A photochemical crossroads: the (<i>Z</i>)/(<i>E</i>) equilibrium determines emission or cyclization of 2′-hydroxychalcone

The molecule 2′-hydroxychalcone (2HC) exhibits unique photoprocesses, including excited-state proton transfer and photoisomerization. While many of the major components of the photochemistry of 2HC have been reported previously, there remains a lack of consensus concerning the importance of proton transfer and its contribution to the isomerization. We employed second-order n-electron valence state perturbation theory (NEVPT2) to investigate the potential energy surfaces of 2HC′s low-lying singlet and triplet excited states with several key reaction coordinates, focusing on the mechanism for photoisomerization and emission. Our results exhibited excellent agreement with previous experimental studies and are qualitatively consistent with previous theoretical works but provide a more accurate modelling of the excited states. Additionally, we investigated the importance of the equilibrium between the (<i>Z</i>)-keto and (<i>E</i>)-keto isomers in the ground state which was not considered in previous works. The excitation of the (<i>Z</i>)-keto and (<i>E</i>)-keto leads to distinct photochemical reactions where the photochemistry of the former involves fluorescence and phosphorescence while the latter involves a photocyclization via a conical intersection. This improves the current understanding of the photochemistry of 2HC that can be extended to spectral tuning to enhance or inhibit fluorescence and cyclization of 2HC and its derivatives.