Optical absorption spectra and monomer interaction in polymers: Investigation of exciton coupling in DNA hairpins

We investigate the effect of exciton coupling on the optical absorption spectra of polymer molecules under conditions of strong inhomogeneous broadening. It is shown that the dependence of the maximum in the rescaled absorption spectrum on the number of monomers is determined by their resonant exciton coupling and is insensitive to inhomogeneous broadening. Thus the absorption spectrum can be used to determine optical interactions between monomers. Using our theory and semiempirical calculations we determine exciton coupling between adjacent AT pairs in DNA to be 0.04 eV and discuss exciton localization in DNA hairpins composed of AT pairs.