There
is substantial urgency to create artificial light-harvesting systems
that are relatively inexpensive and capable of absorbing a significant
fraction of the solar spectrum. Molecular materials possess a number
of attractive characteristics for this purpose, such as their light
weight, spectral tunability, and the potential to use self-assembly
to form large structures capable of executing multiple photophysical
processes required for photoelectric energy conversion. In this work,
we demonstrate that ionically assembled complexes composed of oppositely
charged conjugated polyelectrolytes (CPEs) that function as excitonic
donor/acceptor pairs possess 10 significant potential as artificial
energy transfer antennae. We find that, upon complexation in water,
excitation energy is transferred from the donor to the acceptor CPE
in less than 250 fsa timescale that is competitive with natural
light-harvesting antennae. We further find that the state of CPE chain
extension and thus spatial delocalization of the excited-state wavefunction
can be readily manipulated using the relative polyion charge ratio,
allowing us to tune the emission quantum yield of the CPE in a straight-forward
manner. Collectively, our results point toward the fact that the extension
of a CPE chain upon complexation is a cooperative phenomenon between
multiple chains even at dilute polymer concentrations.
Photosynthetic organisms have mastered the use of "soft" macromolecular assemblies for light absorption and concentration of electronic excitation energy. Nature's design centers on an optically inactive protein-based backbone that acts as a host matrix for an array of light-harvesting pigment molecules. The pigments are organized in space such that excited states can migrate between molecules, ultimately delivering the energy to the reaction center. Here we report our investigation of an artificial light-harvesting energy transfer antenna based on complexes of oppositely charged conjugated polyelectrolytes (CPEs). The conjugated backbone and the charged side chains of the CPE lead to an architecture that simultaneously functions as a structural scaffold and an electronic energy "highway". We find that the process of ionic complex formation leads to a remarkable change in the excitonic wavefunction of the energy acceptor, which manifests in a dramatic increase in the fluorescence quantum yield. We argue that the extended backbone of the donor CPE effectively templates a planarized acceptor polymer, leading to excited states that are highly delocalized along the polymer backbone.
The ability to manipulate
the state of assembly of light-harvesting
molecular materials can be important to control light-induced damage
at high illumination intensities. Disassembly of such materials is
thus an important consideration from a photoprotection perspective.
Here, we show that an artificial light-harvesting antenna based on
an interconjugated polyelectrolyte complex can be disassembled using
ionic surfactants in aqueous solution. We demonstrate that both anionic
and cationic surfactants can do so with comparable efficiency, thereby
shutting electronic energy transfer between the exciton donor and
acceptor conjugated polyelectrolytes. However, the structural evolution
of the aqueous complexes both above and below the critical micelle
concentration differed significantly depending on the sign of the
ionic surfactant and the relative polyion charge ratio.
There is great interest in developing inexpensive, molecular light-harvesting systems capable of efficiently converting photon energy to chemical potential energy. It is highly desirable to do so using self-assembly and...
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