Laser photolysis of 1-(4-nitrophenyl)-3-methylpyrazole and 1-ethyl-3-(4-nitrophenyl)-5-chloropyrazole

The spectral kinetic characteristics of the triplet states of 1-(4-nitrophenyl)-3-methylpyrazole (1) and 1-ethyl-3-(4-nitrophenyl)-5-chloropyrazole (2) were Studied by the laser nanosecond photolysis technique in different solvents. The triplet lifetimes (tau(T)) of molecules 1 and 2 were found to depend strongly oil the solvent nature. Ail increase in tau(T) by approximately two orders of magnitude oil going from nonpolar and polar aprotic solvents (tau(T) <= 15 ns) to aqueous-acetonitrile solutions (tau(T) = 1100 ns for a volume acetonitrile to water ratio of 1 : 3) was analyzed, taking into account the influence of the medium on the relative contribution of the n,pi*- and pi,pi*-configurations to the lowest triplet state.