Internal Charge Transfer ICT

Aoyama has also shown that 5-indolylboronic acid (3) imdergoes fluorescence quenching upon complexation with oligosaccharides [38]. The stability constants of monosaccharides followed the trend observed by Lorand and Edwards [12] however, higher oligomers of saccharides enjoyed increased stabilization relative to lower oligomers due to a secondary interaction with the indole N-H. The observed stability constants (fC pp) for 3 were D-fructose 630 M , D-glucose 7.1 M and D-melibiose 58 in water at pH 9. 

Anthrylboronic acids (1, 2) used by Czamik display only a small fluorescence change. By screening eight aromatic boronic acids it was shown that 4 and 5 produce 

Accordingly, in this chapter the original fluorescence systems have been classified as internal charge transfer (ICT) fluorophores where the acceptor is the boronic acid (these systems have no defined donor). 

Sandanayake et al. made the first systematic attempt to couple a donor and acceptor in the construction of an ICT system using the coumarine boronic acid 7 [44]. Here both fluorescence intensity and wavelength are affected since the nitrogen is directly connected with the chromophore. Sadly, this system shows small shifts in intensity and wavelength, in spite of its clever molecular design. The observed stability constant (f pp) for 7 was 27 for D-fructose in 1 1 methanol-water. 

Lakowicz quickly recognized the importance of stilbene boronic acid 6d and has since prepared several analogous ICT fluorophore systems, including oxazoline 8 [45], chalcones 9a,b [46] and boron-dipyrromethene (BODIPY) 10 [47]. The observed stability constants (Kapp) for 8 were 526 for D-fructose and 27 for D-glucose in 2 1 (v/v) water-medianol at pH 7.0 (phosphate buffer). J pp for 9a,b were 400, 476 M for D-fructose and 29, 33 M for D-glucose in 2 1 (v/v) water-methanol at pH 6.5 (phosphate buffer). K pp for 10 were 1000 for D-fructose and 13.7 M for D-glucose in water at pH 7.5 (phosphate buffer). 

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Arimori S, Bosch LI, Ward CJ, James TD (2001) Fluorescent internal charge transfer (ICT) saccharide sensor. Tetrahedron Lett 42(27) 4553-4555... 

Charge transfer excited states internal charge transfer (ICT) states, metal-to-ligand charge transfer (MLCT) and twisted internal charge transfer (TICT) states... 

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... 

The electronic absorption, fluorescence and excitation spectra of these compounds indicate the presence of an internal charge transfer (ICT) excited state giving rise to a fluorescence band that displays strong solvatochromism. Both the emission wavelengths and the Stokes shifts increase with solvent polarity, in agreement with a large increase in dipole moment in the excited state. As the chain length increases the... 

The use of heterocyclic units as fluorophores introduces electrostatic interactions arising from their dipolar excited states with internal charge transfer (ICT) character. The other partner in such interactions would be the proton-bound receptor which is held a short distance away by the spacer module. Such... 

We explored the strategy of linking an anion receptor to a fluorophore that exhibits emission from an internal charge transfer (ICT) excited state. This allowed us to obtain a high dynainic range of emission response, and in one case,... 

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 mentioned above, the first fluorescent sensor for saccharides was reported by Yoon and Czamik." The internal charge transfer (ICT) sensor 1 consisted of a boronic acid fragment directly attached to anthracene. On addition of saccharide, it was noted that the intensity of the fluorescence emission for the 2-anthrylboronic acid 1 was reduced by 30%. This change in fluorescence emission intensity is ascribed to the change in electronics that accompanies rehybridization at boron. For boronic acid 1 (below its pA a). the nentral sp hybridized boronic acid displayed a strong flnorescence emission (above its pA a) and the anionic sp boronate displayed a reduction in the intensity of fluorescence emission. 

Following the synthetic strategy of the DCSNs, we have demonstrated the possibility to take advantage of the spatial organization and electronic communication between chromophoric units on the surface of silica nanoparticles for the development of a self-organized Zn(II) fluorescent chemosensor [116]. We used a triethoxysilane derivative of TSQ (6-methoxy-(8-p-toluenesulfonamido)quinoline) to realize a multichromophoric network on the surface of preformed silica nanoparticles. TSQ is a widely used fluorescent chemosensor able to bind Zn(II) ions with good selectivity. It is characterized by an off-on response due to an internal charge transfer (ICT) in the Zn(II)TSQ and Zn(II)(TSQ)2 complexes (Fig. 17). 

Figure12.2 Internal charge transfer (ICT) fluorescence sensors.

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