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Luminescent tris(8-hydroxyquinolates) of Bismuth(III)

Abstract

Luminescent homoleptic bismuth(III) complexes have been synthesized by adding several functionalized 8- hydroxyquinolate ligands to bismuth(III) chloride in a 3:1 mole ratio in ethanol or tetrahydrofuran (THF). These complexes have been studied with the use of single X-ray diffraction (XRD) analysis, UV-vis spectroscopy, fluorescence spectroscopy, and density functional theory (DFT) calculations in order to determine the structures and photophysical properties of these complexes. Solution dimerization of the mononuclear tris(hydroxyquinolate) complexes was quantified using UV-vis spectroscopy. Two dimeric compounds were characterized by single crystal X- ray analysis; bis([mu]₂-5-chloro-7-iodo-8- hydroxyquinolinato)tetra(5-chloro-7-iodo-8- hydroxyquinolinato)dibismuthine and bis([mu]₂-7-bromo-8- hydroxyquinolinato)tetra(7-bromo-8- hydroxyquinolinato)dibismuthine 2-tetrahydrofuran. Tris(2- (diethoxymethyl)-8-quinolinato)bismuth and triethylammonium tris(7-iodo-8-hydroxyquinolinato-5- sulfonate)bismuthate were found to be monomers in the solid state. Both compounds exhibit a higher energy emission feature, which DFT calculations assign to a transition between the HOMO ([pi] orbital localized on the quinolate phenoxide ring) and LUMO ([pi] orbital localized on the quinolate pyridyl ring). Both compounds showed a single emission band at low concentrations and dual emission spectra at higher concentrations. Several other compounds exhibit dual emission peaks on UV excitation that show a similar concentration dependence. UV-vis spectra of these compounds show a deviation from Beer's Law, which can be modeled as a monomer-dimer equilibrium. Excitation and emission spectra show that the monomer emits at the shorter wavelength. Electronic structure calculations support the experimental result with the HOMO -LUMO gap of 2.156 eV for the monomer significantly larger than for the dimers (1.772 eV and 1.915 eV) which correlate well with the energies of the emission peaks

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