Disrupting the Photochemical Landscape of a β-Diketone via Electrostatic Perturbation of Ground-State Tautomers

Photochemically triggered control of keto-enol equilibria is of growing importance in rational synthetic strategies. Questions exist, however, about the extent to which the electrostatic effects (i.e., interactions with counterions and solvent) can perturb or direct such photocatalytic approaches. Here, we directly address the question of whether electrostatic tuning via alkali metal binding can affect the photochemistry of the classic β-diketone molecule, avobenzone, which exists in keto and enol forms. A combination of photodissociation mass spectrometry over the ultraviolet (UV) (390-238 nm) and infrared (IR) (800-1800 cm^-1) ranges and quantum chemical calculations (density functional theory (DFT) and SCS-ADC(2)) is applied to isolated (gas-phase) alkali metal-avobenzone complexes for the first time. We find that Na⁺, K⁺, and Rb⁺ binding only modestly perturbs the avobenzone π*-character excited states compared to the bare molecule. However, electrostatic binding has a dramatic overall effect since cation binding reverses the relative stability of the tautomers on the electronic ground state: without cation binding, the enol dominates, whereas upon cation binding, the keto form dominates. This is of critical photochemical importance as the ketone tautomer is the doorway geometry to photoinstability through providing access to optically dark triplet states that are adjacent to bright singlet states. Ion binding therefore opens a route to enhanced photochemical activity and molecular dissociation. Our results demonstrate a paradigm where electrostatic binding, here with a metal cation, perturbs the molecular keto-enol photochemistry through disruption of the ground-state electronic surface.