Chapter 4
Fluorescent Photoinduced Electron-Transfer Sensors
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The Simple Logic and Its Extensions Richard A. Bissell, A. Prasanna de Silva, H. Q. Nimal Gunaratne, P. L. Mark Lynch, Colin P. McCoy, Glenn Ε. M . Maguire, and Κ. R. A. Samankumara Sandanayake School of Chemistry, Queen's University, Belfast BT9 5AG, Northern Ireland
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 1a. 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 intrinsically fluorescent environments. Sensors like 3 in aqueous β -cyclodextrinsolution illustrate the possibilities of this idea. Fluorescence is visual. Its appeal has much to do with the fact that it directly assaults our first sense i.e. vision. When fluorescence is expressed in molecules, we have a phenomenon of some power which is capable of bridging the divide between the world of molecules and that of our own with photons. Unlike many phenomena in chemistry, molecular fluorescence can be easily eliminated by chemical command i.e. quenching (7,2). Thus 'on' and 'off states arise naturally. The possibility of negating or neutralizing the first command with a second results in fluorescence recovery and gives rise to reversible switching between 'on' and 'off states. Selective binding of a chemical species upon molecular recognition can lead to large perturbation of the host environment, especially when the guest is ionic. These perturbations can be exploited in a variety of ways in order to provide the first or second chemical commands that we require for fluorescence modulation. Conversely, the sensitivity of the host fluorescence to the guest occupancy endows the host molecules with chemosensory function. Our pet approach to such sensor molecules
0097-6156/93/0538-0045$06.00/0 © 1993 American Chemical Society In Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by COLUMBIA UNIV on February 20, 2013 | http://pubs.acs.org Publication Date: October 20, 1993 | doi: 10.1021/bk-1993-0538.ch004
46
FLUORESCENT CHEMOSENSORS FOR ION AND MOLECULE RECOGNITION
makes use of the photoinduced electron transfer (PET) principle, a subject which deservedly enjoys sustained popularity (3). Fluorescent PET sensors can be formalized as 'fluorophore-spacer-receptof systems of modular structure (4). The photon- and guest-interaction sites can be chosen to cater for various excitation/emission wavelengths and for various concentration ranges of a given guest (5). This choice has several constraints, however and these constitute the design logic of fluorescent PET sensors. The energy stored in the fluorophore excited state upon photon absorption must be sufficient to, say, oxidize the guest-free receptor and to simultaneously reduce the fluorophore (
