Light-induced phase transitions

We have already seen that in organic molecular crystals, transitions between en-ergehcally nearby phases occur frequently. They can for example be induced by changes in the temperature or by applied pressure. For molecular electronics, such phase transihons could become quite interesting, provided that they can be controlled with light. 

A nice example is provided by the radical-ion salts of DCNQI. We have already pointed out in Sect. 9.5 that these include a great variety of salts which differ in terms of the metal ions or in terms of the substituents on DCNQI (CH3,1, Br and others), but which have essentially the same crystal structures (Fig. 1.7). The minor differences in the intermolecular spacings and the mutual orientations of the molecules within the crystal can however lead to great differences in their physical properties [13]. In Figs. 9.15 and 12.5, it becomes clear that in contrast to normal Cu dimethyl DCNQI, i.e. hs, the deuteration of the six CH3 protons to 4 leads to a Peierls transition in the range of ca. 70 K from a metallic to an insulating phase, reversibly and in a very narrow temperature interval. This behaviour could be made use of for a molecular switch. If, as in hg, the phase transition is suppressed, then this is because the Cu ions act as bridges between the DCNQI stacks and therefore convert the one-dimensional system into a three-dimensional one (c Sect. 9.5). 

In any case, these experiments show that the organic metals introduced here can also serve as switches, in which an optical pulse can be used to change the electrical conductivity of the whole crystal bulk over many orders of magnitude, rapidly and reversibly. 

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]. 

There are other interesting examples of light-induced phase transitions. For example, H. Matsuzaki et al. report on a light-induced transition from the diamagnetic to the paramagnetic phase at room temperature in a radical crystal in Phys, Rev, Lett. 91, 017403 (2003). 

---

Establishment of the theory for such properties of light induced phase transition of N-isopropylacrylamide-chlorophyllin gels the mechanism of the induction of phase transition is a result of local heating of the gel by light... 

Light-induced phase transitions apply to the structures of entire crystals or crystallites. These are thus genuinely macroscopic processes. They wiU be treated in Sect. 12.4. 

No matter what kind of molecules is used, the purpose is to efficiently control the properties of CLCs by light. In the following sections, the different types of light-induced behavior in CLCs such as light-induced phase transition, handedness inversion, pitch length change, and the related reflection color control are introduced. 

LIGHT-INDUCED PHASE TRANSITIONS INVOLVING CHOLESTERIC PHASE... 

Tamaoki and coworkers reported some chiral azobenzenes and diphenylbutadiene-based LC dimers 13-15 exhibiting light-induced phase transition behavior starting from smectic phases [26, 51-55]. For example, dimers 15, which were synthesized by connecting a cholesteryl group with 1,4-diphenylbutadiene unit, were found to experience isothermal phase transition from SmA to N and further to the isotropic state upon UV light irradiation at 366 nm [26]. 

This chapter highlights the photomodulation of CLCs, including light-induced phase transition related to cholesteric mesophase, helix inversion, reflection color control, and creation of mechanical motion in photoresponsive CLCs. The material systems introduced in this chapter are classified into two main groups based on chiral meso-gens and induced CLCs with several subsystems in each. Although the cholesteric phase was initially found with pure mesogenic compounds and the early quest for... 

Suzuki, A. (1994) Light-Induced Phase-Transition of Poly(N-Isopropylacrylamide-Co-Chlorophyllin) Gels, J. Intelligent Mat. Systems and Structures, 5, 112-6. 

Photoproduction of both charge- and spin-carrying species was observed with various ex situ spectroscopies. Further basically similar results also obtained ex situ were reported by Kaneto et al. [1009] and Hattori et al. [1010]. Light-induced phase transitions ( photoswitching ) were observed by Hosaka et al. [1011]. 

---