Structural Studies of Bioinspired Chromophore Aggregates: Exploring the Design Rules for Light-Harvesting Complexes
EMSL Project ID
48608
Abstract
The proposed work will reveal the design principles that determine the light-harvesting and energy transfer efficiency of supramolecular antenna complexes. Such complexes in Nature are optimized to collect solar energy and funnel it to the reaction center where photosynthesis takes place. Using a bio-inspired porphyrin aggregate with properties similar to those of natural antenna complexes, experiments at EMSL will reveal how environmental effects influence the link between structural and optical/electronic properties. In previous work, we have established the hierarchical nature of the tetra-sulfonatophenylporphyrin (TSPP) aggregate. We hypothesize that this aggregate is built up from cyclic N-mer subunits, with N - 16, reminiscent of the LH2 complex of purple photosynthetic bacteria. Water-mediated hydrogen bonds then drive cyclic N-mers to assemble into helical nanotubes, similar to chlorosomes in photosynthetic green bacteria. More recently, we have demonstrated the ability to tune the optical and structural properties of the TSPP aggregate through environmental perturbations such as salts and alcohols. This tunability presents us with a unique opportunity to probe and exploit the link between these properties. The proposed work will lead to better understanding of how the efficiencies of light-harvesting and energy transfer depend on details of the aggregate morphology and lead to improved efficiency of artificial photosynthesis. We pursue the following objectives: Objective 1. Reveal relationships between morphologies of porphyrin aggregates and environment conditions. Helium ion microscopy (HIM), high-resolution STM, and cryo-TEM will reveal the morphology of porphyrin aggregates as a function of environment. The higher resolution afforded by these techniques, compared to what can be achieved at WSU, will enable detailed understanding of aggregate internal structure and provide tight constraints for models for the internal structure.
Objective 2. Use kinetics to explore hierarchical assembly and the potential for control of aggregate morphology. We propose stopped-flow measurements of the optical spectra to resolve and arrest multiple stages of aggregate self-assembly. The evolution of optical properties during assembly will provide insight into the hierarchy of intermolecular couplings that control energy transfer and light harvesting. These measurements, unavailable at WSU, will allow us to time-resolve excitonic coupling during self-assembly.
Objective 3. Elucidate the connection between excitonic coupling and hierarchical aggregate structure. Temperature-dependent studies of aggregate fluorescence and resonance light scattering (RLS) at EMSL will reveal how excitonic coupling depends on hierarchical supramolecular structure. Fluorescence and RLS spectra are strongly influenced by the same excitonic couplings on which light-harvesting and energy transfer depend. EMSL facilities offer the ability to observe these couplings as a function of temperature thereby resolving intermolecular and inter-sub-unit interactions according to their strengths.
Objective 4. Formulate a model for the observed optical spectra which is consistent with structural data from imaging. In work to be done at WSU, the spectroscopic properties of environmentally-perturbed TSPP aggregates will be modeled using Frenkel exciton theory. Observed optical and light scattering spectra, including polarization, will be simulated based on a hypothesized internal structure constrained to reproduce images obtained under Objective 1 and spectra obtained under Objectives 2 and 3.
Project Details
Project type
Exploratory Research
Start Date
2014-11-11
End Date
2015-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Leishman CW, and JL McHale. 2015. "Light-Harvesting Properties and Morphology of Porphyrin Nanostructures Depend on Ionic Species Inducing Aggregation." Journal of Physical Chemistry C 119(50):28167–28181. doi:10.1021/acs.jpcc.5b08849
Leishman C W,McHale J L 2016. "Illuminating Excitonic Structure in Ion-Dependent Porphyrin Aggregates with Solution Phase and Single-Particle Resonance Raman Spectroscopy" Journal of Physical Chemistry C 120(23):12783–12795. 10.1021/acs.jpcc.6b00867
Leishman, C. W.; McHale, J. L. Light-Harvesting Properties and Morphology of Porphyrin Nanostructures Depend on Ionic Species Inducing Aggregation. The Journal of Physical Chemistry C 2015, 119, 28167-28181.