Researchers at Harvard, MIT and Arizona State University have designed a new synthetic structure that can harvest light energy in a way similar to naturally occurring photosynthetic bacteria.
The idea was to develop and excitonic circuit or material to direct absorbed energy,” said Gabriela Schlau-Cohen (MIT), a co-author on the paper and CIFAR Azrieli Global Scholar. “The goal of these types of systems is to get energy as far as possible as efficiently as possible.
The discovery, published in Nature Materials last month, aligns closely with the work being carried out in CIFAR’s Bio-inspired Solar Energy program, which seeks to uncover the nanoscale mechanisms photosynthetic organisms employ to harvest energy from the sun and create artificial devices that can do the same, with equal (near 100 per cent) quantum efficiency.
“This is a keystone paper related to our CIFAR BSE program,” said Alán Aspuru-Guzik (Harvard), a senior fellow in the Bio-inspired Solar Energy program whose research focuses on mimicking photosynthetic organisms by recreating these molecular structures and processes from the bottom up.
By organizing pigments on a DNA scaffold, researchers have designed a light-harvesting material that mimics many elements of bacterial light-harvesting structures. (courtesy of Gabriela Schlau-Cohen)
The synthetic circuit built by Aspuru-Guzik, Schlau-Cohen and their fellow authors organizes molecules to form a supramolecular structure where energy gets delocalized or shared across the entire structure.
“In natural systems, you see energy moving through two different types of mechanisms, a more incoherent or hopping type transport and a more coherent or wave like motion,” said Schlau-Cohen. “We were able to develop a synthetic system that can control the balance of these.”
“This is a keystone paper related to our CIFAR BSE program”
In nature, photosynthetic organisms use light-harvesting pigments and reaction centers to convert light energy from photons into chemical energy. In this process, the photons are captured by the pigments, energizing pigment electrons and turning them into energized versions of themselves called ‘excitons’. Excitons pass this energy from one pigment molecule to another until it reaches a reaction center where the light energy is converted into chemical energy.
Controlling the direction and flow of excitons in synthetic systems is something that scientists have struggled with until recently. In order for the pigments to actually absorb light and carry the excitons, they must be organized in a particular nanostructure.
To address this problem, senior author Mark Bathe (MIT) designed a synthetic DNA “origami” scaffold which allows the researchers to organize a synthetic pigment called pseudoisocyanine into structured clusters reminiscent of structures found in photosynthetic bacteria. Organizing the pigments in this way allowed the team to control the absorption of photons and transportation of excitons along a directed path.
Once the structure had been designed, Schlau-Cohen was part of the team that measured the system’s efficiency using advanced spectroscopy. Schlau-Cohen is also the lead author on a companion paper published in The Journal of Physical Chemistry Letters, which looks at how the efficiency of energy transport changed as a function of the distance traveled.
“When we think about mimicking natural systems an important parameter is getting efficient energy transport over long distances,” said Schlau-Cohen.
Building excitonics systems such as these from the bottom up has been a long-time goal for Aspuru-Guzik, who started the MIT-Harvard Center for Excitonics approximately 10 years ago. Because excitons behave differently in different materials, the centre works to create materials that allow excitons to travel faster and further.
While the team’s findings are not the first report of DNA-supported exciton transport, the research explores factors that haven’t been considered before, such as dye orientation, and uses computational methods to determine optimal structures and pathways for energy transfer.
Aspuru-Guzik warns that this technology is expensive and not at the point where it can be scaled. However, the experiment provides us with the knowledge of how to direct exciton movement.
“What we could learn is how to make a cheap [circuit] in the future that has the same type of bottom up self-assembly,” said Aspuru-Guzik.
The next steps for the researchers are to make more complicated nanostructures that look more like those of photosynthesis.
“This is a proof of principle but now we want to optimize it, we want better dyes, we want better DNA origami structures to improve the efficiency and that’s something that we can address theoretically and experimentally,” said Schlau-Cohen.