New members of fluorescent 1,8-naphthyridine-based BF2 compounds: selective binding of BF2 with terminal bidentate NΛNΛO and NΛCΛO groups and tunable spectroscopy properties

Intensely luminescent 1,8-naphthyridine-BF2 complexes 1-9 containing terminal bidentate (NNO)-N-Lambda-O-Lambda and/or (NCO)-C-Lambda-O-Lambda groups are synthesized and structurally characterized by X-ray diffraction, electrospray ionization mass spectrometry, H-1 and F-19 NMR spectroscopy and elemental analysis. Complexes 1-4 are synthesized from 2-acetamino-1,8-naphthyridine derivatives by a facile route. Selective bonding modes and the chemical stability of complexes 5 and 6 obtained by reacting BF3 center dot Et2O with 1,8-naphthyridine derivatives bearing dual-functional groups ((NCO)-C-Lambda-O-Lambda and (NNO)-N-Lambda-O-Lambda) are investigated by crystal structure analysis and time-dependent density functional theory calculations. The products containing a BF2 core bound to a (NCO)-C-Lambda-O-Lambda chelating group are energetically favorable and can expand the range of derivatives by substitution at the 2-position. In this regard, a free -NH2 group at the 2-position of complex 7 obtained from 5 can be functionalized under a variety of pH conditions to generate complexes 8 and 9, which bear flexible coordination arms that can be used to recognize certain transition metals. The photophysical properties of the complexes are examined in solution and solid state at room temperature. Compared with those of the starting naphthyridine-based compounds, the naphthyridine-BF2 complexes display desirable light-absorbing properties and intense solution and solid-state emission with large Stokes shifts. Complex 4 in solution exhibited an emission quantum yield of 0.98. In complexes 5-9, the binding sites for the BF2 core change from (NNO)-N-Lambda-O-Lambda to (NCO)-C-Lambda-O-Lambda, which leads to red shifts of absorption and emission, excellent chemical stability and high emission quantum yields.