Solvent and substituent effects in aromatic carbonyl compounds: the lowest triplet states of xanthone and xanthone-18O1

The phosphorescence of xanthone and xanthone-18O1 in ethanol-ether-isopentane (EPA) and in 3-methylpentane (3-MP) has been studied between 12 and 125 K. The dual phosphorescence in this ketone is attributed to the existence of two different ground-state conformations, each of which has a characteristic triplet state. The longer-lived phosphorescence is assigned to a more planar conformation than the short-lived species. Because a progression based on the carbonyl stretch frequency appears in both short and long-lived components, we assign some n, π* character to both forms. Thermally populated fluorescence from the lowest 1(n, π*) state of xanthone is also reported for the first time.