Harvesting a Wide Range of Light: Computational chemistry & organic synthesis create a new light-absorbing molecule

Matthew Kegley
By Matthew Kegley May 12, 2019 13:06

Harvesting a Wide Range of Light: Computational chemistry & organic synthesis create a new light-absorbing molecule

Nothing is perfect, but often, nature comes pretty close. Photosynthesis, the process that converts light into electrochemical energy, is something researchers want to emulate. It happens to be the single biochemical process that sustains life on Earth, not least because it has the role of converting carbon dioxide—animal waste—into food and usable oxygen. Studying how photosynthesis captures light allows engineers to improve renewable technologies like solar and hydrogen fuel cells. Consequently, artificial photosynthesis, which mimics this natural process with man-made materials, has long been a scientific goal.

One key component in artificial photosyn­thesis is the light-absorbing material. Re­searchers from the Brudvig and Batista labs at Yale recently synthesized a molecule, a panchromatic dyad, by combining com­putational and experimental approaches. “Panchromatic” refers to the ability to ab­sorb across the spectrum of visible light, while “dyad” means a dye composed of two molecular building blocks. The team’s suc­cess represents another step toward effi­cient artificial photosynthesis.

Absorbing light like a leaf

Yale researchers aimed to mimic a par­ticular phase of photosynthesis that har­vests light and converts it into useful en­ergy. For this phase, plants and bacteria use a protein complex called Photosystem II, which absorbs light and drives elec­tron transfer across a membrane, build­ing up an electrochemical potential that can be used to build molecules. Accord­ing to Shin Hee Lee, the lead researcher of the project, dyes work similarly to Pho­tosystem II. “When light hits the dyad, it excites an electron from the ground state to an excited state,” Lee said. That is to say, energy is transferred from light to the electrons in the molecules.

Light is the energy source in photosyn­thesis. It can be understood to exist as waves of electromagnetic radiation. One important property is its wavelength, the distance between successive wave peaks or troughs, which determines its color. Visible light is composed of light waves over a particular spectrum of these wave­lengths—anybody with a glass prism can demonstrate this. Red light has the lon­gest wavelength, orange the next longest, and so on, and violet has the shortest wavelength.

Every colored material absorbs light—a leaf appears green because it absorbs red, its complementary color. Researchers in artificial photosynthesis are interest­ed in the “absorption spectrum”, which shows how much a material absorbs light of each wavelength. For this project, the researchers wanted a material that cov­ered a wide range of wavelengths. “The goal was to develop a dye that not only absorbs light well, but also a wide range of light,” Lee said. The goal was therefore to study and synthesize a molecule with a wide span of absorption.

Playing LEGO with chromophores

The researchers relied on existing knowl­edge of chromophores to achieve their goal. Chromophores are light-absorbing mole­cules, like chlorophyll in plants. One import­ant chromophore is called porphyrin. This molecule has four carbon-nitrogen rings con­nected together to form an even larger ring. Porphyrins commonly exist in natural pig­ments, exhibiting deep colors since they ab­sorb visible light well. “[Porphyrin] is a very strong light absorber. It is what gives hemo­globin [in red blood cells] its color,” said Gary Brudvig, the Benjamin Silliman Professor of Chemistry and co-corresponding author of this project. The question, then, was how to extend the absorbing capabilities of a porphy­rin ring and make it panchromatic.

One typical approach would be linking the porphyrin with another chromophore mole­cule that absorbs light, forming a “dyad” that combines the strengths of both its compo­nents. In this study, however, the research­ers decided to try something different. “We looked into the possibility of linking a phenazine with a porphyrin,” Brudvig said. Phenazines, by contrast to porphyrins, absorb light rather weakly. Phenazines also have dif­ferent chemical structures—a phenazine unit comprises three adjacent carbon-nitrogen rings. When paired with a porphyrin, prelim­inary studies found that phenazines could en­hance the dyad’s light absorption.

Adam Matula, a researcher from the Batis­ta Lab, combined different porphyrins and phenazines in computational simulations to determine which would give the best ab­sorption spectrum. Each spectrum was based on the resulting dyad’s chemical and ener­getic properties. From computations, the most panchromatic dyad contained dibenzo­phenazine, which is phenazine with two extra six-membered carbon rings. With the dyad components selected by computation, the challenge was now to find the best link.

A contest for connection

The researchers knew that the chemical link between the porphyrin and dibenzo­phenazine would be just as important, if not more so, than the building blocks. Candi­dates included a direct link, an “amidyl link” involving a bond like that between ami­no acids in proteins, as well as an “ethynyl link”, which contains triply-bonded carbon atoms. To decide the winner, rather than making the molecules in the lab and test­ing them, the researchers turned again to computation. In particular, they used densi­ty functional theory (DFT)—a popular tool in computational chemistry—to predict op­tical absorption properties of the relevant molecules. These computations determined the extent of electron mixing, also known as conjugation, between dibenzophenazine and the porphyrin. In light-absorbing mole­cules, more conjugation results in better ab­sorption. DFT showed that the ethynyl link spreads electrons most effectively across the dyad, allowing for greater absorption of red wavelengths of light by the electrons.

Having determined ethynyl to be the opti­mal link, there were still two ways to connect the dyad components. One was connecting the two parts closest together on the ring system, giving the PoZ dyad, and the other further apart, giving the PmZ dyad. The key difference between the two isomers was that PmZ adopts a planar configuration while PoZ does not. The planarity of the dyad can affect how the electrons are mixed. Lee and Matula together determined that mixing oc­curs better in the planar PmZ. With the con­nectivity and planarity sorted out, there was one final consideration.

Ethynyl linkage effect

In PmZ and PoZ, the lowest energy con­formations generally involve some rotation about the ethynyl bond. Analogously, one can think about wringing out a wet towel but the towel is the dyad with the porphyrin and phenazine on opposite sides of the twist. This rotation had to be considered to maximize panchromaticity. Matula calculated with DFT that a rotation of ninety degrees about the link adversely affects the panchromatic qualities of the spectrum. A rotation of zero degrees, with the entire molecule lying flat, maintains the best conjugation, and therefore the widest absorption spectrum. “These two molecules strongly interact by electronic coupling. This coupling is maximized when the molecules are in the same plane,” Brudvig said.

However, because the molecule was synthe­sized at room temperature, the dyad was not restricted to a single rotation angle, and could adopt a variety of energy states. This means the rotation of the dyad could end up anywhere between zero and three-hundred and sixty de­grees, with the main contributor being the low energy conformations. Practically, this means that the measured absorption spectrum would be an average over all the possible rotation states, with various contributions by various angles. This is called a “Boltzmann average” and it is useful because it describes the mole­cule’s overall absorption spectrum without la­beling any particular angle.

Finally, the dyad’s absorption was com­pared against those of its individual compo­nents to assess if an improvement was ob­tained. The porphyrin component has a very high intensity over the blue/violet-wave­length region, and absorbs slightly lower in the orange region. The phenazine has a small absorption range over the violet region. Brudvig notes that typically the absorption of a molecule is the sum of its parts. How­ever, in this case, the combination of the two molecules created an entirely new spectrum with wide-ranging absorption. “That was ac­tually quite surprising and interesting, and may be one of the most novel aspects of the work,” Brudvig said. While other dyads ab­sorb light additively from their constituent parts, this dyad is novel in extending the range of absorption into both short-wave­length violet and long-wavelength red.

With the simulations in hand, they were off to the races— the actual synthesis of the molecules was straightforward. Lee per­formed the synthesis using a palladium/cop­per-catalyzed reaction known as Sonogashira coupling, chosen for its wide use and conve­nience over other methods.

A Stronger Link

The only connection that might be stronger than ethynyl between the two chromophores is that between the lab members among the Brudvig and Batista groups. “One of the most important aspects was the interplay between experiment and theory,” Brudvig said. This interaction was manifest at every step of the project. For instance, before organic synthesis began, Matula’s simulations of the dyad aided Lee in choosing the ethynyl linkage between dibenzophenazine and porphyrin and having the associated properties confirmed.

For Lee, this research project is part of a larger goal by researchers to mimic plants at what they do best—convert sunlight into usable energy. Commonly, artificial photo­synthesis projects apply dyads to solar cells, and work to maximize the solar cells’ per­formance and efficiency in related electro­chemical reactions. However, this project took on a more fundamental responsibil­ity—it sought to modify and enhance the properties of porphyrin molecules to har­vest most of the visible wavelength spec­trum, from red light to blue/violet light. Its success was enabled by the interplay be­tween computational and synthetic chemis­try across two Yale research labs, challeng­ing conventional trial-and-error methods to devise a new, useful material.

Matthew Kegley
By Matthew Kegley May 12, 2019 13:06
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