Push and pull: super-fast photochromic oscillators
Molecular photoswitches are used in a wide range of applications, from optoelectronics to optogenetics. Scientists at IOCB Prague, Comenius University in Bratislava (Slovakia), and Goethe University Frankfurt (Germany) led by Eliška Procházková (NMR Spectroscopy group) and Marek Cigáň have joined forces to design and prepare a new class of extremely fast photochromic oscillators with tunable properties.
The photoswitching of azopyrimidines is based on (Z)/(E) isomerization of the azo double bond. A planar E-isomer is thermally stable and upon UV light illumination isomerizes into a non-planar Z-isomer. For some applications, the Z-to-E thermal relaxation needs to be very fast. However, the fastest systems are based on ionic structures, limiting their solubility in various matrices needed for further application.
To design ultrafast non-ionic systems, DFT calculations were performed to find structures with an extremely unstable Z-isomer allowing very fast thermal relaxation back to an E-isomer in the sub-microsecond region. The calculations proposed azopyrimidines with a strong push-pull character, which decreased the energy barrier significantly while keeping the molecules uncharged. The synthetized compounds were studied using spectroscopic methods and indeed displayed extremely fast thermal relaxation. The half-life can be tuned by eight orders of magnitude. The fastest azopyrimidine with a half-life of two nanoseconds is comparable to the fastest ionic ones.
The results of this study, published in Angewandte Chemie International Edition, offer an opportunity for the design of ultrafast photochromic oscillators (∼100 MHz) as well as for versatile applications in smart materials.
Original paper: Čechová, L., Filo, J., Dračínský, M., Slavov, C., Sun, D., Janeba, Z., Slanina, T., Wachtveitl, J., Procházková, E., Cigáň, M. (2020). Polysubstituted 5‐Phenylazopyrimidines: Extremely Fast Non‐Ionic Photochromic Oscillators. Angewandte Chemie International Edition. DOI: 10.1002/anie.202007065