An ab initio study of electronic spectra and excited-state properties of 7-azaindole in vapour phase and aqueous solution Original Research Article

The geometries of 7-azaindole (7AI), its tautomer (7AT), and 7AI–H2O and 7AT–H2O complexes were optimised in the ground state and some low-lying singlet excited states using the 3-21G basis set. Differences of total energies of the optimised ground and excited states and the vertical excitation energies of these systems were used to explain the observed electronic spectra. Effect of solvation of these systems in bulk water was studied using the polarized continuum model (PCM). The mode of binding of a water molecule in the S2(n–π*) excited state of 7AI was found to be quite different from those in its ground and π–π* excited states. It is shown that crossing of the lowest two singlet excited-state potential surfaces S1(π–π*) and S2(n–π*) of 7AI occurs in the vapour phase under geometry relaxation while on interaction with water, the S2(n–π*) excited state is raised up appreciably going even above the S3(π–π*) excited state. Ground- and excited-state molecular electrostatic potential mapping was carried out, which led to valuable information regarding the nature of excited states of the above-mentioned systems.