Theoretical Study of the Substituent and Solvent Effects on the Molecular Structures, Absorption and Emission Spectra of Open-Form Spiropyrans

The effects of substituents and solvents on the molecular structures, excitation energies and emission energies of a series of donor-acceptor substituted spiropyrans were investigated using the density functional methods. Different donor-acceptor pairs lead to alternations of the molecular structures. A relationship between the strengths of donor-acceptor pairs and the structural parameter BLA (bond length alternation) was examined and discussed. The impact on geometrical parameters induced by the solvents is more significant than that caused by the substituents, as indicated by the larger BLA changes. The substituent and solvent effects on the UV absorption and emission spectra of open-form spiropyrans were studied by the TD-DFT method. The absorption maxima of open-form spiropyrans are expected to red shift, and the emission are expected to blue shift as the strengths of donor-acceptor pairs and the polarities of solvents increase. A simple model was adopted to mimic the substituent and solvent effects, in the framework of AM1, employing Sparkles to create an external electric field on the open-form spiropyran, in order to induce a systematical polarization of the molecular equilibrium geometry and the electronic configuration.