Synthesizing Molecular Aggregates for Solar Energy Applications

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No molecule stands alone—they want others, a minimum of in the case of with the ability to show helpful photophysical, digital, and chemical properties. When particular person molecules mix into an combination, or a fancy of two or extra molecules, they turn into far more than the sum of their particular person elements.

Photoactive molecular aggregates, although—complexes of two or extra chromophores, that are molecules that take up gentle at sure wavelengths, thereby displaying shade—go the place remoted molecules don’t. As a result of favorable interactions between molecules, these aggregates are of curiosity for biomedical, solar-energy-harvesting, and light-generating applied sciences. That’s as a result of—in pure photosynthesis and in bioinspired technological functions—photoactive aggregates are environment friendly at power switch, the transport of photo voltaic power from one place to a different. For example, in pure photosynthesis, essentially the most widespread power conversion system on our planet, aggregates effectively switch the power from the place the sunshine is absorbed to the place it’s transformed into fees for electrical energy or chemical substances for gasoline manufacturing.

Nationwide Renewable Vitality Laboratory (NREL) researchers have synthesized two new compounds and studied how the properties of the person molecules contribute to the—typically sudden—properties of the bigger aggregates. The staff synthesized tetracene diacid (Tc-DA) and a dimethyl ester analogue (Tc-DE) designed to forestall intermolecular hydrogen bonding whereas preserving Tc-DA’s core electronics. The outcomes are described in a Journal of the American Chemical Society paper, “Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs.”

“The goal of this fundamental study was to decipher which molecular properties dictate the eventual emergent properties of the collective ensemble where the whole is greater than the sum of the individual parts, similar to putting seemingly unrelated puzzle pieces together and an unexpected image emerges,” mentioned NREL’s Justin Johnson, senior scientist. “For molecular-based light harvesting architectures that aim to use unconventional mechanisms to more efficiently use the solar spectrum than typical solar cells, it’s the collective properties that determine efficiency.”

“Tc-DA was created to exploit intermolecular hydrogen-bonding interactions at semiconductor surfaces to well-ordered monolayers,” mentioned NREL’s Nicholas Pompetti, postdoctoral researcher. “However, we found that we could control the aggregation of Tc-DA as it approached the surface through solvent and concentration choices. This opened up insights about tetracene-based aggregates and how their size and structure provide promising pathways for their use in light-harvesting applications.”

In a given solvent atmosphere, sturdy intermolecular interactions direct steady and deterministic aggregation. Nonetheless, sturdy however uncontrolled interactions would possibly result in the formation of huge aggregates that will weaken solubility. Then again, weak interactions spur dissociation with molecules appearing as monomers. Happily for Tc-DA, the diploma of aggregation might be finely managed, starting from monomers to steady bigger order aggregates by altering focus or solvent system.

Tetracene and its derivatives are prime candidates for singlet fission (SF), a course of that will enhance photoconversion effectivity by lowering wasteful warmth manufacturing and depends on particular molecular tendencies that aggregates can obtain. Researchers used ¹H nuclear magnetic resonance (NMR) spectroscopy, computational modeling, and concentration-dependent optical conduct to research the possible combination construction of Tc-DA and Tc-DE. Regular-state spectroscopy evaluation allowed them to look at absorption conduct and emission profiles of the aggregates. Computational modeling utilizing density practical principle (carried out by Kori Smyser and Sandeep Sharma on the College of Colorado Boulder), together with the NMR outcomes, knowledgeable the researchers of the possible orientation of molecules inside an combination construction. Researchers then examined the impacts of aggregation on Tc-DA’s excited-state dynamics utilizing transient absorption spectroscopy.

“The excited-state dynamics were surprisingly sensitive to crossing a well-defined threshold of concentration, almost like going through a phase transition for a pure material,” Johnson mentioned.

As the dimensions and construction of the aggregates are necessary to gentle harvesting, researchers systematically diverse solvent polarity and focus in answer to investigate the well-defined tetracene aggregates and their behaviors, together with the possibly necessary singlet fission. The researchers discovered that noncovalent tetracene-based aggregates past a dimer have been stabilized at sure solvent polarities and concentrations, quickly forming cost switch and multiexcitonic states, that are fascinating species for delivering fees (typically a number of items) to an electrode or catalyst.

The mixture of NMR, computational research, and spectroscopic outcomes allowed the researchers to explain combination buildings not typically seen in solution-phase polyacenes.

“Controlling the landscape through molecular design and the associated solvent clearly allows us to dictate what the electrons do when they are photoexcited,” Johnson mentioned. “Nature uses hydrogen bonds in many types of aggregated architectures to tune energy landscapes in a similar fashion, like funneling water to a reservoir. Bringing such principles to artificial light-harvesting systems with the potential for controlling multiexcitons is a logical pursuit that is leading to interesting consequences.”

Study extra about Basic Energy Sciences at NREL and concerning the U.S. Department of Energy Office of Science Basic Energy Sciences program. Learn “Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs” within the Journal of the American Chemical Society.

By Justin Daugherty, NREL.