Temperature dependence of spin-selective charge transfer pathways in donor-bridge-acceptor molecules with oligomeric fluorenone and p -phenylethynylene bridges

The temperature dependence of spin-selective intramolecular charge recombination (CR) in a series of 2,7-fluorenone (FN_1-2) and p-phenylethynylene (PE_1-2P) linked donor-bridge-acceptor molecules with a 3,5-dimethyl-4-(9-anthracenyl) julolidine (DMJ-An) electron donor and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor was studied using nanosecond transient absorption spectroscopy in the presence of a static magnetic field. Photoexcitation of DMJ-An into its charge transfer band and subsequent electron transfer to NI results in a nearly quantitative yield of ¹(DMJ^+•-An-FN_n-NI^-•) and ¹(DMJ^+•-An-PE_nP-NI^-•), which undergo rapid radical pair intersystem crossing (RP-ISC) to produce the triplet RPs, ³(DMJ^+•-An-FN_n-NI ^-•) and ³(DMJ^+•-An-PE _nP-NI^-•), respectively. The CR rate constants, k _CR, in toluene were measured over a temperature range from 270 to 350 K, and a kinetic analysis of k_CR in the presence of an applied static magnetic field was used to extract the singlet and triplet charge recombination rate constants, k_CRS and k_CRT, respectively, as well as the intersystem crossing rate constant, k_ST. Plots of ln (kT^1/2) versus 1/T for PE₁P show a distinct crossover at 300 K from a temperature-independent singlet CR pathway to a triplet CR pathway that is positively activated with a barrier of 1047 ± 170 cm ^-1. The singlet CR pathway via the FN₁ bridge displays a negative activation energy that results from donor-bridge and bridge-acceptor torsional motions about the single bonds joining them. In contrast, the triplet CR pathway via the FN_1-2 and PE_1-2P bridges exhibits positive activation energies. The activation barriers to these torsional motions range from 1100 to 4500 cm^-1 and can be modeled by semiclassical electron transfer theory.