TICT Molecules

Fig. 3 Ground-state and excited-state energies in the planar and twisted configurations of a TICT molecule. Photon absorption usually elevates the molecule to the LE state in the planar configuration, where the angle of intramolecular rotation, cp0, is close to zero. From the LE state, the molecule can return to the ground state with a rate kf, or it undergoes intramolecular rotation with a rate ka. The twisted state is characterized by a larger intramolecular rotation angle

The twisting motion in the TICT mechanism is also a large-amplitude motion which is easily stopped by very high-viscosity conditions, and therefore TICT molecules with various rotating moieties can be used in the development of free volume... 

Aromatic amines are not the only type of molecule to undergo a spontaneous twist in the excited state, but DMABN is the best-studied example. Other such compounds are stilbene-type molecules, where the double bond twists in the excited state. The electronic structure for both cases, twisting double bonds and twisting single or partly double bonds of 7r-donor linked to 7r-acceptor (TICT molecules), can be related to each other and characterized in the framework of quantum-chemical treatment as described in Section III. 

Fig. 9. The energy dependence of the excited states of TICT molecules on the twist angle between the dimethylamino group and the aromatic ring. In the TICT state (at 90°) full charge separation is reached...

The principles of Twisted Intramolecular Charge Transfer (TICT) formation are outlined and many examples given. The most recent developments in the understanding of these states both from the theoretical and the phenomenological viewpoint are reviewed. Emphasis is given to the effect of broadened angular distributions and to the possibility of multiple TICT channels. Application possibilities with respect to dye developments and fluorescent probes are outlined as well as various ways of intermolecular reactivity of TICT molecules. 

Very recently, a further mechanism has been added to the list of additional factors 6. the encounter of two excited TICT molecules to form a species known as bicimer . In this case, both a dependence on concentration and on excitation light intensity is expected. This dependence has been found for DCS, an... 

Table 2. Bimolecular fluorescence quenching rate constants k, (10 1 mol s " for different TICT molecules with various electron donors [138], The oxidation potential of the donor (in Volts) is also given...

There are a number of studies which indicate that certain TICT molecules undergo specific interactions with solvent or quencher molecules. One of the early proposals to explain the dual fluorescence of DM ABN was in fact that of a solute-solvent exciplex (excited complex) [26-28]. The first specific interaction documented as such, however, was that of DMABN (and model compounds) with saturated amines, where a mechanism necessitating a close approach was postulated. This was concluded from the lack of correlation of the quenching reactivity with the ionization potential of the amine quencher. However, a correlation was found between the reactivity pattern and the sterical demands of the saturated amine [140,141]. The complexes formed were non-emissive and were proposed to involve a two-center-three-electron bond between the aromatic and the saturated amino group. 

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