Fluorophore-spacer-receptor

In contrast to the -conjugated probe architecture utilizing an ICT process, the number of anion probes that rely on the fluorophore-spacer-receptor design and an active PET process is abundant [72], Again, anthracene and naphthalene... 

Rurack K, Resch-Genger U, Bricks JL et al (2000) Cation-triggered switching on of the red/near infra-red (NIR) fluorescence of rigid fluorophore-spacer-receptor ionophores. Chem Commun 2103-2104... 

Fluorescent PET (photoinduced electron transfer) sensors are considered to be those molecular systems where the binding of ions and other species leads to the perturbation of the competition between the de-excitation pathways of fluorescence and electron transfer. The early developments in this field are traced and the design logic of these sensors is detailed. A variety of examples drawn from different areas of chemistry are classified according to the fluorophore-spacer-receptor format and their photophysical behaviour is rationalized in terms of fluorescent PET sensor principles. Cases are pointed out where such experimental data are unavailable but desirable. During these discussions, the relevance of twisted fluorophore-receptor systems and the contrast with integrated fluorophore-receptor systems is noted. The utility of the fluorescence on-ofP phenomenon in these PET sensors for the area of molecular photoionic devices is pointed out. 

Several rather basic azamacrocycles act as receptor components within lumo/fluorophore-spacer-receptor systems (29) [82], (30) [83] and (31) [84], which are at least structurally related to PET sensors. However, the lack of any substantial luminescence/fluorescence quenching in the cation-free state suggests that the thermodynamic criteria for PET (Eq. (1)) are not met in some of these cases. The low luminescence quantum yields seen in some variants of (31) [84] are due to the intrusion of metal centered lowest excited states. Related cases, with or without macrocyclic units, (32) [85] and (33) [86] carry the... 

The discussion so far has centred on receptors based on the iminodiacetate element . However, even crown ethers can serve as receptors in sensors where the dicationic alkaline earths can elicit a larger fluorescence response than the alkali monocations. Charge density effects of this sort are particularly noticeable in solvents of relatively low polarity. The best examples are to be found among fluoroionophores [137] with integrated fluorophore and receptor moieties such as (53) [120] and (54) [121]. No PET sensors of this type are currently available, though case (32) [85] is an approximate example of a fluorophore-spacer-receptor system. 

Metal ions such as Na ", K" ", Ca ", etc., which have a close shell electronic configuration (ET precluded) and lack of any redox activity (eT prevented), are photophysically inactive. This makes possible the design of efficient OFF-ON fluorosensors. A classical example is given by de Silva s fluorophore-spacer-receptor... 

These systems are promising as potential labels due to high emission quantum yields and excited-state lifetimes that can be as long as several tenths of a millisecond (108). A cyclen (12-ane-N4) unit connected to a phenanthridine moiety in fluorophore-spacer-receptor conhguration (Fig. 26) exhibit strong Tb(III) based luminescence (109) in the absence of protons and oxygen. Few other luminescent lanthanide complexes are available in the literature (110,... 

The principle of photoinduced electron transfer is combined with the modular system Fluorophore-Spacer-Receptor to develop the phenomenon of cation-responsive fluorescence. pH controlled on-off fluorescence is demonstrated in the case of the dialkylaminoalkyl heterocyclic derivative la. The modular system is then extended in two directions. In the first of these, targetting/anchoring modules are added to allow the investigation of proton fields in microheterogeneous membrane media with high spatial resolution. The sensor family 2a-f is the realization of this approach. The second direction employs phosphorescent (instead of fluorescent) modules with/without protective shields to permit the development of phosphorescent pH sensing in an interference-free manner within... 

Fig. 2 Modular fluorophore-spacer-receptor format, where PET or EET processes can deactivate the excited state (either before or after capture of the target). [View this art in color at www.dekker.com.)...

Fig. 1 The fluorophore-spacer-receptor paradigm. It is expected that the interaction of the desired analyte with the receptor portion drastically modifies the emission of the nearby fluorophore. In the case illustrated, the recognition process restores the quenched fluorescence. Alternatively, the fluorescence before recognition may be on and after recognition be off.

These fluorescent probes are constructed either as fluorophore-spacer-receptor or as integrated fluorescent probes (Figure 14). In the former case, there is a spacer between the receptor and the signaling moieties that prevents the conjugation, whereas in the latter case the receptor is part of a r-electron system of the fluorophore. ... 

Sensor 14 [53] equips a fluorescent fluorophore-spacer-receptor system with a pyridine receptor for H" " which becomes more reducible by protOTiation. So 14 galvanizes fluorescence-quenching PET from the pyrazoUne fluorophore to the pyridine receptor (Mily when H" " arrives. 

Figure 17 Schematic representation of the fluorophore-spacer-receptor design assembly for fluorescent PET sensory systems.

Figure 19 The first fluorescent PET sensor for saccharides to have been rationally designed. In this illustration, the fluorophore-spacer-receptor construction is readily apparent anthracene represents the fluorophore methylene, the spacer and N-methyl-o-(aminomethyl)phenylboronic acid the receptor.

Figure 20 The first rationally designed boronic acid-based fluorescent PET sensor to display selectivity for D-glucose. The receptor-spacer-fluorophore-spacer-receptor assembly requires binding to occur at both receptors in order to restore fluorescence.

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