Photo-excitation of electron donor–acceptor systems can lead to the generation of a charge separated state (CT). Sometimes the charge recombination occurs mainly to the local triplet excited state (T1). How does the spin flip?

We report on experimental high-resolution spectroscopic studies in combination with ab initio computational studies that investigate the excited-state dynamics of methyl-4-hydroxycinnamate thioester and (5-hydroxyindan-(1E)-ylidene)acetic acid, derivatives of the photoactive yellow protein (PYP) chromophore. These studies aim to elucidate (a) how the thioester moiety influences the photophysics and photochemistry of the p-coumaric acid chromophore and (b) to what extent rotation of the single bond adjacent to the phenyl ring is involved in the decay dynamics of the electronically excited states. The experimental studies show that sulfur substitution leads to broad, unstructured excitation spectra that contrast sharply with the well-resolved spectra of compounds with an oxygen-based ester. Furthermore, internal conversion to the lower-lying nπ* state is absent. The absence of this decay channel is rationalized by quantum-chemical calculations that reveal that in the nπ* state of the thio compounds the molecule exhibits a large out-of-plane "kink" at the sulfur atom. Franck-Condon simulations of the excitation spectra of the V(ππ*) state highlight the activity of various vibrational modes in the neutral chromophore and indicate that upon sulfur substitution internal conversion to the ground state occurs at a significantly higher rate. The similarities observed in the excitation spectra and decay dynamics of the locked and unlocked compounds suggest that in the present experiments single-bond torsion does not show up prominently. The conclusion that for the isolated molecule double-bond torsion is dominating the excited-state dynamics is tentatively confirmed by the quantum-chemical calculations.