Spectroscopic and theoretical characterization of strong ferromagnetic interaction in heterospin biradicals.

摘要

Chemists are striving to create devices with molecular level functionality including switches, storage devices, LEDs, magnets, and molecular wires. A thorough understanding of the underlying mechanisms which impart these functionalities is absolutely essential to the rational design of new molecular devices. The possible use of heterospin biradicals as synthons in hybrid inorganic-organic molecular magnetic materials is considered.; To develop heuristics for the design of high spin ligands, a high spin organic donor-acceptor (D-A) biradical, nitronyl nitroxide semiquinone (NN-SQ) is thoroughly characterized. Control of metal-ligand magnetic coupling is explored through characterization of a series of complexes in which the NN-SQ ligand is coordinated to divalent first row transition metals with an ancillary ligand which is either the three coordinate cumenyl methyl tris(pyrazolylborate) (TpCum,Me) or a four-coordinate ligand.; As ground state mechanisms are insufficient, excited-state mechanisms are invoked to explain the unprecedented strength of ferromagnetic interaction in NN-SQ. Detailed computational studies of the donor (SQ) and the acceptor (NN), are undertaken. Computational studies of the NN-SQ biradical, including a broken symmetry approach, are performed. Confidence in the electronic absorption spectral band assignments allows for the development of a valence-bond configuration-interaction (VBCI) model to explain the excited state origin of strong ferromagnetic coupling in NN-SQ. Magnetic exchange parameters obtained via broken symmetry density functional theory (DFT) calculations provide quantitative agreement with experimental data, supporting the VBCI model.; The phenylene bridged diradical NN-Ph-SQ acts as a ground state analogue of the charge-separated D+-B-A- state in bridge mediated electron transfer. Computational studies on NN-Ph-SQ including varying donor-bridge-acceptor torsion angle ϕ shows a Cos4(ϕ) dependence in coupling. The unprecedented magnitude of electronic coupling in NN-Ph-SQ molecules is found to be controlled by energetic, spatial, and torsional factors which affect overlap of donor and acceptor orbitals. A series of molecular derivatives of NN-bridge-SQ is proposed, to test these assertions about magnetic and electronic coupling in D-A biradicals.