Synthetic, spectroscopic, and computational investigation of charge and energy transport through pi stacks

The goal of this thesis is to study the influence of energy levels, electronic coupling, and bridge length on charge and energy transfer through well-defined pi-stacked donor-bridge-acceptor (DBA) molecules. We begin with an investigation of double-stranded DNA, using femtosecond transient absorption spectroscopy to monitor the rate and efficiency of hole transfer from a stilbenediamide acceptor to a stilbenediether donor over a series of base pair sequences. The energy levels of the individual bases have a large effect on hole transport, and by judicuous arrangement of these levels in AmGn diblock sequences we are able to transport a hole over 9 base pairs with an efficiency of >25%.A similar experimetal setup is used to study the rate of Dexter triplet energy transfer from benzophenone to naphthalene across a pi-stacked fluorene bridge with 1-3 bridge units. Because all triplet states in this system have distinct spectroscopic features, we are able to measure or derive all microscopic and macroscopic energy transfer rates in the system. Our detection of a bridge-populated intermediate state is the first such observation for either charge or energy transfer through donor-bridge-acceptor systems. Energy transport in the systems with 2 and 3 fluorenes proceeds by a novel mechanism in which the triplet hops onto and off of the bridge, but not between bridge units. This is explained as a consequence of the driving force term in the Marcus equation dominating the electronic coupling term. We show the first umambiguous crossover from single-step tunneling dominated to multi-step dominated transport for triplet energy transfer.We then use the M06-2X density functional to map the relationship between binding energy and electronic coupling for perylenediimide diimide (PDI) pi-stacked dimers. 20 known PDI crystal structures are screened and several are predicted to show excellent charge mobilities in the solid state. Finally, we synthesize and characterize a PDI cyclophane which is intended as a model complex for studying charge transfer through PDI stacks. This cyclophane is shown to allow charge hopping between the PDIs on a sub-100 ns timescale. Ground- and excited-state spectroscopy indicates high exciton and vibrational coupling between the conformationally-restricted PDI dimer.