Dipolar Cycloaddition of Aurones: Products and Photophysics

Abstract

Aurones are naturally occurring molecules that have intriguing biological and photophysical properties and have become enticing synthetic precursors due to the reactivity of the exocyclic alkene. While several pyrazoles have been made through dipolar cycloaddition using that alkene, their photophysical properties have been poorly explored. To better understand these properties, a series of pyrazoles were prepared by the cycloaddition/ring opening of aurones with ethyl diazoacetate. Their absorption and emission behavior in solvents with a range of polarities and protic natures were examined, showing that the presence of an electron donating group substituent increases the fluorescence response as polarity of the solvent increases. This leads to a significant increase in water, a solvent that typically quenches fluorescent activity. Halogen substituents were also found to have a similar effect in halogenated solvents. Correlations between the structure of each pyrazole and its fluorescence in a variety of solvents have been noted, and a plausible rationale has been proposed.