Disrupting Kasha’s Universal Rule – Pioneering Discoveries on an Enigmatic Blue Molecule

A team of scientists at IOCB Prague have uncovered the unique characteristics of azulene, a fundamental aromatic molecule, which could revolutionize organic chemistry and optimize the capture of light energy. The accompanying image artistically depicts the remarkable nature of azulene. Photo Credit: Tomáš Belloň / IOCB Prague

IOCB Prague’s researchers are the inaugural group to elucidate the reasons behind azulene’s intriguing behavior. This molecule has not only attracted attention for its blue color but also for its exceptional qualities.

The team’s latest work has the potential to significantly alter the bedrock of organic chemistry in future years, and practically, to optimize the utilization of absorbed light energy. The results of their research were recently published in the Journal of the American Chemical Society (JACS).

The intriguing nature of azulene has long been a subject of interest for chemists. The enigma of its blue color was resolved nearly five decades ago by globally renowned scientist Prof. Josef Michl, who had a close affiliation with IOCB Prague.

Dr. Tomáš Slanina now continues this research legacy, aiming to provide solutions to additional puzzles in the field. He and his colleagues have effectively demonstrated why azulene does not conform to the universally accepted Kasha’s rule.

Kasha’s rule outlines how molecules emit light when transitioning between various excited states. To draw an analogy, think of an ascending staircase where the first step, or the first excited state of the molecule, is elevated. Subsequent steps are increasingly lower, making them closer to the preceding one. The molecule tends to quickly descend from these steps to lower energy states, lingering longest on the initial step before emitting light. Azulene, however, deviates from this pattern.

To clarify azulene’s unique behavior, the IOCB Prague team employed the notion of (anti)aromaticity. In layman’s terms, aromatic substances are stable but not necessarily fragrant. Conversely, antiaromatic substances are unstable and seek to exit this state expeditiously. For azulene, the initial excited state is antiaromatic, leading it to rapidly descend within picoseconds, without time to emit light.

However, in the next stage, azulene behaves like a stable aromatic molecule. This is crucial because it can stay in this excited state for a full nanosecond, which is sufficiently long for it to emit light. Therefore, the energy in this excited state is not wasted but is fully converted into a high-energy photon.

Dr. Slanina’s research addresses current demands to ensure the effective use of energy from captured photons, such as solar energy. The objective is to engineer molecules that manage light energy as proficiently as possible. Their study also indicates that azulene’s properties can be appended to the structure of other aromatic molecules, enhancing their characteristics.

Tomáš Slanina commented, “The theories we have developed are straightforward enough to visualize, recall, and implement. We have successfully explained specific molecular behaviors using a simple concept.”

In their study, IOCB Prague researchers utilized specialized software to analyze how electrons in molecules behave in high excited states, a subject still largely unexplored. Their research, supported by experimental evidence, also benefitted from collaboration with Prof. Henrik Ottosson of Uppsala University in Sweden, a leading authority in the field of (anti)aromatic molecules.

It’s worth noting that the impact of azulene extends beyond photochemistry to medical applications. The laboratories of IOCB Prague have previously developed a chamomile oil-based ointment containing an azulene derivative. Over the years, this preparation, known as Dermazulen, has become a staple in first-aid kits nationwide.

Reference: “Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule” by David Dunlop, Lucie Ludvíková, Ambar Banerjee, Henrik Ottosson, and Tomáš Slanina, published on September 13, 2023, in the Journal of the American Chemical Society.
DOI: 10.1021/jacs.3c07625

Frequently Asked Questions (FAQs) about azulene research

More about azulene research

• Journal of the American Chemical Society
• IOCB Prague Official Website
• Introduction to Aromaticity and Antiaromaticity
• Understanding Kasha’s Rule
• Optimizing Light Energy Capture
• Basics of Organic Chemistry
• Solar Energy and Photovoltaics
• Uppsala University Chemistry Department