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) (PE _nP, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN _n, where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ ^+•-An-PE _nP-NI ^-• and DMJ ^+•-An-FN _n-NI ^-•, 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 ^+• to NI ^-•, while ^3*An is produced by electron transfer from NI ^-• to DMJ ^+•, 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 PE _nP, while it occurs by preferential electron transfer when the bridge is FN _n. 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 β = 0.08 Å ^-1 for the PE _nP series and β = 0.03 Å ^-1 for the FN _n series. In the PE _nP 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 FN _n series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.