Synthesis of new photo switchable azobenzene macrocycles and tuning of their half-life for Z to E thermal relaxation: A kinetic approach combined with theoretical study

A new switchable azobenzene macrocyclic derivative has been prepared in 4 steps with an overall yield of 47%. This molecule shows photoisomerization between the E and Z isomers using light of 365 nm and 405 nm respectively.¹ What differentiates this macrocyclic switch from azobenzene is that the λ_max for E (ππ*) and Z (nπ*) are well separated by 99 nm. The photostationary state (PSS) after irradiating by 365 nm LED light (measured at 415 nm) had a Z-selectivity of 81% Z and the PSS after irradiating by 405 nm LED light (measured at 316 nm) had an E-selectivity of 88% E. Both were reached after 90 s. The most unusual feature was that there was no thermal relaxation from Z to E observed at 20 °C over a period of 120 days, although photo switching was facile. Even at elevated temperatures of 70 °C and 90 °C, the thermal half-lives of the Z-isomer were as high as 36.4 hours and 2.35 hours respectively. Unusually, the macrocycle does not only photoisomerize in solution but also in the solid state. Therefore, this macrocycle could be useful for data storage and solar fuels applications.
Nevertheless, though the kinetics of thermal isomerization of azobenzene and its derivatives have been studied during last 70 years, kinetic studies of the isomerization of macrocyclic azobenzenes and tuning of bridge substitution of macrocyclic azobenzenes have been hardly elucidated. We performed a comprehensive kinetic study of Z to E thermal relaxation for 12-membered sulphur and oxygen bridge azobenzene macrocycles in solvents of different polarities. This study reveals that half-lives of Z-isomers of azobenzene macrocycles are increased with increasing the polarity of solvent. Also, half-life of the Z-isomers of oxygen bridged azobenzene macrocycles is higher than that of sulphur bridged azobenzene macrocycles which is further supported by geometrical strain analysis through the judgement of energy distribution and thermal energy barrier analysis of the isomerization process by DFT. Furthermore, the NTO theory confirms nature of π-π* and n-π* transition.

Photoisomerization of oxygen bridge azobenzeene macrocycle (left); Thermal energy barier of oxygen (red line) and sulphur (blue line) bridge azobenzene macrocycles (right)