Long‐Range Intramolecular Electron Transfer between two Naphthalene π‐Moieties Attached to a Rigid Norbornylogous Spacer of Variable Length: An ESR and ENDOR Study

Abstract

AbstractESR, ENDOR, and TRIPLE‐resonance studies are reported on the radical anions of 7(1,0), 7(2,0), and 7(1,1) in which a completely rigid spacer separates two naphthalene π‐moieties by 2, 4, and 6 CC single bonds, respectively. Whereas 7(1,0)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document} has to be considered as a ‘truly’ charge‐ and spin‐delocalized radical anion of a homobinaphthylene‐like system, the frequency of intramolecular electron transfer in 7(2,0)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document} and 7(1,1)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document} is governed by the migration rate of the counterfort (K+) between two equivalent sites at the π‐moieties. Under conditions of tight ion pairing (solvent 2‐methyltetrahydrofuran), the unpaired electron in 7(2,0)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document} and 7(1,1)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document} is localized on one π‐moiety and the pertinent spectra resemble that of the radical anion produced from an analogous mononaphthalene derivative 8. On the other hand, with solvents of higher cation‐solvating power (1,2‐dimethoxyethane or its mixtures with N,N,N′,N′,N″,N′‐hexamethylphosphoric triamide), delocalization of the electron spin over both π‐moieties becomes evident, thus indicating that the electron‐transfer rate exceeds the hyperfine time‐scale (ca. 107s−1). Simultaneous occurrence of ‘localized’ and ‘delocalized’ radical anions is also observed, in particular for 7(1,1)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document}. Previous findings for the radical anion of 6(1) which, like 7(1,1)\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }} $\end{document}, contains two naphthalene π‐moieties separated by 6 CC single bonds are reinterpreted in the light of the results obtained in the present work. Exhaustive reduction of 7(2,0) and 7(1,1) leads to the formation of triplet dianions in which each of the two π‐moieties houses one unpaired electron.