Plants and other photosynthetic organisms use a wide variety of pigments to absorb different wavelengths of light. MIT researchers have now developed a theoretical model to predict the spectrum of light absorbed by aggregates of these pigments, based on their structure.
The new model could help guide scientists in designing new types of solar cells made of organic materials that efficiently capture light and funnel the light-induced excitation
Photosynthesis, performed by all plants and algae, as well as some types of bacteria, allows organisms to harness energy from sunlight to build sugars and starches. Key to this process is the capture of single photons of light by photosynthetic pigments, and the subsequent transfer of the excitation to the reaction centers, the starting point of chemical conversion.
Chlorophyll, which absorbs blue and red light, is the best-known example, but there are many more, such as carotenoids, which absorb blue and green light, as well as others specialized to capture the scarce light available deep in the ocean.
These pigments serve as building blocks that can be arranged in different ways to create structures known as light-harvesting complexes, or antennae, which absorb different wavelengths of light depending on the composition of the pigments and how they are assembled.
These antennae are embedded in or attached to membranes within cell structures called chloroplasts. When a pigment captures a photon of light, one of its electrons becomes excited to a higher energy level, and that excitation is passed to nearby pigments along a network that eventually leads to the reaction center. From that center, the available charge travels further through the photosynthetic machinery to eventually drive the transformation of carbon dioxide into sugar through a cycle of chemical reactions.
The MIT researchers wanted to explore how the organization of different pigments determines the optical and electrical properties of each antenna.
Their new model uses experimental measurements of the spectrum of light absorbed by different pigment molecules and their surrounding proteins. Using this information as input, the model can predict the spectrum of light absorbed by any aggregation, depending on the types of pigments it comprises. The model can also predict the rate of energy transfer between each aggregate.
The model provides, for the first time, a systematic link between the structure of antennae and their optical and electrical properties.
Scientists working on designing materials that absorb light, using quantum dots or other types of light-sensitive materials, could use this model to help predict what kinds of light will be absorbed and how energy will flow through the materials, according to the antenna structure
