A photophysical study of novel silicon and zinc phthalocyanines

Abstract

Phthalocyanines are an important class of compounds with many commercial applications. The core macrocycle is chemically robust, such that a wide variety of substituted species with interesting properties can be synthesised. The work reported in this thesis represents a detailed photophysical study of both known and novel substituted silicon and zinc phthalocyanines.The behaviour of tetrasulfonated zinc phthalocyaniae (ZnPcS(_4)) in aqueous acetonitrile is investigated. This work shows the sensitivity of ZnPcS(_4) to pH, caused by protonation of the phthalocyanine on the bridging azomethine nitrogen atoms. The spectral perturbation that results includes red shifted absorption and emission bands, with a reduced fluorescence lifetime relative to the unprotonated species. AxiaUy substituted silicon phthalocyanines functionalised through bis-esters have highly resolved spectral features due to reduced ring interactions, whilst the light core atom results in excellent fluorescence quantum yields and lifetimes. Electron rich axial substituents (in this case methoxybenzenes) cause quenching of the phthalocyanine fluorescence through an electron transfer mechanism, and the extent of quenching is related to both the oxidation potential of the quencher and the chain length/flexibility. A number of silicon phthalocyanines with tetrathiafulvalene (TTF) containing substituents are also investigated. TTF is known to quench phthalocyanine fluorescence very efficiently, but this current work shows that the degree of quenching is strongly distance dependent. The oxidation potential of TTF is low, and this allows the selective electrochemical oxidation of TTF without affecting the phdialocyanine. It is shown that such oxidation causes a reversal of the electron transfer mechanism. The phenomenon of fluorescent dimers in phthalocyanine chemistry is rarely observed, and most reports arise from spectral and experimental artefacts. However a solketal substituted zinc phthalocyanine and some of its derivatives are conclusively shown to demonstrate this behaviour. At low temperature a broadened absorption profile is observed, with new emission centred at ~750 nm - such spectral behaviour agrees with Exciton Theory for a clamshell dimer. Flash photolysis studies at intermediate temperatures highlights the dynamic nature of the dimer species - absorption of a photon results in the separation of the dimer, followed by fast recombination.