文摘
Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin鈥搒pin interactions in a novel U-shaped electron donor鈥揷hromophore鈥揳cceptor鈥搑adical (D鈥揅鈥揂鈥揜鈥?/sup>) system in which a xanthene bridge holds a tert-butylphenyl nitroxide (BPNO鈥?/sup>) radical in close proximity to a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor. Photoexcitation of the 4-aminonaphthalene-1,8-dicarboximide (ANI) chromophore results in rapid, two-step electron transfer to generate the triradical (D+鈥?/sup>鈥揅鈥揂鈥撯€?/sup>鈥揜鈥?/sup>). The large through-bond distance between A鈥撯€?/sup> and R鈥?/sup> makes their spin鈥搒pin exchange interaction (2JAR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (DAR), which is observed as a set of broad lines in the TREPR spectra of D+鈥?/sup>鈥揅鈥揂鈥撯€?/sup>鈥揜鈥?/sup> in solid toluene solution at 85 K. Transient nutation experiments show that these transitions belong to a species with spin S = 1, whereas experiments on D+鈥?/sup>鈥揅鈥揂鈥撯€?/sup>鈥揜鈥?/sup> in the oriented nematic liquid crystal 4-cyano-4鈥?n-pentylbiphenyl at 85 K demonstrate the anisotropy of DAR.