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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry