Electric field-controlled dissociation and association of porphyrin J-aggregates in aqueous solution

Abstract

The electrooptic effects of porphyrin J-aggregates of tetraphenyl porphyrin tetrasulfonic acid (TPPS) in aqueous solution were studied using electroabsorption (EA) spectroscopy. When the J-aggregates were three-dimensionally distributed, the EA spectra exhibited broadening in the exciton band. When a DC or AC electric field was applied for a long time, the J-aggregates with KCl were dissociated into monomers via N-mers (N = 2–4) as intermediate states, while those without KCl had an increase in aggregation. The EA spectra showed a red shift in the exciton band for N-mers, which indicates that N-mers are isolated microaggregates with a coherent aggregation number N, and isolated microaggregates have not been microscopically or spectrally observed until now. The estimated third-order nonlinear optical susceptibility χ⁽³⁾ for EA spectra in aqueous solution was 10⁴ times larger than that in a polymer film. The molecular rearrangement model was applied to a variety of orientational distributions and the results were explained fairly well. The contribution of the electric double layer is the most probable reason for the large enhancement of χ⁽³⁾ for the solution sample. The dynamic equilibrium between two types of monomers, J-aggregates of various aggregation numbers and cations such as K⁺ and H⁺ was investigated to reveal that K⁺ is more loosely bound to the constituent monomers in J-aggregates than H⁺. Equilibrium equations also show that well-grown aggregates with N > 15 tend to dominate in a solution of J-aggregates, which explains why only well-developed aggregates can be observed spectroscopically.