Ultrafast Intersystem Crossing and Spin Dynamics of Photoexcited Perylene-3,4:9,10-bis(dicarboximide) Covalently Linked to a Nitroxide Radical at Fixed Distances

Time-resolved transient optical absorption and EPR (TREPR) spectroscopies are used to probe the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) ((1)*PDI) with a stable tert-butylphenylnitroxide radical ((BPNO)-B-2*) at specific distances and orientations. The (BPNO)-B-2* radical is connected to the PDI with the nitroxide and imide nitrogen atoms either para (1) or meta (3) to one another, as well as through a second intervening p-phenylene spacer (2). Transient absorption experiments on 1-3 reveal that (1)*PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with, tau congruent to 2 ps to give structurally dependent 8-31% yields of (3)*PDI. Energy- and electron-transfer quenching of (1)*PDI by (BPNO)-B-2* are excluded on energetic and spectroscopic grounds. TREPR experiments at high magnetic fields (3.4 T, 94 GHz) show that the photogenerated three-spin system consists of the strongly coupled unpaired electrons confined to (3)*PDI, which are each weakly coupled to the unpaired electron on (BPNO)-B-2* to form excited doublet (D-1) and quartet (Q) states, which are both spectrally resolved from the (BPNO)-B-2* (D-0) ground state. The initial spin polarizations of D-1 and Q are emissive for 1 and 2 and absorptive for 3, which evolve over time to the opposite spin polarization. The subsequent decays of D-1 and Q to ground-state spin polarize D-0. The rates of polarization transfer depend on the molecular connectivity between PDI and (BPNO)-B-2* and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.