Fluoroionophores
Fluoroionophores are large ligands that are described as supramolecular systems; they con- sist of two components joined together covalently - an ionophore ('ion-lover') molecule linked to a light-sensitive fluorophore (light-lover'). The role of the first unit is to capture metal ions, whereas the role of the second unit is to capture light and emit it at a different wavelength as fluorescence, which can then be detected and measured. It is the impact of a captured metal on the fluorescence process that is at the core of the analytical application. These fluoroionophores are designed to sense metal ions selectively; ion recognition involves the ionophore binding the metal ion through coordinate covalent bonds, whereas the signalling process of the fluorophore depends on photophysics. Typically, the pro- cess involves metal ion control of photo-induced electron transfer (Figure 9.6). Selective metal-ion detection using a fluoroionophore arises because of the variation of colour (or fluorescence wavelength) with metal ion. Even the various alkali and alkaline earth ions can be distinguished.
Phosphorescence or fluorescence of complexes in the visible region has also permitted other developments of note. For example Pt(II). Cu(II). Rh(III) and Ir (III) complexes of polydentate ligands such as aromatic N-donors, and mixed N- and P- or C-donor com- pounds produce emissions in the visible region, and are being developed as optical light emitting diodes (OLEDs) or phosphorescent optical light emitting diodes (PHOLEDs).
Figure 9.6
A cartoon of a fluorescent 'switch', turned on or off (quenched) depending on the absence or presence of a metal ion. The ionophore (the cyclic polyether) is the metal-binding component, the fluorophore (the fused-ring aromatic unit) is the component activated by light. Complexation stops electron transfer that otherwise quenches fluorescence.
Ruthenium complexes used to lead research in photochemistry of metal compounds, but rhodium complexes have recently overtaken them as the key target compounds due to their applications in OLEDs. This is a lively and ever-changing field; for example, over 90% of luminescent iridium(III) complexes have been reported only in the six years to the beginning of 2009. With their luminescence 'tuneable' through ligand choice, iridium complexes are firm candidates for optical display applications.
