The vision system, evolved over millions of years, is highly complex. To make vision sensitive throughout the whole range of visible wavelengths, Nature employs a supramolecular chemistry approach. The visual pigment, cis-retinal, changes its shape upon capturing a photon. This shape transformation is accompanied by changes in the supramolecular organization of the surrounding proteins, subsequently triggering a cascade of chemical signaling events that get amplified and eventually lead to visual perception in the brain.
“Some deep-sea fish have evolved antenna-like molecules capable of absorbing photons in the red wavelength range, whose abundance at great depths is close to zero. After absorbing a photon, this antenna molecule transfers its energy to the nearby retinal molecule, thus inducing its conformational change from the cis to trans-retinal. In synthetic systems, such process would enable using low-energy light for applications in for instance energy storage or controlled drug release,” explains the lead author of the work Prof. Rafal Klajn from the Weizmann Institute of Science.
Inspired by this phenomenon, the researchers developed a superior supramolecular machine capable to efficiently convert widely used synthetic photoswitchable molecules — azobenzenes — from the stable to the metastable conformation with almost any wavelength of visible light. The approach includes a metal-organic cage filled with one azobenzene molecule and one light-absorbing antenna molecule, the sensitizer. In close confinement inside the supramolecular cage, chemical processes that would not take place in normal conditions, become possible.
“A common problem of azobenzenes is that they cannot efficiently undergo photoswitching from the stable trans form to the metastable cis form upon low-energy red and near-infrared light, but the process has to be driven by UV light. This substantially limits their applications in fields such as photocatalysis or photopharmacology. Now, using the supramolecular caging approach we can reach almost quantitative trans-to-cis isomerization with any color of visible range,” says Dr. Nikita Durandin, Academy of Finland Research Fellow in Supramolecular Chemistry of Bio- and Nanomaterials group, who has been working with sensitization approaches in Tampere University for the last 7 years.
“Time-resolved spectroscopic studies done at Tampere University revealed that the photochemical processes triggering the isomerization happen superfast, in the nanosecond range. In other words, almost 1 billion times faster than the blink of your eyes,” continues Dr. Tero-Petri Ruoko, Marie Sklodowska-Curie Fellow in Smart Photonics Materials group, and expert in ultrafast spectroscopy.
“Once you shine light on this supramolecular cage, it quickly converts almost all of the trans isomers into cis isomers. Simple mixing of components and light that matches the absorption profile of the sensitizer is enough to make this machinery work,” he adds.
According to Prof. Arri Priimägi, the leader of Smart Photonics Materials group specializing in light-active materials, the study presents a new approach for activating photoresponsive molecules with low-energy light, pushing them out from their thermodynamic equilibrium utilizing chemistry that only takes place under confinement.
It took millions of years of evolution for the eye of deep-sea fish to emerge. Learning from that, the research led by Rafal Klajn’s group extended these concepts to synthetic materials in less than 5 years.
“We are already working on the next generation of the light-driven supramolecular machines, aiming at applying the developed methodologies in soft robotics and light-activated drug delivery systems,” concludes Priimägi.