A pH-dependent fluorescence

Wolfbeis, O. S. Schipfer, R. Knierzinger, A. pH-dependent fluorescence spectroscopy. Part 12. Flavone, 7-hydroxyflavone, and 7-methoxyflavone. J. Chem. Soc., Perkin Trans. 2 1981, 1443-1448. 

The first pH indicators studied possessed the acid-base site (phenol, aniline, or carboxylic acid) as an integral part of the fluorophore. Structurally, in the most general sense, pH sensitivity is due to a reconfiguration of the fluorophorets re-electron system that occurs on protonation. Consequently, the acid and the base forms often show absorption shifts and also, when the two forms fluoresce, emission shifts or at least, when only one form emits, a pH-dependent fluorescence intensity. This class of compounds has been reviewed 112 and the best structures have to be designed according to the medium probed and the technique used. After a short consideration of physiological pH indicators we will describe the main photophysical processes sensible to protonation. 

All in all, uptake studies with targeted systems carrying a pH-dependent fluorescent label in conjunction with monensin treatment at the end of the incubation period offer the possibility to discriminate between different intracellular trafficking pathways. At 37°C, an increasing fluorescence emission upon addition of monensin points toward accumulation of the targeted drug delivery system within acidic compartments of the cell, such as lysosomes. 

Another path in which the science of cycloparaphenylenes is expanding is via the inclusion of heteroatoms into the CPP backbone. Itami reported the synthesis of [18]CPP with two 2,2 -bipyridyl units replacing four of the phenyl rings in the CPP structure (Fig. 42) [46]. This [14,4]CPPy was synthesized using established methodology developed by the Itami group and shows a pH dependent fluorescence that can be reversibly red shifted by addition of acid. These preliminary results may open a door towards the controllable fluorescence of nitrogen-doped CPPs. 

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See also in sourсe #XX -- [ ]

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