Kinetics and thermodynamics in surface-confined molecular self-assembly Rico Gutzler, * Luis Cardenas and Federico Rosei * Received 3rd August 2011, Accepted 21st September 2011 DOI: 10.1039/c1sc00531f Two-dimensional molecular self-assembly at the liquid/solid interface is a widely employed approach in surface science to pattern surfaces at the nanometre scale. A multitude of supramolecular structures can be realized depending on parameters such as the functionalization of the molecular building blocks, the temperature at which self-assembly takes place, the type of solvent and solute concentration. How these and other parameters influence the kinetics and thermodynamics of the self-assembly process is the subject of this review. Introduction Molecular self-assembly is a widely used strategy to pattern and functionalize surfaces at the nanometre scale with applications such as functional nanomaterials and organic electronics. 1–3 The physisorption of organic molecules onto surfaces and their arrangement into ordered monolayers has been extensively investigated, leading to a myriad of different supramolecular structures with different molecular building blocks, both at the liquid/solid interface and the vacuum/solid interface. 4–15 Despite numerous investigations following the pioneering work of Foster & Frommer 16 and Rabe & Buchholz, 17,18 it is not yet possible to deliberately engineer molecular monolayers or to predict the structure of such a monolayer for a given molecular building unit. To tackle these shortcomings, it is important to understand the mechanisms that underlie surface-confined molecular self- assembly. Polymorphism – the formation of two or more distinct crystalline monolayer structures – is one facet of two-dimen- sional molecular self-assembly that lacks a detailed under- standing, although many parameters have been identified that can be used to control polymorphism and to switch between several different polymorphs. A nonexclusive list of these parameters includes the type of solvent, 19–24 concentration of the molecular building block, 25–27 molecular interactions, 28–34 size and structure of the adsorbing molecule, 30,35–37 type of surface 38–42 and temperature. 43–48 Care must be taken to discriminate between polymorphism in three-dimensional and two-dimensional crystals, the latter type being relevant in self-assembled organic monolayers. For example, concentration- dependent polymorphism in 3D cannot occur due to thermo- dynamic reasons, while it is now a commonly accepted feature in 2D crystallization, as pointed out by Matzger and coworkers. 49 At the liquid/solid interface, interactions between molecular building blocks, solvent molecules and the surface are crucial in defining the structure of a monolayer. These interactions constitute one aspect of a complex thermodynamic description of the self-assembly process, which necessarily also includes parameters such as temperature, entropy, or chemical poten- tials. 13 Attempts have been made to find suitable thermodynamic models that semi-quantitatively describe isolated experimental results, 37,47,48,50,51 but, until now, a conclusive and more global thermodynamic description is still lacking. Furthermore, as in any reaction, kinetic factors influence supramolecular self- assembly. Several studies show that kinetically stabilized phases can form at a surface, which over time transform into thermo- dynamically more stable polymorphs. 49,52,53 Some of the parameters that govern kinetics and thermodynamics of molec- ular self-assembly are elucidated here in view of their importance for the self-assembly process. The complexity of a thorough description of supramolecular self-assembly at surfaces stems from the intricate adsorption process itself. Once the solution is applied to a clean surface, commonly highly oriented pyrolytic graphite (HOPG), the dis- solved molecular building units (analyte) start to adsorb on the substrate. They are free to move on the surface and eventually bind to other adsorbed molecules or desorb back into solution. Stable monolayers can be formed when a sufficiently large number of molecules on the surface assemble into large crystal- line domains. However, a molecule adsorbed in a monolayer is not necessarily fixed in its position and may desorb back into the bulk solution. The monolayer is thus in a dynamic equilibrium with the solution and constant adsorption/desorption is likely to happen on timescales relevant for experiments. Parameters such as adsorption/desorption rates, analyte mobility in solution and on the surface and the nucleation rates of different polymorphs can influence the kinetics of the two- dimensional crystallization. Solvation enthalpy, adsorption enthalpy and entropy changes between the dissolved state and adsorbed state of the analyte describe the energetic contribution of surface-confined molecular self-assembly and hence merge into a thermodynamic description. A microscopic picture of the adsorption/surface diffusion processes can be offered for the Institut National de la Recherche Scientifique, Universit e du Qu ebec, 1650 boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, Canada. E-mail: gutzler@emt.inrs.ca; rosei@emt.inrs.ca This journal is ª The Royal Society of Chemistry 2011 Chem. Sci. Dynamic Article Links C < Chemical Science Cite this: DOI: 10.1039/c1sc00531f www.rsc.org/chemicalscience PERSPECTIVE Downloaded on 31 October 2011 Published on 03 October 2011 on http://pubs.rsc.org | doi:10.1039/C1SC00531F View Online / Journal Homepage