Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Rapid capture of charges by polyfluorenes in pulse-radiolysis experiments at LEAF
Principal Investigators: Andrew R. Cook and John R. Miller
Department of Chemistry, Brookhaven National Laboratory, Upton, NY, 11973 USA
acook@bnl.gov, jrmiller@bnl.gov
Program Scope:
This program applies both photoexcitation and ionization by short pulses of fast
electrons to investigate fundamental chemical problems relevant to the production and
efficient use of energy and thus obtain unique insights not attainable with other
techniques. These studies may play an important role in the development of safer, more
effective, and environmentally beneficial processes for the chemical conversion of solar
energy. Picosecond pulse radiolysis is employed to generate and study reactive chemical
intermediates or other non-equilibrium states of matter in ways that are complementary to
photolysis and electrochemistry and often uniquely accessible by radiolysis. This
program also develops new tools for such investigations, applies them to chemical
questions, and makes them available to the research community. Advanced experimental
capabilities, such as Optical Fiber Single-Shot detection system, allow us to work on
fascinating systems with 5-10 ps time-resolution that were previously prohibitive for
technical reasons.
Current focuses are studies of transport of charges and excitons in conjugated
polymers, which are of interest due to their applications in “plastic” electronics and
organic photovoltaic devices. Pulse radiolysis is almost unique in its ability to rapidly
inject charges into conjugated polymer chains and observe them with optical
spectroscopy.
Recent Progress:
Ion-Pairing by Protons and Avoiding this drag on Mobility. Ionization of
THF, one of the best solvents for conjugated polymers, produces free ions, which move
independently of each other, and a somewhat larger amount of geminate (or spur) ions
consisting of electrons and holes that are separated by 1-7 nm, but are bound by their
mutual Coulomb attraction. When THF was used in past work to investigate electron
transfer and the Marcus theory, the geminate ion-pairs presented only a minor problem
because they annihilate upon recombination. Electrons in long conjugated molecules
behave differently. Experiments with oligomers of fluorene, Fn, n=1-4 gave rise to ion
pairs (Fn -• ,RH2 + ), where the solvated proton, RH2 + , (THF=RH) is the positive counter-ion
in THF, formed by proton transfer from THF radical cations (RH +• +RH� R • + RH2 + ).
The (Fn -• ,RH2 + ) ion-pairs disappear immediately for F1, but have long (~100 ns) lifetimes
for F2 - F4 and longer. The results determine unexpectedly slow rates, kpt, of highlyexoergic
proton transfer within the ion-pairs. They also furnish rates of escape, kesc, to
form free ions. They additionally make comparisons to several smaller molecules. The
dependence of kesc on length of the oligomer gives a valuable first look into the
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