Photoinduced nematic-isotropic phase transition: A case for the random-field Ising model K. L. Sandhya, S. Krishna Prasad, and Geetha G. Nair Centre for Liquid Crystal Research, Jalahalli, Bangalore 560 013, India Received 4 June 2001; published 24 September 2001 Experiments have been carried out on a photoactive guest-nonphotoactive host system exhibiting a photo- induced nematic-isotropic phase transition. It is shown that the phenomenon of photoinduced shift in the transition temperature is similar to the induction of cooperative chiral order in ‘‘sergeant-soldier’’ copolymer systems. An expression resembling the one given by the random-field model proposed for the latter system is seen to quantitatively describe the shift in the transition temperature as a function of the magnitude of the radiation. DOI: 10.1103/PhysRevE.64.041702 PACS numbers: 61.30.-v, 64.70.Md I. INTRODUCTION Reversible shape transformation of photoactive molecules resulting from photoisomerization and consequent effects on phase transitions in liquid crystals has been well studied 1–5. For example azobenzene derivatives can exist as two geometric isomers, the E form and the Z form. In the ground state the azobenzene molecules exist in the E also known as trans form that has a rodlike shape. Upon illumination with uv radiation 360 nm photoisomerization takes place re- sulting in transformation to the Z or cis form with a bent shape. When such azobenzene entities are incorporated into a liquid crystalline medium, either by physical mixing or by chemical bonding, the photoisomerization can lead to spec- tacular results. For example, the E form due to its rodlike shape stabilizes the liquid crystalline phase while the photo- induced Z isomer with its bent shape acts like an ‘‘impurity’’ destabilizing the phase. The destabilization can be significant enough to even cause an isothermal photoinduced transition from a liquid crystalline phase, say, the nematic phase to the isotropic phase. This phenomenon has attracted attention, not only from a basic point of view, but also for possible appli- cations in optical switching and image storage 6. A general observation 1–3 is that in a guest-host system, with the photoactive material being the guest, the shift in the transi- tion temperature (  T ) has a nonlinear dependence on the power of the incident uv radiation.  T increases steeply at lower values of the power, but saturates for higher values. In this paper we show the similarity between this behavior and the cooperative chiral order observed in the ‘‘sergeant- soldier’’ copolymer materials 7. The latter system has been theoretically analyzed 8 using a random-field Ising model and our results show that the expressions similar to that de- rived for this model describe well the variation of  T with uv power. II. EXPERIMENT The experiments were done on a mixture of the host ma- terial E 7, a commercially available room temperature nem- atic material from E Merck, and an uv active dopant p- ( p -Ethoxy phenylazophenyl hexanoate EPH for short from Eastman Kodak serving as the guest compound; the concentration of EPH in the mixture was 3.8% by weight. The resulting mixture, hereafter referred to as Mixture 1, shows the nematic-isotropic NI transition at 67.6 °C, with no crystallization until it reaches room temperature. This guest-host system has been shown to exhibit a large photo- induced shift in the NI transition temperature 2. The depen- dence of the NI transition temperature, T NI , on the intensity of the uv light was determined using a pump-probe beam setup described elsewhere 9. Essentially it consisted of an intensity stabilized uv source with a fiber-optic guide Hamamatsu L7212-01, Japan along with a uv-bandpass fil- ter and an infrared-block filter. The actual power of the ra- diation, I u v , passing through the filter combination, falling on the sample was measured with a uv power meter Hamamatsu, C6080-03 kept in the sample position. The intensity of the probe He-Ne laser beam ( I probe ) transmitted through the sample was monitored using a photodiode. The sample was sandwiched between two polyimide coated, uni- directionally rubbed glass plates with mylar spacers defining the cell thickness. The actual cell thickness was determined using an interferometric technique. The sample cells were mounted in a temperature controlled hot stage INSTEC HS250 for the temperature dependent measurements. III. RESULTS AND DISCUSSION Figure 1 shows representative raw scans of I probe versus temperature for a sample thickness of 39.6  m for different values of the uv power. The temperature at which there is an abrupt change in the intensity corresponds to the NI transi- tion. As expected this temperature shifts to lower values on irradiating the sample; the shift depends strongly on I u v at lower values of the uv power, but becomes saturated at higher powers. A plot of  T as a function of uv power for different thicknesses of the sample is shown in Fig. 2. It is seen that for the different thicknesses both the qualitative trend and the saturated value of  T are the same. However, the growth rate and consequently the uv power required to attain the saturated  T value has a significant dependence on the thickness of the sample. Now, let us look at the mechanism behind the photoin- duced NI phase transition in a guest-host system. The guest photoactive molecules are randomly distributed in the non- PHYSICAL REVIEW E, VOLUME 64, 041702 1063-651X/2001/644/0417024/$20.00 ©2001 The American Physical Society 64 041702-1