Solvation and intramolecular reorganization in 9,9′-bianthryl: Analysis of resonance Raman excitation profiles and ab initio molecular orbital calculations

Abstract

The observations of a clear solvent-dependence of resonance Raman intensities, but an absence of concommitant changes in absorption cross-sections, are reported for the molecule 9,9′-bianthryl (BA). Displacements obtained by analysis of the nonpolar solvent data are found to reproduce the absorption spectra recorded in all solvents studied, but not the resonance Raman intensities in polar solvents. Moreover, transform theory is found to be unsuccessful in reproducing the resonance Raman intensities in any solvent. These observations suggest that ultrafast relaxation dynamics (on the timescale probed by the resonance Raman experiment) are changing the displacements of the intramolecular vibrational modes. The changes in the displacements determined by analysis of the data are consistent with a correlation between the total reorganization energy accompanying the charge transfer and the solvent dielectric properties (i.e., the mode-specific reorganization is found to increase with solvent dielectric properties). In effect, the immediate free energy surface “seen” by the molecule changes dramatically with time and causes significant intramolecular reorganization, at least for the initial stages of evolution of the emissive state. These findings are supported by ab initio molecular orbital (MO) geometry optimization, analytical frequencies, and excited state calculations (CIS/3-21G*, HF/6-31G*). It is shown that most of the normal modes of the S0 state of BA are splittings of corresponding anthracene modes, however, for the relaxed S1 geometry of BA (i.e., gas phase equilibrium geometry), the modes are calculated to be red-shifted and have significantly greater splittings. Furthermore, the dipole moment of this relaxed S1 state is calculated to be 0.099 debye in the gas phase, compared to 0.00 debye for the equilibrium ground state and the vertical, unrelaxed, S1 state. The optimized S1 geometry of BA is found to be a “90°” geometry (i.e., torsion angle between the anthracene ring planes), similar to that of the ground state except for subtle asymmetries in each anthracene ring which lower the symmetry from D2. We suggest that these results provide direct evidence for the importance of solvent-dependent intramolecular reorganization in this molecule.