Scheme 1. The chemical structures of the azobenzene-modified UPy derivative (Azo-UPy), and the schematic illustration of the “photolocking” strategy in the construction of the photoswitchable quadruple H-bonded (a) homodimer and (b) heterodimer. The 
value was used as an expression for the apparent dimerization constant of Z-Azo-Upy, due to the coexistance of tautomers I and II of (Z-Azo-UPy)2 in CDCl3 solution.
