Excited-State Aromatization Drives Nonequilibrium Planarization Dynamics

Excited-state aromaticity is one of the most widely applied concepts in the field of chemistry, often used as a rational guideline for predicting conformational changes of cyclic π-conjugated systems induced by photoexcitation. Yet, the details of the relationship between the corresponding photoinduced electronic and structural dynamics have remained unclear. In this work, we applied femtosecond transient absorption and time-resolved time-domain Raman spectroscopies to track the nonequilibrium planarization dynamics of a cyclooctatetraene (COT) derivative associated with the excited-state aromaticity. In the femtosecond time-resolved Raman data, the bent-to-planar structural change was clearly captured as a continuous peak shift of the marker band, which was unambiguously identified with ¹³C labeling. Our findings show that the planarization occurs after a significant change in the electronic structure, suggesting that the system first becomes aromatic, followed by a conformational change. This work provides a unique framework for understanding the excited-state aromaticity from a dynamical aspect.