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%) c ontaining 1,4,7,10-tetraazacyclododecane (cyclen) and azobenzene units were selectively synthesized under UV irradiation (330 nm < <lambda> < 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 azo benzene 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 (<Delta>G(double dagger), DeltaH (double dagger), and DeltaS(double dagger)) of 8a-c and 9a-c were studied i n nonpolar media (benzene) and polar media (methanol). The cis to trans iso merization rates in the dark for these cyclic compounds increase with ring size or structural flexibility (8a < 8c < 8b < 9a < 9b < 9c). In principle, <Delta>S-double dagger dominates DeltaG(double dagger) in cyclic compounds .