Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation

Abstract

When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. To overcome that problem, we designed 1,8,13-trimercaptomethyltriptycene (T1) and 1,8,13-trimercaptotriptycene (T2) as potential tripodal SAM precursors and investigated their adsorption behavior on Au(111) combining several advanced experimental techniques and state-of-the-art theoretical simulations. Both SAMs adopt dense, nested hexagonal structures but differ in their adsorption configurations and structural uniformity. While the T2-based SAM exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring, all three anchoring groups of T1 are equally bonded to the surface as thiolates, resulting in an almost upright orientation of the benzene rings and large-area structural uniformity. These superior properties are attributed to the effect of conformationally flexible methylene linkers at the anchoring groups, absent in the case of T2. Both SAMs display interesting electronic properties, and, bearing in mind that the triptycene framework can be functionalized by tail groups in various positions and with high degree of alignment, especially T1 appears as an ideal docking platform for complex and highly functional molecular films.

Carbazole‐Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

“…−1 eV) . The adsorption energies of triptycene tripods with three thiol anchors, which account for loss of hydrogen upon binding to a gold surface, were found to be −1.62 eV (for less flexible aryl thiols) and −2.67 eV (for more flexible benzylic thiols) . The tetrapodal anchors perform comparably to the former case despite using thiomethyl anchors rather than thiols.…”

Multidentate Anchors for Surface Functionalization

“…For instance, Reinhoudt have probed this issue of SAMs of β‐cyclodextrin heptathioether derivatives by XPS analysis and found that on average only 4.5 of the 7 attachment points were used . In order to have an ideal homogenous immobilization, the use of a flexible methylene linker between the rigid framework and the contact site would be helpful in some cases …”

“…Very recently,F ukushima and co-workersr eported the interfacial assembly of 1,8,13-trimercaptomethyltriptycene (8)a nd 1,8,13-trimercaptotriptycene (9)o nA u(111)s urface ( Figure 4). [62] In comparison, the monolayers of 9 exhibits al ow degree of order and noticeable deviation from the desired tripodal anchoring; however,t he assembly of 8 results in an almostu pright orientation of the benzene rings and large-area structural uniformity through ap erfect three-point contact. This difference is ascribed to the effect of the flexible methylene linkerso f8.…”

“…Although many multipodal anchors have been reported, few of them can bind to the substrate with large‐area structural uniformity and precise alignment of functional groups. Very recently, Fukushima and co‐workers reported the interfacial assembly of 1,8,13‐trimercaptomethyltriptycene ( 8 ) and 1,8,13‐trimercaptotriptycene ( 9 ) on Au(111) surface (Figure ) . In comparison, the monolayers of 9 exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring; however, the assembly of 8 results in an almost upright orientation of the benzene rings and large‐area structural uniformity through a perfect three‐point contact.…”

“…(b,c) STM images and simulated arrangement of 8 on Au(111) surface (Au/ 8 ). Reprinted with permission from reference . Copyright 2019 American Chemical Society.…”