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

Abstract

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, for the most part, remained elusive. Previously, it was shown that the number of coherently connected chromophores, Coh N , can be determined directly from the ratio ( R S ) of the 0-0 and 0-1 vibronic line strengths in the photoluminescence (PL) spectrum. The relation,where 2 0 λ is the associated Huang-Rhys parameter, was derived in the Frenkel exciton limit. Here, it is shown that R S remains a highly accurate measure of coherence for systems characterized by significant charge transfer interactions (e.g. conjugated π-stacked systems). The only requirement is that the exciton band curvature must be positive, as in a J-aggregate.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

“…S4), in agreement with common notion, but in contrast to our results of a direct calculation. Although largely ignored in the literature, the framework developed by Spano and coworkers (57,59) also allows for the possibility to account for different peak ratios by distinct electron−hole separations for a given coherence length. Consistent with our results, a larger electron−hole distance is predicted to give rise to a decreased peak ratio between the ZPL and the vibrational mode (SI Coherence Length of the Emitting Site).…”

Exciton Delocalization in Indolenine Squaraine Aggregates Templated by DNA Holliday Junction Scaffolds

“…This is a hopping interaction and is referred to in the literature in a number of categorical ways in terms of interaction, hopping, and coupling or in the context of a parameter, shift, or potential. Relative to the former, J m , n is referred to as an exchange interaction, , instantaneous intermolecular interaction, resonant interaction or coupling, − resonant exciton hopping, intermolecular Coulombic interaction, − intermolecular dipole–dipole coupling, long-range dipole–dipole Coulombic interaction, excitonic coupling, and electronic interaction. , In the context of the latter, J m , n is called a hopping parameter, monoexcitonic shift, and point dipole interaction potential . Although not a complete list, the number of ways J m , n is described suggests that a more thorough description may be useful.…”

“…Although not a complete list, the number of ways J m , n is described suggests that a more thorough description may be useful. The excitonic hopping interaction inherent in J m , n enables exciton delocalization and relaxation. ,,,− Where intermolecular distances are less than ∼4 Å (e.g., aggregates with π-stacking or covalently bridged), wave function overlap can induce intermolecular charge transfer (also described as superexchange coupling or Dexter mechanism, coupling or interaction) and can contribute to the J m , n term. − ,,− In our work, it is assumed that intermolecular charge transfer can be neglected. It should be noted that J m , n is often approximated as the coupling between a pair of transition point dipoles and is referred to as such.…”

Exciton Delocalization and Scaffold Stability in Bridged Nucleotide-Substituted, DNA Duplex-Templated Cyanine Aggregates

“…Note that Mass et al recently provided an extensive discussion of previous terminology regarding J m , n . The coupling arising from the J m , n term in the Hamiltonian is responsible for exciton delocalization ,− and is therefore of interest for designing an exciton wire.…”