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 x 10(-6)M; 0.094M HClO4; 20 degrees 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 (D-m, m < 10-12), while the transition energy as well as the bandwidth remains unchanged during aggregate growth (D-N, 20 < N < 500). The second-order rate law of the association of precursor oligomers reveals the molar absorbance in JB peak (epsilon(JB)) decreases with increasing association number <(N)over bar> (N-dependent hypochromism, epsilon(JB)/M(-1)cm(-1) = 0.5 x 10(6) + 1.65 x 10(7)/(N) over bar), which makes possible to determine (N) over bar. Mean association numbers (N) over bar = 400-500 can be predicted for the molar absorbance varying with mean association number (N) over bar.