Chemical Sciences and Technology

Butadiene based photoresponsive soft materials

photoresponsiveThe creation of stimuli responsive materials offers considerable challenges in the area of material science. The use of light as an external stimulus has particular advantages since it can bring about rapid transformations in remote regions in a highly precise manner. Naturally occurring photoresponsive systems provide the motivation for developing corresponding artificial systems using molecular self-assembly to address issues such as quantum efficiency, selectivity and amplification. A practical strategy for developing photoresponsive materials is to utilize molecules which can undergo considerable change in shape on photoisomerization. Although photoisomerization of polyenes between their linear all trans isomer and its bent cis isomers have been extensively investigated in solution and in organized media due to its relevance to naturally occurring photoresponsive systems, its use in developing artificial photoresponsive systems has not been well explored.Donor-acceptor Donor-acceptor substituted butadiene derivatives form a versatile class of materials with multifunctional properties such as photoresponsive liquid crystals and gels with potential for applications in color recording devices and controlled release systems. The sensitivity of the absorption and emission properties of the butadiene derivatives to their state of aggregation provides valuable information on the role of molecular packing in controlling their photophysical and photochemical properties in self-assembled materials. Studies are being undertaken towards understanding the factors that control the efficiency and regioselectivity of the photoisomerization of butadienes and related molecules in self-assembled systems are in progress (Langmuir, 2010, 26, 1598).

In this context, light induced generation of stable blue phase in photoresponsive diphenyl-butadiene based mesogen has been investigated. Blue phases (BP) are an interesting class of frustrated chiral liquid crystals possessing a fluid lattice which self-assemble into three-dimensional cubic defect structures. The periodicity of these defect structures is of the order of the wavelength of visible light, resulting in interesting optical properties such as selective reflection of circularly polarized light.

These properties combined with the ability to reorient the molecules in blue phases using external electric fields make them very attractive for applications such as fast light modulators and tuneable photonic crystals. Such Chiral frustated mesophases could be generated isothermally by photolysis of diphenylbutadiene based liquid crystalline molecules, whose nature and stability of liquid crystalline phase depended upon the nature of the alkyl chain. Photochemically generated cis isomers of the diphenylbutadiene derivative resulted in isothermal phase transitions from smectic A* to stable chiral phases such as the chiral nematic and blue phases depending upon the extent of photoisomerization ( Chem . Commun. 2010, 46, 2796)