Anomalous effect of pressure on spectral solvent shifts in water and perfluoro n -hexane

Abstract

It is well known that when typical aromatic chromophores are transferred from the vapor to a solvent the spectrum usually shifts to the red. This shift is commonly understood to arise from the action of dispersion forces, which lower the energy of the excited state more than that of the ground state. As would be expected from such a theory, the shift is usually larger, the larger the refractive index of the solvent. Since pressure increases the density, and therefore the refractive index, one would expect that pressure would generally cause a further red shift of the spectrum. This is observed with benzene, chlorobenzene, naphthalene, and phenanthrene in most solvents. When the solvent is water, however, pressure causes no further red shift for the weak transitions of benzene and chlorobenzene. (The red shift in water at 1 atm is also smaller than would be expected from its refractive index.) When the solvent is perfluoro n -hexane the red shifts of the weak transitions of benzene, chlorobenzene, and naphthalene are small at 1 atm, as expected from the low refractive index of this solvent. Raising the pressure causes no further red shift at all for naphthalene and causes a shift to the blue for benzene and chlorobenzene. The weak n -π* transition of methyl nitrite shows anomalous behavior in water similar to that of benzene and chlorobenzene. Under pressure the strong transitions of naphthalene and phenanthrene in water show the normal behavior. The strong transition of naphthalene in perfluoro n -hexane shows a less anomalous pressure effect than the weak transition. The anomalous behavior of the weak transitions in water is ascribed to an interaction between the permanent dipole moments of ordered water molecules surrounding the chromophore and the static quadrupole moments of the ground and excited states of the chromophore. We have been unable to devise a simple quantitative explanation for the anomalous behavior in the fluorocarbon solvent. The small polarizability of the fluorine atom is believed to be an important factor because blue shifts in the spectra of benzene and other systems are also observed when helium or neon are present, and these atoms also have very small polarizabilities. A small polarizability of the solvent molecules would reduce the magnitudes of the dispersion interactions that are responsible for the normal red shift and would bring into prominence the contributions of repulsive forces and exchange interactions. Since the overlap of the chromophoric electron and the solvent molecules is presumably greater in the excited state than in the ground state, the repulsive interactions should raise the energy of the excited state more than that of the ground state and thus cause blue shift in the spectrum. This raises the possibility that repulsive interactions may make considerable contributions to spectral shifts in other solvents, which are normally masked by the larger red shift caused by the dispersion interaction. Most current theories of spectral shifts fail to take these repulsive contributions into effect.