Effect of Extended π Conjugation on the Spectroscopic and Electrochemical Properties of Boron Difluoride Formazanate Complexes

The effect of extended π conjugation on the spectroscopic and electrochemical properties of boron difluoride (BF₂) formazanate complexes was studied by the systematic comparison of phenyl- and naphthyl-substituted derivatives. Each of the BF₂ complexes described was characterized by ¹H, ¹³C, ¹¹B, and ¹⁹F NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, UV–vis absorption and emission spectroscopy, and mass spectrometry. X-ray crystallography and electronic structure calculations were used to rationalize the trends observed, including direct comparison of 3-cyano-, 3-nitro-, and 3-phenyl-substituted BF₂ formazanate complexes. In all cases, the wavelengths of maximum absorption and emission were red-shifted as π conjugation was systematically extended (by replacing phenyl with naphthyl), fluorescence quantum yields increased (up to 10-fold), and electrochemical conversion of the formazanate complexes to their radical anion and dianion forms occurred at less negative potentials (easier to reduce)