Photooxidation of 3,5-diaryl-1-phenyl-2-pyrazolines: Experimental and computational studies

Photooxidation of various 2-pyrazolines is studied experimentally and computationally. Experimental results show that the electron-donating/withdrawing substituents increases/decreases the rate of this photoreaction. The proposed light-induced electron-transfer mechanism explains the steric and electronic effects of the substituents, co-planarity of the aryl rings substitutions, orientations of the C-3-aryl ring towards the C-3 = N-2 double bond and the solvent on the irradiation time for completion of reaction. Computational (TD)B3LYP/6-311 + +G(d,p) results, including frontier orbital energies, UV-vis transitions, electrostatic potential, CHELPG charges, are used to find and describe tunneling electron-transfer process and its associated structural relaxations which form the rate-determining step of the reaction, and to justify the effect of substitutions on the rate of reaction. The intermediate complex formed after the first electron transfer proceeds the proton transfer step only in its triplet state. Triplet excited states of 2-pyrazolines may have little contribution to the electron transfer step. Cyclic voltammetric measurements support the photochemical results.