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

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 10(4) 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.