Conversion and Intersystem Crossing in Xanthone

We theoretically investigated vibronic coupling responsible for nonradiative transitions, i.e., internal conversion (IC) and intersystem crossing (ISC), in xanthone. The nonradiative decay pathway of aromatic ketones is often debated because of their fast ISC. Xanthone in the gas phase follows a pathway that obeys El-Sayed’s rule, namely, IC from the 1ππ* to 1nπ* states and ISC from the 1nπ* to 3ππ* states, of which a simple pathway is adequate for analyzing vibronic structures. We employed an expression for the nonradiative rate constant based on Fermi’s golden rule within the mixed-spin crude adiabatic approximation, which has the advantage that both IC and ISC can be considered as equally vibronically-induced transitions. Our calculations showed that the IC from the 1ππ* to 1nπ* was faster than ISC channels because of stronger vibronic coupling and less favorable spin–orbit (SO) coupling to nearby triplets. Additionally, the ISC from 1nπ* to 3ππ* was faster than that from 1nπ* to 3nπ* because of the large structural displacement and small energy gap. This study can provide guidelines for determining whether IC or ISC dominates, depending on the balance between vibronic coupling, SO coupling, and the energy gap, thereby informing molecular design for controlled nonradiative decay. Specifically, for xanthone, the ISC from 1ππ* can dominate its IC counterpart by decreasing the energy gap or increasing the SO coupling between 1ππ* and an energetically close triplet.