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

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 x 10(-5) mol dm(-3) pyrene in the oxygen-free solvent mixture of 90 vol% DMSO and 10 vol% toluene, the radical cation PY.+ (lambda(max) = 450 nm; epsilon(450) = 2.29 x 10(4) dm(3) mol(-1) cm(-1)) was formed with the rate constant k = 4.0 x 10(9) dm(3) mol(-1) s(-1) and decayed by a first-order reaction with k(1) = 1.2 x 10(4) s(-1), forming the dimer radical cation (Py)(2)(.+). Simultaneously, the radical anion Py.- (lambda(max) = 495 nm; epsilon(495) = 2.17 x 10(4) dm(3) mol(-1) cm(-1)) was produced with k = 6.0 x 10(10) dm(3) mol(-1) s(-1) and disappeared by formation of the dimer radical anion (Py)(2)(.-) with k(1) = 4.6 x 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 approximate to 8 x 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 x 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 x 10(10) dm(3) mol(-1) s(-1) and k(1) = 0.9 x 10(4) s(-1), and for Py.- : k = 2.3 x 10(10) dm(3) mol(-1) s(-1) and k(1) = 4, 1 x 10(4) s(-1). (C) 2003 Published by Elsevier Science Ltd.