Spectroscopy of photoinduced charge-transfer reactions between tetrasulfonated aluminium phthalocyanine and methyl viologen

Abstract

Interactions between tetrasulfonated aluminium phthalocyanine (AlPcTS⁴⁻) and methyl viologen (Mv²⁺) have been studied in water and ethanol solutions using several experimental techniques. UV-visible absorption and fluorescence spectroscopies show that ion-pair complexes occur in ethanol, disappearing in more polar environments such as water. Time-resolved fluorescence spectroscopy (picosecond timescale) reveals the existence of several emissive species in ethanol solutions, of which one of the components is attributed to the charge-transfer complex (AlPcTS⁴⁻)(Mv²⁺)₂, another to higher-order aggregates and yet another to the isolated AlPcTS⁴⁻ molecule. The AlPcTS⁴⁻ emission is quenched by Mv²⁺, leading to transient diffusion in the fluorescence decay kinetics. On the other hand, the emissive complex has an exponential decay with a relatively long lifetime (above 1 ns). Time-resolved absorption measurements did not reveal the existence of radicals in aqueous solution, even on the picosecond timescale. The spectra reveal the presence of excited singlet state AlPcTS⁴⁻, which decays via the triplet excited state back to the ground state.