Spectroscopic Properties of Morin in Various CH₃OH–H₂O and CH₃CN–H₂O Mixed Solvents

Abstract

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.

Spectroscopic Properties of Apigenin in Various Bulk Solutions and Aerosol‐OT Reverse Micelles

“…14,25,26 Although many experimental studies have been carried out for various flavonoids due to their strong antioxidant activities, only a few theoretical works regarding the optimal geometrical molecular structure at the various electronic states have been reported on API to date. 14,[25][26][27]43 The optimized dihedral angle of quercetin becomes close to 0 in the gas phase. This angle determines the position of the B ring relative to the γ-pyrone ring (rings A and C).…”

“…14,[25][26][27]43 The optimized dihedral angle of quercetin becomes close to 0 in the gas phase. 21,25,26 The intramolecular hydrogen bonding interaction will be a more important stability feature than steric hindrance in many flavonoid compounds. [43][44][45] It is helpful to further consider the geometrical molecular structure of API to understand its specific fluorescence properties.…”

“…[43][44][45] It is helpful to further consider the geometrical molecular structure of API to understand its specific fluorescence properties. 14,25,26 Since API does not have a C(3) OH group, this intramolecular hydrogen bonding interaction will occur without any interruption in the gas phase and aprotic solvents. The delocalization effect favors planar structures, and steric hindrance is reflected in the C(2) C(1 0 ) interring bond distance.…”

“…The antioxidant activity of flavonoids is mediated via different mechanisms including direct free radical scavenging, modulation of cell signaling, chelation of transition metal ions, inhibition of certain enzymes, and removal of oxidatively damaged biomolecules. 14,21,25,26 However, the structure-activity relationship is not yet well understood, since the antioxidant activity can be determined by many factors. Due to the many substitution patterns on the basic skeleton of the flavone molecule, a large number of flavonoids exist.…”

“…13,[15][16][17][18][19][20] The position and number of OH groups substituted on the basic flavonoid structure exhibits strong influence on the chemical properties and antioxidant activities of these molecules. 14,21,25,26 The AOT/H 2 O/alkane ternary systems are especially suitable as a solvent system for this type of work, since AOT micelles are membrane mimetic, homogeneous, and optically transparent over a wide range of temperatures and concentrations. It is known that flavonoids possess attractive characteristic properties such as dual fluorescence and excited state intramolecular proton transfer (ESIPT).…”