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Contributed by Moungi G. Bawendi, May 30, 2014 (sent for review December 2, 2013)
Significance
The scientific community has been broadly inspired by tiny deep-sea bacteria, the green sulfur bacteria, that are able to harvest minute amounts of incoming sunlight with exquisite efficiency. Nature’s masterpiece consists of soft, cylindrical-shaped, supramolecular structures that are densely packed in superstructures. Only little is known about the fundamental processes that govern nature’s efficiency. Here we unravel, for the first time to our knowledge, the impact of structural complexity through the use of a model system akin to that found in nature, focusing on the properties that are prerequisite for nature’s efficient light harvesting. Our work suggests that the cylindrical geometry presents a rational design that may be key for protecting the system’s quantum properties upon dense packing.
Abstract
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials.
- supramolecular assembly
- self-assembled excitonic nanoscale systems
- photosynthesis
- exciton theory
- light-harvesting antennae systems
Footnotes
- ↵1To whom correspondence may be addressed. Email: Eisele{at}ccny.cuny.edu, Nicastro{at}brandeis.edu, J.Knoester{at}rug.nl, or MGB{at}mit.edu.
Author contributions: D.M.E. and M.G.B. developed and guided the project; D.M.E. prepared shock-frozen samples for cryo-EM/ET, prepared samples for nonlinear spectroscopy, and performed linear spectroscopy experiments; D.H.A. performed linear dichroism and nonlinear spectroscopy experiments and data analysis with input and support from D.M.E. and C.P.S. and guidance from A.T. and K.A.N.; X.F. recorded and processed cryo-EM/ET data; D.M.E., X.F., and D.N. analyzed cryo-EM/ET data; E.A.B. performed spectral simulations in collaboration with D.M.E. supervised by J.K.; R.A.J. built the fast-acquisition absorption spectrometer and performed flash-dilution measurements with D.M.E.; P.R. performed exciton dynamic simulations in collaboration with D.M.E and S.L.; H.E. provided fundamental contributions to the discussion on the structure analysis and simulated illustrations for the bundle structure; all authors provided fruitful discussions and interpretations of the data and analyses; J.K. provided essential interpretation of optical data, particularly in the context of exciton theory; and D.M.E., S.L., J.K., and M.G.B. wrote the paper, with input from all authors.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1408342111/-/DCSupplemental.
Freely available online through the PNAS open access option.
Robust excitons inhabit soft supramolecular tubes
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