Hydroxycinnamate anions: the evolution of gas-phase action spectroscopy to study photoisomerisable molecules

The hydroxycinnamate moiety is a widespread biochromophore in nature, acting as a photoswitch in photoactive yellow protein and as UV-absorbing photoprotective units in the leaves of plants. The α, β-unsaturated carbonyl group supports E and Z geometric isomers, which may photoisomerise following the absorption of light, with this structural change driving subsequent photodynamics or activity in biological systems. This review summarises our current understanding of the intrinsic (gas phase) photophysics of hydroxycinnamates anions, focussing on the most common para structural isomers. While the photochemistry of hydroxycinnamate anions has intrigued scientists for several decades, recent advances in gas-phase instrumentation involving the coupling of electrospray ionisation with mass spectrometry and action spectroscopy techniques have incrementally provided a complete picture of their fundamental photo-induced dynamics. Significantly, the combination of femtosecond time-resolved photoelectron and photoisomerisation action spectroscopy has highlighted the role of excited-state barriers along the isomerisation co-ordinate in defining the photophysics. These barriers are largely suppressed in condensed phases. Overall, the hydroxycinnamates provide a clear example of how gas-phase action spectroscopy experiments have evolved to uncover intrinsic reaction dynamics of isolated molecules.