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

Ultrafast hole transfer from solvent radical cations produced by radiolysis with ∼10 ps, 9 MeV electron pulses to solutes in tetrahydrofuran (THF) was investigated. Because of rapid fragmentation of initially produced THF, solute radical cations are not expected and have not previously been reported. When 9,9-dihexyl-2,7-dibromofluorene (BrF) at 5 to 1000 mM was used, BrF with radiation chemical yields up to = 2.23/100 eV absorbed was observed. While more than half of this was the result of direct solute ionization, the results highlight the importance of capturing holes from THF prior to solvation and fragmentation. The observed data show a time-resolution limited (15 ps) rise in transient absorption of BrF, identical in form to reports of presolvated or dry electron capture in water and a few organic liquids, including THF. The results were thus interpreted with a similar formalism, finding = 1.7 M, the concentration at which 37% of holes escape capture. The yield of solvent hole capture can be accounted for by the formation of solvent holes adjacent to solute molecules reacting faster than they can fragment; however, mechanisms such as delocalized holes or rapid hopping may play a role. Low temperature results find over two times more capture, supporting the speculation that if THF was longer lived, the yield of capture in under 15 ps would have been at least 2 times larger at 1 M BrF, possibly capturing nearly all available holes from the solvent.