The influence of pressure on the electronic absorption spectra of some aromatic molecules and ions

Abstract

Measurements have been made of the effects of raising the pressure from 1 bar to 2000 bar on the frequencies and intensities of the low frequency electronic absorption bands of 57 aromatic molecules and ions in aqueous solution at 25°C. For comparison, measurements have also been made of frequency shifts produced by changing the solvent from water to ethanol at atmospheric pressure.     A remarkable feature of the results is that although compression raises the dielectric constant and refractive index of water, it produces red frequency shifts (shifts to lower frequencies) in the n → π* bands of molecules. These bands are well known to undergo blue shifts when one solvent is changed to another of higher polarity at atmospheric pressure. The apparent anomaly can be explained in terms of McRae's theory of frequency shifts, as arising from a decrease, with increasing pressure, of the energy of interaction between permanent dipoles of the solvent and of the absorbing molecules. The π → π* absorption bands of molecules generally show red shifts with both increasing pressure and increasing solvent polarity: this result is also consistent with McRae's theory. The absorption bands of most of the ions show blue shifts which can be explained by charge delocalization in the excited states. The intensity changes are in general agreement with the predictions of current theories of oscillator strengths in solution.    The results provide indirect evidence of the influence of pressure on the ionization constants of excited states of phenols.