Determining the Conformational Landscape of σ and π Coupling Using <i>para</i>-Phenylene and “Aviram–Ratner” Bridges

Abstract

The torsional dependence of donor–bridge–acceptor (D–B–A) electronic coupling matrix elements (<i><b>H</b></i>_{<i><b>DA</b></i>}, determined from the magnetic exchange coupling, <i><b>J</b></i>) involving a spin S_D = 1/2 metal semiquinone (Zn-<b>SQ</b>) donor and a spin S_A = 1/2 nitronylnitroxide (<b>NN</b>) acceptor mediated by the σ/π-systems of <i>para</i>-phenylene and methyl-substituted <i>para</i>-phenylene bridges and by the σ-system of a bicyclo[2.2.2]­octane (<b>BCO</b>) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D–B–A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, “Ramachandran-type” plots that relate D–B and B–A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (<i><b>J</b></i> ≈ 1–175 cm^–1) and electronic coupling (<i><b>H</b></i>_{<i><b>DA</b></i>} ≈ 450–6000 cm^–1) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings