Electron Spin Dynamics as a Controlling Factor for Spin-Selective Charge Recombination in Donor−Bridge−Acceptor Molecules

Photoinitiated charge separation and thermal charge recombination (CR) in a covalent donor−bridge−acceptor (D-B-A) system consisting of a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, 2,7-oligofluorene bridge (FL n ), and phenothiazine donor (PTZ) (PTZ−FL n −PDI) have been shown to transition from superexchange to charge hopping mechanisms as the D−A distance increases. In work presented here, the spin-selective multiple CR pathways in PTZ−FL n −PDI are studied by a detailed analysis of the magnetic field effect (MFE) on the radical ion pair (RP) lifetime and triplet yield. A kinetic analysis of the MFE gives the spin-selective CR rates and the RP singlet−triplet (S-T) relaxation rates for n = 2−4. When n = 2 and 3, where the S-T splitting (2J) of the RP is large, slow S-T relaxation results in a kinetic bottleneck slowing the observed total CR rate at zero magnetic field. These results show that spin state mixing is an important controlling factor for CR reaction rates in these systems. The CR rate constant for the triplet RP (k CRT) obtained by MFE analysis is about 10 times faster than the corresponding rate for the singlet RP (k CRS) when n = 2−4, indicating that k CRT occurs near the maximum of the Marcus rate vs free energy dependence, whereas k CRS is deep in the inverted region. The distance dependence of both k CRS and k CRT is explained by the crossover from superexchange (n = 1 and 2) to distant independent thermal hopping (n = 3 and 4). A possible mechanism of the S-T relaxation is proposed based on S-T dephasing, which may be induced by fluctuations of 2J resulting from bridge torsional dynamics.