Intersystem crossing mediated by photoinduced intramolecular charge transfer: Julolidine - Anthracene molecules with perpendicular π systems

Time-resolved electron paramagnetic resonance studies show that the primary mechanism of triplet formation following photoexcitation of julolidine-anthracene molecules linked by a single bond and having perpendicular π systems is a spin-orbit, charge-transfer intersystem crossing mechanism (SOCT - ISC). This mechanism depends on the degree of charge transfer from julolidine to anthracene, the dihedral angle (θ₁) between their π systems, and the magnitude of the electronic coupling between julolidine and anthracene. We compare 4-(9-anthracenyl)-julolidine with the more sterically encumbered 4-(9-anthracenyl)-3,5-dimethyrjulolidine and find that fixing θ₁ ≅ 90° serves to enhance SOCT - ISC by increasing the change in orbital angular momentum accompanying charge transfer. Given that the requirements for the SOCT - ISC mechanism are quite general, we expect it to occur in a variety of electron donor-acceptor systems.