| Abstract
| - Vibrational activities in the Raman and resonance Raman spectra of the cationic, neutral, andanionic forms of 4‘-hydroxybenzylidene-2,3-dimethyl-imidazolinone, a model compound for the greenfluorescent protein chromophore, have been obtained from quantum-chemical calculations in vacuo andwith the inclusion of solvent effects through the polarizable continuum model. It is found that inclusion ofsolvent effects improves slightly the agreement with experimental data for the cationic and neutral forms,whose spectra are qualitatively well-described already by calculations in vacuo. In contrast, inclusion ofsolvent effects is crucial to reproduce correctly the activities of the anionic form. The structural effects ofsolvation are remarkable both in the ground and in the lowest excited state of the anionic chromophoreand influence not only the vibrational activity but also the photodynamics of the lowest excited state.CASPT2//CASSCF photoreaction paths, computed by including solvent effects at the CASSCF level, indicatea facile torsional deformation around both exocyclic CC bonds. Rotation around the exocyclic CC doublebond is shown to lead to a favored radiationless decay channel, more efficient than that in gas phase, andwhich explains the ultrafast fluorescence decay and ground-state recovery observed in solution. Conversely,rotation around the exocyclic CC single bond accounts for the bottleneck observed in the ground-staterecovery cycle. It is also speculated that the ultrafast radiationless decay channel would be hampered inprotein for unfavorable electrostatic interactions and steric reasons.
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