Photoelectrochemical studies of thin films of monomer, dimer and trimer stacked-ring silicon phthalocyanines

Thin films of silicon phthalocyanines (SiPc) on SnO₂ and Au substrates have been studied as photoelectrodes. These SiPc molecules were the same monomer, dimer and trimer stacked-ring Pc's (m-SiPc, d-SiPc and t-SiPc) which had been previously characterized by voltammetry in CH₂Cl₂ solutions. As thin solid films, the m-SiPc proved to yield high photocurrent quantum efficiencies for oxidation of hydroquinone and ferrocyanide, up to 10% at surface coverages below 40 equivalent monolayers. Transmission spectra and photoaction spectra, using frontside and backside illumination, showed that the m-SiPc films consisted of at least two distinguishable phases on the SnO₂ surface where as d-SiPc and t-SiPc films appeared to consist of only one phase at all coverages, d-SiPc and t-SiPc films on SnO₂ substrates photoenhanced the reduction of benzoquinone over the oxidation of hydroquinone, while on m-SiPc on SnO₂, the hydroquinone oxidation was preferentially photoenhanced. The two phases of m-SiPc on SnO₂ appear to be first deposited as a porous, ordered photoactive phase of less than 100 equivalent monolayer thickness, and second as a randomly oriented phase, with lower photoactivity. This last phase filters light incident first on the m-SiPc/electrolyte interface, producing a photocurrent strongly dependent upon illumination direction.