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

The wavelength of maximum absorption of tripler xanthone (lambda(max)) is s trongly dependent on the nature of the solvent, shifting to the red as the solvent polarity decreases. Thus triplet xanthone can be employed as a prob e for polarity of microenvironments such as micelles, cyclodextrins and zeo lites. Using nanosecond laser flash photolysis we have measured lambda(max) in a variety of solvents using a Gaussian fitting routine to obtain an obj ective value for the tripler maximum. These data were compared with two dif ferent measures of solvent polarity: the solvent dielectric constant and th e 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 diff erent correlations with I-1/I-3, one with hydrogen bonding solvents and one with non-hydrogen bonding solvents. The triplet lifetime and absorption ba nd width (FWHM) also exhibit some solvent dependence. We propose the triple r xanthone scale as a convenient tool for probing supramolecular systems us ing laser hash photolysis; this tool complements nicely the fluorescence of pyrene, which requires a different experimental technique and probes a sho rter time scale. (C) 1999 Elsevier Science S.A. All rights reserved.