Our paper published in Nature Communications, 9, 2510 (2018) can be read from https://www.nature.com/articles/s41467-018-04928-9.
Simple azobenzenes, spiropyrans, diarylethenes, or hemithioindigos - all of these commonly known photoswitches are one-dimensional. Like a computer they can only decide between 0 and 1, off and on, open and closed, or E or Z isomer. As always nature is already far ahead with photoswitches like retinal, which possesses not only four potential photoisomerizable double bonds but each of them is adjacent to a single bond that can also be isomerized in the photochemical process. However, embedded in an enzyme pocket only one photoisomerization at the 11-C takes place. But what about the adjacent single bond? After it was discovered that the photoreaction of retinal inside rhodopsin is extremely fast, a simple double bond isomerization (DBI) seemed very unlikely because it would require very large volume changes. In 1985 Asato and Liu proposed a volume-conserving mechanism, which they termed “hula-twist” (HT) motion. Here the double bond and the adjacent single bond rotate at the same time to reduce volume changes during the photoreaction. Although many claims for HT photoreactions have been made for different chromophores ever since, no direct experimental evidence could be delivered so far for this complex photoreaction even if fancy measurements on the femtosecond time scale or under ultra-cold conditions were tried. The central problem in all these studies is the occurrence of very fast thermal follow-up reactions leading to further bond rotations and thus clouding identification of the initial photoproducts.
As chemists we don’t think about making spectroscopy faster but about modifying the molecule itself in such a way that the rates of thermal follow-up reactions become extremely small. Then we can follow and analyze the initial photoreaction on its own at room temperature with slower but high resolution techniques like NMR spectroscopy and crystal structure analysis.
To do this we chose a photoswitch based on hemithioindigo (HTI) as the starting point. HTI possesses a rigid molecular structure where only two bonds can isomerize: the central double bond and the adjacent single bond. Thermal rotation around the single bond can be suppressed by introducing a volume demanding tert-butyl substituent at the central double bond of the molecule. By adding two stereoinformations - axial chirality and a sulfoxide stereocenter – four diastereomeric isomers are possible. Luckily, these diastereoisomers are stable at ambient temperature and could be separated easily by common column chromatography and crystallized individually. After solving the crystal structures of each of the four diastereoisomers we could assign the corresponding 1H NMR spectra unambiguously. With this molecular setup we were therefore able to follow all possible photochemical processes in detail using different methods like NMR or UV-vis measurements in solution at high and low temperatures and in organic ices and glasses at 77 or 90 K. It was even possible to obtain quantum yields for each possible photoreaction separately.
To our delight actually all possible photoreactions in this molecular system i.e. DBI, single bond rotation (SBR), and the so far elusive HT were performed. The individual preferences for each photoreaction depend strongly on the environment of the photoswitch and can therefore be controlled by simple changes in e.g. solvent polarity or temperature. In ices we observe 99% of DBI, but the value changes to 82% HT in low temperature liquids.
But what is this HT isomerization exactly? Is it a simple combination of DBI and SBR or a different mechanism on its own? After translation into binary code the former would refer to 11 for the combination of DBI and SBR. However, our investigations showed that the HT reaction is not a combination of two photoreactions or a combination of a photoreaction and a thermal reaction. Thus we need to introduce a quaternary code with 0 for no reaction, 1 for SBR, 2 for DBI and 3 for HT to express this behavior properly. Finally we left the one-dimensionality and received a much more complex system. Therefore we have truly opened the door to complex applications with a lot of possibilities for photochemists and molecular engineers.