Calibration of the triplet–triplet absorption of xanthone as a microenvironment sensor

Abstract

The wavelength of maximum absorption of triplet xanthone ( λ max) is strongly dependent on the nature of the solvent, shifting to the red as the solvent polarity decreases. Thus triplet xanthone can be employed as a probe for polarity of microenvironments such as micelles, cyclodextrins and zeolites. Using nanosecond laser flash photolysis we have measured λ max in a variety of solvents using a Gaussian fitting routine to obtain an objective value for the triplet maximum. These data were compared with two different measures of solvent polarity: the solvent dielectric constant and the pyrene fluorescence band ratio ( I 1/ I 3). There is a modest correlation ( R = 0.89) with dielectric constant. The value of λ max shows two different correlations with I 1/ I 3, one with hydrogen bonding solvents and one with non-hydrogen bonding solvents. The triplet lifetime and absorption band width (FWHM) also exhibit some solvent dependence. We propose the triplet xanthone scale as a convenient tool for probing supramolecular systems using laser flash photolysis; this tool complements nicely the fluorescence of pyrene, which requires a different experimental technique and probes a shorter time scale.