Characterization of chiral H and J aggregates of cyanine dyes formed by DNA templating using stark and fluorescence spectroscopies

A series of studies is presented to characterize the photophysical properti es of a novel type of aggregate formed by the spontaneous noncovalent assem bly 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, firs t synthesized and characterized by Armitage and co-workers (J. Am. Chem. So c. 2000, 122, 9977-9986), are restricted to the width of the dye dimer beca use of steric constraints in the minor groove, though the length of the agg regate can extend essentially for the full length of the DNA template. Thes e unique species exhibit both H- and J-type absorption bands that are shift ed from the absorption maximum of the monomeric dye by +1650 and -1275 cm(- 1), respectively, because of the stacking interactions between the dyes com posing the dimers. Additional splittings are seen because of head-to-head i nteractions between adjacent dye dimers. Here, we present the low-temperatu re (77 K) absorption, fluorescence, and electroabsorption spectra of these aggregates as well as measurements of the fluorescence lifetime of the mono mer and of the J-type emission at 10 degreesC. The electroabsorption measur ements 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 excitatio n (\<(<Delta>mu )over right arrow>\) of between 0.6 and 0.7 D that are some what smaller than that of the monomer (1.1 D). The absorption and fluoresce nce 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 f luorescence decay of the aggregate is roughly a factor of 2 faster. Implica tions of all of these measurements for determining the number of dyes that are excited cooperatively upon light absorption are discussed.