Spectroscopic Properties of Morin in Various CH 3 OH –H 2 O and CH 3 CN –H 2 O Mixed Solvents

The specific fluorescence properties of morin (3,2′,4′,5,7‐pentahydroxyflavone) were studied in various CH 3 OH–H 2 O and CH 3 CN–H 2 O mixed solvents. Although the dihedral angle is large in the S 0 state, morin has an almost planar molecular structure in the S 1 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 S 1 state cannot occur immediately after excitation, S 1 → S 0 fluorescence can be observed. Two conformers, Morin A and B have been known. At the CH 3 OH–H 2 O, Morin B will be the principal species but at the CH 3 CN–H 2 O, Morin A is the principal species. At the CH 3 OH–H 2 O, owing to the large Franck–Condon (FC) factor for S 2 → S 1 internal convernal (IC) and flexible molecular structure, only S 1 → S 0 fluorescence was exhibited. At the CH 3 CN–H 2 O, as the FC factor for S 2 → S 1 IC is small and molecular structure is rigid, S 2 → S 0 and S 1 → S 0 dual fluorescence was observed. This abnormal fluorescence property was further supported by the small pK 1 value, effective delocalization of the lone pair electrons of C(2′)–OH to the A, C ring, and a theoretical calculation.