New Nonlinear Optical Materials: Expedient Topotactic Self-Assembly of Acentric Chromophoric Superlattiees

摘要

Although molecule-based assemblies offer many attractions for the construction of efficient second-order nonlinear optical materials (large nonresonant response, ultrafast response times, low dielectric constants/losses, and intrinsic tailorability),significant synthetic challenges have impeded the. realization of optimum materials. These challenges include maximizing chromophore hyperpolarizability and number density as well as maximizing the degree and thermal/temporal stability of induced microstructural acentricity. An attractive alternative to electric field poled glassy polymers and Langmuir -Blodgett films is a self-assembly approach in which acentric chro-mophoric super lattices are built up in a layer-by-layer fashion through the formationof covalent bonds. In the past, the efficiency of this approach has been limited by the slow rate (days) at which the large hyperpolarizability stilbazolium. chromophore layers could be assembled and the resulting, less than optimum superlattice structural regularity. We report here a new topotactic "molecular layer assembly approach in which the key assembly step is effected in either a solvent-free solid state or chemical vapor deposition mode, which in combination with other building block modifications, allows the expedient assembly of robust chromophqric architectures with high structural regularity and with very large nonlinear optical response. The results suggest that other self-assembly routes to multilayer structures could be achieved in a similar manner.