Halogen bond-driven azo-hydrazone tautomerisation: a computational study

Azo-hydrazone tautomerisation affects the photoswitching behaviour and physical properties of tautomerisable azobenzenes. Non-covalent interactions, such as halogen bonding, can shift the tautomeric equilibrium by stabilising one tautomer over the other. Here, we have used computational methods to study how halogen bonding affects the azo-hydrazone tautomerisation of 2-hydroxy- and 4-hydroxyazobenzenes and their azonaphthalene derivatives. We also studied the effect of alkoxy groups, commonly employed as attachment points when incorporating azobenzenes into functional polymeric systems, on tautomerisation and halogen bonding by systematically replacing ring hydrogens with methoxy groups. In addition, self-complementary halogen-bonded dimers based on 2'-iodo-2-hydroxyazonaphthalene bearing methoxy and nitro groups were studied. Our results show that halogen bonding generally shifts the tautomeric equilibrium towards the hydrazone form. When the azo tautomer is only slightly more stable (∆G = 0-2 kcal mol^-1), halogen bonding can invert the tautomeric preference. External factors such as temperature affect the halogen bonding strength and thereby the tautomeric equilibrium, suggesting that these halogen-bonded systems may offer a tunable platform for sensing applications.

The Gaussian 16 program was used for geometry optimisations, interaction energy calculations, and to generate the wavefunctions. All calculations were performed using the M06-2X/DGDZVP density functional theory method. The counterpoise correction method by Boys and Bernardi was used to correct for the basis set superposition error. The AIMAll program was used to perform the interacting quantum atoms analyses of the wavefunctions.