Lewis-Acid Mediated Reactivity in Single-Molecule Junctions

Abstract

While chemical reactions at a gold electrode can be monitored using molecular conductance and driven by extrinsic stimuli, the intrinsic properties of the nanostructured interface may perform important additional functions that are not yet well understood. Here we evaluate these properties in studies of single-molecule junctions formed from components comprising 4,4'-biphenyl backbones functionalized with 12 different sulfur-based linker groups. With some linkers, we find evidence for in situ S-C(sp³) bond breaking, and C(sp²)-C(sp³) bond forming, reactions consistent with the ex situ transformations expected for those groups in the presence of a Lewis acid. Notably, we also approach the limits of substituent influence on the conductance of physisorbed sulfur-linked junctions. As an illustrative example, we show that a tert-butylthio-functionalized precursor can form both chemisorbed (Au-S) junctions, consistent with heterolytic S-C(sp³) bond cleavage and generation of a stable tert-butyl carbocation, as well as physisorbed junctions that are >1 order of magnitude lower conductance than analogous junctions comprising cyclic "locked" thioether contacts. These findings are supported by a systematic analysis of model thioether components comprising different simple hydrocarbon substituents of intermediate size, which do not form chemisorbed contacts and further clarify the inverse relationship between conductance and substituent steric bulk. First-principles calculations confirm that bulky sulfur-substituents increase the probability of forming junction geometries with reduced electronic coupling between the electrode and π-conjugated molecular backbone. Together, this work helps to rationalize the dual roles that linker chemical structure and metal electrode Lewis character can play in mediating interfacial reactions in break-junction experiments.

Figure 1

(a) A schematic of the model molecular junctions studied here, comprising a 4,4′-biphenyl backbone substituted with different thioether-based electrode linker groups (-SR). (b) Chemical structures of the different junction components investigated (for complete structures see the SI, Figure S1). Through systematic studies that probe the influence of the sulfur substituent on junction conductance, we categorize each linker group as stable (left) or reactive (right). MTh and Tc refer to the explicitly drawn biphenyl compounds comprising locked thioether linkers with 5- or 6-membered rings (n = 1 or 2), respectively.

Figure 2

An illustrative example of (a) 1D conductance and (b) 2D conductance-displacement histograms for a measurement of ^tBu (10,000 traces). We assign the multiple peak features observed to junctions contacted through physisorbed (P) -S(R)-Au or chemisorbed (C) -S-Au linkages. The structures of proposed PP, PC, and CC junctions are provided in the inset to (a). (c) A proposed reaction mechanism for the in situ cleavage of -^tBu groups upon coordination of the Lewis basic sulfur group to a Lewis acidic gold adatom. (d) Chemical structure of ^tBuAc, a molecule that can only form PC or CC junctions. (e) Overlaid 1D conductance histograms (≥9,000 traces) for two different measurements of ^tBu (illustrating experiment to experiment variation), ^tBuAc, and H (a molecule that can only form CC junctions when measured in solution). Conductance peaks for ^tBuAc and H align with the peaks assigned to PC and CC junctions formed from ^tBu, within a factor of 2. Additional histograms from repeated measurements of ^tBu, as well as 2D conductance-displacement histograms for ^tBuAc and H, are provided in the SI, Figure S2.

Figure 3

(a) Overlaid 1D conductance histograms obtained from measurements of Tc-pre, Tc, and Me (4,900–10,000 traces). The lower conductance peak of the multimodal feature observed in the histogram of Tc-pre aligns with the peak observed for Me, while the higher conductance peak feature aligns with the peak observed for Tc. (b) The data in (a) suggests that Tc can be formed from Tc-prein situ. Green/blue protons are lost/gained during the transformation (detailed mechanisms are provided in the SI, Figure S4). (c) Overlaid 1D conductance histograms obtained from measurements of MTh-pre, MTh, Me, and H (5,000–10,000 traces). We observe a distinct shoulder to higher conductance in the histogram of MTh-pre. (d) The data in (c) suggests that MTh can be formed in situ from MTh-pre. As shown in the SI, Figure S3f, histograms obtained for different measurements of MTh-pre vary from experiment to experiment (as also observed for ^tBu) and provide further evidence that suggests it is possible to ring open MTh under these conditions to form junctions of the CC type (such as those formed from H).

Figure 4

(a) Overlaid 1D histograms for biphenyl molecules with different sulfur linker groups, showing an inverse correlation between conductance and steric bulk. (b) Newman projections illustrating variation in the orientation of the S–Au bond with respect to the aryl plane (solid black line) through rotation of the aryl-S bond. When the S–Au bond is misaligned with the aryl π-system, a low conductance (G) is expected as the backbone orbitals will be decoupled from the electrode (and vice versa). (c) An example of the junction models studied in this work using first-principles calculations. Here, a 1,4-phenylene backbone with -S^tBu linker groups is connected to two Au₅ clusters. The Au-S-C-C dihedral angle is constrained to 25° (a dihedral angle of 90° aligns the Au–S bond perpendicular to the aryl plane). (d) Overlaid potential energy curves for each Au₅ junction series. The structures of each molecular model are shown in panel (c) and (e, inset). The torsional energy landscape for P1-Me is relatively shallow, allowing a wide range of dihedral angles to be sampled at room temperature. In contrast, P1-Th and P1-^tBu exhibit a sharp energy increase with variation of the dihedral angle away from 88 and 27°, respectively. (e) Overlaid plots of tunnel coupling as a function of dihedral angle for each series. In contrast to the other models, the tunnel coupling is small for the lowest energy conformations of P1-^tBu.