Fluorescent boron complexes of 25-oxa­smaragdyrins containing axial silyloxy groups

Abstract

A series of fluorescent B^III complexes of meso-triaryl-substituted 25-oxasmaragdyrins containing axial silyloxy groups were prepared in 80–85 % yields by treating B(OH)₂ complexes of meso-triaryl 25-oxasmaragdyrins with different alkyl/aryl chlorosilanes in toluene in the presence of base at reflux temperature followed by simple column chromatographic purification. The eight axially silyloxy-substituted B^III complexes of smaragdyrins were freely soluble in common organic solvents and were characterized by HRMS, 1D and 2D NMR, absorption, fluorescence and electrochemical studies, as well as by X-ray crystallography in the case of one of the complexes. The crystal structure revealed that the smaragdyrin macrocycle is almost planar and that the –OSiMe₃ groups on B^III are oriented in the up and down directions relative to the macrocycle. The absorption studies indicated that the axially silyloxy-substituted B^III smaragdyrins are strongly absorbing in the 710–440 nm region with absorption coefficients higher than those of B(OH)₂-smaragdyrin complexes. The silylation of –OH groups at the B^III centre in the B^III smaragdyrin complex results in five- to six-fold enhancements in the fluorescence quantum yields and in longer singlet state lifetimes. Thus, the axial silyloxy B^III complexes are strongly fluorescent with quantum yields in the 0.65–0.78 range and singlet state lifetimes in the 4.3–4.9 ns range. The complexes are highly stable under redox conditions and each exhibit two reversible oxidations and one reversible reduction.