Institut National de la Recherche Scientifique, Centre Énergie, Matériaux, Télécommunications , 1650 Lionel Boulet Boulevard, Varennes, QC, Canada J3X 1S2.
Department of Chemistry, McGill University , 801 Sherbrooke Street W., Montreal, QC, Canada H3A 0B8.
ACS Nano. 2017 Sep 26;11(9):8901-8909. doi: 10.1021/acsnano.7b03172. Epub 2017 Aug 21.
Two-dimensional (2D) molecular self-assembly allows for the formation of well-defined supramolecular layers with tailored geometrical, compositional, and chemical properties. To date, random intermixing and entropic effects in these systems have largely been associated with crystalline disorder and glassy phases. Here we describe a 2D crystalline self-assembled molecular system that exhibits random incorporation of substitutional molecules. The lattice is formed from a mixture of trimesic acid (TMA) and terthienobenzenetricarboxylic acid (TTBTA), C-symmetric hydrogen-bonding units of very different sizes (0.79 and 1.16 nm, respectively), at the solution-highly oriented pyrolitic graphite (HOPG) interface. Remarkably, the TTBTA substitutes into the TMA lattice at a fixed stoichiometry near 12%. The resulting lattice constant is consistent with Vegard's law prediction for an alloy with a composition TMATTBTA, and the substrate orientation of the lattice is defined by an epitaxial relation with the HOPG substrate. The Gibbs free energy for the TMA/TTBTA lattice was elucidated by considering the entropy of intermixing, via Monte Carlo simulations of multiplicity of the substitutional lattices, and the enthalpy of intermixing, via density functional theory calculations. The latter show that both the bond enthalpy of the H-bonded lattice and the adsorption enthalpy of the molecule/substrate interactions play important roles. This work provides insight into the manifestation of entropy in a molecular crystal constrained by both epitaxy and intermolecular interactions and demonstrates that a randomly intermixed yet crystalline 2D solid can be formed through hydrogen bonding of molecular building blocks of very different size.
二维（2D）分子自组装允许形成具有定制的几何形状、组成和化学性质的明确定义的超分子层。迄今为止，这些系统中的随机混合和熵效应主要与晶体无序和玻璃相有关。在这里，我们描述了一种二维结晶自组装分子体系，该体系表现出取代分子的随机掺入。晶格由均苯三甲酸（TMA）和三噻吩并苯三羧酸（TTBTA）组成，它们是非常不同大小的 C 对称氢键单元（分别为 0.79nm 和 1.16nm），在溶液-高取向热解石墨（HOPG）界面处形成。值得注意的是，TTBTA 以固定的化学计量比（接近 12%）取代 TMA 晶格。所得晶格常数与组成 TMATTBTA 的合金的 Vegard 定律预测一致，晶格的基底取向由与 HOPG 基底的外延关系定义。通过考虑取代晶格的多重性的熵混合的吉布斯自由能，通过蒙特卡罗模拟阐明了 TMA/TTBTA 晶格的吉布斯自由能，通过密度泛函理论计算阐明了混合焓。后者表明，氢键晶格的键焓和分子/基底相互作用的吸附焓都起着重要作用。这项工作深入了解了在分子晶体中由外延和分子间相互作用共同约束的熵的表现，并证明了可以通过非常不同大小的分子构建块的氢键形成随机混合但结晶的二维固体。
