Time-resolved extreme ultraviolet photoelectron spectroscopy of 4-bromophenolate anions in a liquid jet.

Abstract

The dynamics of photo-excited 4-bromophenolate anions in aqueous solution are investigated using femtosecond extreme ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) in a liquid flat jet. Excitation to the first singlet S₁ (1¹ππ*) excited state at 290 nm yields lifetimes of ∼160 fs and 1.1 ps that are attributed to the timescale for the transition from the initially excited S₁ state to the optically-dark ¹πσ* state and the subsequent decay of the ¹πσ* state, respectively. Excitation at 240 nm to the second singlet excited state (S₂, 2¹ππ*) reveals an additional ultrafast component assigned to S₂ → S₁ decay with a lifetime of 60 fs. Subsequent decay components show lifetimes of 130 fs and 1.4 ps, corresponding to the transition from S₁ to the ¹πσ* state and its subsequent decay. These dynamics differ from the phenolate anion, which undergoes S₁ → S₀ internal conversion and hydrated electron formation within ∼20 ps upon S₁ excitation. Instead, photoexcited 4-bromophenolate decays primarily through the ¹πσ* state, which is repulsive along the C-Br bond length and correlates to ˙C₆H₄O˙ (biradical fragment) + Br^-. Quantum chemical calculations suggest that the S₁-¹πσ* state crossing involves a small potential energy barrier that promotes a facile non-adiabatic transition. This study highlights the competing dynamics between photo-oxidation and non-adiabatic relaxation within the framework of anions in aqueous solution.