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 (Br₂F) at 5 to 1000 mM was used, Br₂F^+• with radiation chemical yields up to G = 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 Br₂F^+•, 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 C₃₇ = 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 Br₂F, possibly capturing nearly all available holes from the solvent.