Determining the spatial coherence of excitons from the photoluminescence spectrum in charge-transfer J-aggregates

The importance of spatial coherence in energy and charge transfer processes in biological systems and photovoltaic devices has been hotly debated over the past several years. While larger spatial coherences are thought to benefit transport, a clear correlation has yet to be established, partly because a simple and accurate measure of the coherence length has remained elusive. Previously, it was shown that the number of coherently connected chromophores, N-Coh, can be determined directly from the ratio (S-R) of the 0-0 and 0-1 vibronic line strengths in the photoluminescence (PL) spectrum. The relation N-Coh = lambda S-2(0)R, where lambda(2)(0) is the associated monomeric Huang-Rhys parameter, was derived in the Frenkel exciton limit. Here, it is shown that S-R remains a highly accurate measure of coherence for systems characterized by significant charge transfer interactions (e.g. conjugated pi-stacked systems). The only requirement is that the exciton band curvature must be positive, as in a J-aggregate. (C) 2016 Elsevier B.V. All rights reserved.