Fingerprinting the Excited State Dynamics in Methyl Ester and Methyl Ether Anions of Deprotonated para-Coumaric Acid

Bull, James N, Anstöter, Cate S. and Verlet, Jan R. R. (2020) Fingerprinting the Excited State Dynamics in Methyl Ester and Methyl Ether Anions of Deprotonated para-Coumaric Acid. The Journal of Physical Chemistry A, 124 (11). pp. 2140-2151. ISSN 1089-5639

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Chromophores based on the para-hydroxycinnamate moiety are widespread in the natural world, including as the photoswitching unit in photoactive yellow protein and as a sunscreen in the leaves of plants. Here, photodetachment action spectroscopy combined with frequency- and angle-resolved photoelectron imaging is used to fingerprint the excited state dynamics over the first three bright action-absorption bands in the methyl ester anions (pCEs−) of deprotonated para-coumaric acid at a temperature of ≈300K. The excited states associated with the action- absorption bands are classified as resonances because they are situated in the detachment continuum and are open to autodetachment. The frequency-resolved photoelectron spectrum for pCEs− indicates that all photon energies over the S1(ππ∗) band lead to similar vibrational autodetachment dynamics. The S2(nπ∗) band is Herzberg-Teller active and has comparable brightness to the higher lying 21(ππ∗) band. The frequency-resolved photoelectron spectrum over the S2(nπ∗) band indicates more efficient internal conversion to the S1(ππ∗) state for photon energies resonant with the Franck-Condon modes (≈80 %) compared with the Herzberg-Teller modes (≈60%). The third action-absorption band, which corresponds to excitation of the 21(ππ∗) state, shows com- plex and photon-energy-dependent dynamics, with 20–40% of photoexcited population internally converting to the S1(ππ∗) state. There is also evidence for a mode-specific competition between prompt autodetachment and internal conversion on the red edge of the 21(ππ∗) band. There is no evidence for recovery of the ground elec- tronic state and statistical electron ejection (thermionic emission) following photoexcitation over any of the three action-absorption bands. Photoelectron spectra for the deprotonated methyl ether derivative (pCEt−) at photon energies over the S1(ππ∗) and S2(nπ∗) bands indicate diametrically opposed dynamics compared with pCEs−, namely intense thermionic emission due to efficient recovery of the ground electronic state.

Item Type: Article
Faculty \ School: Faculty of Science > School of Chemistry
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Depositing User: LivePure Connector
Date Deposited: 05 Mar 2020 09:04
Last Modified: 26 May 2020 00:11
DOI: 10.1021/acs.jpca.9b11993

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