Ultrafast intersystem crossing and spin dynamics of ohotoexcited 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) ( ¹PDI) with a stable tert-butylphenylnitroxide radical ( ²BPNO') at specific distances and orientations. The ²BPNO' 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 1PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with T ≅s 2 ps to give structurally dependent 8-31% yields of ³PDI. Energy- and electron-transfer quenching of ¹PDI by ²BPNO 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 ³PDI, which are each weakly coupled to the unpaired electron on ²BPNO to form excited doublet (D ₁) and quartet (Q) states, which are both spectrally resolved from the ²BPNO(D ₀) ground state. The initial spin polarizations of 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 ²BPNO and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.