Materials that emit and manipulate light form the backbone of many modern technologies, from solar energy systems to advanced imaging and sensing devices. Yet, despite decades of research, certain fundamental behaviors in these materials have remained unresolved, continuing to puzzle scientists.
Researchers at Rice University have now addressed one such long-standing mystery in a widely used organic semiconductor, revealing an unexpected insight: tiny structural imperfections can actually enhance how these materials function rather than degrade them. The breakthrough was reported in the Journal of the American Chemical Society, where the team studied 9,10-bis(phenylethynyl)anthracene (BPEA), a model compound used to understand how light energy moves through organic materials.
For years, scientists had observed unusual optical behavior in BPEA, particularly the presence of two distinct absorption and emission signals that could not be explained by existing theories. “This was a long-standing puzzle in the field,” said Colette Sullivan, a doctoral student in Rice’s Department of Chemistry and co-author of the study. “Once we connected the experimental results with theory, it became clear the two signals were coming from completely different processes.”
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To uncover the underlying mechanism, the researchers combined spectroscopy experiments with advanced computational simulations. They discovered that the unusual light absorption arises from interactions between two excited-state systems: excitons, which transport energy through the material, and charge-transfer states, where electrons shift between neighboring molecules.
However, the most surprising discovery emerged when they examined how the material emits light. Instead of originating solely from an ideal, defect-free crystal structure, the lower-energy emission was traced to microscopic structural imperfections. These defects consist of small irregularities where molecules form X-shaped pairs, creating localized regions that trap and redirect energy within the material.
“These defects aren’t just imperfections; they actually create new pathways for energy flow, essentially turning apparent flaws into desirable features,” explained Lea Nienhaus, associate professor of chemistry and member of the Rice Advanced Materials Institute.
Further theoretical analysis led by postdoctoral researcher Jakub Sowa showed that these defect sites do not reduce performance as traditionally expected. Instead, they enhance a process known as triplet–triplet annihilation, which enables the material to convert lower-energy light into higher-energy light. At the same time, they suppress competing energy pathways that would normally reduce efficiency.
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As a result, the material demonstrates improved energy conversion and emission performance—despite the presence of structural disorder. This finding challenges a long-held assumption in materials science that defects are inherently harmful and should be minimized at all costs.
“Our work shows that material defects can actually improve performance, creating a target for materials engineering,” said Peter J. Rossky, the Harry C. and Olga K. Wiess Chair in Natural Sciences Emeritus at Rice University. “By understanding how molecular structure, disorder and electronic interactions work together, we can begin to design materials where these effects are not just tolerated but deliberately used to control how energy moves.”
This new understanding opens promising pathways for the design of next-generation materials. By carefully tuning how molecules pack together and controlling where defects form, researchers may be able to create more efficient systems for solar energy harvesting, optoelectronic devices, and light-based sensing technologies.
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The research was supported by the National Science Foundation, the Camille and Henry Dreyfus Foundation, and the Alfred P. Sloan Foundation, along with computational resources provided by Rice University’s Center for Research Computing.
