Despite the multitude of surface supported monolayer structures already reported for trimesic acid (TMA), new self-assembled structures are still discovered, depending on conditions and environment. This exemplifies the versatility of this archetypical supramolecular building block and justifies its role as a model system. At the interface between 1-phenyloctane (PO), a highly nonpolar solvent, and graphite, a new densely packed and partly hydrogen-bonded TMA structure is observed by means of scanning tunneling microscopy (STM). Normally, the TMA solubility in PO is too low to allow for self-assembly of interfacial monolayers. However, as verified by UV-vis spectroscopy, sonication of solutions with TMA sediment increases the amount of dissolved solute molecules. Consequently, the self-assembly of interfacial monolayers can be observed with these enriched solutions. In contrast to many other structures reported, the observed monolayers are densely packed and composed of partly hydrogen-bonded TMA molecules that form zigzag chains. The proposed structural model is derived from semiempirical quantum chemistry methods, which also provide the basis for STM image simulations by means of a scattering formalism. Solvophobic effects are likely to account for both, low TMA solubility in PO and the high packing density of the interfacial monolayer.
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...
The growth and electronic levels of molecular monolayer structures of helical polyalanine-based peptides (PA) on Au(111) surfaces were investigated by scanning tunneling microscopy (STM), spectroscopy (STS), and optical ellipsometry under ambient conditions. The self-assembled monolayer (SAM) films revealed a high degree of lateral and rotational order. Moreover, because of the formation of Au–S bonds, resulting from the termination of the helix by cysteine, the PA molecules are oriented, and their intrinsic dipole moment is tilted by around 50° with respect to the surface normal. This charge ordering within the SAM facilitates internal electric fields of up to 1 V/nm, which may renormalize the molecular orbital energies along the helix, thus enabling a high conductance through these peptides. The characteristic bonding scheme and ordering, found in this study for chiral and polar PA molecules, will be important to understand the accompanied spin polarization of propagating electrons as well as the spin exchange mechanism at the hybrid interface.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.