Spectra and Mean Association Number of Porphyrin J Aggregate

The mean association number of a porphyrin J aggregate matured in acidic aqueous media (dye concentration, 7.9 × 10−6 M; 0.094 M HClO4; 20 °C) was determined to be 440 by measurements of the sedimentation and diffusion constants on ultracentrifuge. The aggregate forms in a rod-shaped microcrystalline consisting of more than 400 stacked porphyrins which grows for a period of two days in acidic aqueous solutions. MCD (magnetic circular dichroism) B-term extrema and hypochromic reduction observed with J- and H-bands in aggregate were elucidated on the basis of structure of J aggregate. The Stokes shift and bandwidths observed with J bands were also discussed using motional narrowing model. The characteristic J peak appears in an earlier stage of dye association (Dm, m < 10–12), while the transition energy as well as the bandwidth remains unchanged during aggregate growth (DN, 20 < N < 500). The second-order rate law of the association of precursor oligomers reveals the molar absorbance in JB peak (εJB) decreases with increasing association number N (N-dependent hypochromism, εJB/M−1 cm−1 = 0.5 × 106 + 1.65 × 107/N), which makes possible to determine N. Mean association numbers N = 400–500 can be predicted for the molar absorbance varying with mean association number N.