S2 Emission and Conformational Landscapes: System Specific Excited-State Photophysics in 1,2-Dicarbonyls

Photophysics of 1,2-dicarbonyl compounds-benzil being a classic example-exhibiting multiple emissions has been the subject of extensive investigation over the past decades, targeting principally to their interesting, so called, cis-trans photoisomerization. Despite a host of explorations, several issues remained obscured, such as assignment of the high energy fluorescence observed in benzil and α-naphthil, as well as the possible coexistence of multiple conformers in the photoexcited states. The present account, mostly based on a series of our own experiments combining indigenous as well as state-of-the-art steady-state and time-resolved spectroscopic techniques, supported by quantum chemical calculations, offers an in-depth exploration of the complex photophysics of a series of 1,2-dicarbonyl compounds namely, benzil, α-naphthil, 2,2′-pyridil, α-furil, and 9,9′-anthril. The pioneering and ground-breaking features of the report include assignment of the high energy fluorescence of benzil and α-naphthil to originate from the respective S2 states, a challenge to Kasha’s rule; and coexistence of multiple conformers in the lowest excited singlet as well as triplet states. Interestingly, these characteristics are not found to be general behavior of the 1,2-dicarbonyl family but are very much specific to the individual molecular systems, as validated from the potential energy profiles simulated from the DFT and TDDFT based quantum chemical calculations. While unravelling the complex photophysics involving photoisomerization processes of the molecular systems in the series and capturing the intricate excited-state dynamics therein with enhanced clarity, we have cunningly adopted our pioneering experimental technique of freezing the solutions at 77 K independently in the presence and absence of the exciting radiation. Overall, this vivid and progressive account presents a pivotal step forward in understanding the photophysics of 1,2-dicarbonyl compounds.