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 τ = 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 τ = 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 T₀ 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.