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

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 ∼300 K. 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 complex 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 electronic state and statistical electron ejection (thermionic emission) following photoexcitation over any of the three action-absorption bands. The 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.