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

The wavelength of maximum absorption of tripler xanthone (lambda(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 lambda(max) in a variety of solvents using a Gaussian fitting routine to obtain an objective value for the tripler 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 lambda(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 tripler xanthone scale as a convenient tool for probing supramolecular systems using laser hash photolysis; this tool complements nicely the fluorescence of pyrene, which requires a different experimental technique and probes a shorter time scale. (C) 1999 Elsevier Science S.A. All rights reserved.