Helical Superstructures of aC2-Symmetric Molecule Exhibiting Strong Second Harmonic Generation in the Solid-State

Abstract

The design of molecular materials for quadratic nonlinear optical (NLO) applications involves optimization of the structure at both the molecular and the materials level. [1] Molecular design aims at the maximization of the hyperpolarizability (b) as well as the incorporation of structural features that facilitate suitable assembly in the bulk phase. The latter features include hydrogen-bonding functionalities, [2] ionic groups, [3] optimally long alkyl chains, [4] and chirality. [5,6] Of these, only chirality ensures noncentrosymmetric organization, an essential prerequisite for the observation of quadratic NLO effects. However, even though the basic symmetry requirement is satisfied, efficient exploitation of the inherently strong molecular nonlinear response is rarely achieved at the bulk level. An exceptionally successful case is that of N-4-nitrophenyl-(S)-prolinol (NPP). [6] We previously investigated the strategic placement of stereogenic centers in push-pull quinonoid molecules to achieve enhanced second harmonic generation (SHG). [7] A logical extension of this study is that an axial chiral system having a strong b component coincident with the symmetry axis would be a promising candidate. We also envisaged that C 2 -symmetric molecules could potentially form helical assemblies, the chirality of which extends over the whole molecular superstructure. Recent studies have demonstrated enhanced NLO effects arising from supramolecular chirality in polymers, [8] mesoscopic systems, [9] and Langmuir ± Blodgett films.[10] Helical [12] For recent examples of supramolecular systems deposited on surfaces, see a) P. Laitenberger, C. G. 903 ± 913. The sol ± gel process is a synthetic technique for preparing oxide gels, glasses, and inorganic compounds at far lower temperatures than is possible by conventional synthesis. The approach is based on the hydrolysis and condensation of molecular precursors such as metal alkoxides. It has received considerable attention because it possesses a number of desirable characteristics in terms of producing materials of high purity and excellent homegeneity. Moreover, since the sol ± gel approach is a solution-based method, it is readily adaptable to producing thin films and fibres as well as bulk materials. The flexible solution chemistry associated with the synthesis of sol ± gels makes it possible to incorporate a large variety of organic molecules in the inorganic oxide matrix. The optical properties of the molecular dopants are then imparted upon the solid derived from the sol ± gel. Hence this process gives rise to a large number of materials with interesting and unique properties. The interconnected nanopores in sol ± gels are filled with liquid which can provide supramolecular machines with a local solution environment in which large molecular movements (co-conformational changes) can be performed, with the silicate framework providing the macroscopic support. The silicate matrix is also transparent to visible light and this transparency makes it possible to study a phot...