ULTRAVIOLET AND VISIBLE CHARGE-TRANSFER ABSORPTION AND EMISSION-SPECTRA OF SOME POLAR DYES IN AQUEOUS-SOLUTION UNDER HIGH-PRESSURE

The ultraviolet and visible absorption and emission spectra of aqueous solutions of four dyes, 4-amino-4′-cyanobiphenyl, carbostyril 124, coumarin 1, and coumarin 102, and the absorption spectrum of 4-nitroaniline were measured at pressures up to 6 kbar. The compressions produced red shifts or, in one case, virtually no shifts. The variations of vmax with pressure were reproduced by linear equations. The largest red shift was for the absorption of 4-nitroaniline, where the slope was -107 cm-1/kbar. The slopes for the two coumarin dyes were remarkably different, considering the similarity of their structures. Established theory of dielectric solvation effects on Franck-Condon transition energies was modified to apply to solvents of high dielectric constant and predicted pressure effects whose magnitude is small compared to observation. A quasi-thermodynamic approach was more encouraging. Because the charge-transfer excited states are zwitterions, the hypothetical excitation process at equilibrium is attended by a large decrease in volume due to electrostriction of the solvent. Thus there is marked volume strain in the Franck-Condon final state. The decrease in volume was estimated for three of the substrates and accounts for a major part of the pressure shifts. © 1990 American Chemical Society.