Theoretical study of the absorption and emission spectra of indole in the gas phase and in a solvent

The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used in a theoretical analysis of the electronic spectrum of indole. The calculations comprise a large number of singlet and triplet valence and Rydberg excited states with the aim to obtain a full understanding of the electronic spectrum. In addition to the gas-phase spectra, solvatochromic shifts have been computed at the CASPT2 level for the low-lying singlet valence states using a continuum representation of the solvent with the solute in a spherical cavity. The results support the assignments of two low-lying pi --> pi* valence singlet excited states, (1)L(b) and (1)L(a), computed at 4.43 and 4.73 eV in the gas phase. The location of the (1)L(a) band origin 0.23 eV above the (1)L(b) band origin is in agreement with the most recent experimental studies. The most intense feature of the spectrum is obtained at 5.84 eV as a pi --> pi* singlet excited valence state. In total 25 singlet states have been computed, including 2 Rydberg series and 7 valence excited states, and in addition 15 triplet excited states. The solvatochromic shifts in the absorption and emission bands for the (1)L(b) and (1)L(a) states show the large sensitivity of the (1)L(a) state to the polarity of the solvent. The change in the main fluorescing state on going from nonpolar ((1)L(b) state) to polar solvents ((1)L(a) state) is confirmed by the calculations.