Organic solid state and molecular architectures providing nano-or mesoporous space for the inclusion of guest species open up new opportunities for the synthesis of materials showing designed chemical and physical properties. Porous materials are reviewed by taking into account recent progress in inorganic zeolites, metalloorganic and organic networks including carbon nanotubes and condensed cyclodextrins. Various properties requiring a parallel alignment of close-packed guests are addressed. As a particular example of physical property design, spontaneous polarity formation in channel-type inclusion compounds is discussed.

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The synthesis of polar molecular materials with designer properties is a fundamental target of contemporary materials research. Perhydrotriphenylene (PHTP) is a molecular building block that, when co-crystallized with dipolar guest molecules, has yielded polar inclusion compound crystals [1] in 95 % of the 40 cases we have studied. A statistical mechanism described more generally by Markov's theory of stochastic processes [2] has recently been identified as the driving force behind polarity formation during self-assembly. [3,4] Markov's theory is a statistical description in which future events within a system depend on a constant set of probabilities acting on its present state. This concept has been applied to chemical reactions, [2] to the prediction of DNA sequences, [5] and to copolymer [6] and protein [7] formation. In the context of designing efficient nonlinear optical (NLO) materials, [8,9] these probabilities are associated with the orientations of dipoles during crystal growth. Based on this groundwork, we now present the first example of a fully rational synthesis of polar materials from a given set of precursors (PHTP and three different guests); varying the relative proportions of each guest permits us to tune the level of polarity with an unprecedented degree of control.The key to such a rational synthesis lies in recognizing the structural elements required. Whilst polar properties (e.g., pyroelectric, second-order nonlinear, and electro-optic effects) can result from the presence of a polar crystal axis, its creation is often difficult, due to the tendency of adjacent dipolar molecules to align in an antiparallel fashion. Many elegant molecular and crystal engineering techniques have been utilized to overcome this problem, [8,9] including the use of derivatized or chiral molecules, Langmuir±Blodgett films, and encapsulation of the molecules as guests within lattices that influence (sterically or electronically) their alignment, e.g., poled polymers, [8] hydrogen-bonding anionic networks, [10] and channel-type inclusion architectures. [11] The {PHTP±guest} inclusion crystals we have studied consist of dipolar molecules AD (where A and D are acceptor and donor substituents, respectively, attached to linear, p-conjugated frameworks) surrounded by columnar stacks of PHTP molecules oriented parallel to the channel axis (Fig. 1). The guests align as molecular chains within the well-separated channels, with negligible interchannel interactions. The hydrocarbon nature of PHTP results in it having minimal electronic influence over the packing of the guests. Polarity in this system is thus governed by the relative orientations of the guests as they align within the channels, which in turn are controlled by the intermolecular interaction energies of the A and D substituents.[**] This work was supported by the Swiss National Science Foundation through grants NFP 36 (no. 4036-0439932) and NF (no. 20-4316.95). We thank Patrick Reber for the synthesis of the biphenyl derivatives. Fig. 1. Schematic re...

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The refractive indices of structurally well-characterized crystals of the new optically nonlinear organic crystal (S)-3-methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine have been measured from 480 to 660 nm and fitted to four-parameter Sellmeier equations with an accuracy of ±3 × 10-3 to ±5 × 10-3. The value of n y was accurately verified by the measurement of the external angle of incidence required, at the (101) plane, to observe type I phase matching with a fundamental wavelength of 1064 nm. The phase matching locus has been established and is of Hobden class eight. The values of the n x , n y , and n z refractive indices at 532 nm were found from the Sellmeier equations to be 1.690, 1.901, and 1.725, respectively, and their relative magnitudes are correlated with the molecular arrangement in the crystal lattice. This material is a member of a series of pyridinamines that can exhibit the phenomenon of frequency-dependent orientation of their dielectric axes. Since this compound crystallizes in an orthorhombic structure, rotation of the dielectric axes is precluded for symmetry reasons.

In addition to possessing strong optical nonlinearity, the (-)-2-(a-methylbenzylamino)-5-nitropyridine (MBANP) crystal has been shown to have unusual linear optical properties. Two methyl derivatives of MBANP have now been synthesized and grown as large single crystals; their refractive indices have been measured as a function of frequency and second-harmonic generation, induding the determination of the phase matching loci has been investigated. Data is also available on the racemic form of crystalline MBANP. The properties of this family of crystals have been reviewed and an attempt is made to relate the experimental responses of the crystals to their molecular structure and the calculated properties of the molecules.