Azobenzene-LCs

These azobenzene LCs display the liquid crystalline phase only when the azobenzene moiety is in the trans form, and no liquid crystalline phase at any temperature when the azobenzene moiety is in the cis form. In these azobenzene LC system, it was predicted that phase transition should be induced on essentially the same time-scale as the photochemical reaction of the photoresponsive moiety in each mesogen, if the photochemical reactions of a large number of mesogens were induced simultaneously by the use of a short laser pulse (Figure 7).1391 On the basis of such a new concept, the photoresponse of azobenzene LCs with the laser pulse was examined, and it was found that the N to I phase transition was induced in 200 xsJ39 40 This fast response, on the microsecond timescale, had been demonstrated for the first time in NLCs. From the viewpoint of application of LCs to photonic devices, such a fast response is quite encouraging. 

As another approach to fast I-N phase transition, the optical switching behavior of LMW and polymer azobenzene LCs has been explored by means of reflection-mode analysis (Figure 10). 45 On laser-pulse irradiation, it is possible to switch the... 

A unique feature of polymer azobenzene LCs as optical image storage materials has been demonstrated. In nematic glasses (solid state azobenzene LC films retaining an N... 

In addition, more recently, an interesting approach to fast response of photorespon-sive LCs has been reported. Crosslinked PLC networks containing azobenzene molecules were prepared by polymerization of ternary mixtures of monofunctional and difunctional LC monomers together with a LMW azobenzene LC, as shown in Figure... 

Figure 3.18. (a) Photoisomerization of 4,4 -disubstituted azobenzene derivative and (b, c) phase diagrams of the photochemical phase transition of azobenzene/ LC systems (N, nematic I, isotropic). Source Ikeda, 2003. 

A new system has been developed in which every mesogen in the LC or LC polymer is photosensitive (Ikeda, 2001 Tsutsumi et al., 1998b Tsutsumi et al., 1997 Ikeda and Tsutsumi, 1995). For example, the azobenzene moiety could play roles as both a mesogen and a photosensitive moiety (Fig. 3.20) in azobenzene derivatives that form LC phases. These azobenzene LCs show a stable LC phase only when the azobenzene moiety is in the trans form, but do not show an LC phase when all the azobenzene moieties are in the cis form. Examination of these azobenzene LCs revealed that a nematic-isotropic phase transition is induced in the azobenzene LC polymers within 200 ns over a wide temperature range imder optimized conditions (Ikeda, 2001). 

It is known that the trans isomer of azobenzene has elongated rod shape that is favorable for the stabilization of the LC phases, while the cis isomer in bent form is unfavorable and tends to destabilize the LC phases. The trans-cis photoisomerization will decrease the order parameters, if significantly enough, which could lead to the destruction of the ordered LC phase and formation of isotropic state. The reverse transition can be achieved with either visible light irradiation or thermal relaxation as a result of cis-trans isomerization. This principle was initially used to induce the phase transition from nematic to isotropic in azobenzene LCs [39]. Cholesteric mesophase is intrinsically similar to nematic with additional helical arrangement of nematic layers, thus the photoisomerization of azobenzenes can also bring out the phase transition from the cholesteric state to the photoin-duced isotropic (PHI) state. 

The decrease in order parameter in LCs caused by the photoisomerization of azobenzene molecules are also known to induce phase transition from chiral smectic phase to cholesteric phase. This type of transition is mainly observed in chiral mesogenic compounds. In 1999, My et al. conducted a systematic research on the phase transition behavior of a series of chiral azobenzene LCs [44]. A photo... 

Photochemical reaction of photochromic molecules is a very fast process. For instance, trans—cis photoisomerization of azobenzene derivatives takes place in picoseconds. However, the photochemical phase transition of the azobenzene LCs was induced in 200 xs. This difference in the response... 

The stability of the induced I phase at the irradiated site of polymer azobenzene LC strongly depends on temperature. At temperatures above the induced I phase disappeared and the N phase was restored after some time, which depended on temperature. At high temperatures such as 140°C, the I phase disappeared in seconds. At temperatures just above Tg the induced I phase remained fairly stable for some time, but disappeared after a prolonged time. On the other hand, at temperature below Tg the induced I phase remained unchanged even after 1 year and 6 months. The cis-trans back-isomerization took place thermally even at room temperature, and the trans-ioun was restored almost completely in 1 day. Polarized absorption spectroscopy showed that even though the trans-iorm was restored in 1 day at room temperature, orientation of the /ra 5-azobenzene mesogens was different from that of the initial state. Before photoirradiation, the azobenzenes were aligned in one direction to form uniaxial monodomain, but after photoirradiation the cw-form was produced, and N—I phase transition was induced. However, after the trans-ioim was restored thermally, the orientation... 

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