The growing global energy crisis demands that we find environmentally benign methods for artificial light-harvesting as an alternative to fossil fuel usage. One critical function that an artificial panchromatic light-harvesting system should possess is the ability to efficiently move energy of electronic excited states, of excitons, through space. Due to their extended electronic states, sensitivity to their local environment and tunable assembly thermodynamics, inter-conjugated polyelectrolyte complexes (CPECs) are attractive artificial energy-transfer antennae. Electronic energy transfer (EET) between organic semiconductors is commonly described via the celebrated Forster model. This model is appropriate when the exciton donor and acceptor molecules are far enough apart to be considered point transition dipoles. However, within a CPEC, the spatial extent of the CPE’s delocalized electronic wavefunction is comparable to the distance between the donor and acceptor. This leads to the breakdown of the Forster model, and theoretical calculations show that the precise functional form of the π-electronic wavefunction begins to exert a major influence on the EET rate. In this work we aim to elucidate precisely how the EET efficiency within a CPEC is influenced by the wavefunction delocalized along the CPE backbone. To do so, we use Suzuki cross-coupling chemistry to synthesize a chemical series of CPE exciton donors consisting of identical ionic sidechains and a very similar ionic charge density but differing in their conjugated backbone chemical structures. We then use a common exciton acceptor CPE to systematically investigate the role of the donor electronic structure. Varying the sidechain length of the common acceptor CPE allows us to further interrogate the donor/acceptor distance dependence of the EET rate. In this work we employ hybrid DFT wavefunction calculations via time-efficient plane-wave codes for the purpose of relating the dependence of experimental EET rates to the underlying excitonic coupling. We argue that the CPEC assembly allows for an elegant and relatively clean way of determining the fundamental parameters that govern EET between proximal conjugated polymer chains.

Unraveling the influence of the backbone electronic wavefunction on exciton transfer in aqueous inter-conjugated polyelectrolyte complexes

ACS Spring 2022

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