INTRAMOLECULAR PHOTOINDUCED ELECTRON-TRANSFER FROM NITROXYL RADICALS

Intramolecular photoinduced electron transfer was observed from a nitroxide radical to a diimide acceptor in N,N'-bis(2,2,6,6-tetramethyl-N-oxidopiperidin-4-yl)-1,4,5,8-naphthalenediimide (DTDI). Irradiation of this substrate produced a zwitterion in which the diimide monoanion, with a lifetime of >200 mu s, was detected by transient absorbance spectroscopy. Slow charge recombination was attributed to the different symmetry constraints for forward electron transfer to an n,pi* state and back electron transfer from a pi,pi* state. The restricted motion of the nitroxide group prevents efficient overlap with the ring pi system, slowing back electron transfer.