Large Dipolar Spin - Spin Interaction in a Photogenerated U-Shaped Triradical

Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin - spin interactions in a novel U-shaped electron donor - chromophore - acceptor - radical (D - C - A - R^•) system in which a xanthene bridge holds a tert-butylphenyl nitroxide (BPNO^•) 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^+• - C - A^-• - R^•). The large through-bond distance between A^-• and R^• makes their spin - spin exchange interaction (2J_AR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (D_AR), which is observed as a set of broad lines in the TREPR spectra of D^+• - C - A^-• - R^• 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^+• - C - A^-• - R^• in the oriented nematic liquid crystal 4-cyano-4′-n-pentylbiphenyl at 85 K demonstrate the anisotropy of D_AR. (Figure Presented)