Positive Constructs: Charges Localized on Surface-Confined Organometallic Oligomers

Abstract

Assemblies with molecular-level organization based on organic chromophores and a bimetallic palladium complex are presented. A layer-by-layer strategy is employed by alternately coordinating vinylpyridine-terminated chromophores to the metal centers to form cationic oligomers. These new structures are formed from solution on quartz and silicon substrates functionalized with a covalently bound template layer. Twelve consecutive deposition steps result in structurally regular assemblies as demonstrated by linear increases in the ellipsometrically determined thickness and UV−vis optical absorption. The increase in thickness for each additional layer shows that the long-range order of the system is determined by the structure of the chromophores and by the square-planar geometry of the metal centers. Furthermore, the optical properties indicate that the conjugation length of the assembly component does not increase in the surface-bound oligomers with each additional deposition cycle. Structural communication is transferred via the system components, but they remain electronically isolated. This is supported by density functional theory (DFT) calculations.

Designing Surface‐Confined Coordination Oligomers

“…Subtle changes in electronic structure of conjugated oligomers have been previously predicted by DFT calculations. [3b,d, 21, 22] In conclusion, we have demonstrated the ability to tune the band gaps of relatively long (> 10 nm), surface-bound metal-organic oligomers, [7,8,12,23] while preserving the adopted structure and chemical framework. HOMO-LUMO engineering to generate small band-gap materials is an important challenge in building up materials with potential electronic functionality.…”

“…These oligomers were assembled by using our iterative deposition strategy that involves coordinating PdCl 2 and either the bis(pyridylethenyl)benzene (1) or bis(pyridylethenyl)tetrafluorobenzene (2) chromophore to a siloxane-based template layer (Scheme 1 and Scheme S1 in the Supporting Information). [7,8] The resulting films were characterized by UV/Vis spectroscopy, spectroscopic ellipsometry, semicontact atomic force microscopy (AFM), and synchrotron X-ray reflectivity (XRR). The assemblies were found to be low refractive index materials.…”

“…The deposition strategy used had previously resulted in assemblies with roughness values in the range of 0.3-2 nm, with 1-PdCl 2 assemblies being the least rough. [7,8,12,13] The thickness determined from fitting the XRR data is 4.5 nm (Figure 3). Based on the thicknesses of the assembly and template layers (Figure 2) and the length of the oligomers (from DFT calculations, see below), the tilt angle can be approximated as about 208.…”

“…This is significantly lower than for other coordination-based oligomers, such as the 1-PdCl 2 oligomers (n in the range of 1.8-2.0). [7,8] Fluorinated polymers and fluorinated monolayers are known to be low-n materials.…”

Supramolecular Organization of $$\pi $$-Conjugated Oligomers

“…9.23. Siloxide grafting of ligand 43 gave rise to a pyridine ligand furnished surface which was then used to assemble π -conjugated oligomers 44 and 45 or 46 via hierarchical self-assembly of supra-molecular palladium complexes [23]. …”