Formation of radical cations of diaryloxadiazoles

The nature of the formation of the radical cation of the 2,5-bis-(p-diethylaminophenyl)-1,3,4-oxadiazole (PC) in liquid n-butyl chloride and acetonitrile has been investigated by observing excited state fluorescence and transient absorption using nanosecond pulse radiolysis and laser flash photolysis. The formation of solute oxonium ions has also been observed. At concentrations < ∼ 5 x 10-4 mol dm-3 the growth time at which the transient absorption of the radical cation reaches the maximum follows the rise time of the electron pulse (< 10 ns) used. The excitation of PC with laser light of a N2 laser yields the solute radical cation in an acetonitrile solution of 2 x 10-4 mol dm-3 PC via an electronically excited state. Here, the generation time was smaller than 5 ns. The yield of the cation is increased by addition of CCl4. A reaction mechanism is proposed that explains the fast cation formation in terms of an exciplex formed by interaction between an electronically excited state of diaryloxadiazole and the ground state of the solvent. This exciplex yields the solute radical cation. (author)