In conventional materials, BLA = 0.10 angstrom, approximately, and Marder finds that this value is too large to optimize the proper performance of the donor-acceptor dye. Using computational chemistry, he finds that an index of nonlinear activity has maximum value when BLA = 0.04 angstrom. Standard techniques of organic chemistry then permit synthesis of molecules whose bridges show this value.
Commonly used polymers include stilbenes such as dimethylamino nitro stilbene (DANS), with an index value of 466. Marder has prepared counterparts, using thiobarbituric acid acceptors, with an index as high as 19,000. In implementing the Pockels effect, this increase would reduce the necessary electric field from 200 volts to 4 or 5.
For engineering improved bulk crystalline materials, following a suggestion in the literature from Gerald Meredith (now at Dupont), Marder has studied organic salts. He finds particular promise in dimethylamino methylstilbazolium tosylate (DAST); its frequency-doubling efficiency is 20 times that of lithium niobate, a commonly used crystal. Moreover, its electrooptical properties are superb and may lead to entirely new types of devices. This enhancement results from the almost ideal orientation of the molecules in the crystal lattice.
A separate topic in materials science features the search for principles of chemistry that can permit construction of an organic ferromagnet. The rationale lies in observing that physicists today identify some 14 distinct magnetic states of matter, representing modes of ordering of electron spin. Five of these magnetisms are known classically, having the prefixes ferro-, antiferro-, ferri-, para-, and dia-. Most of the rest have been discovered only since about 1975 and merit further research. Because organic chemistry offers great freedom in modifying molecular structures, it suggests the prospect of creating families of related materials that display novel types of magnetism in varying degrees. An organic ferromagnet then represents a first step, an initial problem in this area.
Difficulties exist in the chemical syntheses, and characterization often demands liquid helium temperatures. Nevertheless, standard rules permit selection of candidate molecular radicals that display the fundamental ferromagnetic property of having two unpaired electrons with spin parallel. Dennis Dougherty of the California Institute of Technology, working with colleagues, has tested these candidates' suitability by combining them with trimethylenemethane, which also has spin-parallel electrons. This work shows that m-phenylene and cyclobutane offer particular promise.