Thermal cis-to-trans isomerization mechanism of N-(phenylazo)-substituted nitrogen heterocycles
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Triazenes, compounds containing a diazoamino moiety (–N(1)=N(2)–N(3)<), are known for their reversible cis-trans isomerization character and hence, have the potential to be used in photoswitchable devices and photostorage media. However, little is known about their cis-trans isomerization mechanism. In this thesis, kinetic studies on the thermal cis-to-trans isomerization of N-(phenylazo)-substituted nitrogen heterocycles are presented. It is shown that the isomerization rate constant increases as the size and electron-donating character of the cyclic amine increases, as the electron-withdrawing character of the para substituent group on the phenyl ring increases, and as the polarity of the solvent increases. All these trends are interpreted in terms of a rotational isomerization mechanism involving a dipolar transition state. In addition, photolytic cleavage of the N(2)–N(3) bond of target substrates is shown to be affected as well by the size and electronic character of the cyclic amine, the electronic character of the phenyl ring substituent, and the polarity of the solvent, with the result that the photolysis yield increases as the isomerization rate decreases. Theoretical calculations on target substrates both in the gas phase and various solvents were also performed based on DFT-B3LYP/6-31+G* method. Overall, the cis-to-trans isomerization is predicted to take place through rotation around the N(1)=N(2) bond. Furthermore, the calculated energy barriers are found to be influenced by the size and electronic character of the cyclic amine, the electronic character of the phenyl ring substituent, and the polarity of the solvent, consistent with the effects obtained experimentally from the kinetic studies.