  Understanding the properties of electronically interacting molecular
chromophores, which involve internally coupled electronic-vibrational motions,
is important to the spectroscopy of many biologically relevant systems. Here we
apply linear absorption, circular dichroism (CD), and two-dimensional
fluorescence spectroscopy (2DFS) to study the polarized collective excitations
of excitonically coupled cyanine dimers (Cy3)2 that are rigidly positioned
within the opposing sugar-phosphate backbones of the double-stranded region of
a double-stranded (ss) - single-stranded (ss) DNA fork construct. We show that
the exciton-coupling strength of the (Cy3)2-DNA construct can be systematically
varied with temperature below the ds - ss DNA denaturation transition. We
interpret spectroscopic measurements in terms of the Holstein vibronic dimer
model, from which we obtain information about the local conformation of the
(Cy3)2 dimer, as well as the degree of static disorder experienced by the Cy3
monomer and the (Cy3)2 dimer probe locally within their respective DNA duplex
environments. The properties of the (Cy3)2-DNA construct we determine suggest
that it may be employed as a useful model system to test fundamental concepts
of protein-DNA interactions, and the role of electronic-vibrational coherence
in electronic energy migration within exciton-coupled bio-molecular arrays.
