High pressure study of luminescence from intramolecular CT compounds

Abstract

The effects of high pressure on the luminescence properties of four organic intramolecular charge transfer compounds were investigated. The compounds were studied in a wide range of liquid and polymeric environments, and for one of the compounds, the effect of chemical substitution was also examined. In general, an increase of pressure affects the luminescence in a way which is in some degree similar to an increase of solvent polarity. A single configuration coordinate model was used to interpret the luminescence in terms of excited state interactions. Three of the compounds exhibited only fluorescence: p-(9-anthryl)-dimethylaniline (ADMA), 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and 1-anilino-8-naphthalenesulfonate (ANS). Fluorescence from the first two compounds in nonpolar media is attributed to an excited state relatively unaffected by the charge transfer process. In polar media, fluorescence is believed to originate from another excited state that possesses considerable charge transfer character, and whose energy is strongly influenced by solvent polarity. At high pressure, an apparent fluorescence shift to higher energy was observed in viscous alcohols apparently due to the introduction of fluorescence from the energetically higher lying non-charge transfer excited state. This explanation is supported by the dual fluorescence decay observed from ADMA coincident with the sign reversal in fluorescence peak shift. Three excited states are believed to affect ANS fluorescence, two of them with charge transfer character. The model describing the excited states of ADMA and PRODAN is consistent with ANS fluorescence except in glycerol, whose anomalous behavior is discussed in the text. The fourth compound, nitroaniline, was studied in its three isomeric forms, with and without N-methylation. In addition to affecting the phosphorescence and fluorescence of p-nitroaniline, pressure studies indicate that nπ* singlet and triplet states figure prominently in the radiative and nonradiative processes of o-nitroaniline and m-nitroaniline as well.