INTRAMOLECULAR PHOTOASSOCIATION AND PHOTOINDUCED CHARGE-TRANSFER IN BRIDGED DIARYL COMPOUNDS .7. A SEMIEMPIRICAL MO STUDY OF INTRAMOLECULARCHARGE-TRANSFER IN THE EXCITED SINGLET-STATES OF DINAPHTHYLAMINES

A semiempirical MO study of the intramolecular charge transfer (CT) in the excited singlet states of dinaphthylamines has been carried out w ith the program systems MOPAC and ARGUS. The excited-state energies fo r various conformations of the molecules were obtained, in both the ab sence and the presence of a polarizable medium, by adding the transiti on energies calculated with the INDO 1/S method to the groundstate ene rgies calculated by means of the AM1 method. The CT state corresponds to a twisted geometry in which one naphthalene moiety is conjugated wi th the amino bridge, while the other moiety is perpendicular to the fi rst. The gas-phase energy of this twisted intramolecular CT (TICT) sta te is only slightly greater than that of the lowest excited singlet (S -1) state of smaller dipole moment. In solvent of large dielectric con stant, the TICT state is therefore predicted to be the lowest excited singlet state of the molecule. The computed oscillator strength of the absorption to the TICT state is much smaller than that to the lowest- energy excited state of an isolated molecule, so that the increased CT character of the S-1 state in polar solvents is expected to lead to a decrease in the radiative decay rate of the state. These results are consistent with the experimental observation of a large fluorescence S tokes shift, and a reduction in the S-1 radiative decay rate, of the c ompounds in polar solvents relative to nonpolar solvents.