Abstract
The temperature dependence of spin-selective intramolecular charge recombination (CR) in a series of 2,7-fluorenone (FN1−2) and p-phenylethynylene (PE1−2P) 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 n P−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 n P−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 FN1−2 and PE1−2P 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.
