C-H center dot center dot center dot Br-C vs. C-Br center dot center dot center dot Br-C vs. C-Br center dot center dot center dot N bonding in molecular self-assembly of pyridine-containing dyes

摘要

We have studied a series of closely related N-(5-bromosalicylidene)-x-aminopyridine compounds (x-2, 1; 3, 2; 4, 3), obtained by condensation of 5-bromosalicylaldehyde with the corresponding aminopyridine. H-1 NMR spectroscopy in solution revealed a single structure, at least in CDCl3. According to single crystal X-ray diffraction it was established that the crystal structures of 1-3 each are stabilized by a linear intramolecular hydrogen bond of the O-H center dot center dot center dot N type, formed between the hydroxyl hydrogen atom of the phenolic ring and the imine nitrogen atom. The dihalogen C-Br center dot center dot center dot Br-C and halogen C-Br center dot center dot center dot N(Py) interactions play a crucial role for the formation of supramolecular architectures in the structures of 2 and 3, respectively. The overall geometry of each molecule in the structures of 1 and 2 was found to be almost planar, while a significantly twisted structure was found for 3. Hirshfeld surface analysis showed that the structures of all compounds are mainly characterized by H center dot center dot center dot X contacts as well as by a remarkable contribution from C center dot center dot center dot C and C center dot center dot center dot N contacts which is clearly observed for 1 and 2. Diffuse reflectance spectra of 1-3 each exhibit a mixture of enol, cis-keto and trans-keto forms. A major contribution of the trans-keto form is found for 1 whereas a moderate fraction is detected for 2, and only traces of the trans-keto form were observed for 3. Contrary to expectations based on dihedral angle phi considerations, 1 exhibits negative photochromism, although it was expected to be only thermochromic. Both 2 and 3 are not photochromic, whereas 3 was expected to be photochromic (phi > 25 degrees). Highly favoured C center dot center dot center dot C, C center dot center dot center dot N and O center dot center dot center dot Br intermolecular contacts as well as the absence of Br center dot center dot center dot Br dihalogen interactions in the structure of 1, are presumably mainly responsible for the photochromic behaviour. Furthermore, significantly impoverished H center dot center dot center dot C and H center dot center dot center dot N contacts further support the observed negative photochromism of 1.