A series of studies is presented to characterize the photophysical properties of a novel type of aggregate formed by the spontaneous noncovalent assembly of numerous cofacial dimers of cyanine dyes (DiSC(3+)(5)) to the minor groove of poly(dI-dC) DNA. The dimensions of these helical aggregates, first synthesized and characterized by Armitage and co-workers (J. Am. Chem. Soc. 2000, 122, 9977-9986), are restricted to the width of the dye dimer because of steric constraints in the minor groove, though the length of the aggregate can extend essentially for the full length of the DNA template. These unique species exhibit both H- and J-type absorption bands that are shifted from the absorption maximum of the monomeric dye by +1650 and -1275 cm(-1), respectively, because of the stacking interactions between the dyes composing the dimers. Additional splittings are seen because of head-to-head interactions between adjacent dye dimers. Here, we present the low-temperature (77 K) absorption, fluorescence, and electroabsorption spectra of these aggregates as well as measurements of the fluorescence lifetime of the monomer and of the J-type emission at 10 degreesC. The electroabsorption measurements yield values of the average difference polarizability on excitation, < Delta alpha > respectively. These are between 2 and 6 times larger than that of for the H and J bands of -74 and -34 Angstrom (3) the monomer. Both bands exhibit similar values for the difference dipole moment on excitation (\<(mu )over right arrow>\) of between 0.6 and 0.7 D that are somewhat smaller than that of the monomer (1.1 D). The absorption and fluorescence experiments show that the line width of the J band is similar to4 times narrower than the experimental fwhm of the DiSC(3+)(5) monomer while the fluorescence decay of the aggregate is roughly a factor of 2 faster. Implications of all of these measurements for determining the number of dyes that are excited cooperatively upon light absorption are discussed.