Quantum Chemistry Investigation of Equilibria of Molecular Switches in Solution: Thermodynamics and UV/Visible Absorption Spectra

A composite high-level wave function approach has been designed to calculate the differences in energy between the different forms of molecular switches. This approach to calculate the reaction Gibbs free energies includes (i) MP2 determination of the equilibrium geometries and of the thermal corrections, (ii) subsequent calculation of CCSD(T) energies obtained by enacting the resolution of the identity and the pair natural orbital schemes, (iii) estimated corrections for the PNO scheme, and subsequent (iv) inclusion of the implicit solvent effects from the RI-MP2 method combined with the perturbation theory on energy and density. The reliability of this approach has been illustrated by considering two families of molecular switches, (i) <i>N</i>-salicylideneanilines, displaying enol-imine/keto-enamine tautomerism, and (ii) spiropyrans switching between a spiropyran and a merocyanine form, while the latter form can also adopt different conformations. Subsequently, the comparison between the simulated and recorded UV/vis absorption spectra is used as an adequate substantiation tool of these energy differences since the spectra vary substantially between the different forms. These spectra have been simulated at the TDDFT level after taking into account the Maxwell-Boltzmann weights of the different forms of the switches, including implicit solvent effects and accounting for the vibronic structure within the vertical gradient approximation.