Structural and thermochemical properties of a photoresponsive spiropyran and merocyanine pair: Basis set and solvent dependence in density functional predictions

We investigate structural and thermochemical properties of photoresponsive closed (SP) and open (MC) form moieties of nitro-substituted spirobenzopyran (1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitro-spiro[2H-1-benzopyran-2,2′-[2H]indole]) in different solvents. Density functional calculations reveal that in addition to the solvent, polarization and diffuse functions in the computational basis set exert a significant influence. Unlike in vacuum, in water all MC conformers are more stable than SP. The higher entropies and thus lower free energies of the flexible open forms strongly stabilize them in comparison to SP. The partial atomic charges, which are amplified in solvents, and the bond lengths are consistent with a quinoid structure of stable MC conformers.