Self-assembled thin films of semiconducting zinc (II) and terbium (III) chelates for optoelectronics applications

Abstract

Metallorganic thin films with semi-conducting properties, based on zinc chelation with 8,8'-dihydroxy-5,5'-bisquinoline were fabricated via a reactive self-assembly technique. An accelerated film growth was observed with assemblies done using organo-zinc (ZnEt 2, Zn(CH3COO)2, and Zn(CF3COO) 2) salts, compared to a slower growth with ZnCl2, and ZnBr 2. The film growth properties were characterized through various spectroscopic and microscopy techniques. The accelerated growth observed is presently explained via a secondary self-assembly process of labile zinc-precursors stabilized within cavities formed in the primary structure. Light emitting diodes were fabricated from the zinc chelates, which presently exhibit limited quantum efficiencies. A metal-chelating polyurethane-urea based on 2,6-diaminopyridine and 1,6-diisocyanato hexane and chain extended with polyethylene oxide was assembled with a terbium salt to produce luminescent multilayer chelate films, via a novel anhydrous layer-by-layer self-assembly process. UV-Vis spectroscopy and spectroscopic ellipsometry indicate a monotonic but complex increase in film thickness relative to the number of layers. These assemblies show a strong green emission, characteristic of luminescent terbium (Tb3+) chelates. Light emitting diodes from a terbium chelating fully conjugated polyurea, poly(1,4-phenylene-2,6-pyridylurea) were fabricated and electrical properties characterized. The photo-physical properties of these metal chelating pyridyl-based aromatic were also investigated. In their solution state and upon prolonged exposure to 365 nm UV-radiation, these low and high molecular weight compounds were found to cleave the urea linkage nearly quantitatively, generating corresponding amine terminated subunits and CO2 NMR, UV/Vis absorption and fluorescence spectroscopy characterization of the photo-cleavage processes were delineated as a function of UV dose, O2 and water. Experimental results support a light-assisted hydrolysis of the urea bond as the most plausible photo-cleavage mechanism.