Circularly Polarized Luminescence as a Probe for Long-Range Interactions in Molecular Aggregates

The extreme sensitivity of circularly polarized luminescence (CPL) to long-range excitonic interactions inside a helical aggregate is investigated. It is found to persist even in the presence of strong energetic disorder and coupling of the exciton to molecular vibrations, when the emitting exciton is localized to only a few chromophores. The CPL dissymmetry, g(lum), is found to depend on a modulated sum over the excitonic couplings, Sigma(n,s)J(n,n+s) s sin(phi s), where J(n,n+s) is the coupling between molecules separated by s lattice spacings and 4, is the pitch angle between adjacent chromophores. The validity of this relation is confirmed through full-scale numerical simulations of helical MPOV4 aggregates using the disordered Holstein Hamiltonian. In addition, an analytical expression for g(lum) is obtained for a helical chain containing a single, energetically detuned chromophore to represent strong disorder. Subsequently, the resulting expression is generalized to include full distributed disorder. Our results demonstrate that the spatial dependence of extended interactions can be extracted from experimental spectra, without having details on disorder or exciton-vibrational coupling.