Optical line shapes of dynamically disordered ring aggregates

We study the absorption line shape caused by Frenkel excitons in one-dimens ional ring-shaped molecular aggregates, such as circular light-harvesting s ystems, subjected to dynamic disorder with a finite correlation time. We fo cus on dichotomic noise and show that for arbitrary orientations of the mol ecular transition dipoles relative to the ring, the absorption spectrum may be calculated exactly by solving two very similar sets of 2(N) coupled lin ear equations of motion, where N is the number of molecules in the ring. Th ese sets are a factor of N smaller than in methods developed previously, wh ich allows us to evaluate the exact line shapes for N up to 12 and study si ze dependence of the spectra. Previous exact calculations were limited to h examers (N = 6). Moreover, in contrast to earlier work, we take into accoun t long-range dipolar transfer interactions between all molecules in the rin g. We find that the dipole orientation and the long-range interactions stro ngly affect the dependence of the spectrum on the ring size. This holds tru e particularly for the exchange narrowing of the linewidths in the fast-flu ctuation regime. (C) 1999 American Institute of Physics. [S0021-9606(99)706 04-5].