Structural studies of meso-CF3-3(5)-aryl(hetaryl)- and 3,5-diaryl(dihetaryl)-BODIPY dyes by 1H, 13C and 19F NMR spectroscopy and DFT calculations

The synthesis of fluorophores of the family of meso-CF3-substituted BODIPY 1a–e obtained from the corresponding dipyrromethanes 2a–e involves the oxidation of the latter followed by the complexation of the dipyrromethenes formed to boradiazaindacenes. The transformation of dipyrromethanes 2a–e into BODIPY 1a–e is accompanied by the characteristic high-frequency shifts of the 13C NMR signals of pyrrole ring carbons in the same position relative to the nitrogen atoms and substituents. Acceptor character of the BF2 group relative to the pyrrole moiety is also manifested in the significant increase of the 13C chemical shifts of carbon atoms of the aryl or hetaryl substituents in the positions 3 and 5 of the BODIPY complexes in comparison with those of the initial dipyrromethanes. In the thienyl derivatives of BODIPY, a prominent high-frequency shift of the 1H resonance of the proton in position 3 of the thiophene ring, induced by the spatial proximity of the fluorine atoms of the BF2 group, is revealed. Such a proximity is the cause of the effective transmission of the long-range 19F–13C spin–spin coupling via the intermediate CH bond. The calculations in the framework of density functional theory (DFT) of the thienyl derivative of meso-CF3-BODIPY show the preference in energy for the anti conformation of the thienyl ring with respect to the boradiazaindacene core, so that the hydrogen atom in position 3 of the thienyl ring and one of the fluorine atoms of the BF2 group turn out to be spatially close to each other by a distance less than the sum of their van der Waals radii. Topological analysis of the F⋯H interaction parameters within the Quantum Theory of Atoms In Molecules (QTAIMs) and the orbital interactions in considering of the Natural Bond Orbitals (NBOs) formalism reveal that this interaction is much weaker than the conventional intramolecular hydrogen bond, although it is quite sufficient to explain the observed spectral effects.