Synthesis and Isomerization Studies of Cyclotrisazobiphenyl

摘要

We report an efficient synthesis of cyclotris[(E)-3′-(biphenyl-3-yldiazenyl)] compounds (CTBs). An unsubstituted CTB molecule is accessible in four steps in 10?% yield overall, whereas a hexa(methoxymethyl ether) CTB analogue was prepared in nine steps (26?% yield). The final macrocyclization step was accomplished in up to 80?% yield by using a metal-template effect. Furthermore, the photochromic properties were investigated, and all four isomers were detected and characterized by NMR spectroscopy. A strong influence from the solvent and the irradiation wavelength on the switching process was observed. Irradiation in pyridine yielded the highest amount of the all-Z?isomer in the photostationary state. For a full conversion to the all-E?isomer, the reaction has to be heated to 45?°C. The isomerization to the all-E?isomer is slow at room temperature, with a half-life time of the all-Z?isomer of more than nine days in dimethyl sulfoxide (DMSO). Conditions were established to access each possible isomer as the major component in the photostationary state.