Fluorescent internal charge transfer

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

Fluorescent Internal Charge Transfer Sensory Systems... 

Fluorescent Internal Charge Transfer Sensors Incorporating the Ortho-(Aminomethyl)Phenylboronic Acid Fragment... 

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

Bosch LI, Mahon MF, James TD (2004) The B-N bond controls the balance between locally excited (LE) and twisted internal charge transfer (TICT) states observed for aniline based fluorescent saccharide sensors. Tetrahedron Lett 45(13) 2859-2862... 

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... 

TICT state The acronym derives from Twisted Internal Charge Transfer State, proposed to be responsible for strongly Stokes-shifted fluorescence from certain aromatics, particularly in polar medium. 

On the other hand, the fluorescence of ThT has found many applications. In viscous media new emissive species are formed from locally excited states by twisting around the central single C-C bond and their formation correlates with viscosity, thus oflering an opportunity to direct observe the formation of TICT (twisted internal charge transfer) states. ... 

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.

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). 

James and co-workers have prepared 12a, a monoboronic acid fluorescent sensor that shows large shifts in emission wavelength on saccharide binding [50]. The dual fluorescence of 12a, can be ascribed to locally excited (LE) and twisted internal charge transfer (TICT) states of the aniline fluorophore [51]. When saccharides interact with sensor 12a in aqueous solution at pH 8.21 the emission maxima at 404 nm (TICT state) shifts to 362 nm 274 nm, LE state). The band at 404 nm is due to the TICT state of 12a containing a B-N bond i.e. the lone pair is coordinated with the boron and perpendicular to the jt-system. The band at 274 nm (LE state) corresponds to the situation where the B-N bond in 12a has been broken with formation of the boronate (Scheme 12.3). 

The photophysics of fluorophores undergoing photoinduced charge transfer and/or internal rotation(s) is often complex. Time-resolved fluorescence experiments, transient absorption spectroscopy measurements, quantum chemical calculations, and comparison with model molecules are helpful in understanding their complex photophysical behavior. 

This chapter is restricted to intermolecular photophysical processes2). Intramolecular excited-state processes will not be considered here, but it should be noted that they can also affect the fluorescence characteristics intramolecular charge transfer, internal rotation (e.g. formation of twisted charge transfer states), intramolecular proton transfer, etc. 

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