A new series of proton/charge transfer molecules: Synthesis and spectral studies of 2-(5-aryl-1-carbomethoxy-1H-pyrazol-3-yl)phenols

The carbomethoxyhydrazone of 2'-hydroxyacetophenone was trilithiated with e xcess lithium diisopropylamide and C-acylated with a variety of benzoate es ters followed by acid cyclization of the intermediates to 2-(5-aryl-1carbom ethoxy-1H-pyrazol-3-yl)phenols [3-(2-hydroxyphenyl)-1H-pyrazoles]. The prod ucts were characterized by Fourier transform-IR, H-1 NMR, C-13 NMR, UV-visi ble absorption and fluorescence. All the derivatives in n-heptane have an a bsorption maximum at similar to 304 nm and an extremely weak (Phi(f) = 10(- 4)) fluorescence with maxima in the range of 335-460 nm. The broad range of fluorescence maxima and fluorescence quantum yields is attributed to varyi ng contributions of charge transfer that are dependent on both the identity of the substituent and solvent polarity. A phenomenally large Stokes-shift ed fluorescence maximum at 620 nm was observed for 2-(1-carbomethoxy-5-[4-d imethylaminophenyl]-1H-pyrazol-3- yl)phenol in n-heptane and attributed to excited-state intramolecular proton transfer. As a result, competitive exci ted-state proton/charge transfer properties have been observed in the pyraz oles studied, of which the spectral properties can be fine tuned by substit uent as well as solvent effects.