posted on 2020-03-10, 19:25authored byJames N. Bull, Cate S. Anstöter, Jan R. R. Verlet
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.