Scanning Tunneling Microscopy Investigation of Two-Dimensional Polymorphism of Structural Isomers

Abstract

Monolayers of two pairs of structural isomers were deposited on Au(111) and observed via scanning tunneling microscopy in ultrahigh vacuum. We observe exclusively cyclic pentamers of isatin (1H-indole-2,3-dione), whereas its structural isomer, phthalimide (isoindole-1,3-dione), self-assembles primarily into close-packed arrays, with alternate structures that include kinetically locked disordered clusters and tetramer networks. Removal of the phthalimide NH group and its replacement with a CH2 group produces 1,3-indandione, which, despite the loss of the hydrogen-bond donor site, self-assembles into similar structures: close-packed areas and tetramer networks. The equivalent analog for isatin, 1,2-indandione, does not form pentamers and instead forms only close-packed areas and disordered regions. By iteratively altering the chemical structure, we demonstrate the influence that the chemical structure has on the resulting two-dimensional self-assembly.

D–π–A‐Type Triazatruxene‐Based Dopant‐Free Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells

“…Such large aggregates, observed exclusively on the CI‐B1 film, may be formed due to the hydrogen bonding ability of 1,3‐indandione end capping acceptor unit. [ 38,39 ] Similarly, despite CI‐B2 resulted in a more homogeneous morphology and better surface coverage than CI‐B1, it has a substantial presence of pin holes that could deteriorate the device performance. Consequently, the poor film coverage for CI‐B1 and CI‐B2 could be the main factors for the low V OC and FF achieved for these systems, suggesting that end‐capped acceptors could greatly affect the surface morphology and have important consequences for the photovoltaic performance.…”

Methylisatin Structural Isomers Have Different Kinetic Pathways to Self-Assembly

“…The H···A distances calculated for the CH···O hydrogen bonds for these three molecules are relatively short; weak hydrogen bonds are generally considered to have an H···A distance of 2.2–3.0 Å . Previously, we show experimentally that pentamer formation is precluded when chemical susbtitutions to isatin are made which disrupt the NH···O and CH···O hydrogen bond formation. , Figures b, c show that the substitution of a methyl group remote to these hydrogen bond contacts does not significantly alter the geometry of the pentamer; that is, the 7-position proton and the 3-position carbonyl group are still in close enough proximity to feasibly hydrogen bond.…”

“…There is great interest in gaining an understanding of how small changes in molecular structure, including position of functional groups − and molecular geometry, − lead to changes in self-assembled structure. Forging this connection between molecular structure and extended structure is not trivial, especially since one molecule may form multiple different 2D morphologies, , and structural isomers may exhibit vastly different self-assembled structures. ,,, In this work, our focus is on self-assembly in two dimensions (which is essentially a simplified process of crystallization with reduced dimensionality) at the vacuum–solid interface. We systematically vary sample preparation conditions as well as alter the functional groups of the starting molecule to demonstrate which factors drive the formation of particular two-dimensional assemblies.…”

Complex Structures Resulting from Carboxylic Acid Self-Assembly: Comparison of 2-Naphthoic Acid to Quinaldic Acid and 3-Quinoline Carboxylic Acid

“…23 Self-assembly based on COOH dimerization has been used to create linear structures on the nanoscale as well as a wide variety of branched networks. 24,25 Recent work from our laboratory 26,27 has focused on the expansion of the COOH group beyond a simple synthon, showing that structures significantly more complex than dimers can arise when a molecule has additional hydrogen-bond donor or acceptor groups adjacent to COOH. This is the case even when the hydrogen-bonding activity of these groups is relatively weaker than average.…”