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

Abstract

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.

Sub-picosecond Production of Solute Radical Cations in Tetrahydrofuran after Radiolysis

“…Because both presolvated electron capture and the current results have a similar sudden production of solute ions, we will use the same description. This approach worked well for the description of previous sub-15 ps hole capture in chloroform . From eqs and we can derive: This gives expressions for the yield of holes scavenged by Br 2 F from THF +• , G scav .…”

“…In general, one expects slow capture of holes, as they are typically considered as solvent molecular ions that diffuse at typical rates for molecules. The first clear examples of much faster than diffusion, and likely presolvated hole capture, include our previous work in chloroform where solvent holes are long-lived, 6 work by Mostafavi's group using water, 7,36,42−44 where holes (H 2 O +• ) are very short-lived, ∼ 40 fs, 45,46 and work by Kondoh et al, showing ultrafast solute oxidation in dodecane attributed to a highly mobile excited state of the dodecane radical cation. 47,48 The 0.5 ps lifetime of THF +• in the work described here sets an upper limit for the time scale of capture of holes.…”

“…This approach worked well for the description of previous sub-15 ps hole capture in chloroform. 6 From eqs 2 and 3 we can derive:…”

“…There is a fair body of work that suggests that oxidation by the short-lived solvent holes in fragmenting solvents may occur. Previous work in our lab examined pulse radiolysis in chloroform, with a surprising observation that a large fraction of solute radical cations were produced in under 15 ps, and were the result of hole transfer from solvent radical cations at larger distances than possible by ordinary diffusion . We suggested that the proposed mechanisms are likely active in other media.…”

“…Previous work in our lab examined pulse radiolysis in chloroform, with a surprising observation that a large fraction of solute radical cations were produced in under 15 ps, and were the result of hole transfer from solvent radical cations at larger distances than possible by ordinary diffusion. 6 We suggested that the proposed mechanisms are likely active in other media. It was not clear how much faster than 15 ps this occurred, or whether it might compete with the THF +• 0.5 ps lifetime.…”