Thin-film characterization of neutral π-associated assemblies incorporating putative pseudo-rotaxanes

Thin films of 1:1 and 1:2 stoichiometric mixtures of bis(1,5-naphtho)-38-crown-10 (1) with both N,N'-di-n-alkylpyromellitic diimide (2) and N,N'-di-n-alkyl-1,4,5,8-naphthalenetetracarboxylic diimide (3) have been prepared using both Langmuir-Blodgett (LB) and thermal evaporation techniques. For thermal evaporation, the substituted n-alkyl chains were C6H13 (2a and 3a, respectively), while for LB deposition, they were C14H29 (2b and 3b, respectively). LB monolayers of different stoichiometric mixtures of both 1.2b and 1.3b were studied using UV-visible spectroscopy, Brewster-angle microscopy, and angle-dependent X-ray photoelectron spectroscopy and revealed that in both cases a stoichiometric mix of electron-donating and electron-accepting aromatic moieties was necessary to achieve good film-forming properties. 1:2 mixtures of 1.2a and 1.3a were then deposited as thin films using thermal evaporation techniques and studied using atomic force microscopy. This showed that films of 1.2a consisted of large spheres less than or equal to ca. 300 nm in diameter whereas films of 1.3a were found to be relatively uniform, with the maximum surface roughness being less than a third of the total film thickness. Additional electrical conduction measurements made on both LB and thermally evaporated prepared metal/organic/metal devices resulted in only evaporated films of 1.3a giving any measurable resistance. Furthermore, films of this material displayed a Poole-Frenkel conduction mechanism. These experiments have shown that rudimentary electrical devices can be prepared from neutral pi-associated assemblies.