TICT state

From the excited-state conversion rates (ka and kd), it is possible to calculate the ratio of the quantum yields between emission from the twisted and LE states. The ratio of the steady-state quantum yields from the locally excited state (f>LE and the TICT state r/>TI, T for DMABN and other aminobenzonitrile derivatives is described in (1) [12],... 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

Rettig W, Lapouyade R (1994) Fluorescence probes based on twisted intramolecular charge transfer (TICT) states and other adiabatic photoreactions. Topics in fluorescence spectroscopy 4 109-149... 

Gregoire G, Dimicoli I, Mons M, Donder-Lardeux C, Jouvet C, Martrenchard S, Solgadi D (1998) Femtosecond dynamics of TICT state formation in small clusters the dimethyla-minobenzomethyl ester acetonitrile system. J Phys Chem A 102(41) 7896-7902... 

Bulgarevich DS, Kajimoto O, Hara K (1995) High-pressure studies of the viscosity effects on the formation of the twisted intramolecular charge-transfer (TICT) state in 4,4 -diaminodi-phenyl sulfone (DAPS). J Phys Chem 99(36) 13356—13361... 

Herbich J, Dobkowski J, Rulliere C, Nowacki J (1989) Low-temperature dual fluorescence in 9-morpholinoacridine picosecond TICT state formation J Lumin 44 87-95... 

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... 

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). 

Letard JF, Lapouyade R, Rettig W (1993) Synthesis and photophysical study of 4-(N-monoaza-15-crown-5) stilbenes forming TICT states and their complexation with cations. Pure Appl Chem 65 1705-1712... 

In addition to the fluorescence band due to emission from the LE state ( normal band), an emission band corresponding to emission from the TICT state is observed at higher wavelengths ( anomalous band) (Figure 3.4). 

Rettig W. (1982) Application of Simplified Microstructural Solvent Interaction Model to the Solvatochromism of Twisted Intramolecular Charge Transfer (TICT) States, /. Mol. Struct. 8, 303-327. 

Rettig W. and Lapouyade R. (1994) Fluorescent Probes Based on Twisted Intramolecular Charge Transfer (TICT) States and Other Adiabatic Reactions, in Lakowicz J. R. (Ed.), Topics in Fluorescence Spectroscopy, Vol. 4, Probe Design and Chemical Sensing, Plenum Press, New York, pp. 109-149. 

The formation of a TICT state is often invoked even if no dual fluorescence is observed. For donor-acceptor stilbenes (PCT-2 and PCT-3), the proposed kinetic scheme contains three states the planar state E reached upon excitation can lead to state P (non-fluorescent) by double-bond twist, and to TICT state A by singlebond twist, the latter being responsible for most of the emission. 

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