Andrew S. Ichimura

Building blocks for molecule-based magnets: Radical anions and dianions of substituted 3,6-dimethylenecyclohexane-1,2,4,5-tetrones as paramagnetic bridging ligands

We have prepared four tetraaryl derivatives of 3,6-dimethylene-1,2,4,5-tetraoxocyclohexane (aryl = Ph; 4-MeOPh; 4-Me(2)NPh; and 3,5-(t-Bu)(2)-4-MeOPh) with guidance from an earlier reported ab initio analysis (Misiolek, A. W.; Jackson, J. E. J. Am. Chem. Soc. 2001, 123, 4774-4780). These electron acceptors may be chemically or electrochemically reduced to the mono- and dianions desired as building blocks for the assembly of molecule-based magnets. Cyclic voltammetry shows that the potential of the first reduction wave depends on the electron donor ability of the aryl ring substituents, ranging from -0.28 V for the tetraphenyl derivative to -0.78 V for the p-dimethylamino substituted analogue (vs ferrocene/ferrocinium(+) at 0.46 V). Spin density distributions in the semiquinone moieties were elucidated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) observations of hyperfine couplings to internal (1)H sites and bound alkali metal cations. X-ray diffraction studies of the sodium and potassium salts of the octa-t-butyltetramethoxy derivative reveal the structure of the monoanion and its tendency to self-assemble with metal cations into one-dimensional chains in the solid state. Within the chains the anions display the expected bridging and chelating mode of coordination; SQUID magnetometry revealed weak intermolecular spin-spin couplings of 2J = -0.2 and approximately 0 K for the sodium and potassium salts, respectively. NIR transitions in the electronic spectra of the monoanions in solution are consistent with the expected low energy gap between frontier orbitals and its tunability by substituent variations. EPR studies of the free dianions and monoradical analogues indicate diradical localization into separate triphenylmethyl-like monoradicals via twisting of the diarylmethylene termini.

Oxidation of Electron Donor-Substituted Verdazyls: Building Blocks for Molecular Switches

Species that can undergo changes in electronic configuration as a result of an external stimulus such as pH or solvent polarity can play an important role in sensors, conducting polymers, and molecular switches. One way to achieve such structures is to couple two redox-active fragments, where the redox activity of one of them is strongly dependent upon environment. We report on two new verdazyls, one subsituted with a di-tert-butyl phenol group and the other with a dimethylaminophenyl group, that have the potential for such behavior upon oxidation. Oxidation of both verdazyls with copper(II) triflate in acetonitrile gives diamagnetic verdazylium ions characterized by NMR and UV-vis spectroscopies. Deprotonation of the phenol-verdazylium results in electron transfer and a switch from a singlet state to a paramagnetic triplet diradical identified by electron spin resonance. The dimethylaminoverdazylium 9 has a diamagnetic ground state, in line with predictions from simple empirical methods and supported by density functional theory calculations. These results indicate that verdazyls may complement nitroxides as spin carriers in the design of organic molecular electronics.

Nonlinear self-action of light through biological suspensions

It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.

Structural and magnetic properties of vanadyl dichloride solvates: from molecular units to extended hydrogen-bonded solids

The preparation, structural characterization and magnetic properties of three solvent adducts of VOCl(2), trans-VOCl(2)(THF)(2)(H(2)O) (1), trans-VOCl(2)(H(2)O)(2).2Et(2)O (2) and cis-VOCl(2)(MeOH)(3) (3) are described. In these solids, hydrogen bonding among the inorganic complexes is the critical determinant of the formation of extended magnetic networks. Compound forms one-dimensional double chains where alternating monomers from the two branches of the chain are hydrogen bonded via the V-Cl ... H-O-V network (with an axial water molecule and equatorial chloride ions). Magnetic studies indicate no interaction among the vanadyl centers. The paramagnetism of 1 is consistent with the extension of the network from the hydrogen donor site of the axial water, which is orthogonal to the d(xy) magnetic orbital. Compound 2 forms one-dimensional chains with water molecules of adjacent monomers held together by hydrogen bonds to ether molecules (V-O-H ... O(ether) ... H -O-V). The chain network radiates only through the equatorial plane of the complex where the water molecules are located. The presence of the intervening solvent molecule between hydrogen bonds of the primary coordination sphere magnetically insulates metal centers and compound is also a simple paramagnet. Removal of the solvent turns on the magnetic interaction and neighboring spin centers couple antiferromagnetically. Compound 3 forms a layered structure via V-Cl ... H-O-V hydrogen bonding, where all the hydrogen donor sites participate in the formation of the network. The vanadyl spin centers, at distances of 5.5 and 6.5 A from each other, couple antiferromagnetically (J/k=-0.7 K). Thus, magnetic coupling among metal centers is achieved when the hydrogen bond network directly radiates from the coordination plane containing the magnetic orbital. These results further support the utility of hydrogen bond as a viable design element in the construction of low dimensional, magnetic solids.