This Molecule Violates Chemistry. And It's Going to Help Us Harness Limitless Energy.
One of the major engineering challenges of the climate change era is finding ways to make solar panels ever more efficient.
A new study from the Institute of Organic Chemistry and Biochemistry (IOCB) Prague has explored the strange properties of azulene, a blue light-emitting molecule that flouts one of the fundamental rules of photochemistry.
Understanding how molecules like azulene convert energy through fluoresence could help build molecules that are more efficient at converting the Sun’s photons into usable electricity.
When Charles Fritts built the first solar cell in 1883 by coating the mineral selenium with gold, it achieved less than one percent efficiency. That means that out of all of the photons hitting Fritts’ world-changing invention, only a tiny fraction were actually being converted into electricity. Thankfully, times have changed in the past 140 years, and the world’s current most efficient solar cell—constructed by the U.S. National Renewable Energy Laboratory in 2020—converts a stellar 47.1 percent of the Sun’s energy.
But those huge efficiency gains just don’t just magically happen. They require years of intense engineering and leveraging the very latest understanding of photochemistry—the branch of science concerned with the chemical effects of light. The more we understand light and its various excited states, the more scientists and engineers can tick that efficiency percentage higher and higher. And that means solving some of photochemistry’s most persistent mysteries.
One of those mysteries is a fundamental aromatic molecule known as azulene. This molecule was a mystery for decades because scientists couldn’t even figure out why it was blue (hence azul). Thankfully, that puzzle was solved in the 1970s, but that wasn’t the end of the molecule’s mysterious story.
That’s because azulene also runs counter to the photochemical idea known as Kasha’s rule, named after American molecular spectroscopist Michael Kasha. Kasha’s rule explains how molecules emit light in various excited states.
Now, scientists from the Institute of Organic Chemistry and Biochemistry (IOCB) Prague have finally figured out why this azulene violates this well-established law of chemistry. The results were published in the journal American Chemical Society.
“It’s based on the aromaticity and the antiaromaticity of that molecule in different excited states,” lead author Tomáš Slanina said in a press statement. “We can think of aromaticity as a kind of internal stabilization of that molecule. When that molecule is aromatic, it’s happy, it’s stable. When it’s antiaromatic, it’s trying its best to escape that state somehow.”
In the case of azulene, the molecule is aromatic—or stable—in its ground state, but is antiaromatic in its first excited state. Because azulene will do anything it can to escape this “unhappy” state, it drops down to a second level excited state within a few picoseconds (a picosecond is 0.000000000001 seconds). This is so fast that the molecule doesn’t even have time to emit light.
However, azulene is aromatic in this second level excited state for a full nanosecond—which, by our measure, is still an incredibly short amount of time. But it’s more than enough time for the molecule to emit the energy of this excited state as a high-energy photon. In other words, it emits light.
“We’re looking at molecules that can absorb and store light energy and they don’t waste that energy, they can manage it well,” Slanina says in a press statement. “So they can pass it on as photons of fluorescence or … transfer the energy to another molecule.”
A neat trick—and one that’ll come in handy as we can continue to devise ways to harness all of the Sun’s free and practically limitless energy.
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