Receptor,—Spacer—Receptor2 Systems

Almost all the examples discussed so far use covalent bonds to hold the receptor at a reasonable distance from the lumophore. However, alternative strategies are available. For instance, receptor2 selective for the guest ion of interest may be covalently linked to another receptor, which is chosen for its ability to selectively bind a lumophore. This approach has the reward of versatility. Once synthesized, the receptor,-spacer-receptor2 system may be combined with any one of several different lumophores with a range of optical properties. Any enthusiasm this may generate must be tempered because this approach can also have its dangers due to 

Lanthanide ion lumophores have also been included in receptor,-spacer-receptor systems outside of the PET design principle. Nocera has targeted ionic or nonionic aromatic guests.187-189 The aromatic character of the guest is critical since receptor2 (as we as receptor,) is chosen to be optically transparent in this scheme. Since the guest serves to receive photonic excitation for subsequent EET to the lanthanide lumophore, the guest switches the luminescence on.  

Organic lumophores can also be used in conjunction with receptor,-spacer-receptor2 systems. Aminoalkyl cyclodextrins are particularly suitable in this regard since amines and cyclodextrins show almost mutually exclusive receptor properties. 

The predictive power of the luminescent PET sensor principle is again apparent here. Further, the benzocrown ether and the amine receptors would selectively bind Na+ and H+, respectively. A remarkable feature here is that no molecular wiring is needed to allow the human operation of this two-input molecular device. The device self-selects its own ion inputs into the appropriate signal channels by means of the chemoselective receptor modules. Since the output signal is fluorescence, even a single molecule can interface with detectors in the human domain, including the dark-adapted eye. Tanaka s 45133 is another example where fluorescence quenching is achieved only when Ba2+ and SCN are present. This was mentioned in Section 6. Similarly, several sensor systems—1,17, and 21—could be employed 

LUMOPHORE-SPACERi-RECEPTORi-SPACER2-RECEPTOR2 SYSTEMS 

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