Local exciton ground states in disordered polymers

Exciton localization in conjugated polymers with weak conformational disorder is investigated via the Anderson model with Gaussian off-diagonal disorder, σ. We show that a small fraction of the low-energy eigenstates are spatially localized, nonoverlapping, and space filling. We term these states ``local exciton ground states'' (LEGS). The LEGS exhibit an almost Gaussian density of states, an average localization length L~σ^-2/3, and an inhomogeneous optical linewidth ~σ4/3. Their transition dipole moments scale with their localization length in a way consistent with a lowest energy excitation confined to a region ~O(L). The length scale over which the LEGS are confined is an effective low-energy conjugation length or spectroscopic segment. The appendix describes an efficient and accurate technique for calculating the density of states of the Anderson model.