The fluorescence and phosphorescence emission studies have been performed to understand the photochemistry of o-nitrobenzaldehyde in solution upon photoexcitation to the first excited singlet state (S1). The presence of an energy barrier in the S1-state hydrogen transfer potential energy surface is determined experimentally through the observed dual fluorescence emission, and the barrier height is estimated to be about 84–87 kcal mol−1 by gradually decreasing the excitation energy. The phosphorescence spectra of o-NBA in five different solvents are observed for the first time, providing direct experimental evidence on the existence of triplet manifold in the photoexcitation of o-NBA. Furthermore, the relative phosphorescence quantum yield is found to be solvent dependent. To get further insights into the dynamic process involved in the photochemistry of o-NBA, excited state calculations at the TD-B3LYP/6-311+G(d,p) level of theory with the PCM model are accomplished, which strongly support our experimental findings.
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