Laser-flash-photolysis experiments show that, in MeCN at 20". perylene (P) undergoes three distinct electrontransfer reactions: a) IP* + MeCN 4 P'+ + MeCN'b) ' P * t P 4 P ' + + P ' c) 3P* + 3P* 4 (P.P)* ~ P" + P' These processes originate probably from the thermally relaxed excited states of P Introduction.
-The photoinduced electron-transfer processes of perylene (P) in polar solvents have been the object of several investigations [ 1-31 which have led to contradictory conclusions. It is established that in flash-photolysis experiments both radical ions P" and P'are formed in MeCN, and this was attributed to electron transfer within the excimer or collision complex according to Eqn. 1.
D
Here, (P . P)* refers to an excimer or collision complex of indeterminate multiplicity, (P--.P'-)s to the solvated contact ion pair and Pi+, Pito the separated, solvated ions. Such a mechanism implies that the reactive state 'P* is the thermally equilibrated singlet excited state of P and that the electron-transfer process is, therefore, in competition with the deactivation of 'P* by fluorescence and intersystem crossing:
The validity of Eqn. I was subsequently questioned on the basis of quenching experiments: 0 . 8 ~ PhBr was reported to quench the fluorescence of P without affecting the ion yield and this observation would indeed rule out the relaxed excited states 'P* or 'P* as precursors in the electron-transfer reaction.
On the basis of this and similar observations a general mechanism of photoionization of aromatic molecules in liquid solvents was proposed . Ionization is seen as an electron I ) 'P** is used for P excited to an upper electronic and/or vibrational state ')