E→Z photoinduced isomerization and hydrogen bonding in the peri-acetamido substituted (1H-pyrrol-2-ylmethylene)benzocycloalkanones

An unusual effect of hydrogen bond strengthening upon coordination of the basic center with another H-bond donor, recently observed for OH⋯O⋯HN bifurcated system in (2Z)-7-hydroxy-2-(1H-pyrrol-2-ylmethylene)indan-1-one was examined for NH⋯O⋯HN intramolecular bifurcated H-bonds in the first synthesized N-[3-oxo-2-(1H-pyrrol-2-ylmethylene)-2,3-dihydro-1H-inden-4-yl]acetamide and N-[8-oxo-7-(1H-pyrrol-2-ylmethylene)-5,6,7,8-tetrahydronaphthalen-1-yl]acetamide. FT-IR, UV–vis, NMR and theoretical calculations including QTAIM analysis were applied to study the UV-induced reversible E⇆Z isomerization of the synthesized compounds. Depending on the strength and length determined by the structure of the molecule, the intramolecular AcNH∙∙∙O hydrogen bond in the E-isomers is either strengthened or weakened in the Z-isomers. The conclusion is made that weak hydrogen bonds may show unusual mutual reinforcement, whereas strong hydrogen bonds reveal antibate dependence of the lengths and energies of the components.