The photo-induced isomerization of an azobenzene unit not only induces a phase transition between two different phases but also brings about marked changes of physical properties in solids and liquid crystals. We prepared a homologous series of symmetric liquid crystal trimers that possess an azobenzene unit, 4,4′-bis{9-[4-(5-hexylpyrimidin-2-yl)phenyloxy]alkyloxy}azobenzene (I-n: n = 7–11). We investigated their phase transition properties and those of their binary mixtures with a host liquid crystal. Trimers I-8, I-9, I-10 and I-11 exhibited nematic phases whereas trimer I-7 showed no liquid-crystalline properties. Trimers I-9, I-10 and I-11 were found to produce chiral conglomerates in the achiral smectic phase of their mixtures with the host liquid crystal under UV irradiation. Furthermore, odd-membered liquid crystal trimers I-9 and I-11 induced grain boundaries between smectic blocks. We explain the phase transition phenomena in terms of guest–host interactions between the photo-induced cis-trimer and its surrounding host liquid-crystalline molecules.