Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 15, Issue 13, Pages 3173-3185Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200802087
Keywords
anions; copper; crown compounds; dyes/pigments; polymethines
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Three mid-chain rigidified heptamethine cyanine dyes substituted with dimethylamino or cyclam groups at the meso position of the polymethine chain have been prepared and studied both experimentally and theoretically with respect to their spectroscopic response toward protons and various metal ions. Despite a potentially rich conformational ground-state chemistry, the spectroscopic studies revealed that the behaviour of the dyes is in agreement with Kuhn's classic polymethine dye theory. Coordination of metal ions and protonation of the alkyl amino groups in the cyclam moiety accordingly lead to moderate bathochromic shifts in absorption and reduced Stokes shifts. In contrast, protonation of the dimethylamino-substituted derivatives occurs at the polymethine chain, transforming the heptamethine into a trimethine-type chromophore with concomitant gigantic spectral blueshifts. In the presence of Cu-II in buffered aqueous solution, the cyclam-substituted dye selectively forms near-infrared absorbing aggregates. The latter allow the determination of citrate under realistic conditions by changes in absorption at wavelengths greater than 800 nm, which results from a disruption of the aggregates when citrate binds to Cu-II. The copper ions thus act as mediators in a signalling reaction that constitutes a colourimetric displacement assay.
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