Stark spectroscopy (electroabsorption) is used to study the variation of electronic properties with the size of helical H-aggregates that are formed by the spontaneous noncovalent assembly of co-facial dimers of the cyanine dye (DiSC(2)(5)) into the minor groove of double-helical DNA. The unique and important property of these aggregates, first synthesized and characterized by Armitage and co-workers (J. Am. Chem. Soc. 1999, 121, 2987), is that their size is controlled by the properties of the DNA template. Specifically, the length of the aggregate formed is determined by the length of the DNA template and its width along the g stacking dimension is restricted to that of the dye dimer due to steric constraints in the minor groove. Results for aggregates consisting of 1, 2, 5, and similar to35 adjacent dimers bound to DNA are presented here. The absorption maxima of these species exhibit a large blue shift (1750 cm(-1)) from that of the monomer due to the face-to-face interactions within the dimers. Relatively weak (330-650 cm(-1)) secondary splittings are also seen that arise from end-to-end interactions between adjacent dimers on the chain. The average change in polarizability on excitation () is found to double when two dyes form a stacked dimer whereas no further increase in is seen-as the chain length is increased. Semiempirical (INDO-SCI) calculations yield exciton coupling-energies that are consistent with experiment. However, is predicted to increase toward more positive values on dimerization while the reverse trend is seen experimentally. Nonetheless, both experiment and theory find that is unaffected by higher aggregation.