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).