The J- and H-bands of organic dye aggregates

Certain molecular aggregates consisting of organic dyes are remarkable in exhibiting an intense and very narrow absorption peak, known as a J-band, which is red-shifted away from the region of monomer absorption. Apart from those dyes showing the J-band on aggregation, there are also dyes where the absorption maximum is shifted to higher energies. The width of the resulting absorption band (called an H-band) is comparable to that of the monomeric dyes and shows a complicated vibrational structure. Following our analysis of the J-band spectra of polymer aggregates using the CES approximation [A. Eisfeld, J.S. Briggs, Chem. Phys. 281 (2002) 61], a theory that includes vibrations explicitly, we show that the same approximation can account for measured H-band spectra. Using simple analytical forms of the monomer spectrum the origin of the widely different shapes of H- and J-bands is explained within the CES approximation.