Photochemical flip

Fig. 2 Photochemical flip of polarization in ferroelectric liquid crystals.

T. Sasaki, T., Ikeda, and K. Ichimura, Photochemical control of properties of ferroelectric liquid crystals photochemical flip of polarization. J. Am. Chem. Soc. 116, 625-628 (1994). 

A.4.2. Photochemical Flip of Polarization and Alignment Change in Ferroelectric LCs. Photochromic reactions can also strongly influence the... 

Much attention has been paid to molecular design of effective guest molecules to induce the photochemical flip of polarization in ferroelectric LC systems. An azobenzene derivative with a chiral cyclic carbonate group (13) was designed to induce a large value of polarization and examined as a chiral dopant to induce a... 

The photochemical dissociation of a molecule AB often leads to the formation of a pair of radicals A 4- B, e.g. as in Figure 4.33. If the reaction takes place from the lowest triplet excited state of AB, the radicals will have parallel spins and cannot recombine unless a spin flip takes place to bring them to the singlet state of the geminate radical pair. 

Photochemical polarization flip was immediately extended to antiferroelectric LCs (AFLCs), and it was found that in azobenzene guest/AFLC host systems, polarization flip could be induced photochemically and effectively, as shown in Figure 3.1331 Novel photochromic molecules to induce polarization flip effectively have been explored, and molecular design on the basis of large Ps resulting from chiral cyclic carbonates has been found to be quite effective. An azobenzene dopant containing a... 

Photochemical E —> Z isomerization produces a sterically strained state in which the methyl substituents are positioned in close proximity. The strain is released when the methyl groups and the naphthyl moieties switch their positions. This second step is followed by back-isomerization from the Z to the E form of the double bond and again steric hindrance provides an energetically unfavorable state which can reach an energetically more favorable state by a second flip of the sub-... 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

Photochemical control of properties of SmC LC phase was achieved by doping azobenzene A-4 possessing a chiral carbon atom to a ferroelectric LC A-5 [61]. When the SmC LC is in the surface stabilized state, the bulk dipole moment can be flipped by an external electric field. As the hysteresis curve for the Z form is narrower than that of E form, irradiation of UV light to cause E-... 

Figure 5 Polarization flip of ferroelectric LC (A-5) containing A-4 by photochemical ElZ isomerization of A-4.

There are three mechanisms of spin flipping solvent-induced spin relaxation (spin-lattice relaxation), spin-orbit coupling (SOC) and hyperfine coupling (HFC) (2,31. While the first of these mechanisms is quite slow in the absence of paramagnetic impurities, HFC is important in biradicals in which the two radical centers are relatively far apart (1,6-biradicals and longer) [4], and SOC dominates in short biradicals, which are observed as intermediates in numerous photochemical reactions. For these systems, the order of magnitude of the SOC element is about 0.1-5 cm , which is much larger than that of the typical HFC, which is about 0.001 cm. We will therefore concentrate on the SOC mechanism only. 

Catalysis by (6—4) photolyase must accomplish two chemical tasks cleavage of the C6—C4 sig a bond, and transfer of the OH (or —NH2) group from the C5 of the 5 base to the G4 of the 3 base. Because formation of the (6—4) photoproduct is presumed to proceed through a four-mem-bered oxetane or azetidine intermediate, it has been proposed that (6—4) photolyase first converts the open form of the (6—4) photoproduct to the four-membered ring by a thermal reaction, and then the four-mem-bered ring is cleaved by retro [2+2] reaction photochemically (Kim et al, 1994 Zhao et aL, 1997). A site-directed mutagenesis study has identified two histidine residues in the active site that may participate in conversion of the (6-4) photoproduct to the oxetane intermediate by general acid-base catalysis (Hitomi et al, 2001). A current model for catalysis by (6-4) photolyase is as follows (Fig. 8) The enzyme binds DNA and flips out the... 

The reaction mechanism can be summarized as follows. In a dark reaction, the enzyme binds to DNA and flips out the pyrimidine dimer from the double helix into the active cavity. After the photochemical repair, the reaction partners are moved out of the cavity. As shown in Scheme 8.7, MTHF (or alternatively 8-HDF) is converted into an excited state, MTHF, upon absorption of a photon. 

Similar to the phase transition from nematic to isotropic phase induced by azobenzene molecules, the trans-cis isomerization also destabilize the SmC phase composed of calamitic mesogens and lower the Curie point, which is a transition temperature where the SmC will transform from ferroelectric to non-ferroelectric. Some examples have been reported with an early demonstration by Ikeda et al. [130-135]. For example, a photoresponsive SmC was formulated by doping 3 mol % of 4,4 -disubstituted azobenzene 29 into a FLC host 27 and the UV irradiation at 260 nm resulted in the lowering of Curie point and the coercive force Ec required to switch the SSFLC due to the destabilization of bent shape cis isomers [130] (Fig. 5.23). When the electric field was close to Ec before irradiation, the flip of polarization of SmC was achieved. It is noteworthy that its response time 500 ps is much faster than normally observed for photochemical N-I phase transitions [131]. 

We present here the first experimental demonstration of photochemical bistability in an open reactor. This bistable reaction results from the non-linear properties of a photochromic system the dimer of the triphenylimidazyl radical in chloroform. Hysteresis is observed on the plots of the stationary states of the system over a wide range of flow rates. Within this region, the system is bistable and can be made to flip from one state to the other by an external manipulation. One of the stable states is characterized by a high concentration of violet radicals 2 while in the other the violet radicals are replaced by highly fluorescent compounds. Mechanistic studies showed that this bistability was due to a positive feedback loop. This was thought to arise from the screening effect of the violet radicals 2 with respect to the irradiation of the triphenyl imidazole 3 in combination with an inhibition of the violet radicals 2 by the products of photolysis of triphenylimidazole 3. 

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