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

Abstract

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.