Nonlinear optical (NLO) responses from organic dyes can be maximized when the dyes are aligned in appropriate manners in bulk materials. The use of restricted nanospaces provided by interlayer spacing of inorganic layered materials is a promising strategy for imposing suitable molecular alignments for NLO materials on dyes. The hybrid materials thus obtained exhibit salient NLO responses owing to the improved molecular orientation. In some cases, extension of the π-electron system as a consequence of improved molecular planarity, obtained by the intercalation of a dye into the 2-dimensional interlayer space of an inorganic layered material, is also observed as a factor that enhances NLO responses of chromophores at the molecular level. This review focuses on recent progress in the strategies for controlling the molecular orientation of NLO-phores by employing clay minerals, which are one of the typical inorganic layered materials. In addition, development of a means for fabricating composites that satisfy the properties of an optical material, such as a sufficient size and thickness, a flat surface, and low light-scattering characteristics is required to utilize the superior NLO properties observed for clay/dye hybrid materials for practical applications. A novel means for obtaining such a hybrid material is also outlined.

We systematically investigated enhancement of the molecular two-photon absorption (TPA) cross sections (σ (2) ) of organic dyes confined in the interlayer space of clay films. Several possible mechanisms have been proposed to explain the enhanced TPA of dyes in the interlayer spaces of clays. The present study comprehensively investigates two of these mechanisms, namely, (1) enhanced molecular planarity and the consequent extension of the π-electron system and (2) enhanced molecular orientation. We fabricated transparent dyeÀclay films consisting of a synthetic saponite clay and four cationic dyes. Two of these dyes are expected to have enhanced planarity when they are hybridized with a clay mineral. The other two dyes are considered to retain the planarity of their conjugated π-electron systems when they are in solution or in the interlayer space of a clay. We experimentally measured the σ (2) of these dyes in a clay film and in solution and found that the σ (2) of all four dyes is enhanced in a clay film but that this enhancement is greater for the former two dyes. These results are explained in terms of the effects of (a) extension of the π-conjugated system due to the enhanced planarity, (b) reduced detuning energy due to the enhanced planarity, (c) enhanced molecular orientation, and (d) the occasionally hydrophobic environment of the interlayer space of clay minerals. The highest enhancement of σ (2) was observed for a porphyrin derivative, tetrakis(1-methylpyridinium-4-yl)porphyrin ptoluenesulfonate, for which σ (2) in a hybrid film was 13 times greater than that in a dimethyl sulfoxide solution. Therefore, hybridization of cationic dyes with a clay mineral is an effective design strategy for TPA materials.

No abstract

Review: 45 refs.