Pulse radiolysis of pyrene in aprotic polar organic solvents: simultaneous formation of pyrene radical cations and radical anions

Abstract Simultaneous formation of pyrene radical cations Py · + and radical anions Py · − was observed by pulse radiolysis of pyrene in DMSO or sulfolane solutions containing 10 vol % toluene. Using 5×10 −5 mol dm −3 pyrene in the oxygen-free solvent mixture of 90 vol % DMSO and 10 vol % toluene, the radical cation Py · + (λ max =450 nm ;e 450 =2.29×10 4 dm 3 mol −1 cm −1 ) was formed with the rate constant k=4.0×10 9 dm 3 mol −1 s −1 and decayed by a first-order reaction with k 1 =1.2×10 4 s −1 , forming the dimer radical cation ( Py ) 2 · + . Simultaneously, the radical anion Py · − (λ max =495 nm ; e 495 =2.17×10 4 dm 3 mol −1 cm −1 ) was produced with k=6.0×10 10 dm 3 mol −1 s −1 and disappeared by formation of the dimer radical anion ( Py ) 2 · − with k 1 =4.6×10 4 s −1 . The first-order rate constants ( k 1 ) for the decay of the species Py · + and Py · − with formation of the dimer radical ions ( Py ) 2 · + and ( Py ) 2 · − increased with raising the substrate concentration. By scavenging all radiolytically generated electrons with oxygen (k≈8×10 8 dm 3 mol −1 s −1 in 90 vol % DMSO and 10 vol % toluene), the formation of the Py · − transients was suppressed without interfering with the generation of the radical cations Py · + . In oxygen-free solutions of pyrene (5×10 −5 mol dm −3 ) in 90 vol % sulfolane and 10 vol % toluene the corresponding rate constants of the transient formation and the decay were for Py · + :k=1.5×10 10 dm 3 mol −1 s −1 and k 1 =0.9×10 4 s −1 , and for Py · − :k=2.3×10 10 dm 3 mol −1 s −1 and k 1 =4,1×10 4 s −1 .