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

Abstract

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.