The specific fluorescence properties of morin (3,2',4',5,7-pentahydroxyflavone) were studied in various CH3OH-H2O and CH3CN-H2O mixed solvents. Although the dihedral angle is large in the S0 state, morin has an almost planar molecular structure in the S1 state owing to the very low rotational energy barrier around the interring bond between B and the A, C ring. The excited state intramolecular proton transfer (ESIPT) at the S1 state cannot occur immediately after excitation, S1 → S0 fluorescence can be observed. Two conformers, Morin A and B have been known. At the CH3OH-H2O, Morin B will be the principal species but at the CH3CN-H2O, Morin A is the principal species. At the CH3OH-H2O, owing to the large Franck-Condon (FC) factor for S2 → S1 internal convernal (IC) and flexible molecular structure, only S1 → S0 fluorescence was exhibited. At the CH3CN-H2O, as the FC factor for S2 → S1 IC is small and molecular structure is rigid, S2 → S0 and S1 → S0 dual fluorescence was observed. This abnormal fluorescence property was further supported by the small pK1 value, effective delocalization of the lone pair electrons of C(2')-OH to the A, C ring, and a theoretical calculation.
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