Lighting the way to switch chemical reaction pathways
Shedding a little light on certain starting materials can nudge them down one of two reaction pathways. Such a light switch could be used to get past the diffraction barrier to print tiny components on electronic circuits with nanometre resolution.
Nanometer resolution : The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers. A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lower bound to the mesoscopic scale for most solids.
Hannes Houck, Filip Du Prez and Christopher Barner-Kowollik have explored the possibility of a light switch for chemical reactions. Being able to control the outcome of a reaction with an external switch is, they suggest, a ‘key frontier within the realm of chemical reaction control’. They have demonstrated proof of principle with reactions of triazolinediones. These species are useful coupling agents in biomedical and polymer research and can undergo Diels–Alder reactions and addition reactions.
"The reaction of triazolinediones (TADs) and indoles is of particular interest for polymer chemistry applications, as it is a very fast and irreversible additive-free process at room temperature, but can be turned into a dynamic covalent bond forming process at elevated temperatures, giving a reliable bond exchange or ‘transclick’ reaction. In this paper, we report an in-depth study aimed at controlling the TAD–indole reversible click reactions through rational design of modified indole reaction partners. This has resulted in the identification of a novel class of easily accessible indole derivatives that give dynamic TAD-adduct formation at significantly lower temperatures. We further demonstrate that these new substrates can be used to design a directed cascade of click reactions of a functionalized TAD moiety from an initial indole reaction partner to a second indole, and finally to an irreversible reaction partner. This controlled sequence of click and transclick reactions of a single TAD reagent between three different substrates has been demonstrated both on small molecule and macromolecular level, and the factors that control the reversibility profiles have been rationalized and guided by mechanistic considerations supported by theoretical calculations."
"Diels–Alder reactions:
an addition reaction in which a conjugated diene reacts with a compound with a double or triple bond so as to form a six-membered ring."
The team reports that ultraviolet light will induce a Diels–Alder reaction with photocaged dienes. Conversely, in the dark at ambient temperature, an addition reaction with alkenes occurs. However, if visible light is shone on the reaction mixture, a photopolymerisation takes place that causes quantitative photodeactivation that stops all reactions. Dim the lights, however, and the switch can be flipped again.
" dienes:
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds. Dienes occur occasionally in nature. "
"A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. ... The result of photo curing is the formation of a thermoset network of polymers."
"medical Definition of photoreactivation. : repair of DNA (as of a bacterium) especially by a light-dependent enzymatic reaction after damage by ultraviolet irradiation."
This phenomenon could be exploited in connecting small molecules to compounds of biomedical interest with fine control, as well as opening up the possibility of designing super high-resolution photoresists for 3D laser lithography. Toggling between on and off states of reactions was always possible, but relied on temperature as the controlling factor. Light allows a much finer control not only in time but also spatially given that a light can be switched on and off and back on again instantaneously. Moreover, specific areas can be masked to keep them in shadow while other areas are bathed in light.
"Nanoscribe 3D laser lithography. Back. By tightly focusing the light of an ultra-short pulsed laser, the intensity within the very focus is sufficiently high to expose the photoresist by two-photon absorption. This process causes a chemical and/or physical change of the photoresist within a small pixel (“voxel”)."
Related reactions that respond to different wavelengths, or colour, of lights would add another level of control. ‘The results of the study have the potential to establish super resolution photoresists that break the diffraction barrier to print a few nanometre wide lines with visible light – a distant dream today,’ says Barner-Kowollik. ‘It is critical that we also establish more systems capable of functioning as colour-guided photoswitches to have an entire library available.’
"A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface, which is crucial in the whole electronic industry."
"A photoswitch, or photo-electric switch, is a sensor that detects the presence in or change of light. A popular example is that of azobenzene."
‘Clever use is made of an old reaction where a dienophile, which is also an ene reactant, is converted by visible light into an inactive polymeric form. However, the polymer thermally reverts to the active form. Thus, a Diels–Alder reaction (based on a photoenol generated by UV light) and a thermal ene reaction can be turned off or on by prior visible light irradiation or a dark period,’ explains Amilra De Silva of Queen’s University Belfast. ‘The usual goal of photochemists is to use light to make reactions happen. Switching off a chemical reaction with light isn’t easy, especially if it is to be done reversibly. But if this can be done, possibilities of sub-micron photo-patterning arise,’ he adds.
"The ene reaction (also known as the Alder-ene reaction) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift."
"· It was well known that o- quinodimethanes, which are the photoenol product of ... applications in the synthesis of well-defined block copolymers and dendrimers."
Researchers from Queensland University of Technology (QUT), Karlsruhe Institute of Technology (KIT) and Ghent University have stepped towards this by pioneering a system that modulates visible, coloured light to change the reactions of a powerful chemical coupling agent.
Researchers used green laser light to control the reactivity of triazolinediones (TADs), coupling agents which swiftly create bonds with other chemicals, necessary to make materials
Under green light the TADs stopped reacting; when the light was switched off, the TADs became highly reactive again
The light-switching process could be repeated multiple times
The experiment showed two different products can be created from the same set-up simply by switching coloured light on and off
"Two or more light switches can be interconnected to allow control of lighting from, for example, two ends of a long hallway or landings at the upper and lower landings of a flight of stairs. Multiway switching is done using special switches that have additional contacts."
Professor Barner-Kowollik said the ability to use visible light as a remote-controlled on/off chemical reaction switch opened up possibilities for future industry applications in chemical and advanced manufacturing, including computer chip fabrication.
"At the moment, ultraviolet (UV) light, which has shorter wavelengths than light in the visible spectrum, is used in industry to drive chemical processes," he said.
"Industrial processes making use of less harmful visible light are scarce - a sharp contrast to what occurs in nature.
"For plants, visible light plays a critical role in chemical processes. Trees harvest light during the day and use this as an energy source to grow, releasing oxygen in the process. At night, however, when light is no longer available, the chemical process is altered and plants release carbon dioxide.
"We've been inspired by such natural processes and designed a completely light-switchable chemical reaction system for the first time."
The research team said their system may be applied to create light-sensitive materials for 3D laser lithography, allowing printing of very small structures that could be used in such things as computer chip fabrication. 3D laser lithography is a type of 3D printing with direct laser light, and used for creating very precise structures in the micro range, such as scaffolds for cells.
QUT is exploring 3D laser lithography applications with leading KIT physicist Professor Martin Wegener.
"Contemporary chip fabrication is a complex, and expensive, system of chemical processes," Professor Barner-Kowollik said. "Here, due to the so-called light diffraction limit, radiation with short wavelengths - which is very harsh UV light - is used.
"Wafer fabrication is a procedure composed of many repeated sequential processes to produce complete electrical or photonic circuits. Examples include production of radio frequency (RF) amplifiers, LEDs, optical computer components, and CPUs for computers."
"But what if we could use visible light to reversibly switch certain chemical processesand get around the light diffraction limit and print very, very small structures, for example five nanometres wide?
"Being able to switch chemical reactivity within 3D laser lithography could revolutionise chip printing, and make it cheaper, simpler and safer.
"There are substantial barriers to overcome, but potentially the system we have devised with visible light as a chemical deactivation mechanism could provide an avenue to achieve that.
The reaction of triazolinediones (TADs)
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