Circularly polarized (CP) light is encoded with information through its photon spin and can be utilized in applications such as low-power displays, encrypted communications and quantum technologies. Organic light emitting diodes (OLEDs) produce CP light with a left or right “handedness”, depending on the chirality of the light-emitting molecules used to create the device.

While OLEDs usually only emit either left- or right-handed CP light, researchers have now developed OLEDs that can electrically switch between emitting left- or right-handed CP light – without needing different molecules for each handedness.

“We had recently identified an alternative mechanism for the emission of circularly polarized light in OLEDs, using our chiral polymer materials, which we called anomalous circularly polarized electroluminescence,” says lead author Matthew Fuchter from the University of Oxford. “We set about trying to better understand the interplay between this new mechanism and the generally established mechanism for circularly polarized emission in the same chiral materials”.

Light handedness controlled by molecular chirality

The CP light handedness of an organic emissive molecule is controlled by its chirality. A chiral molecule is one that has two mirror-image structural isomers that can’t be superimposed on top of each other. Each of these non-superimposable molecules is called an enantiomer, and will absorb, emit and refract CP light with a defined spin angular momentum. Each enantiomer will produce CP light with a different handedness, through an optical mechanism called normal circularly polarized electroluminescence (NCPE).

OLED designs typically require access to both enantiomers, but most chemical synthesis processes will produce racemic mixtures (equal amounts of the two enantiomers) that are difficult to separate. Extracting each enantiomer so that they can be used individually is complex and expensive, but the research at Oxford has simplified this process by using a molecule that can switch between emitting left- and right-handed CP light.

The molecule in question is a helical molecule called (P)-aza[6]helicene, which is the right-handed enantiomer. Even though it is just a one-handed form, the researchers found a way to control the handedness of the OLED, enabling it to switch between both forms.

Switching handedness without changing the structure

The researchers designed the helicene molecules so that the handedness of the light could be switched electrically, without needing to change the structure of the material itself. “Our work shows that either handedness can be accessed from a single-handed chiral material without changing the composition or thickness of the emissive layer,” says Fuchter. “From a practical standpoint, this approach could have advantages in future circularly polarized OLED technologies.”

Instead of making a structural change, the researchers changed the way that the electric charges are recombined in the device, using interlayers to alter the recombination position and charge carrier mobility inside the device. Depending on where the recombination zone is located, this leads to situations where there is balanced or unbalanced charge transport, which then leads to different handedness of CP light in the device.

When the recombination zone is located in the centre of the emissive layer, the charge transport is balanced, which generates an NCPE mechanism. In these situations, the helicene adopts its normal handedness (right handedness).

However, when the recombination zone is located close to one of the transport layers, it creates an unbalanced charge transport mechanism called anomalous circularly polarized electroluminescence (ACPE). The ACPE overrides the NCPE mechanism and inverts the handedness of the device to left handedness by altering the balance of induced orbital angular momentum in electrons versus holes. The presence of these two electroluminescence mechanisms in the device enables it to be controlled electrically by tuning the charge carrier mobility and the recombination zone position.

The research allows the creation of OLEDs with controllable spin angular momentum information using a single emissive enantiomer, while probing the fundamental physics of chiral optoelectronics. “This work contributes to the growing body of evidence suggesting further rich physics at the intersection of chirality, charge and spin. We have many ongoing projects to try and understand and exploit such interplay,” Fuchter concludes.

The researchers describe their findings in Nature Photonics.

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Physicist, molecular biologist, neuroscientist: Francis Crick’s scientific career took many turns. And now, he is the subject of zoologist Matthew Cobb’s new book, Crick: a Mind in Motion – from DNA to the Brain.

Born in 1916, Crick studied physics at University College London in the mid-1930s, before working for the Admiralty Research Laboratory during the Second World War. But after reading physicist Erwin Schrödinger’s 1944 book What Is Life? The Physical Aspect of the Living Cell, and a 1946 article on the structure of biological molecules by chemist Linus Pauling, Crick left his career in physics and switched to molecular biology in 1947.

Six years later, while working at the University of Cambridge, he played a key role in decoding the double-helix structure of DNA, working in collaboration with biologist James Watson, biophysicist Maurice Wilkins and other researchers including chemist and X-ray crystallographer Rosalind Franklin. Crick, alongside Watson and Wilkins, went on to receive the 1962 Nobel Prize in Physiology and Medicine for the discovery.

Finally, Crick’s career took one more turn in the mid-1970s. After experiencing a mental health crisis, Crick left Britain and moved to California. He took up neuroscience in an attempt to understand the roots of human consciousness, as discussed in his 1994 book, The Astonishing Hypothesis: the Scientific Search for the Soul.

Parallel lives

When he died in 2004, Crick’s office wall at Salk Institute in La Jolla, US, carried portraits of Charles Darwin and Albert Einstein, as Cobb notes on the final page of his deeply researched and intellectually fascinating biography. But curiously, there is not a single other reference to Einstein in Cobb’s massive book. Furthermore, there is no reference at all to Einstein in the equally large 2009 biography of Crick, Francis Crick: Hunter of Life’s Secrets, by historian of science Robert Olby, who – unlike Cobb – knew Crick personally.

Nevertheless, a comparison of Crick and Einstein is illuminating. Crick’s family background (in the shoe industry), and his childhood and youth are in some ways reminiscent of Einstein’s. Both physicists came from provincial business families of limited financial success, with some interest in science yet little intellectual distinction. Both did moderately well at school and college, but were not academic stars. And both were exposed to established religion, but rejected it in their teens; they had little intrinsic respect for authority, without being open rebels until later in life.

The similarities continue into adulthood, with the two men following unconventional early scientific careers. Both of them were extroverts who loved to debate ideas with fellow scientists (at times devastatingly), although they were equally capable of long, solitary periods of concentration throughout their careers. In middle age, they migrated from their home countries – Germany (Einstein) and Britain (Crick) – to take up academic positions in the US, where they were much admired and inspiring to other scientists, but failed to match their earlier scientific achievements.

In their personal lives, both Crick and Einstein had a complicated history with women. Having divorced their first wives, they had a variety of extramarital affairs – as discussed by Cobb without revealing the names of these women – while remaining married to their second wives. Interestingly, Crick’s second wife, Odile Crick (whom he was married to for 55 years) was an artist, and drew the famous schematic drawing of the double helix published in Nature in 1953.

Stories of friendships

Although Cobb misses this fascinating comparison with Einstein, many other vivid stories light up his book. For example, he recounts Watson’s claim that just after their success with DNA in 1953, “Francis winged into the Eagle [their local pub in Cambridge] to tell everyone within hearing distance that we had found the secret of life” – a story that later appeared on a plaque outside the pub.

“Francis always denied he said anything of the sort,” notes Cobb, “and in 2016, at a celebration of the centenary of Crick’s birth, Watson publicly admitted that he had made it up for dramatic effect (a few years earlier, he had confessed as much to Kindra Crick, Francis’s granddaughter).” No wonder Watson’s much-read 1968 book The Double Helix caused a furious reaction from Crick and a temporary breakdown in their friendship, as Cobb dissects in excoriating detail.

Watson’s deprecatory comments on Franklin helped to provoke the current widespread belief that Crick and Watson succeeded by stealing Franklin’s data. After an extensive analysis of the available evidence, however, Cobb argues that the data was willingly shared with them by Franklin, but that they should have formally asked her permission to use it in their published work – “Ambition, or thoughtlessness, stayed their hand.”

In fact, it seems Crick and Franklin were friends in 1953, and remained so – with Franklin asking Crick for his advice on her draft scientific papers – until her premature death from ovarian cancer in 1958. Indeed, after her first surgery in 1956, Franklin went to stay with Crick and his wife at their house in Cambridge, and then returned to them after her second operation. There certainly appears to be no breakdown in trust between the two. When Crick was nominated for the Nobel prize in 1961, he openly stated, “The data which really helped us obtain the structure was mainly obtained by Rosalind Franklin.”

As for Crick’s later study of consciousness, Cobb comments, “It would be easy to dismiss Crick’s switch to studying the brain as the quixotic project of an ageing scientist who did not know his limits. After all, he did not make any decisive breakthrough in understanding the brain – nothing like the double helix… But then again, nobody else did, in Crick’s lifetime or since.” One is perhaps reminded once again of Einstein, and his preoccupation during later life with his unified field theory, which remains an open line of research today.

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