Ordering Magnetic Molecules within Nanoporous Crystalline Polymers

The development of molecule-based materials and devices requires that functional molecules can be organized on appropriate substrates and/or embedding media, in an ordered manner. In this perspective, the possibility of using nanoporous crystalline polymers as a framework for active molecules in alternative to inorganic or metalorganic or gel host structures is appealing, as it takes advantage of their intrinsicproperties (transparency, manufacturing capability, flexibility). In recent years, two nanoporous crystalline phases of syndiotactic polystyrene, s-PS , have been obtained that are capable of hosting in their cavities several low -molecular-mass guest molecules. This leads to the formation of different kinds of co-crystals and induces on the guest molecules different kinds of long-range order. In particular, for s-PS co-crystals defined as intercalates, the host molecules stay inside layers intercalated with ac layers of alternated enantiomorphic helices (Figur e 1). Films formed with s-PS/active-guest co-crystals have been proposed as advanced chromophore, fluorescent, nonlinear optical, photoreactive, and magnetic materials. The last are particularly appealing guests, because the current trend in molecular magnetism is directed toward the use of substrates for organizing molecules, for instance, single molecule magnets, SMM, in environments that are structured like crystals but allow possibilities of addressing individual molecules. A macroscopic polymer film hosting iso-oriented magnetic molecules would be the dream of many researchers, allowing to easily measure the anisotropy of isolated molecules. The paper deals with the exploration of s-PS polymer films use to obtain organized arrays of magnetic molecules by studying the simple and well-characterized nitroxide radical compound, 2,2,6,6tetramethyl-piperidinyl-N-oxyl (TEMPO). In this communication, we show that indeed TEMPO can be ordered in a s-PS film, opening perspectives for ordered organizations of magnetic molecules. The data succeed to show how a simple and low-cost polymer as s-PS can act as host for magnetic molecules, keeping the features of a crystal for high concentrations of guests. Even if 1 D character has been previously suggested for TEMP O molecules in phosphazene nanochannels, this is the first time the anisotropic behavior typical of low-dimensional magnetic materials can be directly probed for paramagnetic molecules in a porous host. Another exciting feature of s-PS , which we started to exploit, is that of providing the possibility, unique among polymers, of three different kinds of uniplanar orientations of the crystalline phase. In particular, it is possible to impose, by polymer processing, three different orientations with respect to the film plane, of the high density ac layers, formed by close-packing of alternate d enantiomorphous s-PS helices. Two of these uniplanar orientations can be also combined with the axial orientation (common to all semi-crystalline polymers) producing uniplanar-axial orientations, which should give in principle the opportunity to achieve for the magnetic guest molecules not only the axial order with respect to the polymer stretching direction (shown in the present paper) but also a 3 D orientational order in the whole macroscopic film.