Planar intramolecular charge-transfer state

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). 

Figure 10 Potential energy diagram for emission from normal planar (NP) and twisted intramolecular charge transfer (TICT) excited states. The charge-separated state is stabilized by twisting and by the polarity of the environment.

The next chapter by Ohshima, Kajimoto and Fuke reviews some results obtained with linked systems, in which charge transfer is facilitated by torsional motion around a single bond. Picosecond time-resolved experiments allowed the direct measurement of the decay of the LE state and the rise of the CT state fluorescence. In some cases a third excited state, also characterized as a charge-transfer state, was observed. It was assigned to a charge-transfer state in which the two connected moieties (anthracene and aniline in this case) are twisted from the planar conformation characteristic of the ground state, but not to a fully perpendicular conformation. The photophysics of twisted intramolecular charge-transfer (TICT) states, were studied extensively in the gas phase and in solution (see the previous chapter by Herbich and Brutschy). 

In addition to these photophysical processes, twisted intramolecular charge transfer (TICT) process (non-radiative process) also takes place to return to ground state. Molecules of compounds such as dimethyiaminobenzonitrile (DMABN) are flexible. These are planar in the ground state, but are twisted in excited state. The twisted conformation is able to transfer its full electronic charge from one part to another in polar solvents and thus deactivated to ground state (Fig. 6.5) [4]. 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

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