Near-field spectroscopy of water-soluble and water-insoluble porphyrin J-aggregates

Mesoscopic properties of J-aggregates of water-soluble [tetrakis(4- sulfonatophenyl)porphyrin] and water-insoluble [tetrakis(4-methoxyphenyl) porphyrin and tetraphenylporphyrin] tetraphenylporphyrin derivatives in thin films have been investigated by scanning near-field optical microscopy. From surface topography, the sample films were found to be composed of planar J-aggregate microcrystallines. The transmittance spectra of the water-insoluble samples are site specific, while those of the water-soluble samples are not. This observation might reflect the spatial inhomogeneity of the thin films in mesoscopic scales and is related to the broad J-bands of water-insoluble samples. Excited state lifetimes, obtained from time-resolved, pump-probe measurements in small domains, were in the 30-70 ps range for 4-methoxy substituted and in the 40-100 ps range for unsubstituted tetraphenylporphyrin, which are of the same order as the reported far-field results for 4-sulfonated compound in water.