PET photoinduced electron transfer cation sensors

PET (photoinduced electron transfer) cation sensors 10.3.2.1 Principles 

PET fluorescent sensors for cations have been highly developed (Bissell et al., 1993). The principle is the same as for PET pH indicators, described in Section 10.2.2.5. 

Most PET fluorescent sensors for cations are based on the principle displayed in Eigure 10.7, but other photoinduced electron transfer mechanisms can take place tvith transition metal ions (Eabbrizzi et ah, 1996 Bergonzi et ah, 1998). In fact, 3d metals exhibit redox activity and electron transfer can occur from the fluorophore 

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Figure 16.3 Principle of the PET (photoinduced electron transfer) chemosensor. (a) The HOMO level of the unbound receptor acts as an electron donor and effectively quenches the fluorescence of the sensor, (b) Upon coordination of the substrate, the energy of the HOMO level of the receptor is decreased due to electrostatic or coordinative interaction with cationic species and PET quenching is no longer possible...

This type of probe, often called fluorescent photoinduced electron transfer (PET) sensors, has been extensively studied (for reviews, see Refs. 22 and 23). Figure 2.2 illustrates how a cation can control the photoinduced charge transfer in a fluoroiono-phore in which the cation receptor is an electron donor (e.g., amino group) and the fluorophore (e.g., anthracene) plays the role of an acceptor. On excitation of the fluorophore, an electron of the highest occupied molecular orbital (HOMO) is promoted to the lowest unoccupied molecular orbital (LUMO), which enables photoinduced electron transfer from the HOMO of the donor (belonging to the free cation receptor) to that of the fluorophore, causing fluorescence quenching of the latter. On... 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

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