Role of the deposition temperature on the self-assembly of the non-planar molecule benzene-1,3,5-triphosphonic acid (BTP) at the liquid-solid interface.

Benzene-1,3,5-triphosphonic acid (BTP) contains three non-planar phosphonic acid groups which enable three-dimensional hydrogen bonding. Because of these versatile 3D functional groups, BTP is an interesting intermediate to design both 2D and 3D supramolecular hydrogen-bonded architectures and organic-inorganic hybrid frameworks. However, the adsorption of BTP has surprisingly not been the subject of scanning tunneling microscopy (STM) investigations so far. Here a STM study of the adsorption pattern of BTP as obtained from deposition out of a solution in undecanol on an interface to highly-oriented pyrolytic graphite (HOPG) is presented. Furthermore, the influence of the substrate temperature during the deposition from solution on the self-assembly is investigated. High-resolution STM images reveal that the BTB molecules usually form various structures by co-adsorption with undecanol and that the BTP molecules as parts of self-assembled aggregates adsorb with their benzene ring planes tilted with respect to the substrate plane. The specific supramolecular pattern and the 2D packing density of BTP can be precisely tuned by adjusting the initial substrate temperature during deposition. The experimental results are compared to corresponding model structures obtained from semi-empirical simulations and explained by the influence of temperature on the concentration at the solution-solid interface and the kinetics of the self-assembly process. Based on these results, the control of the deposition substrate temperature has been proven to be a versatile tool to control the polymorphism of molecular patterns deposited out of solutions.