| Abstract
| - Absorption and resonance Raman spectra have been measured over a wide range of solvents for twomerocyanine dyes containing the indoline (“Fischer” base) electron donor group with different acceptinggroups. One appears to be near the cyanine limit (equal contributions of the neutral and zwitterionic resonanceforms to both ground- and excited-state structures) based on electrooptic absorption data showing a verysmall dipole moment change upon electronic excitation. The resonance Raman spectra of both moleculesshow significant frequency shifts and intensity redistributions that evolve monotonically with increasing solventpolarity and are consistent with increasing zwitterionic character of the ground-state structure. The vibrationalreorganization energies of both molecules, obtained by simulating the absorption band shapes, are smaller inpolar solvents than in nonpolar or weakly polar ones, consistent with a more cyanine-like structure at highersolvent polarities. However, the vibrational reorganization energies of both molecules exceed 700 cm-1 in allsolvents, larger than in many true cyanine dyes, and the optical absorption maxima do not correlate well witheither solvent polarity or vibrational reorganization energy. This indicates some limitations to the structuralconclusions that can be reached from the two-state model for π-conjugated donor−acceptor systems.
|
