Fluoro-substituent-participating hydrogen bonding in the non-coplanar arrangement of aromatic rings in cis -1,2-bis(2-fluorophenyl)-3,8-dimethoxyacenaphthene-1,2-diol in solution

The involvement of fluoro substituents in both classical and non-classical intramolecular hydrogen bonds was investigated using nuclear magnetic resonance (NMR) spectroscopy and structural characterisation. The title compound, cis-1,2-bis(2-fluorophenyl)-3,8-dimethoxyacenaphthene-1,2-diol, contains two 2-fluorophenyl groups attached at the cis-1,2-positions of the acenaphthene framework, forming a non-coplanar arrangement of accumulated aromatic rings. One of fluoro substituents participated simultaneously in two types of intramolecular hydrogen bonds, namely a classical O–H…F–(sp2)C hydrogen bond and a non-classical C(sp2)–H…F–(sp2)C hydrogen bond. The effects of the solvent environment on these intramolecular hydrogen-bonding interactions were evaluated through variable-temperature NMR experiments in different solvents. Comparison of the NMR spectra with those of two homologous compounds, in which one or both hydroxy groups at the 1,2-positions of the acenaphthene ring were methylated, revealed distinct conformational distributions. Analysis of the conformer populations enabled identification of the spatial arrangements associated with the fluoro-substituent-participating hydrogen bonds. The solution-state structure elucidated by NMR spectroscopy was compared with the X-ray crystal structure of the title compound, with particular emphasis on similarities in the non-covalent interactions governing the solution and crystalline states. These findings provide useful information for understanding and designing high-performance polymers and functional polymers bearing fluoro substituents.