Two-dimensional (2D) molecular self-assembly leads to 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 been associated with glassy phases and imperfect crystals. Here, we describe a 2D crystalline molecular self-assembled system that incorporates random substitutional defects. The system comprises two C3-symmetric hydrogen bonding units with different sizes: terthienobenzenetricarboxylic acid (TTB) and 1,3,5-benzene tricarboxylic acid (TMA). Both of these molecules are well-studied, and self-assemble on highly oriented pyrolitic graphite (HOPG) surfaces to form hydrogen-bonded porous meshes. When combined, the TTB and TMA can spontaneously form an intermixed lattice, where a lattice comprising predominantly TMA also includes a small number of TTB-substituted sites. This system is analogous to the substitutional solid solutions formed in some alloys of metals.<br/><br/>By combining density functional theory (DFT) calculations with Monte Carlo simulations, we are able to elucidate the thermodynamics of the system. This work provides insight into minimization of Gibbs free energy in an epitaxial molecular system, where molecule-substrate interactions, molecule-molecule interactions and the multiplicity of the available lattice configurations all play an important role.

Entropic intermixing in a binary molecular system: A two-dimensional hydrogen-bonded molecular substitutional solid solution

253rd National Meeting (2017)

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