1 H NMR‐based method for the determination of complexation equilibrium parameters and chemical shifts in a hydrogen‐bonded system with dynamic composition

A photoswitchable self‐complementary hydrogen bond system with an azoheteroaromatic backbone is presented. The trans ‐isomer ( T ) is the most stable form designed to dimerize through 6 hydrogen bond interactions. The dimerization constant ( K T · T ) and the dimer structure in the solid state have been obtained by conventional methods. When the cis ‐isomer ( C ) is generated through photoisomerization, the dynamics of the complex structures in solution change. The less stable cis ‐isomer can engage in dimerization, and a trans ‐ cis complex may form with remaining trans ‐isomer (whose concentration increases as cis ‐ trans thermal reversion takes place). A mathematical approach to calculate the trans ‐ cis complexation constant ( K T · C ) along with an estimation of the cis ‐isomer dimerization constant ( K C · C ) from phenyl analogues allows a description of the species distribution in solution to be generated.