Understanding the tautomerism in azacalixphyrins

Understanding the chemical nature and spectroscopic signatures of a new class of organic molecules remains a strong challenge. Azacalixphyrin, the first member of a family of strongly aromatic macrocycles absorbing in the near infrared domain, can exist in several tautomeric forms. Here, we use DFT calculations and NMR measurements to propose the first in-depth investigation of proton exchanges occurring in two forms of azacalixphyrins (non-protonated and protonated). Our results reveal, on the one hand, a very effective solvent-assisted tautomerism in the non-protonated form whereas the intramolecular proton transfer is less probable, and, on the other hand, the presence of a mixture of almost isoenergetic tautomers differing in both their aromaticity and absorption profiles. This clearly indicates that smartly-designed chemical substitutions could alter the relative weights of the different tautomers, and consequently tune the optical signatures of these new macrocycles in a versatile and efficient way. For the protonated form, rotations of the NH2 groups take place rather than the chemical exchange.