Preferential solvation of meso -methyl BODIPYs with pyridine via pseudo -hydrogen-bonds.

This study explored unexpected pseudo -hydrogen bond interactions between meso -methyl BODIPYs and pyridine or acridine. NMR spectral evidence indicated that the meso -methyl group and BF 2 core of BODIPYs formed C-H⋯N and C-H⋯F-B pseudo -hydrogen bonds with pyridine, respectively. The weak binding strength was attributed to the preferential solvation of pyridine in the vicinity of meso -methyl BODIPYs in cyclohexane. The observations were explained by the formation of pseudo -hydrogen bonds based on the quantum theory of atoms in molecules (QTAIM) formalism. In contrast, acridine binds to BODIPY with a moderate binding strength. QTAIM formalism suggested the existence of the complementary pseudo -hydrogen bonds, which superficially seemed to rationalise the experimental observations. However, extensive NMR experiments have found no discrete geometry for the complex, indicating considerable geometric freedom. This discrepancy suggests that the static pictures based on the QTAIM analyses conflict with the enthalpy-entropy compensation principle in essential thermodynamics.