Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid

Bull, James, da Silva, Gabriel, Scholz, Michael S., Carrascosa, Eduardo and Bieske, Evan J. (2019) Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid. Journal of Physical Chemistry A, 123 (20). pp. 4419-4430. ISSN 0022-3654

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Abstract

Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E → Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest ππ* transition. For the E isomer of the carboxylate deprotomer, a one-way photoinitiated proton transfer generates the phenoxide deprotomer through a mechanism postulated to involve an excited-state enol–keto tautomerism followed by a series of ground-state rearrangements including a second proton transfer. This mechanism is supported by experiments in which the relevant intermediate keto isomer is prepared and spectroscopically probed and through master equation modeling of possible ground-state isomerization processes. The Z isomer of the carboxylate deprotomer shows a weak Z → E photoisomerization response that occurs in competition with photodestruction (presumably electron detachment), demonstrating that the E and Z isomers undergo different processes in their excited states. The study highlights the utility of isomer-selective spectroscopy for characterizing the photochemistry of isolated anions possessing multiple deprotonation sites.

Item Type: Article
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: LivePure Connector
Date Deposited: 16 May 2019 11:30
Last Modified: 03 Nov 2020 00:57
URI: https://ueaeprints.uea.ac.uk/id/eprint/71025
DOI: 10.1021/acs.jpca.9b02023

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