Metallochromic merocyanines of 8-hydroxyquinoline series
Introduction
In recent years, Kubo et al. have published a number of papers on metallochromic near-IR dyes of the indoaniline type containing a metal-binding fragment, 5-substituted 8-hydroxyquinoline residue [1], [2], [3], [4]. Interaction of these dyes with metal salts resulted in high bathochromic shifts of their absorption maxima, along with a significant increase in absorption intensities, an obvious advantage over the other types of ionophores [5], [6], [7], [8], [9].
The idea of binding the polymethine chromophore to the 8-hydroxyquinoline nucleus is not entirely new. Phillips et al. prepared some samples of this group of compounds much earlier. A number of merocyanines based on 5- and 7-substituted 8-hydroxyquinoline were synthesized and studied as analytical reagents [10], [11], [12], [13], [14]. However, a knowledge of their spectral and metallochromic properties and the effect of metal ions on the dye colour is inadequate. Ionofluorophoric properties of the above-mentioned dyes were not studied at all, except for the investigation on the ionofluorochromism of spiropyranes containing the 8-hydroxyquinoline core [15].
Since the chromophore structure of indoaniline dyes is close to that of cyanines, we have studied, in detail, the merocyanine dyes based on 5- and 7-substituted 8-hydroxyquinoline; the dye series under study was somewhat extended and their structure was modified as compared to Refs [10], [11], [12], [13], [14]. Spectroscopic investigation of their interaction with transition metal salts has been performed. Our main attention has been paid to the investigation of the effect of metal ions on the absorption spectra of these dyes.
Section snippets
Experimental
Absorption spectra were recorded on a Shimadzu UV-3100 spectrophotometer, and fluorescence spectra on a Hitachi-850 fluorescent spectrophotometer. Proton NMR spectra were obtained with a Varian VXR-300 instrument at 300 ÌHz using tetramethylsilane as internal standard. The purity of metal salts used for the spectral experiments was >99%. 5-Formyl-8-hydroxyquinoline 1 and 7-formyl-5-chloro-8-hydroxyquinoline 3 were prepared according to the published procedures [16], [17]. The yields, melting
Results and discussion
The merocyanine dyes used in this study were prepared starting from 5- and 7-formyl derivatives of 8-hydroxyquinoline 1 and 3.
The synthesis of merocyanines from 5-formyl-8-hydroxyquinoline 1 was performed via the condensation of aldehyde 1 with the Fischer base or 2,3-dimethylbenzothiazolium p-toluenesulfonate in ethanol (2a and 2b, respectively), whereas 2c was obtained from 1,2-dimethylquinolinuim iodide in pyridine in the presence of the equimolar amount of piperidine (Scheme 1).
Merocyanines
Conclusion
Thus, the merocyanine dyes containing the fragment of 8-hydroxyquinoline have been found to display significant metallochromic properties and can be used as indicators for the presence of transition metal cations. However, bathochromic effects are observed only for the derivatives substituted at the C-5 position and containing end nuclei of low basicity. This is in agreement with the results obtained for the structurally analogous compounds of the indoaniline type [1], [2], [3], [4]. Remarkable
References (17)
- et al.
Anal. Chim. Acta
(1968) - et al.
Chem. Lett.
(1987) - et al.
J. Chem. Soc. Perkin Trans. I
(1989) - et al.
J. Chem. Soc. Perkin Trans. I
(1990) J. Chem. Soc. Perkin Trans. I
(1994)- et al.
Chem. Ber.
(1980) - et al.
Chem. Ber.
(1981) - et al.
Acc. Chem. Res.
(1985)