The air we breathe is not always as it seems. Within its familiar molecules lies a hidden, reactive counterpart that can power life—or destroy it.
Think of oxygen as an element with a split personality. On one hand, there's the stable, life-sustaining oxygen we breathe daily. On the other, there's its volatile, highly reactive twin—singlet oxygen—capable of damaging cells and degrading batteries with equal efficiency. For over half a century, scientists have grappled with a fundamental question: what causes this "evil twin" to appear when oxygen molecules are formed in chemical reactions?
This isn't just a story for chemists—it affects everything from how our cells avoid damage to how we might build better batteries for a sustainable future. The solution emerged not from discarding old theories, but from applying a classic concept from the 1960s to a mystery that had persisted even when hundreds of scientists gathered at the Conference on Singlet Molecular Oxygen (COSMO 84) in Clearwater Beach, Florida 3 5 .
In January 1984, the scientific community converged on Clearwater Beach, Florida, for a landmark event: the Conference on Singlet Molecular Oxygen (COSMO 84). This interdisciplinary gathering brought together researchers from photochemistry, photobiology, photomedicine, laser physics, and atmospheric science, all united by a common theme—understanding the properties and behavior of singlet molecular oxygen 3 .
Date: January 4-7, 1984
Location: Clearwater Beach, Florida
Focus: Singlet Molecular Oxygen
Disciplines: Photochemistry, Photobiology, Photomedicine, Laser Physics, Atmospheric Science
The conference was prompted by several recent developments, including the direct detection of singlet oxygen through its characteristic emission at 1.27 microns, which allowed scientists to measure its lifetime and reactions in different environments 3 . This technical advancement opened new avenues for studying this elusive molecule.
The conference proceedings noted enthusiastic participation throughout the event, with attendance on the final morning matching that of the opening session—a rare occurrence in scientific meetings that signaled the vibrant interest in the topic 3 .
Among the presentations was work on the detection of singlet oxygen emission in enzymatic processes, foreshadowing later discoveries about its role in biological systems 6 . The questions raised at COSMO 84 would continue to resonate through laboratories for decades, setting the stage for a mystery that would take nearly forty years to solve completely.
To understand the significance of this discovery, we must first grasp oxygen's fundamental duality. Ordinary oxygen gas (O₂) exists in what chemists call the "triplet" state (³O₂). In this stable form, the molecule's two most energetic electrons occupy separate orbitals and spin in the same direction 2 . This configuration makes triplet oxygen relatively unreactive—it's the version that sustains our lives through respiration without immediately damaging our tissues.
Ground State
Excited State
Singlet oxygen (¹O₂), however, is oxygen's excited, highly reactive counterpart. Here, the two outer electrons share the same orbital and spin in opposite directions 2 . This leaves an empty orbital desperately eager to snatch electrons from any nearby molecule—whether in cellular components or battery materials.
"The way these electrons are distributed makes all the difference. In singlet oxygen, the empty orbital is very eager to snatch additional electrons from any organic molecule that crosses its path." — Professor Stefan Freunberger 2
What makes singlet oxygen particularly important is its role as a Reactive Oxygen Species (ROS). In biological systems, ROS can cause oxidative stress and damage to proteins, lipids, and DNA . Yet, at the same time, our cells have learned to harness these reactive molecules for signaling purposes, regulating processes from inflammation to cell growth 2 .
The central mystery that persisted long after COSMO 84 concerned the origins of singlet oxygen. Scientists knew that singlet oxygen often forms from superoxide (O₂⁻), a reactive oxygen species that's the closest chemical state to molecular oxygen 1 2 . Superoxide plays a pivotal role in oxygen chemistry, involved in everything from cellular respiration to battery operation.
In particular, researchers were focused on a process called superoxide disproportionation (also known as dismutation), where two superoxide molecules react together—one gets reduced to peroxide while the other gets oxidized to form either triplet or singlet oxygen 1 2 .
But what determined whether this handshake produced the "good" triplet oxygen or the "bad" singlet oxygen? For decades, this question sparked what Freunberger describes as "a fierce debate about whether or not it [singlet oxygen] forms in the environment of cells" 4 .
The answer had profound implications—if scientists could understand what controls this process, they might learn to control it, potentially preventing damage in biological systems and energy technologies.
"If two superoxide molecules 'shake hands,' one gets reduced to peroxide and the other one gets oxidized to oxygen." — Soumyadip Mondal, Ph.D. 2
| Species | Chemical Symbol | Description | Reactivity |
|---|---|---|---|
| Triplet Oxygen | ³O₂ | Ground state molecular oxygen | Relatively unreactive |
| Singlet Oxygen | ¹O₂ | Electronically excited state | Highly reactive |
| Superoxide | O₂⁻ | Anion radical, one unpaired electron | Reactive oxygen species |
| Peroxide | O₂²⁻ | Fully reduced oxygen form | Moderate reactivity |
The solution arrived not through discarding old theories, but by applying a well-established concept from the 1960s—Marcus theory—to this persistent problem. Developed by Rudolph Marcus, who would receive the Nobel Prize in Chemistry in 1992, this theory describes how electron transfer reactions occur 1 .
Increasing driving force speeds up reaction rate
Maximum reaction rate achieved
Further driving force slows reaction rate
At the heart of Marcus theory is a counterintuitive prediction: increasing the driving force behind a reaction (its "push") initially speeds it up, but only up to a point. Beyond that point, further increasing the driving force actually slows the reaction down—a phenomenon known as the "Marcus inverted region" 1 7 .
The research team realized that the formation of triplet and singlet oxygen from superoxide might follow separate Marcus-type trajectories. They proposed that as the driving force for superoxide oxidation increases, the production of triplet oxygen would initially dominate, then slow down as it entered the inverted region. Meanwhile, singlet oxygen production would take over at higher driving forces 1 7 .
"This behaviour is characterized by a parabolic relationship between the logarithm of the kinetic constant and the driving force." — Research team in their 2025 Nature paper 1
The intersection of these two parabolas—one for triplet oxygen formation and one for singlet oxygen—would mark the transition point where singlet oxygen becomes dominant.
To test their hypothesis, Freunberger's team designed a series of elegant experiments that would systematically track oxygen formation under controlled conditions. Their approach allowed them to observe the phenomenon across an unprecedented range of driving forces.
The team chose acetonitrile as their solvent—a critical decision because acetonitrile remains stable under highly oxidizing conditions, allowing them to test reactions with large driving forces that would degrade other common solvents 1 .
Using a combination of mass spectrometry and specialized chemical traps, the researchers could precisely measure how much triplet versus singlet oxygen was produced in each reaction 1 . This dual measurement approach was crucial—previous techniques often struggled to detect singlet oxygen reliably due to its short lifetime.
They monitored the reaction rates using ultraviolet-visible (UV-Vis) spectroscopy, allowing them to track how quickly the reactions occurred under different conditions 1 .
The experimental results perfectly matched the predictions based on Marcus theory. The team observed that at low driving forces, triplet oxygen formation dominated. But as the driving force increased beyond a threshold, triplet oxygen production slowed down (entering the Marcus inverted region), while singlet oxygen production became increasingly favorable 1 7 .
Even more remarkably, the researchers discovered that acidity controls the driving force for superoxide disproportionation. In basic environments (high pH), the driving force remains low, favoring triplet oxygen formation. But as the environment becomes more acidic (low pH), the driving force increases, making singlet oxygen formation dominant 2 .
| Environment | pH Level | Driving Force | Dominant Oxygen Form | Biological Context |
|---|---|---|---|---|
| Basic | High (∼8.0) | Low | Triplet Oxygen | Mitochondria (cell powerhouses) |
| Acidic | Low (∼4.7) | High | Singlet Oxygen | Lysosomes (cell degradation centers) |
This pH dependence has profound biological implications. Our mitochondria—the powerhouses of our cells—maintain a basic environment (pH ∼8.0), which suppresses singlet oxygen formation and protects cells from damage 2 . Conversely, lysosomes—the cell's degradation centers—are acidic (pH ∼4.7), creating conditions where singlet oxygen might form more readily 7 .
"There was really truly a fierce debate about whether or not it forms in the environment of cells. Up to now it has never been clarified." — Professor Stefan Freunberger 4
The findings reconcile previously contradictory observations about when and where singlet oxygen appears.
Studying singlet oxygen requires specialized tools and approaches. Here are some key components of the modern researcher's toolkit for investigating this elusive molecule:
A highly stable organic solvent that can withstand strong oxidizing conditions without decomposing, enabling experiments with large driving forces 1 .
Used to detect and quantify triplet oxygen production by measuring molecular oxygen directly 1 .
Specialized compounds like 2,2,6,6-tetramethyl-4-piperidone hydrochloride that react selectively with singlet oxygen, allowing researchers to confirm its presence and measure its concentration 1 .
A workhorse technique for tracking reaction kinetics in real-time by measuring how light absorption changes during the reaction 1 .
Often used in combination with EPR to accurately detect short-lived singlet oxygen molecules in complex environments .
The discovery that Marcus kinetics controls whether superoxide transforms into triplet or singlet oxygen represents more than just the solution to a decades-old chemical mystery. It provides a powerful new framework for controlling oxygen chemistry across diverse fields.
In biology, this understanding helps explain how cells naturally protect themselves from singlet oxygen damage by maintaining specific pH environments in different organelles 2 7 .
"Biological systems know how to defend themselves from singlet oxygen. Whether we are doing basic chemistry or developing batteries, we must take inspiration from biology to keep the reaction's driving force low." — Soumyadip Mondal 2
In energy storage, the findings suggest practical strategies for preventing the battery degradation caused by singlet oxygen. By carefully selecting electrolytes and cations to control the driving force for superoxide oxidation, researchers may develop longer-lasting, more reliable batteries 1 2 .
Beyond these immediate applications, the research opens new avenues in fields ranging from green energy production to medicine. As Freunberger points out, "How singlet oxygen formation impacts the efficiency of water splitting and potentially degrades the electrolyzer's carbon carrier remains to be investigated. With our present knowledge, we might soon be able to tame the bad oxygen in various applications" 2 .
The journey that began with the COSMO 84 conference in 1984 and culminated in the 2025 Nature paper demonstrates how scientific understanding evolves through decades of persistent investigation. What started as an interdisciplinary gathering to share findings about a mysterious molecule has now yielded a fundamental understanding that promises to shape technologies for decades to come.
As we learn to tame oxygen's "evil twin," we gain not just control over a destructive chemical species, but also a deeper appreciation of the elegant electron dance that underlies one of nature's most essential elements.