Organic Spin Clusters: Ferromagnetic Spin Coupling through a Biphenyl Unit in Polyarylmethyl Tri-, Penta-, Hepta-, and Hexadecaradicals

Abstract

Polyarylmethyl high-spin polyradicals are designed to possess a structure corresponding to simple spin clusters, where component spins (S‘) arise from strong ferromagnetic coupling through 1,3-phenylene units and weak ferromagnetic spin coupling between the component spins is mediated via 3,4‘-biphenylene (or 3,5,4‘-biphenylyne) units. This rational design permits not only modular and highly convergent synthesis of very high-spin molecules but also detailed analysis of their magnetic data. A series of polyether precursors for the corresponding polyarylmethyl tri-, penta-, hepta-, and hexadecaradicals are prepared. The polyradicals are generated and characterized in frozen tetrahydrofuran (or tetrahydrofuran-d8) solutions, using bulk magnetization studies. The measured values of S, from fits of magnetization vs magnetic field data to Brillouin functions at low temperatures, indicate high-spin ground states; e.g., S = 7.2 vs theoretical S = 8 for hexadecaradical. Estimated yields per site for generation of “unpaired” electrons are as high as 98% for the best samples of penta-, hepta-, and hexadecaradicals. The magnetization vs temperature data are fit to the Boltzman distribution of energy levels, obtained from Heisenberg Hamiltonian; the fits produced ferromagnetic coupling constants (J/k) through 3,4‘-biphenylene units. For tri- and pentaradical, for which exact analytical solutions to the Heisenberg Hamiltonian can be obtained by the vector model, values of J/k ≈ 90 K are obtained. Hepta- and hexadecaradical, which could not be exactly solved by the vector model, are approximated as a dimer of the S‘ = 5/2 and 1 component spins and a trimer of the S‘ = 5/2, 3 and 5/2 component spins, respectively; the corresponding values of J/k, 13 and 4 K, are found to be scaled by the fraction of component spin directly connected through the weak spin coupler (3,4‘-biphenylene unit).