thermotropic or lyotropic liquid crystallinity is discussed. LC phases of di-LCPAs
are assigned through observation of polarized optical microscope (POM) and
differential scanning calorimeter (DSC) and measurements of X-ray diffraction
(XRD). Using schematic energy levels of ground and low-lying excited states of
non-, mono-, and di-PAs, the origin of the emission of substituted PAs is elucidated, and fluorescent trends including emission color are investigated. It is found
that the macroscopically aligned films of the di-LCPAs emit LPL by virtue of the
functionalities associated with liquid crystallinity and fluorescence. The aligned
structures of the di-LCPAs are characterized in terms of main chain and side chain
type alignments through XRD measurements of the macroscopically aligned
polymer films. The mechanism of the LPL of the di-LCPAs with respect to the
polymer structure, alignment type, and emission color is elucidated.
Keywords
Liquid crystallinity · Conjugated polymers · Substituted polyacetylenes · Linear
polarization · Luminescence
Introduction
Polymers are generally considered insulators. However, it has been demonstrated
that conjugated polymers can become electrically conductive. Among these conjugated polymers, polyacetylene (PA) has attracted much attention due to the discovery of its metallic conductivity in the doped form. When doped with iodine, pristine
PA film has a metallic luster with a black surface and a high electrical conductivity
on the order of 10
4
–10
5 S/cm (Naarrmann and Theophilou 1987; Akagi et al. 1989;
Tsukamoto et al. 1990). This discovery accelerated research on conjugated polymers
and led to developments in polymeric light-emitting diodes (PLEDs), plastic
electronics, polymer battery cells, polymer photovoltaics, and other novel technologies (Skotheim and Reynolds 2007; Perepichka and Perepichka 2009; Mullen
et al. 2014).
However, pristine PA film is insoluble in organic solvents and quickly loses its
electrical conductivity when exposed to atmospheric conditions. Introduction of an
alkyl substituent into the polymer main chain increases the solubility in organic
solvents depending on the length of the alkyl chain. However, the electrical conductivity of the substituted PA is significantly lower than that of non-substituted
PA. This phenomenon is due to decreased coplanarity of the main chain, which
arises from steric repulsions between the substituents, a higher ionization potential
and lower electron affinity. The main chain in the substituted PA remains randomly
oriented, which suppresses the electrical conductivity of the polymer.
The addition of a liquid crystalline (LC) moiety to the polymer main chain makes
the polymer soluble in organic solvents and facilitates alignment by the spontaneous
orientation of the LC group. For example, mono-substituted liquid crystalline PA
(mono-LCPA), which is prepared by introducing a LC moiety into the side chain
342
K. Akagi
are assigned through observation of polarized optical microscope (POM) and
differential scanning calorimeter (DSC) and measurements of X-ray diffraction
(XRD). Using schematic energy levels of ground and low-lying excited states of
non-, mono-, and di-PAs, the origin of the emission of substituted PAs is elucidated, and fluorescent trends including emission color are investigated. It is found
that the macroscopically aligned films of the di-LCPAs emit LPL by virtue of the
functionalities associated with liquid crystallinity and fluorescence. The aligned
structures of the di-LCPAs are characterized in terms of main chain and side chain
type alignments through XRD measurements of the macroscopically aligned
polymer films. The mechanism of the LPL of the di-LCPAs with respect to the
polymer structure, alignment type, and emission color is elucidated.
Keywords
Liquid crystallinity · Conjugated polymers · Substituted polyacetylenes · Linear
polarization · Luminescence
Introduction
Polymers are generally considered insulators. However, it has been demonstrated
that conjugated polymers can become electrically conductive. Among these conjugated polymers, polyacetylene (PA) has attracted much attention due to the discovery of its metallic conductivity in the doped form. When doped with iodine, pristine
PA film has a metallic luster with a black surface and a high electrical conductivity
on the order of 10
4
–10
5 S/cm (Naarrmann and Theophilou 1987; Akagi et al. 1989;
Tsukamoto et al. 1990). This discovery accelerated research on conjugated polymers
and led to developments in polymeric light-emitting diodes (PLEDs), plastic
electronics, polymer battery cells, polymer photovoltaics, and other novel technologies (Skotheim and Reynolds 2007; Perepichka and Perepichka 2009; Mullen
et al. 2014).
However, pristine PA film is insoluble in organic solvents and quickly loses its
electrical conductivity when exposed to atmospheric conditions. Introduction of an
alkyl substituent into the polymer main chain increases the solubility in organic
solvents depending on the length of the alkyl chain. However, the electrical conductivity of the substituted PA is significantly lower than that of non-substituted
PA. This phenomenon is due to decreased coplanarity of the main chain, which
arises from steric repulsions between the substituents, a higher ionization potential
and lower electron affinity. The main chain in the substituted PA remains randomly
oriented, which suppresses the electrical conductivity of the polymer.
The addition of a liquid crystalline (LC) moiety to the polymer main chain makes
the polymer soluble in organic solvents and facilitates alignment by the spontaneous
orientation of the LC group. For example, mono-substituted liquid crystalline PA
(mono-LCPA), which is prepared by introducing a LC moiety into the side chain
342
K. Akagi