Photophysical Investigations Into Oppositely-Charged Conjugated Polyelectrolyte Complexes

Conjugated polyelectrolytes (CPE) offer intriguing possibilities as light-harvesting scaffold in artificial photosynthetic systems. They are water soluble, can be modified synthetically to change their optical properties, have facile exciton migration, have a tendency to self-assemble, are relatively low cost, and can be cast into high quality films. However, the structure – function relationship between a CPE’s conjugated backbone and its photophysics means that these compounds will necessarily see changes in their optical and electronic properties upon complexation with an oppositely charged partner. The question becomes whether those changes can be understood and controlled in an aqueous environment. This dissertation tackles that question by investigating a model donor-acceptor CPE complex. The principle tools in this work are steady-state optical absorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, time-resolved fluorescence anisotropy, and isothermal titration calorimetry. The major findings of this work as it relates to the donor-acceptor complex are as follows: thermodynamically allowed electronic energy transfer (EET) is observed upon complexation; complexation leads to emergent bright states in the donor when excited directly which are not existent outside of the complex; the donor is “unwound” from its native coiled state upon complexation, and the extent of that unwinding and straightening is dependent on presence of a molar excess of acceptor; EET proceeds on ~240 fs time-scale, comparable to natural photosynthesis; in contrast to non-conjugated polyelectrolytes, heat is at times-required to form CPE complexes; the degree to which heat is needed depends upon the extent of non-covalent intra-chain interactions within the complexing CPEs – the larger those interactions the more “protein – like” the CPE behaves