Effect of Ring Size on Photoisomerization Properties of Stiff Stilbene macrocycles

A series of stiff stilbene macrocycles have been studied to investigate the possible impact of macrocycle ring size on their photo-dynamic properties. The results show that reducing the ring size counteracts the photoisomerization ability of the macrocycles. However, even the smallest macrocycle studied (stiff stilbene subunits linked by a six carbon chain) showed some degree of isomerization when irradiated. DFT calculations of the energy differences between the E- and Z- isomers show the same trend as the experimental results. Interestingly the DFT study highlights that the energy difference between the E- and Z- isomers of even the largest macrocycle (linked by a twelve carbon chain) is significantly higher than that of the stiff stilbene unit itself. In general, it is indicated that addition of even a flexible chain to the stiff stilbene unit may significantly affect  its photochemical properties and increase the photostability of the resulting macrocycle.