MS-CASPT2 study of the low-lying electronic excited states of di-thiosubstituted formic acid dimers

The suitability of di-thiosubstituted derivatives of formic acid dimer, both in hydroxyl and carbonyl position, as possible hydrogen-bonded electron transfer linkers in a hypothetical donor–acceptor dyad for photovoltaic cells and artificial photosynthesis reactors has been studied from a theoretical point of view. To this purpose, the valence singlet electronic excited states of the four possible di-thiosubstituted isomers have been characterized through multi-state complete active space second-order perturbation theory (MS-CASPT2). These hydrogen-bonded systems present electronic spectra consisting of nπ* and ππ* excitations, both intra- and intermonomer. The eventual comparison of the calculated spectroscopic characteristics of the isolated hydrogen-bonded linkers with the experimental spectrum of the chromophore in a donor–acceptor dyad could allow establishing whether the linker would compete with the electron donor in the photon absorption process. Additionally, the analysis of the structural changes undergone by these species upon electronic excitation to the S1 would allow determining whether the population of this state of the linker upon UV–vis light absorption could compromise the formation of the charge transfer complex, key in the performance of photovoltaic devices.