When Clare Mahon decided to do her laundry one day in August 2022, she had no idea it would lead to the discovery of a vibrant reaction between two everyday consumer items.
“I had a white T-shirt that bore a somewhat greasy stain, and after washing it several times, I couldn’t eliminate this blemish,” she recalls. “Since it was a white T-shirt, I hung it on my washing line and sprayed it with bleach, and it instantly turned bright red!”
This unusual accident was quite a serendipitous event for an organic chemist, especially one heading an industrial collaboration studying the science of persistent fabric stains. “I snapped a picture of my T-shirt and sent it to my colleague Andrew Beeby, who specializes in spectroscopy, saying, ‘This is super weird,’” Mahon explains.
A little digging online revealed that this wasn’t an isolated incident, as Mahon discovered users on the Mumsnet forum expressing frustration over similar vivid stains. The anecdotal findings suggested that sunscreen was reacting with household bleach, but she needed to identify the specific chemical responsible.
“We purchased a variety of sunscreens and started conducting bleach tests with them,” she shares. “The students became really engaged because it presented a fascinating challenge for them to figure out what was happening.”
Mahon and her team at Durham University, UK, evaluated 11 commercial sunscreens. Seven of these turned red, all of which contained a shared ingredient known as diethylaminohydroxybenzoyl hexyl benzoate (DHHB). Among the four sunscreens that didn’t change color, only one had DHHB.
DHHB acts as an organic ultraviolet (UV) filter; it absorbs harmful sunlight and protects the skin. Organic sunscreens have become more popular compared to older mineral-based options, which can ruin the aesthetic with a pasty white residue. However, some organic compounds are susceptible to chlorination, which a study had indeed identified for DHHB back in 2013.
This earlier research found that one of the aromatic rings in DHHB experienced double substitution by chlorine, leading to the substituents adopting distinct positions on the ring, meta to each other. Despite this, the absorbance profile of the resulting molecule predominantly lies in the UV spectrum, resulting in minimal color manifestation. Mahon and her team hypothesized that chlorination could yield additional compounds, a suspicion swiftly validated through nuclear magnetic resonance (NMR) experiments.
“We immediately recognized from the proton NMR that it wasn’t the meta-dichloro compound since we had lost the aromaticity,” Mahon states. “[That] was when we involved our computational colleagues to assist in determining what it could be.”
Pooling their expertise, the team discovered an uncommon ipso-dichlorination of DHHB, where the two chlorine atoms occupy the same position on the ring. Simulations indicated strong absorption of short- and mid-wavelength visible light, allowing only long wavelengths to produce the striking red color they observed. Mahon points out that mass spectrometry cannot distinguish between the meta- and ipso-dichlorinated versions, which may explain why the latter had gone undetected until now.
It appeared they had identified the source, but to further substantiate their findings and validate their mechanism, the researchers modified DHHB to prevent ipso-dichlorination. Crucially, recognizing that the reaction necessitated the formation of a ketone, they replaced the hydroxyl group on the aromatic ring with an ethoxy group. The modified DHHB was indeed resistant, and its UV absorbance profile was nearly identical to that of the original, raising the intriguing potential for it to be utilized as a stain-resistant UV filter. Nevertheless, significant additional research would be required to assess its applicability and safety in sunscreen formulations.
Vasilios Stavros, who studies sunscreen photochemistry at the University of Birmingham, UK, expressed surprise at the findings. “From my experience with UV filters, this is one of the rare instances I’ve encountered that interact with bleach and result in a red stain… The more you investigate these filters, the more side effects you uncover.”
The positive aspect is that the ipso form is merely metastable and will convert to the meta form over time, taking the concerning red stains with it. Mahon recommends employing what Mumsnet contributors refer to as “the sunshine trick” to accelerate the fading process. Fortunately, she reveals that her T-shirt has recovered, although she hasn’t worn it since: “It came to the lab with me and ultimately became ‘Specimen A.’”