Size distribution of pseudoisocyanine (PIC) J-aggregates studied by near-field absorption spectroscopy

Near-field absorption spectra of the J-aggregates of pseudoisocyanine bromide have been measured at 50×50 pixels with a scanning near-field optical microscope at room temperature. The coherent size distribution of the aggregates was generated from the difference in the spectral peak position at each measured pixel. The size distribution fitted well to the log-normal distribution function when the average size was 30±5 molecules. The inhomogeneous absorption linewidth of the aggregates was estimated to be between 92 and 116 cm−1, where the peak of a broader spectrum had higher energy. A one-dimensional zigzag model for the J-aggregates composed of two molecules in a unit cell is shown to reproduce the polarized absorption spectra of the highly oriented J-aggregates.