Photo-switching and nonlinear optical behaviors of center linked bent-core azobenzene liquid crystalline polymers

A new class of two series (I and II) of center linked bent-core azobenzene liquid crystalline polymers were prepared. They were prepared from two different spacer lengths (2 and 10) between polymer backbone and bent-core mesogen. The bent-core mesogen possesses photoactive linking group with variable terminal chains (7, 8, and 9). The synthesized precursors, monomers, and polymers were characterized by FT-IR, H-1 NMR, and C-13 NMR spectroscopy. Thermal stability of polymers was examined by thermogravimetric analysis. The mesophase transition of monomers and polymers were observed through polarized optical microscopy, and confirmed by differential scanning calorimetry. Among the two series of polymers, the second series of polymers possesses liquid crystalline properties. The photo-switching properties of bent-core azo polymers were investigated using UV-Vis spectroscopy, trans to cis isomerization occurs around 35 s in chloroform and 65 s in thin film, where as reverse processes take place almost 32 h in chloroform. The photo-switching processes of polymer IIa precedes faster and also longer time thermally stable when compared with recently reported similar polymers. Negative optical nonlinear refractive index and optical limiting properties of the polymers were measured using Nd-YAG laser.