Sensing photoinduced electron transfer

The scientists from Hong Kong reported83 on a sol-gel derived molecular imprinted polymers (MIPs) based luminescent sensing material that made use of a photoinduced electron transfer (PET) mechanism for a sensing of a non-fluorescent herbicide - 2,4-dichlorophenoxyacetic acid. A new organosilane, 3 - [N,V-bis(9-anthrylmethyl)amino]propyltriethoxysilane, was synthesized and use as the PET sensor monomer. The sensing MIPs material was fabricated by a conventional sol-gel process. 

Designing a conjugated polymer sensor based on FQ, however, is not only a matter of making a fluorescent polymer for which the photoinduced electron transfer reaction is energetically favorable. There are other important factors that must be considered and requirements that must be met to rehably detect any analyte of interest, including TNT, from the vapor phase. In the broadest sense, these considerations distill to the two primary considerations for any sensing system, sensitivity and selectivity. 

Figure 6 Sensing by a photoinduced electron transfer (PET) mechanism. A ntt or charge transfer excited state is repositioned relative to fluorescent nn upon interaction of analyte A with a lone pair of electrons proximate to a reporter site.

Artificial Sugar-sensing Systems utilizing Photoinduced Electron Transfer (PET) I 291... 

Cyclodextrins have been used for enhancing the phosphorescence of guests by encapsulation [12]. de Silva et al. employed cyclodextrins as transparent shields to protect the phosphor molecule sterically from contact with its environment (especially O2) while allowing access to photons. Sensing remains viable because the proton receptor module is not enveloped (Figure 9) [13]. The authors used this system as a phosphorescent PET (photoinduced electron transfer) chemosensor for... 

The majority of the research on the photochemistry of porphyrins linked to other moieties has been in the area of photoinduced electron transfer, and the systems studied are all in some sense mimics of the photosynthetic process described above. The simplest way to prepare a system in which porphyrin excited states can act as electron donors or acceptors is to mix a porphyrin with an electron acceptor or donor in a suitable solvent. Experiments of this type have been done for years, and a good deal about porphyrin photophysics and photochemistry has been learned from them. Although these systems are easy to construct, they have serious problems for the study of photoinduced electron transfer. In solution, donor-acceptor separation and relative orientation cannot be controlled. As indicated above, electron transfer is a sensitive function of these variables. In addition, because electron transfer requires electronic orbital overlap, the donor and acceptor must collide in order for transfer to occur. As this happens via diffusion, electron transfer rates and yields are often affected or controlled by diffusion. As mentioned above, porphyrin excited singlet states typically have lifetimes of a few nanoseconds. Therefore, efficient photoinduced electron transfer must occur on a time scale shorter that this. This is difficult or impossible to achieve via diffusion. Thus, photoinduced electron transfer between freely diffusing partners is confined mainly to electron transfer from excited triplet states, which have the required long lifetimes (on the micro to the millisecond time scale). 

Photoinduced electron transfer studies have been performed in a more limited number of cases, due to the fact that the simultaneous presence of two reaction partners with appropriate characteristics is necessary. The study of suitably functionalized dendrimers can furnish information on the accessibility of the inner shell or core to external reagents (such as molecular oxygen or other small molecules) and on interchromophoric interactions within the dendritic structures. Electro-inactive dendrimers can also be important in regulating the electron transfer reactions between hosted partners in this sense dendrimers may also constitute a peculiar medium to investigate electron transfer reactions in restricted spaces. 

Photoinduced Electron Transfer in a Calixarene-based Supermolecule Designed for Mercury Ion Sensing [10]... 

The exploitation of photoinduced electron transfer (PET) (Fig. 1) has led to the development of an enormous range of molecules capable of sensing a wide array of analytes [8], Judicious alteration of these sensors has stimulated the production of molecules capable of displaying characteristics reminiscent of logic operations. 

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... 

Scheme 10 Rationale for the photoinduced electron transfer (PET) sensing mechanism.

Kuhn [24, 26, 34], Mobius [28, 35], and others [36, 37] have studied the distance dependence of the rate of photoinduced electron transfer in LB films. Their observed dependence agrees in a qualitahve sense with the other experimental [38-48] and theorehcal [29] results described by Eq. (11) and Eq. (12). As they did, the distances between the three functional moieties, that is. A, S, and D, within LB monolayers can be closely controlled at known values. 

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