ATRP synthesis of oligofluorene-based liquid crystalline conjugated block copolymers

A series of well-defined conjugated-liquid crystalline (LC) block copolymers containing oligofluorene and side-chain liquid crystalline polymers with cyanobiphenyl moieties were successfully synthesized by atom transfer radical polymerization. The block copolymers were prepared with number-averaged molecular weights (M_n) ranging from 8000 to 16 000 and narrow molecular weight distribution less than 1.20. The chemical structures of these block copolymers were confirmed by ¹H NMR, ¹³C NMR, and FTIR studies. All of the block copolymers exhibited the smectic mesophase as illustrated by differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. A bilayer structure of mesogens was formed in the smectic layer of block copolymers with a thickness of 3.5 nm. The isotropization of the smectic phase increased with the molecular weight and leveled off at M_n =14 000. The optical properties of these block copolymers in solution and solid-films were investigated comparatively by UV spectroscopy, photoluminescence, and photoluminescent excitation characterization. The results suggest that energy transfer from the LC mesogens to the conjugated oligomer occurs both in dilute solution and in the solid state, which was more efficient in solid state due to higher local chromophore density.