Unprecedented high fluorescence anisotropy in protic solvents: hydrogen bond induced solvent caging?

Atypical high fluorescence anisotropy has been revealed in protic solvents for excited state intramolecular proton transfer (ESIPT) prone 3-hydroxyflavone and some of its hydroxy derivatives. In non-protic solvents, however, the anisotropy values are low. The low and alike anisotropy values of 3-methoxyflavone in all the studied solvents and other experimental evidences ascribe the formation of hydrogen bond induced cage-like structures in case of the hydroxy compounds involving the probe and the solvent molecules to be responsible for the high values of fluorescence anisotropy in protic media