A new class of molecular switches, which work like the pedals of a bicycle, have been developed in Spain and Holland and could pave the way for the development of functional molecular systems which are useful in small spaces.
Molecular cars – nano-sized cars which exhibit quasi-mechanical movement – are well known, and now researchers have developed a molecular switch that is fuelled by light and acts like the pedals of a bicycle.
Molecules that are switched by light to alter their molecular structure are essential building blocks for photoresponsive molecular nanotechnology. However, many of the molecular switches currently available need a relatively large free volume to transpose between their two structural states.
Prototype examples include molecules in which isomerization of a double bond occurs, as seen in the rotor molecules which won Ben Feringa a Nobel Prize in Chemistry in 2016. Isomerisation is a process which involves one molecule transforming into different one while retaining the same atoms.
In many practical applications - such as drug delivery, catalysis or molecular computers – there is just not enough space for large-scale movements, so finding novel chemical motifs to enable switching in a minimal volume are of great interest in this fast-emerging arena.
A new class of azodicarboxamide-based molecular switches has recently been proposed by Professor Jose Berna from the Department of Organic Chemistry at the University of Murcia in Spain. They are derived from a modification in the azo-moiety in azobenzene, a widely used component in 'light switchable' materials. In contrast to azobenzene, the new systems are planar, and it was therefore expected that they would demonstrate different types of movement upon irradiation with light. However, investigation of the actual motion taking place was out of reach - until now.
Dr. Saeed Amirjalayer from the Van't Hoff Institute for Molecular Sciences (HIMS) at the University of Amsterdam, Holland set out to investigate the exact method of operation of these azodicarboxamide-based molecular switches. He measured their vibrational frequencies by employing extremely short pulses of infrared light – pulses with a span of less than a trillionth of a second. The frequencies act like a fingerprint of the molecular structure and offer a direct method to establish precisely how the structure of the molecule changes after being activated by light.
Analysis revealed that these switched do show an utterly different switching mechanism when compared to standard switches. The latter demonstrate large-scale rotation around one bond, whereas the new molecules work like the bottom bracket and the pedals of a bicycle. Rather than performing a full revolution like bicycle pedals would, the switches exhibit a back and forth motion. Advanced quantum chemical calculations helped establish that the molecules became planar by light absorption, and crank back when returning to their ground state.
In Angewandte Chemie International Edition, the team "report on a novel switch that has azodicarboxamide as its photo-triggerable element. Time-resolved UV-pump/IR probe spectroscopy in combination with quantum-chemical calculations shows that the azodicarboxamide functionality, in contrast to other azo-based chromophores, does not undergo trans-cis photoisomerization. Instead, a photoinduced pedalo-type motion occurs, which because of its volume-conserving properties enables the design of functional molecular systems with controllable motion in a confined space."
The striking pedalo motion characteristic is accompanied by minimal displacement of the atoms involved – the molecule stays fixed in space more or less, and needs only a minimum switching volume. This could present the opportunity for applications where motion at a molecular level is severely restricted, such as in the solid state, on surfaces and when embedding in polymers.
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