Effects of chemical substitutions on the properties of azacalixphyrins: a first-principles study

Azacalixphyrins are particularly stable macrocycles absorbing light up to the near infrared region, which makes them good candidates for applications in a wide panel of fields. However, to date, only a very limited number of substituted azacalixphyrins are experimentally known. Herein, aiming to drive the synthesis towards the most promising substitution patterns, we explore with first principles approaches the impact of adding substituents on both the aromaticity and absorption spectrum of these macrocycles. Both neutral and dicationic species are considered. For the former, we show that the substituents have an impact on the relative stabilities of the different tautomers, which can lead to significant tuning of the properties. Depending on the electron donating or electron attracting character of the chemical groups added at the periphery, the first absorption band in the near infrared region can be remarkably red or blueshifted, respectively. Our calculations also predict that replacing the macrocycle phenyl rings with pyridine rings leads to a more planar structure - due to the reduction of the steric stress at the centre - and to more equalised bond lengths. This results in an increase of the aromatic strength of the macrocycle by ca. 20%, at the cost of a large blueshift of the first absorption band. Compared to the neutral forms, the latter dicationic species are found to be more aromatic. Flatter and therefore more aromatic macrocycles can also be obtained for the neutral species by replacing the peripheral NH₂/NH groups with OH/O groups/atoms, owing to the interaction between the oxygen lone pairs and the vicinal hydrogen atom. It is found that the combination of nitrogenation and oxidation allows doubling the aromatic character strength compared to the parent compound. This work therefore paves the way to the control of the properties of azacalixphyrins by purpose-designed substitutions.