Polymorphism Driven by Concentration at the Solid–Liquid Interface

Abstract

To tune the structures and properties of self-assembled architectures of organic molecules different building strategies like functional groups; 2À12 substrate; 14À19 temperature; 20À22 chemical nature of solvent (aromatic interaction, 13,23 oddÀeven or parity effect, 24À26 saturated and unsaturated solvent, 27 alkyl chain length 1,2,10 ); properties of solvent (solubility, hydrophilic, and hydrophobic properties, 28 polarity, 29 chirality, viscosity 30 ); and so on have been used. Out of all of these, functional groups have been widely exploited because they give an excellent tunability over the strength and symmetry of the structures that one would like to design. 31,32 Trimesic acid (TMA) with threefold symmetric carboxylic acid functionality is a suitable candidate for the formation of hydrogen-bond networks because it can act as both hydrogen-bond donor and acceptor at the same time; therefore, it forms stable supra-molecular structures via hydrogen-bonding with other TMA or with different molecules.Typically, TMA assembles on crystalline substrate into two porous networks from fatty acid solutions, the low-packing density "chicken-wire" (0.8 molecules/nm 2 ) and higher-packing density "flower" structures (1.1 molecules/nm 2 ) 1 as depicted in Figure 1. Both structures are governed by intermolecular hydrogen bonding and exhibit periodic cavities of ∼1.0 nm diameter. These structures are found to form at both solidÀliquid interface and at vacuumÀsolid interface (deposited in UHV) on different substrates. 1,33,34 The building blocks for chicken-wire and flower structures are dimers or dimers and trimers, respectively. TMA also coassembles with other molecules (1, 3, 5-tris(4-pyridyl)-2,4,6-triazine (TPT), terephthalic acid (TPA), coronene), which eventually form different patterns (honeycomb packing motif; bone-shaped packing motif, etc.). 35,36 Lackinger et al. showed a solvent-induced polymorphism of TMA in different alkanoic acids (C nÀ1 H 2nÀ1 COOH) from butyric (n = 4) to nonanoic acid (n = 9) on graphite (0001). 1 These STM experiments revealed that in long-chain fatty acids (n = 7, 8, 9) TMA forms a lowpacking density chicken-wire structure, and in short-chain length fatty acids (n = 4, 5, 6, 7), it forms the high-density flower structure. Under electrochemical control depending on an applied electrode potential, TMA may also form high-packing density structures with its molecular plane perpendicular to the substrate plane at solidÀliquid interface. 37 Under UHV condition, different polymorphs of TMA are obtained by varying the coverage of TMA molecules on Au(111) surface. 19 The authors have demonstrated a variety of structures including the flower and chicken-wire structures and also shown at high deposition rate the formation of a rather densely packed phase (1.34 molecules/nm 2 ). 19 Tahara et al. showed that network formation is affected by the concentration-dependent surface coverage of DBA (hexadehydrotribenzo[12]annulene) derivatives at the TCB (1,2,4-trichlorobenzene)/graphite interface. DBA st...

Bias-induced conformational switching of supramolecular networks of trimesic acid at the solid-liquid interface

“…Both networks were present at the same time when heptanoic acid was used as a solvent. Hietschold et al 33,36 reported that by tuning the molecule concentration by sonication, TMA selfassembled in flower, filled-flower, and dodeca-rim structures in heptanoic acid, while the chickenwire structure was not observed for these conditions. In contrast to Lackinger et al, 31 both chickenwire and flower structures could be observed in octanoic acid.…”

“…So far, this structure has never been reported for TMA dissolved in nonanoic acid. 31,33 (highlighted in yellow) and cyclic trimeric (highlighted in blue) hydrogen bonds. The unit cell parameters for the flower structure were found to be a = b = 2.5 nm and θ = 60 • , which match the values reported in the literature.…”

“…1), a planar molecule with 3-fold a) Author to whom correspondence should be addressed: m.a.stohr@rug.nl symmetry, which consists of three carboxyl groups (-COOH) attached to a central benzene ring, has been widely investigated at the solid-liquid interface. [28][29][30][31][32][33][34][35]41 Previous STM experiments at the solid-liquid interface for TMA on highly oriented pyrolitic graphite (HOPG) showed that the type of TMA structure that forms can be controlled by the solvent choice, [30][31][32]34 by the concentration of TMA in solution, 33,36 and by the temperature. 34 Furthermore, it was shown that the structure of TMA networks can be changed by the addition of metal nitrites to the TMA solution.…”

Principles of molecular assemblies leading to molecular nanostructures

“…Careful control over concentration can aid in selecting a specific structure, especially when two or more polymorphs are possible that differ in packing density or adsorption energy [30][31][32][33][34][35]. One of the first examples of concentration-dependent self-assembly was discovered in the case of alkoxylated DBA derivatives [30].…”

“…A number of experimental parameters have already been identified that control the appearance of a certain type of polymorph over another. A non-comprehensive list of these parameters includes the type of solvent [23][24][25][26][27][28][29], the concentration of molecules [30][31][32][33][34][35], the type of substrate [36][37][38][39][40][41], the temperature at which the self-assembly takes place [42][43][44][45] and the thermal history of the sample [46]. How these parameters affect the self-assembly of organic building blocks is the subject of this review, with a special focus on the liquid-solid interface.…”