To Isomerize or not to Isomerize? E/Z Isomers of Cyclic Azobenzene Derivatives and Their Reactivity Upon One-Electron Reduction.

Azo compounds are efficient electron acceptors. Upon one-electron reduction they generally isomerize forming the thermodynamically most stable radical anion. Herein we show that the size of the central ring in 1,2-diazocines and diazonines has a ruling influence on the configuration of the one-electron reduced species. Markedly, diazonines, which bear a central nine membered heterocycle, show light-induced E/Z isomerization, but retain the configuration of the diazene N=N moiety upon one-electron reduction. Accordingly, E/Z isomerization is not induced by reduction.

Substituted nitrogen-bridged diazocines.

Novel nitrogen-bridged diazocines (triazocines) were synthesized that carry a formyl or an acetyl group at the CH2NR-bridge and bromo- or iodo-substituents at the distant phenyl ring. The photophysical properties were investigated in acetonitrile and water. As compared to previous approaches the yields of the intramolecular azo cyclizations were increased (from ≈40 to 60%) using an oxidative approach starting from the corresponding aniline precursors. The Z→E photoconversion yields in acetonitrile are 80-85% and the thermal half-lives of the metastable E configurations are 31-74 min. Particularly, the high photoconversion yields (≈70%) of the water-soluble diazocines are noteworthy, which makes them promising candidates for applications in photopharmacology. The halogen substituents allow further functionalization via cross-coupling reactions.

Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths.

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.

The Dynamic Covalent Chemistry of Amidoboronates: Tuning the rac5 /rac6 Ratio via the B-N and B-O Dynamic Covalent Bonds.

Amidoboronates were prepared as a mixture of up to three isomers (rac5 , meso5 and rac6 ) from the reductive coupling of N-aryl iminoboronates with either cobaltocene or decamethylcobaltocene in acetonitrile. The interconversion of rac5 and rac6 isomers via rearrangement of their dynamic covalent B-N bonds was investigated in solution by redissolving isolated crystals. The aniline para substituent and catechol within the amidoboronates tuned the rac5 /rac6 ratio; the rac6 isomer predominated for amidoboronates based on pyrocatechol, ranging from a ratio of 0 : 1 with electron-withdrawing Cl substituents to 0.5 : 0.5 for electron-donating NMe2 substituents. No interconversion was observed for the rac5 isomers of amidoboronates based on tetrachlorocatechol. Furthermore, the rac5 /rac6 distribution was altered by catechol exchange of pyrocatechol for tetrachlorocatechol exploiting the dynamic covalent B-O bonds and the rac5 isomer was the major isomer following exchange.

The Dynamic Covalent Chemistry of Amidoboronates: Tuning the rac5 /rac6 Ratio via the B-N and B-O Dynamic Covalent Bonds.

Invited for this month's cover is the group of Anna J. McConnell and Christian Näther from Christian-Albrechts-Universität zu Kiel, Germany. The cover picture shows an amidoboronate puzzle divided into four quarters that represent four possible amidoboronates. How do these amidoboronates interconvert? The right combination of aniline (yellow circles) and catechol (grey and purple circles) pieces solves this conundrum and the amidoboronates interconvert via dynamic covalent B-N (left to right) and B-O bonds (top to bottom). More information can be found in the Research Article by Anna J. McConnell and co-workers.