Action spectroscopy of deprotomer-selected hydroxycinnamate anions

Tandem ion mobility mass spectrometry-coupled laser excitation is used to record photodetachment, photoisomerization and photodepletion action spectra for a series of deprotomer-selected hydroxycinnamate anions, including deprotonated caffeic, ferulic and sinapinic acids. This molecular series accounts for most hydroxycinnamic moieties found in nature. Phenoxide deprotomers for para and ortho structural isomers have similar photodetachment action spectra that span the 350 – 460 nm range with the maximum response occurring between 420 and 440 nm. None of the phenoxide deprotomers showed evidence for E→Z photoisomerization. In contrast, photoexcitation of the carboxylate deprotomers of caffeic and ferulic acids and the meta-phenoxide deprotomer of caffeic acid initiates intramolecular proton transfer to give the para-phenoxide deprotomer. Photoexcitation of the carboxylate deprotomer of sinapinic acid and ortho-coumaric acid does not result in intramolecular proton transfer, presumably due to substantial barriers for rearrangement. For deprotonated meta-coumaric acid, interconversion between the phenoxide and carboxylate deprotomers occurs in the ion mobility spectrometer drift region where the effective ion temperature is Teff≈299 K.