Chiroptical properties and photoalignment of monodisperse glassy liquid crystalline oligofluorenes.

摘要

Monodisperse glassy liquid crystalline conjugated oligomers are promising for application to photonics and electronics devices. At temperatures above glass transition, liquid crystal mesomorphism contributes to spontaneous molecular self-organization in the liquid state. These ordered liquid films can be frozen into the solid state via cooling through glass-transition temperature without encountering crystallization. In this thermal processing protocol, surface anchoring is critical to molecular orientation across the film thickness. While mechanical rubbing has been widely practiced, it inevitably generates dust particles and electrostatic charges; it may also cause physical damage to the alignment layer. This thesis research has furnished (i) fundamental understanding of the origin of chiroptical effects in glassy-cholesteric oligofluorene films, and (ii) new insight into the photoalignment of nematic fluid and glassy-nematic oligofluorene films. Major findings are recapitulated in what follows.; The chain length of chiral oligofluorenes plays an important role in solid morphology. Pristine spin-cast films are amorphous but exhibit substantial circular dichroism and circularly polarized fluorescence, suggesting the presence of hitherto unidentified chiral assemblies. Glassy-cholesteric films emerge from thermal annealing, showing an order-of-magnitude increase in circular dichroism and a handedness reversal in circularly polarized fluorescence in comparison to the amorphous pristine films. The huge chiroptical activities observed in glassy-cholesteric films of nonafluorenes with a varying extent of pendant chirality originates from light absorption, emission, and propagation in a cholesteric stack, as revealed by a theory with independently evaluated model parameters.; The orientations of both a nematic liquid crystal fluid and a series of monodisperse glassy-nematic oligofluorenes were characterized on photoalignment layers comprising a polymethacrylate backbone with coumarin pendants. A transition from a parallel to a perpendicular orientation with reference to the polarization axis of UV-irradiation was observed at a sufficiently high extent of coumarin dimerization. The UV-Vis and FTIR spectroscopic analyses revealed photostability, thereby excluding photodegradation as the basis for crossover. A kinetic model was used to interpret the photodimerization of coumarin and to evaluate the relative importance of three factors affecting the crossover behavior: the relative abundance of coumarin monomers and dieters, their evolving orientational order, and the energetics of their interactions with overlaying liquid crystal molecules.