Scientists from the Moscow State University together with colleagues from Germany have found that a derivative of [3]-radialene, a molecule known to the science for nearly 30 years, can be used to create organic semiconductors. Dmitry Ivanov, the Head of the Laboratory of Materials Engineering at the Department of Fundamental Physics and Chemical Engineering, Moscow State University, believes that the achievement will greatly contribute to the development of organic electronics and, in particular, to fabrication of organic light emitting diodes and new classes of organic solar cells.

Organic or “plastic” electronics is a relatively young scientific field, which came to life about 15-20 years ago. Its purpose is the development of electronic devices based on organic materials. This type of devices is yet inferior to the standard silicon-based electronics in terms of performance, and also less durable. But it also has advantages — lightness, thinness, flexibility, transparency, and most importantly — plastic electronics is much cheaper than silicon. The main applications of organic electronics include solar cells, which will be much cheaper than the silicon-based modules (high cost is one of the reasons, which restricts the latter from covering large areas and thus make better use of the sun energy). Also, organic electronics can be used to design organic light emitting devices and field-effect transistors.

The molecule in question is the so-called dopant, whose addition to a semiconducting polymer substantially increases its electrical conductivity. The dopants for inorganic semiconductors are widely used already for decades, however, according to one of the article’s co-authors, Dmitry Ivanov, for organic conductors this field is far less studied. Currently, the most commonly used are fluorinated dopants. In combination with different organic semiconductors they are able to dramatically increase their electrical conductivity, but not all polymers that are used today in the “plastic” electronics are suitable.

‘Together with our Drezden colleagues we decided to design a completely new type of low molecular weight dopant for the organic semiconductor,’ says Dmitry Ivanov. ‘And here it was important to choose a molecule that it was not only suitable in its energy levels, but, importantly, the dopant must be well mixed with the polymer, so that in contact with the polymer it does not segregate in a separate phase, eventually crystallizing and, in fact, losing contact with the polymer’.

The main contribution of Ivanov’s laboratory in this work consisted in exploring the physics of the phase transitions, physics of mixing in such binary systems, in the other words — in finding a suitable candidate in terms of polymer physics.

And such a candidate was found. It happened to be a derivative of [3]-radialene. This is a small planar molecule in which the carbon atoms are connected to form a triangular structure. Among other potentially interesting compounds, [3]-radialene has the most energetically suitable LUMO level, i.e. the lowest unoccupied molecular orbital. This means that with its help the electrons are easily extracted from the semiconducting polymer matrix, becoming free charges and thereby increasing the conductivity of the doped material. [3]-Radialene thus becomes the strongest dopant for the organic semiconductor among those that are known in the scientific literature.

The experiments with the [3]-radialene, also confirmed by the results of quantum-chemical calculations, show that the substance is well mixed with a semiconducting polymer and allows to increase the electrical conductivity of the polymer by several tens and even hundreds of times. It has been found that up to 50 percent of the dopant’s content in the polymer the phase separation does not occur, but the crystalline structure of the polymer is gradually changed. This meant that the dopant molecules are included in the polymer crystalline lattice and form the so-called co-crystal. And the formation of co-crystals, according to Ivanov, is precisely one of the main reasons for the high efficiency of the new compound.

The described new dopant, as well its fluorinated and currently the most popular analogues, belongs to the category of electron-deficient organic dopants, Dmitry Ivanov tells. ‘Fluorine substituents are known to strongly pull the electrons away from the central part of the molecule, which increases the whole conductivity of the doped polymer. In the present work, the chemical structure of the dopant is completely different, and, in fact, appears to be even better. Perfect mixing of our dopant with the polymer matrix is, I think, the key to its performance. This could pave the way to fabrication of new organic solar cells with improved characteristics. We also think about production of organic field-effect transistors. I think it will give a significant boost to the development of organic electronic devices.’

“Our study shows that the environmental light-dark cycle is important for health,” says Johanna Meijer of Leiden University Medical Centre in the Netherlands. “We showed that the absence of environmental rhythms leads to severe disruption of a wide variety of health parameters.”

Those parameters included pro-inflammatory activation of the immune system, muscle loss, and early signs of osteoporosis. The researchers say that the observed physiological changes were all indicative of “frailty” as is typically seen in people or animals as they age. But there was some more encouraging news, too.

“The good news is that we subsequently showed that these negative effects on health are reversible when the environmental light-dark cycle is restored,” Meijer says.

To investigate the relationship between a loss of the light-dark cycle and disease, Meijer and colleagues, including Eliane Lucassen, exposed mice to light around the clock for 24 weeks and measured several major health parameters. Studies of the animals’ brain activity showed that the constant light exposure reduced the normal rhythmic patterns in the brain’s central circadian pacemaker of the suprachiasmatic nuclei (SCN) by 70 percent.

Strikingly, the disruption to normal light and dark patterns and the circadian rhythm led to a reduction in the animals’ skeletal muscle function as measured in standard tests of strength. Their bones showed signs of deterioration, and the animals entered a pro-inflammatory state normally observed only in the presence of pathogens or other harmful stimuli. After the mice were returned to a standard light-dark cycle for 2 weeks, the SCN neurons rapidly recovered their normal rhythm, and the animals’ health problems were reversed.

The findings suggest that more care should be taken in considering the amount of light exposure people get, particularly those who are aging or otherwise vulnerable. That’s important given that 75 percent of the world’s population is exposed to light during the night. Constant light exposure is very common in nursing homes and intensive care units, and many people also work into the night.

“We used to think of light and darkness as harmless or neutral stimuli with respect to health,” Meijer says. “We now realize this is not the case based on accumulating studies from laboratories all over the world, all pointing in the same direction. Possibly this is not surprising as life evolved under the constant pressure of the light-dark cycle. We seem to be optimized to live under these cycles, and the other side of the coin is that we are now affected by a lack of such cycles.”

The bottom line, according to the researchers is “light exposure matters.”

They say they now plan to perform more in-depth analysis of the influence of distorted light-dark cycles on the immune system. They’d also like to investigate possible health benefits to patients exposed to more normal conditions of light and dark.