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Issue 10, 2013
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Structural and photophysical studies on gallium(III) 8-hydroxyquinoline-5-sulfonates. Does excited state decay involve ligand photolabilisation?

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Abstract

Multinuclear (1H, 13C and 71Ga) magnetic resonance spectroscopy (1D and 2D), DFT calculations and luminescence techniques have been used to study 8-hydroxyquinoline-5-sulfonate (8-HQS) and its complexes with Ga(III) in aqueous solutions. The study combines the high sensitivity of luminescence techniques and the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to obtain a complete understanding of the complexation between the Ga3+ ion and 8-HQS, and how this influences the luminescence behaviour. A full speciation study has been performed on this system and three complexes detected, with (metal : ligand) 1 : 1, 1 : 2 and 1 : 3 stoichiometries, the results being consistent with those previously found for the system Al(III)–8-HQS. Complexation in these systems is relevant to their potential biomedical, sensing and optoelectronic applications. On binding to Ga(III), a marked increase is seen in the intensity of the 8-HQS fluorescence band, which is accompanied by changes in the absorption spectra. These support the use of 8-HQS as a sensitive fluorescent sensor to detect Ga3+ metal ions in surface waters, biological fluids, etc., and its metal complexes as an emitting or charge transport layer in light emitting devices. However, the fluorescence quantum yield of the Ga(III)–8-HQS 1 : 3 complex is about 35% of that of the corresponding system with Al(III). Although this may be due in part to a heavy atom effect favouring S1 → T1 intersystem crossing with Ga3+, this does not agree with transient absorption measurements on the triplet state yield, which is lower with the Ga(III) system than with Al(III). Instead, it is suggested that photolabilisation of ligand exchange plays a major role in nonradiative decay of the excited state and that this is more efficient with the Ga3+ complex. Based on these results, suggestions are made of ways of enhancing fluorescence intensity in metal complexes with 8-HQS by inhibiting ligand exchange using surfactant complexation for applications in either sensing or optoelectronics.

Graphical abstract: Structural and photophysical studies on gallium(iii) 8-hydroxyquinoline-5-sulfonates. Does excited state decay involve ligand photolabilisation?

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Publication details

The article was received on 08 Oct 2012, accepted on 10 Dec 2012 and first published on 10 Dec 2012


Article type: Paper
DOI: 10.1039/C2DT32587J
Citation: Dalton Trans., 2013,42, 3682-3694
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    Structural and photophysical studies on gallium(III) 8-hydroxyquinoline-5-sulfonates. Does excited state decay involve ligand photolabilisation?

    M. L. Ramos, A. R. E. de Sousa, L. L. G. Justino, S. M. Fonseca, C. F. G. C. Geraldes and H. D. Burrows, Dalton Trans., 2013, 42, 3682
    DOI: 10.1039/C2DT32587J

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