Ditopic sensors

Ditopic sensors take advantage of two non-equivalent recognition sites. If one is a boronic acid unit, selectivity can be introduced for saccharidic species such as uronic acids, amino sugars and so forth. To achieve this, the second recognition unit must display, and be positioned, such that effective 

The fluorescent PET sensors 94 and 95 prepared by Cooper demonstrated selectivity for an ammonium terminus at the monoazacrown ether receptor and selectivity for saccharides at the boronic acid receptor. This dual-targeted approach induced D-glucosamine hydrochloride selectivity within the system. 

Curiously, the azacrown ether imparts little if any additional stability to the complex on binding. Nevertheless, binding at the azacrown ether is a prerequisite of generating a fluorescent response as it directly participates in PET, the system requiring both recognition units to complex for fluorescence to be restored. The observed stability constants (. fobs) for sensors 94 and 95 with d-glucosamine were 18 and 17 respectively, in 33.2 wt% ethanol/water at pH 7.18 (triethanolamine buifer). 

Boronic acid-based fluorescent PET sensors developed for the recognition of simple monosaccharides have been extended to include ditopic recognition sites and so introduce selectivity for a diverse range of guest species. 

Developed around the D-glucose sensor 80 the allosteric diboronic acid biscrown ether 96 prepared by James displayed a fluorescence increase upon binding D-glucose. Addition of metal cations with a similar ionic diameter to potassium favour a 1 1 (metal ion/sensor) binding motif, causing the 15-crown-5-ether receptors to sandwich the metal ion. The metal-induced conformational 

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A related anthracene-based ditopic sensor (11.28) has been constructed, which can recognise diammonium cations along the same lines as 3.103 (Section 3.12.3). Use of the anthracene-derived group as a spacer as in 11.27 gives fluorescent recognition of 11 N1 (Cl I NI I3+ guests as a function of spacer length.20... 

The boronic acid, guanidinium appended fluorescent PET sensor 98 developed by Wang and co-workers displayed selectivity for o-glucarate. As with other ditopic sensors both receptor units must be occupied for fluorescence to... 

Koskela SJM, Fyles TM, James TD (2005) A ditopic fluorescent sensor for potassium fluoride. Chem Commun 7 945-947... 

Compared to systems based on a single interaction, cooperative sensors based on multiple, concerted, inter-molecular interactions between the components exhibit higher discrimination capabilities and enhanced functions. As examples for this class of sensors, two systems, both based on the absorption/luminescence properties of the porphyrin nucleus are illustrated in Fig. 5. Structure (a) represents a ditopic receptor where cooperative binding of a cation by the amide-appended calixarene and of an anion by the Zn-porphyrin allows recognition of complete binary metal salts. Structure (b) incorporates two cyclodextrin cavities and relies on the modification of the Fe(III) complex spectroscopic properties by inclusion of guests in these hydrophobic sites.This species is able to sense the presence of benzylmercaptane and 1-adamantanecarboxylate in four... 

Fig. 5 Two examples of porphyrin-based cooperative optical sensors, (a) A ditopic receptor for binary metal salts. (h) A cyclodextrin-capped porphyrin system able to sense and report in four distinguishable association modes. ...

As in the above case of the binding of diammonium substrates to the macrotricyclic coreceptors, the present chain length selection also describes a linear recognition process based on structural complementarity between the dianionic substrates and the coreceptors in a ditopic binding mode (see (28)). In both cases, the receptor molecule acts as a discriminating sensor of molecular length. 

The ditopic fluorescent sensors 100 and 101 have been developed by Koskela as reversible AND logic gates with selectivity for potassium fluoride.The binding of fluoride with boronic acids is well documented. In this case the sp hybridised boronic acid, which is a hard Lewis acid, interacts strongly with the fluoride anion, which is a hard Lewis base, to become sp hybridised. The potassium cation is held in situ partly by the crown ether and partly by the electrostatic interaction with the fluoride anion. This co-operative... 

Scheme 30 Schematic representation of the ditopic fluorescent PET sensors 100 and 101, demonstrating selective and reversible binding of potassium fluoride with concurrent modulation of fluorescence intensity. The initial unbound complex is off , addition of KF generates a large increase in fluorescence intensity turning the system on , removal of potassium by addition of [2.2.2]-cryptand (4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo[ 8.8.8 J-hexacosane)...

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