Internal charge transfer sensing

The PET systems of the aminoalkyl aromatic type discussed so far display a very simple behavior in that luminescence intensity (or quantum yield) is the only variable. Such systems are very user-friendly as a result and tolerate a wide variety of communication wavelengths. However these simple systems could be adapted to include an additional absorptiometric sensing channel which can confirm the results of ion density (pH say) obtained via luminescence. Of course, such increased user-confidence is only attained with a proportionate reduction in simplicity. Now excitation needs to be done at the isosbestic wavelength. These systems, e.g. 11 and 12, use a push-pull fluorophore with electron donor and acceptor substituents which give rise to internal charge transfer (ICT) excited states. In contrast, the simple PET systems employed aromatic hydrocarbon fluorophores with essentially pure nn excited states. The charge separation in ICT states can cause electrostatic... 

Another mechanism used to manipulate fluorescence characteristics of a fluorophore is called internal charge transfer (ICT). Principles of ICT were first applied in an effort to rationalize increased acidity of phenol." However, until Valeur s reports, generalization of these ideas and systematic exploitation in metal sensing were not realized. 

As a result of the Y-fragment relaxation towards the internal equilibrium also with the X-fragment, the charge transfer may spontaneously occur in Y, which in turn will influence the hardness (the chemical potential) of the X-fragment in the sense stated by the principle AIM-HDl ... 

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