Novel nonafluorenes with a varying extent of pendant chirality were synthesized for an investigation of the origins of chiroptical activities in neat films. Thermal annealing of 4-microm-thick sandwiched films and of 90-nm-thick spin-cast films, all on surface-treated substrates, produced monodomain glassy films characterized as a right-handed cholesteric stack with a helical pitch length ranging from 180 to 534 nm and from 252 to 1151 nm, respectively. The observed strong circular dichroism (CD) and g(e) as functions of helical pitch length in single-substrate monodomain glassy cholesteric films were quantitatively interpreted with a circularly polarized fluorescence theory accounting for light absorption, emission, and propagation in a cholesteric stack. Although intertwined molecular helices were likely to be present, cholesteric stacking of rodlike molecules seemed to be the predominant contributor to the strong chiroptical activities. All the cholesteric stacks comprising a polydomain glassy film on an untreated substrate were found to contribute to CD and g(e) largely to the same extent as in a monodomain film. A circularly polarized blue organic light-emitting diode containing a nonafluorene film resulted in a g(e) of 0.35 with a luminance yield of 0.94 cd/A at 20 mA/cm(2), the best performance to date.

The first series of monodisperse chiral oligofluorenes was synthesized and characterized. Chain length was found to play an important role in solid morphology. Whereas dimer through tetramer are amorphous, pentamer through hexadecamer all show cholesteric mesomorphism with varying degrees of morphological stability against crystallization. Pristine spin-cast films, approximately 90 nm in thickness, are amorphous but exhibit pronounced circular dichroism and highly efficient circularly polarized fluorescence, suggesting the presence of chiral assemblies that remain to be experimentally characterized. A nonamer with two sets of the 2S-methylbutyl group replaced by the 3S,7-dimethyloctyl group was prepared and shown to be capable of forming a monodomain, glassy cholesteric film with thermal treatment. The cholesteric film is responsible for an order-of-magnitude increase in circular dichroism and a handedness reversal in circularly polarized fluorescence as compared to the amorphous pristine film. Molecular dynamics simulation furnished new insight into the molecular origin of the observed chiral optical properties in neat films.

The first series of monodisperse glass-forming, nematic oligofluorenes was synthesized following a divergent−convergent approach. Both chain length and pendant structure were found to affect solid morphology and phase transition temperatures. With optimized structural parameters, a T g close to 150 °C and a T c beyond 375 °C were achieved. This material class is characterized by its ability to form monodomain glassy nematic films via spin-coating, thermal annealing at 10 to 20 °C above T g for 15−30 min, and cooling to room temperature without encountering crystallization. The absorption and emission dichroic ratios of 80−90-nm-thick films increase from 7 to 17 with an increasing molecular aspect ratio, while the photoluminescence quantum yield varies erratically from 43 to 60%. Superior stability of emissive color and of emission dichroic ratio and that against thermally activated crystallization were also demonstrated through prolonged heating of a film at 10 °C above T g under argon.