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Conical Intersections and the Electronic Structure of the Excited States of N-Phenyl Pyrrole - a Prototype Molecule Exhibiting Dual Fluorescence

Haas Y., Zilberg S., Dick B.

Polish Journal of Chemistry

2008

Vol. 82, nr 4

773-793

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 S2 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.

Polarity, Polarizability and Molecular Structure of Autocomplexes on the Basic of 1,4-Naphthoquinone

Prezhdo V.

Polish Journal of Chemistry

2001

Vol. 75, nr 10

1521-1531

Experimental and theoretical investigation of molecular dipole moments and molar Kerr constants of 1,4-naphthoquinone based autocomplexes has been carried out. The results indicate the presence of intramolecular charge transfer of the n-_* type. A correlation of the experimental values of molar Kerr constants with the _0-constants of substituents to the arylamine fragment of the autocomplexes has been established. Substituents to the electron donor fragment of the autocomplexes systematically affect the angle between the donor and acceptor fragments. Ab initio quantum-chemical calculations support the n-_*-charge transfer structure and show notable changes in the charge on the nitrogen atom of the donor fragment due to complex formation.