Excited state internal charge transfer

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... 

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). 

Scheme12.3 Locally excited (LE) and twisted internal charge transfer (TICT) states of 12a.

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... 

Z. R. Grabowski, K. Rotkiewicz, W. Rubaszewska, and E. Kirkor-Kaminska, Spectroscopy and kinetic of the twisted internal charge-transfer (TICT) excited state formation in p-substituted dialkylanilines, Acta Phys. Pol. A 54, 767 (1978). 

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... 

In addition to the processes just discussed that yield vibrationally and rotationally excited diatomic ions in the ground electronic state, vibrational and rotational excitations also accompany direct electronic excitation (see Section II.B.2.a) of diatomic ions as well as charge-transfer excitation of these species (see Section IV.A.l). Furthermore, direct vibrational excitation of ions and molecules can take place via charge transfer in symmetric ion molecule collisions, as the translational-to-internal-energy conversion is a sensitive function of energy defects and vibrational overlaps of the individual reactant systems.312-314... 

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... 

Twisted internal charge transfer TtCT) Intramolecular, photoinduced charge transfer between chromophores interconnected by a single bond leading to an excited state (a TICT state) in which the chromophores interact only weakly because of a considerable twist about the interconnecting bond. 

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