Cyclic Voltammetric Study of 3,5-Diaryl-1-phenyl-2-pyrazolines

Abstract

Cyclic voltammetry is used to derive HOMO energies of the 1-phenyl-2-pyrazolines containing electron-donating or electron-withdrawing substituted phenyl rings and or naphthalenyl substitution on the C₃- or C₅-positions of the heterocyclic ring to investigate the steric and electronic effects of the aryl substitutions and the type of aryl system on their electrochemical behaviors. The optical HOMO-LUMO gaps needed for the calculation of LUMO (excited state) energies of these compounds are obtained from their UV-vis spectra. Results show that the substitution on the C₃-aryl ring has significant effect via its π-donor/acceptor ability, compared to the σ-donor/acceptor ability of the C₅-aryl ring, on the CV oxidation peak and onset potentials. Comparative analysis showed very good agreement between the experimentally obtained HOMO and (apparent) LUMO energies and the (TD)DFT/6-311++G(d,p) calculated ground and excited states energies. These computational results indicate also that for all chloro- and methoxy-substituted 2-pyrazolines, the HOMO → LUMO is the most intense transition. While the strong acceptor NO₂ substitution on all positions of either C₃- or C₅-aryl rings, except for one compound, increases the intensity of the HOMO → LUMO+1 (LUMO+2) transition significantly, the first (the first two) transition(s) HOMO → LUMO (and HOMO → LUMO+1) has (have) much smaller or negligible intensity (intensities).