Solvent Effects on the Structure and Spectroscopy of the Emitting States of 1-Phenylpyrrole

Theoretical calculations of absorption and fluorescence properties of 1-phenylpyrrole have been performed, at the CASPT2//CASSCF level, in the gas phase and in acetonitrile solution, using in the latter case the ASEP/MD method. In addition to a locally excited state, it was also possible to identify a candidate intramolecular charge transfer state that could explain the second red-shifted fluorescence band that appears in polar solvents. In the gas phase, the charge transfer state is found to lie higher in energy than the locally excited state and the Franck-Condon absorption state, making it unlikely to be reached under these conditions. In acetonitrile solution, the charge transfer state is stabilized and lies much closer to the locally excited state, becoming accessible after absorption. The results indicate that the free-energy surface of the charge transfer state is very flat in solution, and several geometries are possible, ranging from almost planar to twisted and bent. Solvent caging and transition probabilities favor emission from structures with a small twist angle between the rings and without a pyramidal atom.