Conical intersections and the electronic structure of the excited states of N-phenyl pyrrole - A prototype molecule exhibiting dual fluorescence

The mechanism leading to dual fluorescence in substituted benzene derivatives is discussed using N-phenyl pyrrole (PP) as a test molecule. A model based on the quasi-Jahn-Teller distortion of the charge transfer (CT) state of PP, similar to that of the benzene anion radical, is presented. The model helps to locate stationary points on the S₁ singlet excited state potential surface, and to construct an energy level diagram. The energy and structure of the quasi Jahn-Teller degeneracy which is between S₃ and S ₂ are computed, as well as those of three conical intersections connecting S₁ and S₂. The findings indicate a complex nature of the S₁ state electronic state, containing several local minima, stationary points and surface crossings. The major structural change responsible for the stabilization of the CT state is the ring quinoid deformation which is a main component of the energy difference gradient vector in the branching space of all S₁/S₂ conical intersections. In the resulting charge transfer state (which is of quinoid structure) the pyrrole ring can either rotate to form a stabilized twisted species, or remain planar depending on substitution. This result holds also for DMABN and other molecules in which the donor is a substituted amine moiety. It is concluded, in line with a recent study on DMABN (Gomez et al., J. Amer. Chem. Soc., 127, 7119 (2005)) that the TICT and PICT models are not mutually exclusive, but two different manifestations of the same physical reality.