This Molecule Can Store Sunlight as Heat — Then Boil Water on Command

Drop 107 milligrams of a clear liquid into half a millilitre of water, add a splash of acid, and stand back. That is roughly what chemists at UC Santa Barbara and Brandeis University did in a new paper — and the water boiled in under a second. The liquid in question is a Dewar pyrimidone, a small molecule engineered to capture UV sunlight and lock that energy into its own strained chemical bonds, where it can sit for months or years before being released as heat on demand. The team's compound, detailed in a confidential submitted manuscript, achieves a gravimetric energy storage density of 1.65 MJ/kg — by a wide margin the highest ever recorded for a molecular solar thermal (MOST) energy storage system.

Heating Is the Dirty Secret of the Energy Transition

Electricity gets all the headlines. Solar panels, wind turbines, battery packs — the clean-energy conversation is almost entirely about electrons. But heating accounts for nearly half of global energy demand, and roughly two-thirds of it still comes from burning fossil fuels. Hot water, space heating, cooking: these are energy services that electricity can cover, but not always efficiently or cheaply, especially away from the grid. The MOST concept has been kicking around since at least the 1970s, when norbornadiene–quadricyclane systems first attracted serious attention. The basic idea is elegant: a molecule absorbs sunlight and rearranges into a higher-energy shape, holding that energy in strained chemical bonds. Add a catalyst later, and the molecule snaps back to its original form, releasing the stored energy as heat. No combustion, no emissions, and — in principle — the same material can be reused indefinitely.

Why Every Previous Solar Fuel Fell Short

The problem has always been density — or rather, the lack of it. Norbornadiene, the field's longtime workhorse, stores about 0.97 MJ/kg. That sounds reasonable until you compare it to heating oil, which delivers around 40 MJ/kg through combustion. Azaborinines, a newer class of MOST compounds, pushed molar energy densities higher but needed bulky chemical substituents to prevent side reactions, which dragged their gravimetric performance back down to around 0.65–0.99 MJ/kg. Nearly every competitive system also requires organic solvents, which cuts effective energy density further and rules out mixing with water. The field had essentially hit a ceiling — and no one had managed to demonstrably transfer enough stored heat to do anything as viscerally useful as boiling water.

How Does a DNA Lesion Become a Solar Battery?

Here is the part that is genuinely surprising. The inspiration for this molecule came not from energy research but from cancer biology. When UV light hits DNA, it can produce a defect called a (6-4) lesion — a distorted patch where two nucleobases fuse into a warped, high-energy structure called a Dewar isomer. The body has a dedicated enzyme, (6-4) photolyase, that reverses this damage. Grace Han's group at UC Santa Barbara looked at that chemistry and asked: what if you designed a molecule to do the same thing on purpose? The answer was 2-pyrimidone, a nitrogen-containing ring that, when irradiated at 300–310 nm, undergoes a dramatic rearrangement into a bicyclic Dewar isomer — fusing two highly strained four-membered rings and losing its aromaticity entirely in the process. That complete loss of aromaticity, combined with the strain in the fused ring structure and the presence of a high-energy carbon–nitrogen bond at the reactive site, is what drives the energy density so far above anything achieved before. Crucially, the team optimised the substituent pattern — landing on 1,4,6-trimethyl-2-pyrimidone as their champion compound — through both experiment and detailed quantum mechanical calculations. The material can be synthesised in one step from cheap, commercially available precursors, and the team has already made it on a multi-gram scale.

From the Lab Bench to a Boiling Beaker

Most MOST papers stop at calorimetry. This one didn't. The team dissolved their Dewar pyrimidone in water — something most competing systems cannot do — and added hydrochloric acid as a catalyst. The acid protonates a nitrogen atom in the bridgehead position of the strained molecule, cutting the activation barrier roughly in half and triggering spontaneous ring-opening at room temperature. What followed, captured by infrared thermal imaging, was a temperature spike of 76 °C in under one second: the water reached 100 °C and visibly boiled. For context, the best previously published macroscopic heat-release demonstration, using quadricyclane flowing over a cobalt catalyst in toluene, achieved a maximum temperature rise of 63 °C over 2.5 minutes, delivering 27 J to the medium. The pyrimidone transferred 153 J to the aqueous solution — faster, in a greener solvent, from a smaller mass of material. The team also ran the system through more than 20 charge–discharge cycles with negligible degradation, suggesting the material can be reused as intended.

The Questions That Still Need Answering

There are real gaps here, and the authors are upfront about them. The photoisomerization quantum yields are low — between 0.9% and 7.8% — meaning a lot of UV photons get wasted during the charging step. The compounds also absorb primarily in the UV-A/B range, which makes up only about 5% of the solar spectrum reaching Earth's surface. That is a significant practical constraint; most of the sun's energy simply passes through without doing anything. The acid catalyst used to trigger heat release is homogeneous, requiring a neutralization step afterward that produces salt waste and reduces the net energy density of the system. The team acknowledges that a heterogeneous catalyst — something the Dewar solution flows past in a fixed bed, rather than something dissolved into it — would be far more practical, and they name that as a priority for future work. The scale of the boiling demonstration is also modest: half a millilitre of water, 107 mg of compound. Getting from that to anything resembling a residential heating application involves engineering challenges that chemistry papers don't address.

• Record energy density — At 1.65 MJ/kg, this compound stores more than 70% more energy per gram than any previous MOST material, clearing a benchmark the field has chased for decades.
• Water compatibility matters — The ability to operate in aqueous solution rather than organic solvents is not just an environmental nicety; it is what allowed the team to demonstrate direct, usable heat delivery to a practical medium.
• Solar absorption is still the bottleneck — Until MOST molecules can efficiently harvest visible light — not just UV — the technology will depend on artificial light sources or concentration optics, which limits its off-grid appeal.

"These advances help point the way toward decentralized solar heat storage and off-grid energy solutions." — Nguyen, Han et al., Submitted Manuscript, 2026.