5-Arylvinylene-2,2′-bipyridyls: Bright “push–pull” dyes as components in fluorescent indicators for zinc ions

• 2,2′-Bipyridyl (bipy) is used as an e-acceptor in push–pull type fluorophores.
• Zn(II) binding leads to emission red shifts of bipy-containing push–pull dyes.
• Fluorescent heteroditopic ligands respond to Zn(II) over a large concentration range.
• FRET-based probes have enhanced photostability and narrow bands of emission.

This article reviews the zinc(II)-dependent photophysical properties of arylvinylenebipyridines (AVBs), a class of fluoroionophores in which 2,2′-bipyridyl and an aryl moiety are electronically conjugated. Zinc(II) binding of an AVB may lead to an emission bathochromic shift of the fluoroionophore without diminishing its fluorescence quantum yield. This observation can be explained using the excited state model of electron donor–π bridge–electron acceptor “push–pull” fluorophores, in which the bipy moiety acts as an electron acceptor, and zinc(II)-coordination strengthens its electron affinity. The spectral sensitivity of bipy-containing fluoroionophores, such as AVBs, to zinc(II) can be exploited to prepare fluorescent indicators for this ion. In several cases, AVB moieties are incorporated in fluorescent heteroditopic ligands, so that the variation of zinc(II) concentration over a relatively large range can be correlated to fluorescence changes in either intensity or color. AVB fluoroionophores are also used to introduce an intramolecular Förster resonance energy transfer (FRET) strategy for creating zinc(II) indicators with high photostability and a narrow emission band, two desired characteristics of dyes used in fluorescence microscopy.

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