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 similar to 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 S-1(pi pi*) band lead to similar vibrational autodetachment dynamics. The S-2(n pi*) band is Herzberg-Teller active and has comparable brightness to the higher lying 2(1)(pi pi*) band. The frequency-resolved photoelectron spectrum over the S-2(n pi*) band indicates more efficient internal conversion to the S-1(pi pi*) state for photon energies resonant with the Franck-Condon modes (similar to 80%) compared with the Herzberg-Teller modes (similar to 60%). The third action-absorption band, which corresponds to excitation of the 2(1)(pi pi*) state, shows complex and photon energy-dependent dynamics, with 20-40% of photoexcited population internally converting to the S-1(pi pi(*)) state. There is also evidence for a mode-specific competition between prompt autodetachment and internal conversion on the red edge of the 2(1)(pi pi(*)) 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 S-1(pp*) and S-2(n pi*) bands indicate diametrically opposed dynamics compared with pCEs(-), namely, intense thermionic emission due to efficient recovery of the ground electronic state.