Large Dipolar Spin–Spin Interaction in a Photogenerated U‑Shaped Triradical

Abstract

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 <i>tert</i>-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 (2<i>J</i>_AR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (<i>D</i>_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 <i>S</i> = 1, whereas experiments on D^+•–C–A^–•–R^• in the oriented nematic liquid crystal 4-cyano-4′-<i>n</i>-pentylbiphenyl at 85 K demonstrate the anisotropy of <i>D</i>_AR