Time-resolved EPR studies of charge recombination and triplet-state formation within donor-bridge-acceptor molecules having wire-like oligofluorene bridges

Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10- bis(dicarboximide) (PDI), PTZ-FL_n-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI (³*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. ³*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T₀ mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.