Abstract
The results of quantum chemical, quantum mechanical and classical molecular
dynamics simulations of 1-phenylpyrrole are presented and compared to experimental
data from the literature.
In the first part ab initio results for the geometry of 1-phenylpyrrole
are obtained. Vertical, emission, and non-vertical excitation energies
have been computed using multiconfigurational second-order perturbation
theory by means of the CASPT2 method. In the S0 and
S1 states the most stable conformations were determined
to be twisted structures with dihedral angles between the planes of the
rings of 41.4oand 25.5o, respectively. The lowest-excited
state, S1 (11B) is
identified as the ``locally excited'' (LE) state; the charge transfer state
(CT) S3 (21B) has
a higher energy.
Based on these ab initio points, parameters for analytic potential
functions are fitted in the second part. Vibrational eigenfunctions are
computed for these torsional potentials. The Franck-Condon Spectra calculated
for the S0 (11A)-->
S1(11B) transition
compare well with the experimental fluorescence excitation spectrum with
a fluorescence maximum of 4.07 eV in the gasphase. An additional fluorescence
feature is found at 3.72 eV in acetonitrile from a ``twisted intramolecular
charge transfer'' (TICT) state in a perpendicular conformation.
In the third part the molecular potentials and partial charges derived
from the ab initio calculation are used as parameters in a classical
molecular dynamics simulation. The effect of a solvent (acetonitrile) on
the torsion and on the stabilization of the charge transfer state is investigated.
As a consequence of the large dipole of the CT state in comparison with
LE state, the energy of the higher excited CT state in the gas phase drops
below the energy of the LE state in the solvent, giving rise to near-degenerate
dual fluorescence for both CT and LE states in good agreement with the
experimental fluorescence spectrum. Different time scales are found for
the relaxation with respect to the torsion and the reorientation of the
solvent molecules after charge transfer.
From the overall study it is concluded that the 1-phenylpyrrole system
can be considered as an example where the TICT occurs in the solvent but
not in the gas phase. |
