Computational study on the anomalous fluorescence behavior of isoflavones

Area of Science:

• Photochemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• Isoflavones exhibit solvent-dependent fluorescence intensities and large Stokes' shifts.
• Excited state deprotonation explains some, but not all, isoflavone fluorescence anomalies.
• Neutral isoflavones display anomalous fluorescence behavior not fully understood.

Purpose of the Study:

• To computationally and experimentally investigate the anomalous fluorescence of neutral isoflavones.
• To elucidate the underlying mechanisms responsible for solvent-dependent fluorescence in isoflavones.
• To use daidzein as a model compound for understanding these phenomena.

Main Methods:

• Computational investigation of isoflavone electronic states.
• Experimental fluorescence measurements in various solvents.
• Analysis of excited state properties and molecular geometry changes.

Main Results:

• Absence of fluorescence in aprotic solvents and weak fluorescence in protic solvents explained by a change in the order of lowest singlet states.
• In water, a fluorescent charge-transfer state lies below a nonfluorescent locally excited state, unlike in acetonitrile.
• Large Stokes' shifts are partly attributed to significant rotation around the chromone-phenyl bond in the excited state.

Conclusions:

• The solvent-induced alteration of electronic state ordering is the primary cause of anomalous fluorescence in neutral isoflavones.
• Molecular rotation in the excited state contributes to the observed large Stokes' shifts.
• This study provides a mechanistic explanation for the fluorescence behavior of daidzein and potentially other isoflavones.