Optical spectra and stokes shift in double-stranded helical supramolecular assemblies.

We study the photoluminescence from helical MOPV4 aggregates using a model that includes excitonic coupling, exciton-phonon coupling, and spatially correlated disorder in the chromophore transition energies. The helical aggregates consist of stacked dimers of MOPV4 chromophores. We have modeled these helical stacks as double-stranded aggregates, allowing us to investigate the effect of correlated disorder within the dimers on emission. We have studied the dependence of the Stokes shift, the emission line widths, and the ratio of the 0-0 to 0-1 emission peaks on the aggregate size and disorder. Our findings show that this peak ratio is quite insensitive to the aggregate size if the latter exceeds the coherence length of the emitting exciton. This makes this ratio a reliable probe for both disorder and the coherence length of the emitting exciton. We have found only a weak dependence of this peak ratio on the degree of correlation between the transition energies within each dimer, whereas such correlation has a large effect on the aggregate-size-dependent Stokes shift. By comparison with experiment, we have estimated the coherence length of the emitting exciton to be only one lattice spacing along the stacking direction. From our analysis of the Stokes shift we conclude that the exciton diffusion length is in the range 6-13 nm.