Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor-bridge-Acceptor Molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethy1-4-(9-anthraceny1)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+center dot)-PI-center dot or DMJ(+center dot)-NI-center dot quantitatively in 1-4 (tau(CS) <= 10 ps), and that charge recombination occurs with tau(CS) = 268 and 158 ps for 1 and 3, respectively, and with tau(CS) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+center dot) and PI-center dot or NI-center dot, giving 2J> 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3*)An upon charge recombination occurs by spin orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 21 determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires, the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.