On the unusual fluorescence properties of xanthone in water

Photo-excited xanthone is known to undergo ultrafast intersystem crossing (ISC) in the 1 ps time domain. Correspondingly, its fluorescence quantum yield in most solvents is very small (similar to 10(-4)). Surprisingly, the quantum yield in water is 100 times larger, while ISC is still rapid (similar to 1 ps), as seen by ultrafast pump probe absorption spectroscopy. Temperature dependent steady state and time resolved fluorescence experiments point to a delayed fluorescence mechanism, where the triplet (3)n pi* state primarily accessed by ISC is nearly isoenergetic with the photo-excited (1)pi pi* state. The delayed fluorescence of xanthone in water decays with a time constant of 700 ps, apparently by internal conversion between the (3)n pi* state and the lowest lying triplet state (3)pi pi*.