ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL STUDIES ON WELL-DEFINED SILICON PHTHALOCYANINE STACKED-RING OLIGOMERS.

Abstract

The results of solution electrochemical and photoelectrochemical studies on a series of well-defined silicon phthalocyanine (SiPc) stacked-ring oligomers are presented. These molecules consist of one (monomer), two (dimer), and three (trimer) SiPc rings which are axially stacked through a O-Si-O backbone with t-butyldimethylsilyl "end cap" groups. The interplanar spacing in the dimer and trimer SiPc is about 3.4 Å which facilitates the through-space molecular orbital overlap that gives them unique spectroscopic and electrochemical properties intermediate between those observed for other systems in which the electroactive centers are non-interacting or have exclusively through-bond interaction. There is a blue-shift in both the Q- and Soret absorbance maxima which is accompanied by an increased oscillator strength as more SiPc subunits are added per molecule. In dichloromethane solution, the cyclic voltammograms of these molecules exhibit multiple, one-electron, chemically reversible oxidations and reductions. The number of oxidations and reductions observed for each molecule increases with the addition of more SiPc rings and the energy difference between successive electron transfers decreases. In addition, there is a large cathodic shift of 0.52 V in the first oxidation potential between the monomer and trimer SiPc indicating a net stabilization of the dimer and trimer towards oxidation with respect to the monomer SiPc. These electrochemical results are shown to correlate well with Ultraviolet Photoelectron Spectroscopic (UPS) and UV-visible absorption spectroscopic data and energy level diagrams for the monomer, dimer, and trimer SiPc as well as higher-order polymeric SiPc are developed. Extensive photoelectrochemical studies on SiPc-modified electrodes are also reported. The effects of the chemical nature, E⁰, and concentration of the solution redox couple, as well as the influence of changing the electrode substrate and incident light intensity and wavelength on the photoresponse characteristics of these electrodes are presented and discussed. A solid-state band model for the dyesensitization process is discussed that treats the SiPc layer as a photoconductor that is capable of causing Fermi level pinning to occur at the SiPc/SnO₂ interface, resulting in an open-circuit photovoltage of about 200 mV which is independent of the solution E⁰. A molecular model is also developed that considers the specific molecular interactions which occur between the SiPc and the substrate, between adjacent SiPc molecules in the dye layer, and between the SiPc molecules and the solution redox species. Photoexcitation of the SiPc layer results in the formation of excitons in which the excited-state is delocalized over an aggregate containing several SiPc molecules.