Rapid “Step Capture” of Holes in Chloroform during Pulse Radiolysis

The fundamental process of hole capture in solution was investigated following pulse radiolysis with polyfluorene and 4-cyano-4″-pentyl-p-terphenyl scavengers. Contrary to expectation, a large fraction of holes were captured in experimental time-resolution limited ∼20 ps steps, by a process much faster than diffusion of the initially formed solvent molecular cation. At the highest concentrations, 1.92 mM for a 52 unit long polyfluorene and 800 mM for 4-cyano-4″-pentyl-p-terphenyl, 66% and 99%, respectively, of the initially formed holes were captured by 20 ps, with radiation chemical yield G = 1.2 × 10^–7 and 1.7 × 10^–7 mol J^–1. The data can be explained by capture of presolvated holes, analogous to presolvated electrons, possibly possessing extended wave functions, high mobilities, or excess kinetic energy for the first few picoseconds after their creation. Such a process is not generally known in solution; however, the observed step capture as a function of solute concentration is shown to be well explained by this model. In addition to understanding the capture process in solution, the very large step yields formed in 20 ps will provide the ability to resolve subsequent hole transfer on the polymers with >2 orders of magnitude better time resolution than expected.