Photoinduced phase transition

A photoinduced phase transition has been observed in several cyano-bridged metal complexes [39]. They exhibit bistable electronic states at room temperature. The energy barrier between these bistable states maintains a photoproduced state even after photo-irradiation is terminated. Also, the state can easily be reset either via optical irradiation or temperature control. Moreover, typical phase transitions are excited in a cascaded manner, meaning that they exhibit high quantum efficiencies. 

In their typical optical characterizations, these materials are prepared in bulk form. However, for our transcription purposes, especially for the proof-of-principle experiments shown earlier, it is important to spatially distribute these materials so that we can evaluate the optical responses individually from each material. Therefore, in this paper, we mixed the materials with a dispersant based on an ester surfactant and dispersed the materials as single crystals in the solution. SEM images of single crystals are shown in Fig. 2.9b. The mean size of the single crystals was 1 xm (horizontal) x 1 xm (vertical) x 500 nm (thick). 

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A Mamada, T Tanaka, D Kungwatchakun, M Irie. Photoinduced phase transition of gels. Macromolecules 23 1517-1527, 1990. 

Figure 23. Temperature dependence of the photoinduced phase transition, (a) time courses of absorbance change at various temperature, (b) temperature dependence of apparent initial rate of phase transition.

Figure 24. Schematic model of photoinduced phase transition of the cast bilayer film.

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

Photoinduced spin-related phenomena are a particularly important field of the solid-state photophysics, because fast spin switching is a prospective basis for applications in the field of spintronics. An illustrative example is the production of the metastable state of the iron propyltetrazole (ptz) complex [Fe(ptz)6](BF4)2 by laser light-induced excited spin-state trapping (LIESST) and the determination of the resulting structure by steady-state X-ray photodiffraction [68]. In another example, steady-state X-ray photodiffraction at cryogenic temperatures was successfully utilized to study photoinduced phase transition due to spin crossover in the tris(a-picolylamine)iron(II) complex [69]. The phase transition is accompanied by... 

Naumov P, Hill JP, Sakurai K, Tanaka M, Ariga K (2007) Stmctural study of the thermally induced and photoinduced phase transitions of the l,3,5-trithia-2,4,6-triazapentalenyl (TTTA) radical. J Phys Chem A 111 6449-6455... 

Guerin L, Collet E, Lemee-Cailleau M-H, Buron-Le Cointe M, Cailleau H, Plech A, Wulff M, Koshihara S, Luty T (2004) Probing photoinduced phase transition in a charge-transfer molecular crystal by 100 picosecond X-ray diffraction. Chem Phys 299 163-170... 

We would like to emphasize that, whereas photoinduced phase transitions to isotropic are fairly well known in the literature,reports of photoinduced phase transitions toward the ordered phases are very unusual and not well explained, to our knowledge. We are currently investigating such phenomena. 

Y. Kawanishi, T. Tamaki, and K. Ichimura, Reversible photoinduced phase transition and image recording in polymer-dispersed liquid crystals, J. Phys. D Appl. Phys. 24, 782-784 (1991). 

PHOTOINDUCED PHASE TRANSITION IN LIQUID CRYSTALLINE POLYMERS... 

We chose a polyacrylate with liquid crystalline side chains as shown in Fig. 4. The family of this polymer with different alkyl spacers has been prepared by Ringsdorf and coworkers(25). Recently, they reported an interesting application of the following polymers for image recording(26-28) by means of the photoinduced phase transition principle. 

Photoinduced Phase Transition in Liquid Crystals (S. Tazuke and S. Kurihara)... 

Both phase transitions can be triggered by optical pulses [16]. The crystal is excited by an 80 fs pulse in the region of its CT absorption at temperatures between 77 and 105 K, thus near the transition temperature. The phase transition is detected by the changes in the reflection spectrum of the crystal. It is found that the phase transitions occur with a delay of 500 to 800 fs. Evidently it requires this time for an optically-excited local CT state to be converted into a macroscopic ionic I or neutral N phase. Here, cooperative electron-electron and electron-lattice interactions are presumably the driving forces for these photoinduced phase transitions. This is demonstrated by the dependence of the transition on the hght intensity. The phase transitions are shown schematically in Fig. 12.8. For further details of this process, which is still not understood in all its aspects, we refer the reader to the original literature [16]. 

Mamada A, Tanaka T, Kungwatchakun D, Irie M. 1990. Photoinduced phase transition of... 

Wang, G. Zhang, M. Yang, Q. Liu, R Cheng, Z. Guo, R. Yang, H. Photoinduced phase transitions in chiral hinaphthyl-diol-doped smectic liquid crystals by a photochromic azobenzene. Chem. Lett. 2010, 39, 1144—1145. 

FIGURE 7.4 Photoinduced phase transition occurs in PLCP materials upon light irradiation since usually the trans-azohenzene acts as a rod-like mesogen while the bent cw-azobenzene usually has a soft shape, not compatible with liquid crystalline order. 

It is important to point out that molecular cooperative motion allows for a significant amplification in the response of azobenzene-based LC dopants. It is possible in PLCP systems containing even a small amount of photoresponsive moieties to show reversible switching and optical control. This behavior extends to photoinduced phase transition in copolymer or LCP blend (guest-host). As shown in Figure 7.5, when a... 

FIGURE 7.5 Photoinduced phase transition of PLCP in copolymer or LCP blend (host/guest) systems. 

In reality, the liquid crystalline-to-isotropic phase transition temperature Tc) of the copolymer or LCP mixture with the cw-form Tec) is lower than that with the trans-isomer (Tct). The presence of the cw-azobenzene moieties depresses the transition temperature of the host liquid crystalline phase based on the concentration. If the temperature of the mixture T) is set between Tct and Tec and the sample is irradiated to cause trans-to-cis photoisomerization of azobenzene molecules, Tc decreases with an accumulation of the number of cw-isomer. When Tc becomes lower than the irradiation temperature, the liquid crystalline-to-isotropic phase transition is induced. The photoinduced phase transitions are interpreted in terms of a change in the phase transition temperature of PLCP systems on accumulation of one isomer of the photochromic molecule. As expected from Figure 7.5, AT (= Tct - Tec) is one of the most important parameters in the phase transition [1], When the temperature of systems is set below Tec, no phase transition is induced. When the temperature is set close to Tct, the amount of cw-azobenzene needed to lower Tc is relatively small, enabling one to effectively induce phase transition upon UV irradiation [10]. 

FIGURE 7.6 Schematic illustration of photomechanical effect of crosslinked PLCPs. Anisotropic change in volume can be caused upon photoinduced phase transition. 

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