Theoretical study of the twisted intramolecular charge transfer in 1-phenylpyrrole

Ab initio results for the electronic spectra of 1-phenylpyrrole are presented. Vertical, emission, and nonvertical excitation energies have been computed using multiconfigurational second-order perturbation theory by means of the CASPT2 method. In the S-0 and S-1 states the most stable conformation was determined to be a twisted structure with a dihedral angle between the planes of the rings of 41.4 degrees and 25.5 degrees, respectively. The most intense feature of the absorption spectrum is predicted to be the S-0 (1(1)A) - S-1 (2(1)A) transition placed in vacuo at 5.07 eV. The lowest-excited state, S-1 (1(1)B), identified as the "locally excited" (LE) state, is calculated in vacuo to lie vertically 4.39 eV above the ground state, with the position of the 0-0 transition at 4.16 eV and a fluorescence maximum of 4.07 eV. An additional fluorescence feature is found at 3.72 eV in acetonitrile from a "twisted intramolecular charge transfer" (TICT) state in a perpendicular conformation. The absence of the low-energy emission in vacuo is rationalized in terms of the different shapes of the S-0 and S-1 hypersurfaces. From the overall study it is concluded that the 1-phenylpyrrole system can be considered as an example where the TICT occurs.