Characterization of crystalline and solution-processable phthalocyanine assemblies by electrochemical, photoelectrochemical, and surface spectroscopic techniques

Abstract

Ultrathin organized films of organic electronic materials, such as phthalocyanines (Pc), are promising for both fundamental and applied studies due to their special optical, electronic and photoconductive properties. The studies presented in this dissertation include fabrication of ultrathin molecular assemblies by molecular beam epitaxy and Langmuir-Blodgett techniques. The degree of molecular order, extent of charge transfer and the morphology within these films, assessed by methodologies, such as photoelectrochemistry, electrochemistry, surface analysis and optical spectroscopy were discussed and characterized. Under high vacuum condition, a wide range of ordered structures of some trivalent metal phthalocyanines, such as GaPc-Cl, InPc-Cl and AlPc-F, can be fabricated. These materials exhibit "layer-by-layer" growth on the single crystal SnS₂ surface when deposited by molecular beam epitaxy (MBE). The MBE technique allows for closer packing of these highly ordered phthalocyanines than in self-assembled (SA) or Langmuir-Blodgett (LB) thin films, due to the lack of hydrocarbon side chains which are necessary for control of molecular architecture during SA or LB depositions. Several new solution processable substituted phthalocyanines are introduced, which due to their strong self-assembled tendency, may be suitable for the formation of well organized thin films by SA and LB techniques. It is found that the types of the substituents attached to the Pc rings play a significant role in determining both the aggregation tendency and the electrochemical properties of Pcs. Surface pressure-area isotherms of these substituted phthalocyanines show that there can be one or two stable phase transition regimes for monomolecular film at the air/water interface. On-trough spectroscopic studies of benzylalkoxy substituted phthalocyanines show that in the pressure-area region prior to the formation of the first stable phase extensive aggregation has occurred. Electrochemical studies of fully compressed films of substituted phthalocyanines on certain substrates show the presence of multiple electroactive domains, controlling the oxidation or reduction process of the Pc rings. Spectroelectrochemical studies of LB films of CuPcOC₂OBz suggest that the presence of both monomer and aggregates leads to the two separate oxidation processes.