Photophysical properties and morphology of fluorene‐alt‐benzene based conjugated polymers

Abstract

A series of fluorene-alt-benzene based conjugated main chain polymers attached chemically with different lengths of alkyl side chains on phenylene rings were designed and synthesized by a palladium catalyzed Suzuki coupling reaction. The UV-vis absorption and fluorescence spectra, thermal stability of spectral property, phase transition behavior and morphology of the synthesized polymers were investigated. With increasing the length of the alkyl side chain, the UV and fluorescence spectra exhibit obvious blue shift compared with those of the unsubstituted polymer. The alkyl substitution improves the thermal stability of the spectra due to the steric hindrance of the alkyl side chains and thus the effective separation of the main chain backbones. The phase transition behavior is closely related to the length of alkyl side chains attached on the phenylene rings. The annealed polymer films display characteristic nematic liquid crystalline texture. Transmission electron microscopy (TEM) observations indicate that solvent-cast thin deposits of all the polymers show typical fibrillar morphology.

Side‐chain effects on the morphology and properties of fluorene‐based alternating copolymers

“…4, the polymer chains self‐assemble into many fibrils with typical width of about a few nanometers and length from several hundred nanometers to several micrometers. This is in agreement with previous reports that PF copolymers generally self‐organize into fibrillar nanostructures 1, 9, 13, 29. Detailed studies of microscopic morphology and molecular arrangement in thin films are under investigation 30…”

“…A weak stepwise drop was observed for PDHFP at about 106.2 °C in the first heating scan, which was assigned as the glass transition temperature ( T g ). The endothermic peak of PDHFP at ca 134 °C indicates the phase transition (T N → I ) from nematic liquid crystalline phase (N) to isotropic melt state (I) 13 . T g of PDHFDOOP is observed at ca 58 °C, which is much lower than that of PDHFP, owing to the introduction of the flexible side chain.…”

“…Three different fluorene‐based alternating copolymers coupled with various substituted benzene molecules were synthesized and characterized. These copolymers are the members of a series of PF copolymers previously reported by our group 12–14…”

Star‐like polyfluorenes based on 2,4,6,8, 10,12‐hexabenzyl‐2,4,6,8,10,12‐hexaazaisowurtzitane caged‐core: toward non‐aggregating, blue‐light‐emitting polymers

“…It is known that films of polyfluorenes containing flexible alkyl chains exhibit poor spectral stability upon exposure to heat 5, 26. To examine the effect of incorporating HBIW core on the spectral stability of the materials, the polymer films were thermally treated in air for different times on a hot plate at 150 or 200°C, respectively.…”

“…Although it is easy to make structure modification at C‐9 positions of PFs and achieve relatively high luminescent efficiencies, these materials are still encounter the problem of poor spectral stability, which is commonly associated with the troublesome excimer formation in solid states 4. An additional emission band between 500 and 600 nm may appear and become more pronounced upon thermal annealing or passage of current 5, 6…”