title: Understanding the tautomerism in azacalixphyrins.
abstract: 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.
authors.affiliation: CEISAM, UMR CNRS 6230, Université de Nantes, BP 92208, 2, Rue de la Houssinière, 44322 Nantes, Cedex 3, France. Gabriel.Marchand@univ-nantes.fr.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Marchand
authors.forename: Patrick
authors.initials: P
authors.lastname: Giraudeau
authors.forename: Zhongrui
authors.initials: Z
authors.lastname: Chen
authors.forename: Mourad
authors.initials: M
authors.lastname: Elhabiri
authors.forename: Olivier
authors.initials: O
authors.lastname: Siri
authors.forename: Denis
authors.initials: D
authors.lastname: Jacquemin