Fluorescent boron complexes of 25-oxa­smaragdyrins containing axial silyloxy groups

Ganapathi, Emandi ; Chatterjee, Tamal ; Ravikanth, Mangalampalli (2015) Fluorescent boron complexes of 25-oxa­smaragdyrins containing axial silyloxy groups European Journal of Inorganic Chemistry, 2015 (29). pp. 4810-4818. ISSN 1434-1948

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/ejic.20...

Related URL: http://dx.doi.org/10.1002/ejic.201500718


A series of fluorescent BIII 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 BIII 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 BIII are oriented in the up and down directions relative to the macrocycle. The absorption studies indicated that the axially silyloxy-substituted BIII 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 BIII centre in the BIII smaragdyrin complex results in five- to six-fold enhancements in the fluorescence quantum yields and in longer singlet state lifetimes. Thus, the axial silyloxy BIII 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.

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