Using circularly polarized luminescence to probe exciton coherence in disordered helical aggregates

Circularly polarized emission from helical MOPV4 aggregates is studied theoretically based on a Hamiltonian including excitonic coupling, exciton phonon coupling, and site disorder. The latter is modeled via a Gaussian distribution of site energies. The frequency dependence of the circularly polarized luminescence dissymmetry g(lum)(omega) contains structural information about the low-energy-neutral (excitonic) polaron from which emission originates. Near the 0-0 emission frequency, g(lum)(omega) provides a measure of the exciton coherence length, while at lower energies, in the vicinity of the sideband frequencies, g(lum)(omega) probes the polaron radius. The present work focuses on how the 0-0 dissymmetry, g(lum)(0-0), relates to the emitting exciton's coherence function, from which the coherence length is deduced. In the strong disorder limit where the exciton is localized on a single chromophore, g(lum)(0-0) is zero. As disorder is reduced and the coherence function expands, /g(lum)(0-0)/ increases more rapidly than the sideband dissymmetries, resulting in a pronounced surge in g(lum)(omega) near the 0-0 transition frequency. The resulting spectral shape of g(lum)(omega) is in excellent agreement with recent experiments on MOPV4 aggregates. In the limit of very weak disorder, corresponding to the motional narrowing regime, the coherence function extends over the entire helix. In this region, g(lum)(0-0) undergoes a surprising sign reversal but only for helices which are between n+12 and n+1 complete turns (n = 0,1,...). This unusual sign change is due to the dependence of the rotational line strength on long-range exciton coherences which are also responsible for a heightened sensitivity of g(lum)(omega) to long-range excitonic coupling.