Intramolecular hydrogen bonding enforced by crystal packing and vice versa in nitrophenyl-, nitropyridyl-, and nitropyrimidyl-bearing carbazoles.

This study concerns the synthesis, structural analysis, and optoelectronic properties of N-aryl and N-heteroaryl-9H-carbazoles in which any nitrogen atoms in the heteroaryl rings are positioned ortho- to the nitrogen atom of the carbazole. In principle, these N-atoms can display intramolecular hydrogen bonding (IHB) with the H1/8 protons of the carbazole. This IHB is apparent in the structures determined using X-ray crystallography, and the nitropyrimidyl compound in particular shows significant IHB stability with an almost perfectly planar structure. Here we show that when a substituent is present on the other ortho-site of the aryl moiety, any IHB in the crystal phase is being enforced primarily by molecular crystal packing by comparison with the calculated structure of their ground and first singlet excited state geometries in the gas phase. This is supported by the results of solution phase 1H-NMR spectroscopy, cyclic voltammetry, and UV-vis spectroscopy. A notable observation for the planar nitropyrimidyl compound is that it has a relatively wide HOMO-LUMO gap due to changes in the orbital distribution around the carbazole N-atom. These insights offer pathways for modulating optoelectronic behaviors in π-functional materials and highlight the impact of IHB and steric interactions in carbazole-based chromophores.