Molecular photoswitches that can both convert and store energy could be used to make solar energy harvesting more efficient. A team of researchers has used a quantum computing method to find a particularly efficient molecular structure for this purpose. The procedure was based on a dataset of more than 400,000 molecules, which the researchers screened to find the optimum molecular structure for solar energy storage materials. An open-access paper on the study is published in Angewandte Chemie International Edition.

At present, solar energy is either used directly to generate electricity, or indirectly via the energy stored in heat reservoirs. A third route could involve first storing the energy from the sun in light-sensitive materials and then releasing it as needed.

The EU-backed project MOST (“Molecular Solar Thermal Energy Storage”) is exploring molecules such as photoswitches that can absorb and store solar energy at room temperature to create entirely emission-free utilization of solar energy a reality.

The MOST project aims to develop and demonstrate a zero-emission solar energy storage system based on benign, all-renewable materials. The MOST system is based on a molecular system that can capture solar energy at room temperature and store the energy for very long periods of time. This corresponds to a closed cycle of energy capture, storage and release. The MOST project will develop the molecular systems as well as associated catalysts and devices to beyond state-of-the-art performance and scale. Specifically, hybrid solar collectors utilizing up to 80% of incoming solar energy will be designed and tested together with heat release devices, that combine MOST with thermal energy storage (TES) enabling rapid temperature ramp-up cycles delivering large temperature gradients. Ilustration by Daniel Spacek/Neuron Collective

The research teams of Kurt V. Mikkelsen at the University of Copenhagen, (Denmark) and Kasper Moth–Poulsen at the Technical University of Catalonia, Barcelona (Spain), have taken a closer look at the photoswitches best suited for this task.

They studied molecules known as bicyclic dienes, which switch to a high-energy state when illuminated. The most prominent example of this bicyclic diene system is known as norbornadiene quadricyclane, but a vast number of similar candidates exist. The chemical space consists of approximately 466,000 bicyclic dienes that we have screened for their potential applicability in MOST technology, the researchers said.

Screening a database of this size is typically done by machine learning, but this requires large amounts of training data based on real-world experiments, which the team did not have. Using a previously developed algorithm and a novel evaluation score, “η” (eta), the screening and evaluation of the database molecules yielded a clear result: all six of the top scoring molecules differed from the original norbornadiene quadricyclane system at a crucial point in the structure.

The researchers concluded that this structural change, an expansion of the molecular bridge between the two carbon rings in the bicyclic part, allowed the new molecules to store more energy than the original norbornadiene.

The researchers’ work demonstrates the potential for optimizing solar energy storage molecules. However, the new molecules must first be synthesized and tested under real conditions.

Even though the systems can be synthetically prepared, there is no guarantee that they are soluble in relevant solvents and that they will actually photoswitch in high yield or at all, as we have assumed in η.

Despite this, the team developed a new, large set of training data for machine learning algorithms and have thus shortened the arduous research step prior to synthesis for chemists tackling such systems in the future. The authors envision this much larger repository of bicyclic dienes coming into its own for research into photoswitches for a variety of applications, potentially making it easier for molecules to be tailored to specific requirements.

Resources

• Andreas Erbs Hillers-Bendtsen, Jacob Lynge Elholm, Oscar Berlin Obel, Helen Hölzel, Kasper Moth-Poulsen, Kurt V. Mikkelsen (2023) “Searching the Chemical Space of Bicyclic Dienes for Molecular Solar Thermal Energy Storage Candidates” doi: 10.1002/anie.202309543