Photo-induced host-guest interactions produce grain boundaries between smectic blocks

Photo-induced reorientation of molecules not only induces phase transition between two different phases but also brings marked changes of physical properties in solids. We prepared a symmetric trimer possessing an azobenzene unit, 4, 4’-bis{9-[4-(5-octylpyrimidin-2-yl)phenyloxy]nonyloxy}azobenzene (I), and investigated the phase transition behaviour of trimer I and that of its binary mixture with a host liquid crystal. Trimer I exhibited three crystalline phases on cooling. Photo-induced crystal–liquid transition occurred only in the highest temperature crystalline phase through the trans-cis isomerization of the azobenzene unit. A binary mixture of trimer I (20 wt%) and a host LC (80 wt%) was found to exhibit grain boundaries consisting of nematic liquid crystals between smectic C blocks during a cooling process under UV irradiation. We explain the phase separation in terms of host-guest interactions between the photoinduced cis-trimer and its surrounding host LCs.