J. Phys. Chem. B 2007, 111, 345-350
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Nonequilibrium Liquid Crystalline Layered Phase Stabilized by Light S. Krishna Prasad,* Geetha G. Nair, and Gurumurthy Hegde Centre for Liquid Crystal Research, Jalahalli, Bangalore 560013, India ReceiVed: September 29, 2006; In Final Form: NoVember 13, 2006
The ability of light to alter/stabilize a particular thermodynamic phase is a power tool to investigate condensed matter from a new dimension. This field of photoinduced phase transitions is currently an important area of research. Being elastically soft and having subtle changes between its many phases a liquid crystal material is an attractive medium to investigate such light-driven phase transitions. The attraction is partly due to the large birefringence changes accompanying these transitions that are useful in developing photonic devices. In all of the cases reported to date, the photoinduced transition always leads to a phase that in any case exists in the thermal cycle. Recently we reported the first exception to such an established phenomenon (AdV. Mater. 2005, 17, 2086). The guest-host ternary mixture consisting of the photoactive azobenzene guest molecules does not exhibit smectic A phase in the absence of UV radiation. However, the smectic A phase is induced and stabilized only in the presence of UV light. In this paper, we map out hitherto unexplored temperature versus UV intensity phase diagrams for various mixtures, which illustrate that light mimics, in a limited sense, the role of a thermodynamic parameter such as, for example, pressure. The threshold UV intensity required to photodrive the appearance of the smectic A phase is seen to have a strong concentration dependence. Our studies also suggest the possibility of observing a double critical point by employing the UV intensity as a fine-tuning parameter.
1. Introduction One of the recent topics of significant interest in condensed matter science is to control the electronic, magnetic, and optical states of matter by light.1 Of special interest is the reversible photoswitching in systems containing photoactive molecules owing to their potential for memory storage applications.2 Nonequilibrium phases can be generated from an equilibrium phase by external stimulations; photoirradiation is one of them and sometimes allows the emergence of a new phase, which cannot be reached by simply changing temperature or pressure.3 The phenomenon of reversible photoinduced shape transformation of chromophoric molecules, such as azobenzenes, has been extensively studied.4 The presence of such azobenzene molecules in a liquid crystal environment brings out interesting effects, the principle behind which is outlined in the following. Upon UV irradiation (around 360 nm, corresponding to the π-π* band of the azo group), the energetically more stable E configuration with an elongated rodlike molecular form changes into the bent Z configuration. The reverse transformation can be brought about by illuminating with visible light (around 450 nm, corresponding to the n-π* band). This latter change can also occur spontaneously in the “dark” by a process known as thermal back-relaxation. The liquid crystalline phase is stabilized by the rodlike E form but is destabilized by the bent Z form. Therefore, the E/Z change generally leads to a lowering of the phase transition temperature. The resulting photoinduced isothermal transition has been observed for a variety of systems exhibiting different phases. In all of the photoinduced phase transitions reported to date,5-8 a general feature observed is that the photoinduced phase would, in any case, occur in the thermal cycle. Recently we reported the first exception to such an * Author to whom correspondence should be addressed. E-mail: skpras@ gmail.com.
established principle.9 The employed guest-host mixtures containing photoactive azobenzene guest molecules do not exhibit the smectic A (Sm-A) phase in the absence of UV radiation. However, the Sm-A phase is induced and stabilized only in the presence of UV light. This work was in fact partially motivated by the realization of dynamic self-assembly in other soft condensed matter systems.10 To explain the dynamic selfassembly of these molecules into a layered phase from a simple orientationally ordered fluid phase, we proposed an argument based on photoinduced nanophase segregation6 and a frustrated spin-gas model.11 The present paper describes detailed studies on various mixtures, some exhibiting Sm-A phase and some not showing Sm-A phase in the thermal cycle. By employing a UV beam of very low magnitudes (
