Numerical self-consistent reaction field study of intramolecular charge transfer in p-(dimethylamino)-benzonitrile

The numerical self-consistent reaction field (SCRF) implementation of the semiempirical program package VAMP 5.5 allows to include the solute-solvent interaction in the iterative solution of the Hartree-Fock equations. In going beyond the widely applied Onsager model, the solvent cavity is thereby approximated by a polyhedral, carrying a charge density proportional to the gradient of the molecular potential at the cavity surface due to the charge distribution in the solute molecule. The method was modified to allow the calculation of absorption and emission spectra using a nonequilibrium description of solvation. With this approach, the free enthalpy of solvation can easily be divided into the contribution originating from ultrafast electronic polarisation and from slower orientational relaxation. The newly developed program was applied to the problem of dual fluorescence of p-(dimethylamino)-benzonitrile (p-DMABN). It was found that planarisation of the amino group leads to a stabilisation of the polar excited state independently of the assumed torsional angle theta. In solvents, that are more polar than diethylether (epsilon=4.335), rotation of the planar dimethylamino group connected with the formation of a twisted intramolecular charge transfer state (TICT) is found energetically feasible since the activation barrier is lower than 7 kJ mol(-1). The variation of the calculated transition energies from the TICT state to the corresponding Franck-Condon ground state is in very good agreement with the slope found in the Lippert-Mataga plot of p-DMABN. Calculated dipole moments also agree with experimental values. The numerical SCRF method turned out to give a description of excited state properties of p-DMABN in solution that is superior to previously applied theoretical methods based on the Onsager and the solvaton model. (C) 1997 Elsevier Science S.A.