Hydrogen bonding in electronically excited states: a comparison between formic acid dimer and its mono-substituted thioderivatives

A multi-state complete active space second order perturbation theory (MS-CASPT2) study on the valence singlet electronic excited states of formic acid dimer is presented. The electronic spectrum of this dihydrogen bonded system is dominated by nπ* and ππ* intramonomer and charge transfer excitations and consists of a very intense ππ* transition at 8.25 eV and three weaker nπ*(2×) and ππ*(1×) electronic excitations at 6.21 eV, 9.13 eV, and 9.93 eV, respectively. The characteristic nπ*-nπ*-ππ*-ππ*... pattern found in the formic acid dimer electronic spectrum is altered when a sulfur atom is introduced in the molecule. Furthermore, carbonyl-by-thiocarbonyl or hydroxyl-by-thiohydroxyl substitution is predicted to strongly affect the intensity of the above electronic transitions. The effect of oxygen-by-sulfur substitution on the geometry of the first excited state (S 1 ) has been investigated at the CC2 and CASSCF levels of theory. Although the two methods qualitatively predict the same geometrical changes upon nπ* excitation, the geometries of the S 1 state are found to differ considerably between the two levels. Oxygen-by-sulfur substitution, in the carbonyl or hydroxyl position, leads to changes in the electronic absorption spectrum of formic acid dimer which might have important implications in the design of novel photovoltaic materials.