Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules

The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PEnP, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FNn, where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+center dot)-An-PEnP-NI-center dot and DMJ(+center dot)-An-FNn-NI-center dot, respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3)*An and (3)*NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3)*NI is produced by hole transfer from DMJ(+center dot) to NI-center dot, while (3)*An is produced by electron transfer from NI-center dot to DMJ(+center dot), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3)*NI and (3)*An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PEnP, while it occurs by preferential electron transfer when the bridge is FNn. Over time, the initial population of (3)*NI decreases, while that of (3)*An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3)*NI to An is weakly exponential with a decay parameter beta = 0.08 angstrom(-1) for the PEnP series and beta = 0.03 angstrom(-1) for the FNn series. In the PEnP series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FNn series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.