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

The observations of a clear solvent-dependence of resonance Raman intensiti es, but an absence of concommitant changes in absorption cross-sections, ar e reported for the molecule 9,9(')-bianthryl (BA). Displacements obtained b y analysis of the nonpolar solvent data are found to reproduce the absorpti on spectra recorded in all solvents studied, but not the resonance Raman in tensities in polar solvents. Moreover, transform theory is found to be unsu ccessful in reproducing the resonance Raman intensities in any solvent. The se observations suggest that ultrafast relaxation dynamics (on the timescal e probed by the resonance Raman experiment) are changing the displacements of the intramolecular vibrational modes. The changes in the displacements d etermined by analysis of the data are consistent with a correlation between the total reorganization energy accompanying the charge transfer and the s olvent dielectric properties (i.e., the mode-specific reorganization is fou nd to increase with solvent dielectric properties). In effect, the immediat e free energy surface "seen" by the molecule changes dramatically with time and causes significant intramolecular reorganization, at least for the ini tial stages of evolution of the emissive state. These findings are supporte d by ab initio molecular orbital (MO) geometry optimization, analytical fre quencies, and excited state calculations (CIS/3-21G*, HF/6-31G*). It is sho wn that most of the normal modes of the S-0 state of BA are splittings of c orresponding anthracene modes, however, for the relaxed S-1 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 S-1 state is calculated to be 0.099 debye in the gas phase, compared to 0.00 debye for the equilibrium ground state and the ver tical, unrelaxed, S-1 state. The optimized S-1 geometry of BA is found to b e a "90 degrees" 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 D-2. We suggest that these results provide direct evidence for the importance of solvent-depende nt intramolecular reorganization in this molecule. (C) 1999 American Instit ute of Physics. [S0021-9606(99)00422-5].