Charge transfer, intramolecular sensors

Because of their intrinsic higher sensitivities and selectivities, the luminescent sensors are considered to be convenient. The signaling events rely on emission quenching or enhancement related to photoinduced electron transfer (PET sensors), on new emissions originating by excimer or exciplex formation, on changes in quantum yield, and lifetime of the luminescence by intramolecular charge transfer (PCT sensors). 

Figure 8 Structures of boronic acids for sugar sensing, (a, b) Intramolecular charge transfer (ICT) sensors, (c-e) Photoinduced electron...

Molecular rotors are fluorophores characteristic for having a fluorescent quantum yield that strongly depends on the viscosity of the solvent [50], This property relies on the ability to resume a twisted conformation in the excited state (twisted intramolecular charge transfer or TICT state) that has a lower energy than the planar conformation. The de-excitation from the twisted conformation happens via a non-radiative pathway. Since the formation of the TICT state is favored in viscous solvents or at low temperature, the probability of fluorescence emission is reduced under those conditions [51]. Molecular rotors have been used as viscosity and flow sensors for biological applications [52], Modifications on their structure have introduced new reactivity that might increase the diversity of their use in the future [53] (see Fig. 6.7). 

In fac-(bpy)Re(I) (CO)3-A (where bpy is 2,2 -bipyridine and A is an aromatic amine), the d-7t(Re)—>jr (bpy) MLCT fluorescent excited state is strongly quenched via intramolecular aniline-Re charge transfer leading to a nonfluorescent LLCT state. By incorporating the donor amino group belonging to the A moiety into a crown-macrocycle, Schanze and Mac Queen(137) have provided a new luminescent cation sensor whose quantum yield of fluorescence raises from 0.0017 (without cation) to... 

K. Hamasaki, H. Ikeda, A. Nakamura, A. Ueno, F. Toda, I. Suzuki, andT. Osa, Fluorescence sensors of molecular recognition. Modified cyclodextrins capable of exhibiting guest-responsive twisted intramolecular charge transfer fluorescence, /. Am. Chem. Soc. 775,5035-5040(1993). 

There are various potential applications of photophysical phenomena in analytical chemistry. The relatively short lifetimes of most excited states, however, is a serious drawback to the construction of practical devices but studies which focus on finding ways to extend triplet lifetimes have now been described by Harriman et al. Kneas et al. have examined new types of luminescent sensor on polymer supports, and both Neurauter et al. and Marazuela et al. have designed sensors based on the ruthenium(II) polypyridine complex for the detection of carbon dioxide. A system, based on the formation of twisted intramolecular charge transfer states, has been devised for measuring the molecular weight of polymeric matrices (Al-Hassan et a/.), and the chemical reactivity at the interface of self-assembled monolayers has been assessed using fluorescence spectroscopy (Fox et al). 

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