{"id":677534,"date":"2026-07-06T23:29:19","date_gmt":"2026-07-06T23:29:19","guid":{"rendered":"https:\/\/www.newsbeep.com\/uk\/677534\/"},"modified":"2026-07-06T23:29:19","modified_gmt":"2026-07-06T23:29:19","slug":"how-a-chance-discovery-overturned-one-of-photochemistrys-oldest-dogmas-research","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/uk\/677534\/","title":{"rendered":"How a chance discovery overturned one of photochemistry\u2019s oldest dogmas | Research"},"content":{"rendered":"

Ten years ago, a team of researchers made a chance discovery that would overturn one of the most fundamental rules in photochemistry: that light-activated reactions work best when the wavelength used matches a photoactive molecule\u2019s absorption maximum.<\/p>\n

The absorption maximum is essentially a photoactive molecule\u2019s favourite wavelength of light. Here, it can \u2018capture\u2019 the biggest number of photons and, as the logic goes, therefore do the most amount of work. \u2018It\u2019s ingrained belief, no one\u2019s questioned it,\u2019 says photophysicist Sarah Walden<\/a> from Griffith University in Australia.<\/p>\n

At least not until a team of chemists and physicists \u2013 including Walden and headed by Christopher Barner-Kowollik<\/a> \u2013 uncovered that this rule didn\u2019t hold true. Many reactions, including well-known ones such as [2+2] photocycloadditions and photopolymerisations, work better when irradiated with longer, red-shifted wavelengths.\u00a0<\/p>\n

The discovery could have profound implications for photochemistry, and open doors to applications in tissue engineering and 3D printing. \u2018We can control material properties, we can potentially use molecules in biological systems that we didn\u2019t think were possible, we can save lots of energy,\u2019 Walden says. \u2018It\u2019s really exciting.\u2019\u00a0<\/p>\n

But convincing chemists to take the results seriously and let go of a dogma they\u2019ve clung onto for nearly 200 years took the better part of a decade. \u2018We still, to this day, get criticised that we did something wrong,\u2019 Walden says. The team\u2019s journey involved buying eye-wateringly expensive equipment, doing cross-continent replication experiments \u2013 and, crucially, chemists and physicists working together. \u2018None of this would have been possible without this collaboration,\u2019 says physicist Joshua Carroll<\/a> who\u2019s part of Barner-Kowollik\u2019s group at Queensland University of Technology (QUT) in Australia.\u00a0<\/p>\n

Now the tide seems to finally be turning: Barner-Kowollik and his colleagues have to counter fewer incredulous and dismissive comments at conferences. Other researchers have started to uncover the same absorptivity\u2013reactivity mismatch in their own experiments. And years of chipping away at the phenomenon\u2019s mechanism has finally revealed that molecules\u2019 solvent \u2018habitats\u2019 might be behind it.\u00a0<\/p>\n

An unexpected result\u00a0<\/p>\n

In 2016, Barner-Kowollik\u2019s teams at QUT and at Germany\u2019s Karlsruhe Institute of Technology, together with Georg Gescheidt<\/a>\u2019s group at the Graz University of Technology in Austria, started exploring the mechanism behind some well-known photoinitiators, molecules that start polymerisation reactions when exposed to light. Their reactions quickly turn liquid monomers into solid polymers, which makes them popular for a huge range of uses from dental fillings to protective coatings.<\/p>\n

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We still, to this day, get criticised that we did something wrong<\/p>\n

<\/p>\n

The oxime ester photoinitiators the team was working with are colourless \u2013 indicating that their absorption maxima are in the ultraviolet range. That\u2019s why chemists tend to use UV light to kick off their reactivity. At longer wavelengths, the compounds\u2019 ability to absorb light drops off dramatically, reaching essentially zero for light in the visible range.<\/p>\n

Nevertheless, one day, David Fast<\/a>, a member of Gescheidt\u2019s team, decided to try one of these photopolymerisation reactions with visible light instead. He didn\u2019t expect it to work. Yet it did \u2013 in fact, better than with UV light. In one case, visible light pushed the conversion to nearly 10%, compared with less than 3% at the photoinitiator\u2019s UV absorption maximum.\u00a0<\/p>\n

\u2018When [Fast] came to me with these results I was, frankly speaking, not really confident about them at all,\u2019 Gescheidt recalls. \u2018The first reaction that you have as a supervisor to this is to say: \u201cWell, maybe something\u2019s gone wrong, maybe you didn\u2019t use the right compound, maybe you used the wrong LED,\u201d\u2019 adds Barner-Kowollik.\u00a0<\/p>\n

Full screen in popup<\/p>\n

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Photochemistry is the branch of chemistry that studies chemical reactions and processes that are caused by the absorption of light<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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For nearly 200 years, chemists assumed that using the wavelength where a molecule absorbs most strongly is the best for driving a reaction. As a result, reactions are typically performed at the absorption maximum (\u03bbmax)<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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A team of researchers around Christopher Barner-Kowollik and Georg Gescheidt studying polymerisations using UV light to activate radical photoinitiators found the reaction actually progresses better at a longer, visible-light wavelength<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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That was an unexpected result. Was it experimental error? Or was there something else going on?<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

\u2018My first thoughts were that we were overlooking something, something really simple that we were not thinking of,\u2019 Fast says. The initial assumption was that longer-wavelength light simply travels deeper into the reaction solution. But calculations showed that the reaction vials were simply too small for this light-penetration effect to make any difference.\u00a0<\/p>\n

Checking for errors\u00a0<\/p>\n

What followed were many months of tests to rule out any other sources of error. \u2018We questioned everything, a lot,\u2019 Gescheidt explains. \u2018This was not a straightforward, simple procedure; it was almost painful. It\u2019s admirable how David and Andrea [Lauer, who headed the experimental work with Fast] coped with that because they kept going and they didn\u2019t get frustrated.\u2019\u00a0<\/p>\n

\u2018We looked at whether different polymer types were formed at different wavelengths,\u2019 Barner-Kowollik says. \u2018Or maybe the radical decomposes in a different way and makes [the reaction] more efficient. All that turned out to be a negative.\u2019 To make sure it wasn\u2019t just something about the equipment that caused the effect, they even ran their experiments in parallel at the team\u2019s labs in Germany and Australia.\u00a0<\/p>\n

Eventually, they decided to splurge over \u20ac100,000 (\u00a386,500) on a laser that allowed them to run their reactions with a precise number of photons of an exact wavelength, to track how conversion changed in minute detail.\u00a0\u2018We spent a lot of money on [this laser] \u2013 with sweaty hands, because we still didn\u2019t know whether [the effect] was going to be confirmed on small molecule systems as well,\u2019 Barner-Kowollik recalls. \u2018Obviously, it was a risk \u2013 but no risk, no fun,\u2019 Gescheidt laughs.\u00a0<\/p>\n

Eventually, \u2018the mass of evidence became overwhelming,\u2019 says Walden, who was part of Barner-Kowollik\u2019s QUT team at the time. Although what was happening at the molecular level remained a mystery, in 2017, the team decided to get their results published.\u00a0\u2018I remember it was quite an experience going through the [first paper\u2019s1] reviewing process,\u2019 Barner-Kowollik says. \u2018We convinced [the journal] Macromolecules and the reviewers with the quality of the data. We asked: \u201cTell us what\u2019s wrong with the data?\u201d And they couldn\u2019t. So it got published without an explanation.\u2019\u00a0<\/p>\n

But one does not simply tell the chemistry community that everything they know about absorption maxima and reactivity is wrong. At conferences, Barner-Kowollik got a lot of pushback: \u2018Challenging questions were asked like: \u201cAre you doing the physics of the lasing right?\u201d\u2019 In 2022, the team got invited by a high-ranking journal to write an essay about their findings. \u2018We wrote it and the reviewers trashed us: \u201cThis is nonsense, this can\u2019t be true, they must have made a mistake,\u201d\u2019 remembers Walden.\u00a0<\/p>\n

Over the next few years, the team found that the absorptivity\u2013reactivity mismatch went far beyond the photo-initiated polymerisations in which Fast had first observed the effect. One of the most common organic chemistry photoreactions, the [2+2] photocycloaddition,2 also was a rule-breaker with wavelengths longer than the absorption maximum giving better yields.<\/p>\n

Nevertheless, the fact that the results only ever came from their team gnawed at them. \u2018Christopher [Barner-Kowollik] would call me right before plenary talks and he\u2019d be like: \u201cDid we make a mistake, is there any way this could be wrong?\u201d\u2019 Walden recalls.<\/p>\n

Independent corroboration\u00a0<\/p>\n

In 2021, Richmond Sarpong<\/a> and his team at the University of California, Berkeley, US, published their groundbreaking skeletal editing method, which squeezes out nitrogen from a ring with blue light<\/a>.3 \u2018When we looked at a UV\/Vis spectrum, where we got the maximum absorbance didn\u2019t match with where we saw the best reactivity,\u2019 Sarpong recalls. \u2018At the time, we didn\u2019t understand it. We explained it away.\u2019<\/p>\n

A year later, Sarpong\u2019s colleague Sojung Kim<\/a>, who had been involved in the skeletal editing project, stumbled upon one of Barner-Kowollik\u2019s papers. \u2018We felt vindicated and validated,\u2019 Sarpong recalls. \u2018Although we were kind of puzzled why a lot of the community did not seem to talk about this.\u2019 Kim, Sarpong and their collaborators ended up publishing a detailed investigation of their ring-contraction\u2019s strange photochemical behaviour.4<\/p>\n

Walden recalls how the entire team breathed a sigh of relief when they saw the first independent reports of the absorptivity\u2013reactivity mismatch: \u2018We all kind of collectively went \u201cYes!\u201d because it wasn\u2019t just us anymore. Now there\u2019s about 20 other examples, and it\u2019s encouraging.\u2019<\/p>\n

Full screen in popup<\/p>\n

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They perform months of tests to rule out measurement errors, including cross-lab validation in Germany, Austria and Australia<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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The team buys an expensive laser to have precise control over wavelength and photon number<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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Despite lacking a clear explanation, the team publishes its findings<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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The results are widely discussed and questioned at conferences<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

What was still missing was an explanation of why the mismatch happens. The team entertained many possibilities \u2013 and discarded them one by one when experiments turned up nothing. \u2018There wasn\u2019t a smoking-gun explanation,\u2019 Sarpong says.<\/p>\n

Wavelength dependence eliminated\u00a0<\/p>\n

In 2023, \u2018Josh [Carroll] came into my office and said: \u201cI think we should consider adapting the red-edge effect from fluorescence spectroscopy to photochemical reactivity,\u201d\u2019 Barner-Kowollik says.<\/p>\n

Carroll had come across a decades-old review paper about the red-edge effect,5 which describes how fluorescent molecules seem to break spectroscopic rules when they are embedded in viscous solvents. Molecules in viscous solvent are so slow to rearrange themselves around the fluorophore once it absorbs a photon that each fluorophore\u2019s environment is slightly different. This creates an apparently rule-breaking effect.<\/p>\n

The team suspected that something similar might happen in their [2+2] photocycloaddition. Their breakthrough came when they decided to tether together the reacting molecules \u2013 essentially making them into one big molecule whose ends react with one another. \u2018This completely eliminated the wavelength dependence,\u2019 says Fred Pashley-Johnson<\/a>, who was part of Barner-Kowollik\u2019s QUT team.\u00a0<\/p>\n

The solvent molecules seem to create \u2018microenvironments\u2019 that are different for each individual molecule, similar to the red-edge effect.6 \u2018Every molecule sits in its very own private environment,\u2019 Barner-Kowollik explains. \u2018That leads \u2013 ultimately \u2013 to longer excited-state lifetime at longer wavelengths. And if the molecules are excited longer, it\u2019s easier for them to meet a reaction partner and actually give you the product.\u2019\u00a0<\/p>\n

Some think microenvironments aren\u2019t the whole story. \u2018Certainly, there is still space for development,\u2019 Gescheidt says. \u2018It motivated me to think about additional experiments you could do and I will certainly talk about this with Christopher.\u2019\u00a0<\/p>\n

From pushback to possibility<\/p>\n

With more examples coming from different teams and the first mechanistic explanation out in the world, the chemistry community seems to be slowly warming up to the absorptivity\u2013reactivity mismatch as a general phenomenon. Questions at conferences have started turning from confrontational to excited, Walden says: \u2018It became more \u201cThat\u2019s really cool, we should check our own molecules.\u201d And: \u201cHow can we use this?\u201d\u2019\u00a0<\/p>\n

Being able to use longer-wavelengths of light than was previously thought to be possible could save a lot of energy, particularly in large-scale, industrial settings, Walden says. And there are also some exciting possibilities for photochemistry in biological systems, she adds. Many molecules that were assumed to only be active in the UV region can actually react with blue light. This makes them much more attractive for tissue engineering and photodynamic therapies, where UV rays can do lots of unintentional damage.<\/p>\n

\u2018There\u2019s also the materials design side of things where we can control different material properties using different colours of light,\u2019 Walden says. Pashley-Johnson is imagining using the effect for volumetric printing, a type of 3D printing that creates an entire object at once inside a vat of photosensitive resin.\u00a0<\/p>\n

\u2018I think this really invites all of us to go back and revisit some of these powerful but low-yielding photochemical transformations that were reported in the 1950s, 60s and 70s,\u2019 Sarpong says. \u2018I think there\u2019s an opportunity to hone in on [individual] photoreactive groups like carbonyl groups in complex molecules,\u2019 which often have multiple of the same functional group.<\/p>\n

Full screen in popup<\/p>\n

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As photopolymerisations are complex, the team decide to investigate the effect in small-molecule systems. They observe the effect in a wide array of other reactions, including in [2+2] photocycloadditions<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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Other researchers, including those in Richmond Sarpong\u2019s group working on skeletal editing, observe similar mismatches but initially dismiss them<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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Sarpong\u2019s team relates its findings to the initial papers by Barner-Kowollik\u2019s team and realise they\u2019ve noticed the same effect<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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Meanwhile, more and more independent examples start to emerge from groups around the world<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

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Barner-Kowollik\u2019s team publishes their microenvironment theory on [2+2] cycloadditions, and proposes that the observed effects are likely to be global across a range of other molecules and reaction types where excited-state lifetimes are important for the success of the reaction.
\nTop: Sarpong\u2019s team relates its findings to the initial papers by Barner-Kowollik\u2019s team and realise they\u2019ve noticed the same effect
\nMiddle: Interactions in solution cause a distribution of different molecular microenvironments, each absorbing light at different energies
\nBottom: Some molecular microenvironmenets can be selectively excited with lower energy light. These molecules can have longer excited lifetimes, giving them greater photochemical reactivity than others<\/p>\n

Source: \u00a9 Son of Alan\/Folio Art<\/p>\n

When thinking about what the discovery has taught him, it\u2019s that \u2018there\u2019s value in looking at things that have already been done,\u2019 Barner-Kowollik says. \u2018You never know what you might find if you apply a different lens.\u2019\u00a0<\/p>\n

And none of this would have been possible without scientists from different fields learning to work together . \u2018We always talk about wanting to do interdisciplinary research, but that doesn\u2019t happen overnight,\u2019 Walden says. \u2018It\u2019s something that requires a lot of patience and trust and just spending a lot of time in the room together having those open discussions.\u2019<\/p>\n

\u2018Always be open to looking into something that isn\u2019t part of your field,\u2019 concludes Carroll. \u2018You might be surprised and it might be really exciting.\u2019<\/p>\n","protected":false},"excerpt":{"rendered":"Ten years ago, a team of researchers made a chance discovery that would overturn one of the most…\n","protected":false},"author":2,"featured_media":677535,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[59,90,56,54,55],"class_list":["post-677534","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-gb","tag-science","tag-uk","tag-united-kingdom","tag-unitedkingdom"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/677534","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/comments?post=677534"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/677534\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media\/677535"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media?parent=677534"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/categories?post=677534"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/tags?post=677534"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}