Statistically Controlled Self-Assembly of Polar Molecular Crystals

Abstract

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...

Stochastic polarity formation in molecular crystals, composite materials and natural tissues

“…compounds (Roth et al, 1998;Kö nig et al, 1997). Furthermore, MC simulations demonstrate the promotion of primary reversal due to the addition of acceptor-type symmetric components.…”

Peculiar orientational disorder in 4-bromo-4′-nitrobiphenyl (BNBP) and 4-bromo-4′-cyanobiphenyl (BCNBP) leading to bipolar crystals

“…A key experiment for an experimental confirmation of polarity formation due to orientational disorder can be set up from the following theoretical prediction: Monte Carlo simulations of polarity evolution in solid solutions (A--D) 1 À x (D--D) x predict (Roth et al, 1998) an inverted final state for a system A--D undergoing a reversal transition. This means that a significant addition of D--D to the nutrient can reverse the polarity of the bipolar state without significant uptake of D--D by solid solution formation.…”

Spatially resolved heat conduction in polar perhydrotriphenylene inclusion compounds studied by means of thermal waves

“…From previous work we know [2][3][4][5] that in PHTP inclusion compounds, the mean polarization ͑when averaged over the entire volume͒ vanishes, therefore, the second term can be neglected. The remaining first term represents the deviation of the polarization from its mean value.…”

“…Recently, [2][3][4][5] we have shown that the pyroelectric effect allows one to study the polar domain structure of PHTP inclusion compounds by means of scanning pyroelectric microscopy ͑SPEM͒. A prerequisite for a proper analysis of SPEM results is the knowledge of the thermal properties of the crystals, since the effective local volume wherein the pyroelectric response is generated, strongly depends on the thermal diffusivity K and the radial frequency of the temperature variation , which both determine the thermal diffusion length , according to ϭͱ2K/.…”