Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO-Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical-Triplet Interaction

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3)*PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1)*PDI -> TEMPO+*-PDI-* occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3)*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3*PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3)*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T-0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3)*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism. which requires that the radical and attached triplet are in the weak exchange regime.