Theoretical study of solvent effects on the conformational equilibrium and electronic spectra of 2,2′-bipyridine derivatives

Solvent effects on the s-trans/s-cis conformational equilibrium and electronic spectra of a simple 2,2′-bipyridine derivative have been analyzed on the basis of quantum mechanical computations using a solvent dielectric continuum model and time-dependent density functional theory (TD-DFT). Experimental data for the UV-spectra are also reported. The predicted spectra for the two conformations present significant differences in particular for the band appearing at higher wavelength, which corresponds to the HOMO → LUMO (π → π∗) electronic transition. Since experimental data for the UV-spectra display only small solvent effects, one can conclude that the population of the less stable s-cis conformation is small, even in a polar media such as water. This conclusion is confirmed by computations for s-trans/s-cis free energy differences as a function of the dielectric constant of the medium but contrasts with previously reported experimental estimations based on dipole moment measurements.