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Light abysses

Synthesis of molecules that absorb photons should facilitate the production of optical devices and films

EDUARDO CESARMesoionic compounds (red powder): control over the passage of lightEDUARDO CESAR

Seeing the world in colors is something so natural that we hardly ask ourselves how the brain distinguishes blue, green or red, the basic colors that make up all the others. The perception of light takes place thanks to a substance present in the eye’s retina called retinal. When absorbing light, the retinal undergoes a sort of deformation, and a message is sent to the brain, through the optic nerves. A similar phenomenon occurs with mesoionic compounds, a group of organic substances made from end to end, from the computer project to synthesis, by a group of chemists and physicists from the Federal Universities of Pernambuco (UFPE) and of Paraíba (UFPB).

When these compounds absorb light, a rearrangement of their electrical charges takes place, which changes their optical properties. The researchers discovered that the mesoionics are capable of absorbing two photons – particles of light – at just one go and far quicker than other substances endowed with the same property. This is how they opened up the way for the development of applications for a more suitable material than the glass and polymers normally used in optical devices, like films and protective glasses for laser rays.

“The experimental results have confirmed the theoretical forecasts, and we now have compounds capable of absorbing two photons in a way that is comparable to the best that has so far been described by other groups”, rejoices Alfredo Mayall Simas, from UFPE’s Basic Chemistry Department, who decided eight years ago to design mesoionic compounds, a class of compounds synthesized for the first time in 1935, in Australia. On the computer, the chemist did the calculations for 10,584 molecules – made up of atoms of carbon, nitrogen, oxygen, hydrogen and sulfur – and selected the most promising structures. Next, Joseph Miller’s team, from UFPB, synthesized about 40 of them, resulting in a brick red colored powder. Twelve molecules have now been submitted to experiments of non-linear optics, the part of physics that deals with optical phenomena that depend on the intensity of the light.

Previously, the organic compounds most used in this area were the polyenic compounds, made up of carbon atoms joined up between themselves, alternating single and double links with hydrogen atoms. The polyenic compounds are formed by long chains, while the mesoionics are cyclical (closed), in the shape of a ring. Simas and Miller decided to bet on the mesoionics because of one of the properties of these molecules: they have a distorted electronic density, with one side positive and the other negative. “This characteristic ensures that these substances have, naturally, a fundamental state of the push-pull kind, in which one group of atoms pushes the electrons and another pulls them, which is very important for the purposes of non-linear optics”, explains Simas.

Furthermore, the mesoionics absorb light of shorter wavelengths than the polyenic ones. “They are more transparent”, explains the chemist from UFPE. Physicist Cid Araújo, from UFPE’s Physics Department, who is analyzing the optical behavior of the molecules, adds: “As they show greater optical non-linearities, the mesoionic compounds may also be used in devices based on lower powered lasers”.

Compared with retinal, the mesoionics show a more rapid response. Retinal is present in the cones, the cells of the retina that detect light. There are three kinds of cones: one is sensitive to red, another to green, and the third to blue. When we look at the yolk of an egg, the cones that are sensitive to red and to green (the colors that make up yellow) send signals to the brain. The detection of light by the cones that results in the perception of colors is a sort of switching, comparable to the on-off switch used to switch on a light. The brain perceives the color as a mixture of the basic colors and the signal that it decodes reflects the different sensitivities of the cones.

The mesoionic compound can be used as an optic switch, the equivalent of an on and off switch, except that its purpose to let light pass through. This is one of the main applications envisaged for the mesoionic compounds. The group intends to develop films of micrometric dimensions (one micrometer is equivalent to one millionth of a meter), called thin films, which could be used not only in optic switches, but in other devices for optical communications and signal processing.

The optic switching system, in which a beam of light is used to send codified messages, can be compared with one used in primitive times by the American Indians, using a fire and a blanket: the messages depended on the number of puffs of smoke that they produced. The smoke was the light that they let through – hence, modulated light, transmitted in pulses. Another application is in glasses for protection against lasers, in which the films – actually, a very sensitive light detector – act as optic limiters. Coated with this film, the glasses would suddenly go dark, when exposed to the laser, and thus prevent accidents to the eyes.

The simultaneous absorption of two photons is also used in the nanomachining – the manipulation of nanometric devices (one nanometer is equivalent to one billionth of a meter) – of polymeric resins. If light penetrates uniformly, the whole of resin, which is liquid, will harden. Otherwise, focusing the light on a single spot, where the two photons are going to be absorbed, only this spot will be polymerized. In May, researchers from Osaka University, in Japan, announced an experimental process for hardening resin with biomedical applications, which avails itself precisely of the technique of absorbing two photons.

“The best compounds are still to come”, assures Simas. As early as in 1996, he, his student Gustavo Moura and Miller published an article in the Dutch magazine Chemical Physics Letters, revealing the potential of the mesoionics for applications in non-linear optics. But, if there are already inorganic devices, more resistant and with the same functions, why think about organic ones? “Although they have a shorter span of life, the mesoionics have advantages that compensate for their being organic”, Araújo explains. “One of them is that they are easier to be perfected than the glass or crystals normally used in optical devices”.

The study of mesoionics began in 1935, when Australian chemists Alan Mackney and John Campbel Earl (1890-1978) synthesized the first compound of the category, baptized as Sydnone, in honor of the city of Sydney. At the time, their proposition was that it was a bicyclical structure, with two closed chains of atoms. Fifteen years later, British researchers David Ollis (1925-1999) and Wilson Baker studied once again the structure of the mesoionics, with the proposition that they were aromatic compounds, not bicyclical ones.

Contrary to what the name suggest, in chemistry, aromatic compounds do not necessarily smell. “Being aromatic is a property associated with thermodynamic stability, much stronger than expected, of certain organic molecules”, Simas notes. The term was born of an experiment carried out in 1825, when British scientist Michael Faraday (1791-1867) discovered benzene when pyrolyzing (a sort of burning) whale oil. The typical fragrance of benzene and its derivatives led these compounds to be classified as aromatics. Nowadays, this classification has a chemical meaning related to structure, but not to smell.