Selective synthesis and kinetic measurement of 1:1 and 2:2 cyclic compounds containing 1,4,7,10-tetraazacyclododecane and azobenzene units

1:1 cyclic compounds 8a-c (51-55%) and 2:2 cyclic compounds 9a-c (20-49%) containing 1,4,7,-10-tetraazacyclododecane (cyclen) and azobenzene units were selectively synthesized under UV irradiation (330 nm < λ < 380 nm) and in the dark. Synthesis depended on the wavelength of irradiation light and the length of methylene chains of the linker between the cyclen and azobenzene units. A study of NMR and UV-vis spectra indicated that properties of 8a-c and 9a-c are closely related to their structural flexibility. Rate constants (k) and thermodynamic parameters (ΔG^‡, ΔH^‡, and ΔS^‡) of 8a-c and 9a-c were studied in nonpolar media (benzene) and polar media (methanol). The cis to trans isomerization rates in the dark for these cyclic compounds increase with ring size or structural flexibility (8a < 8c < 8b < 9a < 9b < 9c). In principle, ΔS^‡ dominates ΔG^‡ in cyclic compounds.