Effects of doped dialkylazobenzenes on helical pitch of cholesteric liquidcrystal with medium molecular weight: utilisation for full-colour image recording

The effects of small amounts of achiral photochromic dopants, 4,4'-di-n-alk ylazobenzenes, with various lengths of substituted alkyl chain (CnH2n+1:n = 4, 7, 11, 12, 13, 14, 15, 16) on the helical pitch of a cholesteric liquid crystal (CLC) with ca. 1000 molecular weight were investigated in order to utilise the cholesteric reflected colour of these combinations for a rewri table colour image recording. The (E)-azobenzenes (similar to2 wt%) in the host CLC induced pitch changes dependent on temperature, the concentration of the azobenzenes and the length of substituted alkyl chain on the azobenz enes. In the low region of the cholesteric temperature (below 100 degreesC) , some (E)-azobenzenes remarkably lengthened the pitch. Contrary to this, u pon irradiation, E-Z photoisomerization of all the azobenzenes in the CLC p hase induced consecutive pitch shortenings in response to the formation of the Z isomers. The changed pitch (reflected colour) could be fixed in chole steric glassy solids. In the glassy solid state, the photo and thermal isom erization of the azobenzenes could not influence the cholesteric pitch any longer, therefore, the colour was stably stored. However, since the molecul ar arrangement of the CLC and the conformation of the doped azobenzenes com pletely returned to the initial state upon re-heating to isotropic temperat ures, the stored colour could be easily erased. Based on these fundamental investigations, it is suggested that the mixed systems of the medium-molecu lar-weight CLCs and the 4,4'-dialkylazobenzenes will be effective materials for a rewritable full-colour image recording in a photon mode.