Intramolecular deactivation of photoexcited anthracenes by aromatic ketones. Part 2.—Charge-transfer deactivation of ω-(9-anthryl)ketones in different solvents and binary solvent mixtures and at different temperatures

Abstract

The solvent and temperature dependence of the excited state properties of anthracenes non-conjugatively linked to various aromatic phenones have been investigated by determination of the quantum yields of fluorescence and intersystem crossing. In a series of substituted bichromophoric ω-(9-anthryl)propiophenones, the degree of intramolecular quenching of the anthracene fluorescence by the phenones was not compensated for by increasing intersystem-crossing efficiency in the anthracene chromophore, but was found to be associated with the standard free-energy change for intramolecular charge separation, the solvent polarity and the temperature. In contrast, in monochromophoric anthracene reference compounds neither remarkable solvent polarity nor temperature dependence of the fluorescence emission was detected. For example, for one bichromophoric compound [ω-(9-anthryl)-4′-methoxypropiophenone] an analysis within the framework of the Marcus electron-transfer theory was performed and a dependence of the nuclear Franck–Condon factor for the intramolecular charge-separation reaction on the solvent polarity was found. Shortening the linking non-conjugative bridge in one bichromophore [ω-(9-anthryl)propiophenone] resulted in an increased charge-separation rate constant by more than one order of magnitude in ω-(9-anthryl)acetophenone.