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

Photodissociation of pyrene (Py) dimer radical cation (Py 2 ∙+ ) giving pyrene radical cation (Py ∙+ ) and Py and subsequent regeneration of Py 2 ∙+ by association of Py ∙+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py 2 ∙+ 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 Py 2 ∙+ proceeded via a one-photon process to give the ground-state Py ∙+ (D 0 ) and Py in the quantum yield (Φ diss ) 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 Py 2 ∙+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py 2 ∙+ , even when Py 2 ∙+ was excited to the higher excited state. The difference between the Φ diss value of Py 2 ∙+ and that previously reported for naphthalene dimer radical cation (Np 2 ∙+ ) is discussed.