Twisted states

Two phenomena need to be examined to understand the interaction of a molecular rotor with its environment. First, changes of the ground-state and excited-state energies between LE and twisted states need to be examined, and second, the... 

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

Fig. 4 Ground-state and excited-state energies of the TICT complexes thioflavin T (a) and 9-(dicyanovinyl)-julolidine (DCVJ) (b) as a function of the intramolecular rotation angle (data from Stsiapura et al. [13] and Allen et al. [14]). In both cases, energy levels were determined by quantum mechanical simulations. For thioflavin T, the energy difference between Si and S0 corresponds to approximately 400 nm in the planar state and 470 nm in the twisted state. In the case of DCVJ, the energy differences correspond to 310 and 960 nm, respectively. The DCVJ energy levels reflect a rotation around the vinyl double bond...

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... 

Thus, the PNS structure becomes looser and more disordered by mutation, causing the twisting motion of the chromophore more random and the twisted state formation considerably slower, demonstrating the supreme importance of the well-ordered PNS for the ultrafast and highly efficient twisting reaction of the chromophore in PNS. 

TABLE 2. Free-energy barriers to rotation through the 90° twisted state (A (71. kcal mol 1) and... 

The barriers to passage of the 90° twisted state in case 1 type push-pull ethylenes have been shown to correlate roughly with Ectr of the acceptors46 and the nitro group should be a better acceptor than acyl and thioacyl groups. Consequently, push-pull systems with two nitro groups as acceptors, like 11 and 12, show practically perpendicular acceptor parts with 0 = 89.0 and 86.9°, respectively47. 

In principle, this mode of operation creates the possibility of producing high-information-content displays due to the short frame times associated with bistable displays, since they are basically a memory effect and only new information must be changed. Unfortunately, metastable twist states of intermediate twist, which degrade the optical performance of the device, form around dust particles in cells with a cell gap below a certain value (d < 20 m). Therefore, since the response time is proportional to d, very long response times are observed ( 1 s) for LCDs with a cell gap above this critical value. These optically disruptive metastable twist states also form at the interface with spacers used to generate a uniform cell gap. 

This species, with no 7t-bonding, can be identified with the twisted state in which there is a minimum of overlapping of the p orbitals which extends at right angles... 

Zimmt has studied the twisted state of tetraphenylethylene by a picosecond light pulse technique using an optically detected calorimeter. The method provides an interesting new experimental approach to this subject. Other systems examined by conventional techniques include p-cyano-N,N-dialkylaniline,... 

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