A stabilized lamellar liquid crystalline phase with aggregation-induced emission features based on pyrrolopyrrole derivatives

Aggregation-induced emission (AIE) overcoming the drawback of aggregation-caused quenching (ACQ) makes it possible to access the fluorescence emission of chromophores in the solid state. The development of AIE-active liquid crystals has attracted extensive attention in recent years. Herein, a series of novel (A-D-A)-type AIE-active dyes with different lengths of flexible alkyl chains based on pyrrolopyrrole cores were prepared. Among these compounds, TPPP-C1,2,3 with shorter alkyl chains possessed a more excellent AIE performance; however, they were unable to form a liquid crystalline (LC) phase. Upon increasing the length of alkyl chains, TPPP-C4,5,6,7,8,12 exhibited declining fluorescence emission efficiency in the aggregation states, and only TPPP-C6,7,8,12 with even longer alkyl chains exhibited the LC properties. Thus, all the nine TPPP compounds were AIE-active dyes, whereas TPPP-C6,7,8,12 were AIE-active liquid crystals. Polarized optical microscopy and differential scanning calorimetry experimental results indicated that the LC phases of the four AIE-active liquid crystals were only formed during the cooling process. However, once formed, the LC phases were very stable and could be retained for several months. This might be due to the freezing of intertwined alkyl chains during the cooling process; this was supported by grazing incidence wide-angle X-ray scattering measurements. The models of molecular organization in the LC states were built, and they showed well-organized lamellar structures.