Variations in the elastic constants of a fulgide-doped liquid crystal system

The dye material thiophene fulgide was doped into the nematic liquid crystal E7 to a low concentration (2% m/m) to form a photochromic guest-host mixture. The photochromic thiophene fulgide has a thermally stable UV-induced isomer; the isomerism induces changes to the steric, dipolar and electronic conjugation properties of the species. The bend and splay elastic constants of this novel photochromic guest-host mixture have been determined and compared with the properties of the undoped liquid crystal. The bend elastic constant was determined from the threshold magnetic field of the Freedericksz transition measured optically, whilst the splay elastic constant was found from the variation of birefringence at fields above this threshold field. Both elastic constants of the guest-host mixture were observed to be approximately 10% lower than those of pure E7 at equivalent reduced temperatures. However, irradiation of the photochromic material within the guest-host mixture causing isomerism of the guest did not change the elastic constants of the bulk system. The reproducibility of these results is considered indicating an interesting alignment degradation phenomenon relating to the lecithin surface treatment used.