Photodissociation of pyrene dimer radical cation during the pulse radiolysis–laser flash photolysis combined method

Abstract

Photodissociation of pyrene (Py) dimer radical cation (Py2∙+) giving pyrene radical cation (Py∙+) and Py and subsequent regeneration of Py2∙+ by association of Py∙+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py2∙+ was excited at the local excitation band with the 532-nm laser flash, the rapid growth and decay of monomeric Py∙+ were observed at 460 nm. The dissociation of Py2∙+ proceeded via a one-photon process to give the ground-state Py∙+(D0) and Py in the quantum yield (Φdiss) of (2.9 ± 0.9) × 10−3. It was shown that Py∙+ decayed with a time constant of several tens of nanoseconds, indicating that the association of Py∙+ with Py regenerating Py2∙+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py2∙+, even when Py2∙+ was excited to the higher excited state. The difference between the Φdiss value of Py2∙+ and that previously reported for naphthalene dimer radical cation (Np2∙+) is discussed.