Solvent and intramolecular effects on the absorption spectrum of betaine-30

The effect of solvent electrostatics and solute torsional modes on the abso rption spectrum of betaine-30 in acetonitrile is examined. Combined quantum /classical molecular dynamics ground state simulations are used to calculat e the electronic absorption spectrum in acetonitrile. The model for betaine -30 includes the electronic degrees of freedom of the pi system of the mole cule and their interactions with the electric field of the solvent, treatin g the electronic wave function at the level of Pariser-Parr-Pople semiempir ical electronic structure theory. The absorption intensity, width, and maxi mum of the S-0 to S-1 band are well reproduced by the model. In solution, t he So molecular dipole moment is found to be strongly enhanced due to solve nt-induced electronic reorganization. The width of the absorption band in a cetonitrile is found to be a function of solvent orientational fluctuations and is not correlated with conformational changes caused by torsional moti on in the molecule. This fact, combined with the good agreement between the classical reorganization energies inferred from the simulated and experime ntal spectra indicates that, at least in acetonitrile, the classical compon ent of the reorganization energy is fully determined by solvent orientation al polarization. The spectral band maximum of the lowest energy transition is found to be blue shifted over 7000 cm(-1), compared to a calculation in which the coupling of the betaine-30 electronic structure to the solvent mo lecules is eliminated, in agreement with the shift found experimentally for betaine-30 in acetonitrile compared to alkanes. However, in contrast to th e result found in acetonitrile, the transition energy in the absence of sol vent interactions is found to be strongly correlated with the central pheno late-pyridinium dihedral ring angle. This contrasting behavior implies that in nonpolar solvents, the classical reorganization energy does have a cont ribution from that torsional mode. Correspondingly, this difference in beha vior with solvent indicates that the assumption of a solvent independent in tramolecular contribution to the reorganization energy is questionable.