360
M. Funahashi
18.1 Introduction
Molecular crystals consisting of molecules with extended π-conjugated units function as semiconductors, and they have been applied to organic electronic devices, such
as field-effect transistors (FETs). Electronic charge carriers are transported between
π-conjugated units via orbital overlaps and carrier mobilities strongly depend upon
the supramolecular aggregation states of the crystals [1]. In addition, liquid crystals
having extended π-conjugated units also work as semiconductors and applications
to FETs, electroluminescence devices, and solar cells [2].
The theme of this chapter is to create new soft materials to couple of the electronic charge carrier transport with ionic polarization in nanosegregated liquid crystalline (LC) systems [3]. In contrast to molecular crystals, most characteristic feature
of liquid crystals is a soft dynamic structure [4]. In liquid crystal materials, functional domains can be integrated in liquid crystal phases based on nanosegregation
to produce multi-functional systems [5]. Aggregates of electro- and redox-active πconjugated units and hydrophilic mantles are self-assembled in columnar or lamellar
orders to form soft mixed conductors. In mixed conductors, ionic species polarize to
form local electric fields interacting with electronic charge carriers by the application of the external DC bias, resulting in electro- and redox-active functions of the
materials [6].
For this purpose, ‘side chain engineering’ is indispensable [7]. Electro- and redoxactive materials have extended π-conjugated systems, which indicate strong π –π
stacking interaction. In order to maintain the solubility and flexibility of thin films,
rigid active units should be connected to flexible side chains. Moreover, the side
chains also promote nanosegregation to form flexible nanostructures to induce electronic and electrochemical functions. The author found that oligosiloxane moieties
do not only increase solubilities of LC materials but also promote formation of
nanostructures to enhance electronic and electrochemical functions [8].
In the electrochromism, the electrical double layers formed at the cathode and
anode promote hole or electron injection in the active layer, resulting in reduction
or oxidation of the redox-active units [9]. Thin films of crystals have no nanopores
through which ionic species penetrate and polarization based on the formation of
electrical double layers therefore does not occur. In contrast, thin films of the nanostructured LC mixed conductors have nanochannels, in which ionic species are mobile.
By the DC bias application in the presence of electrolytes, electrical double layers
are immediately formed.
In the doping of nanostructured π-conjugated liquid crystals, ionic reductants
diffuse through the ion channels of the LC thin films and reduce the electron deficient
π-conjugated cores to increase the electron density. Dopant ions are separated from
the electron-transporting π-stacks, and this polarized structure is categorized as an
interstitial doping [10]. This is quite different from the substitution doping in the
organic molecular crystals in which dopant molecules occupy the lattice points of
the host crystals [11].
M. Funahashi
18.1 Introduction
Molecular crystals consisting of molecules with extended π-conjugated units function as semiconductors, and they have been applied to organic electronic devices, such
as field-effect transistors (FETs). Electronic charge carriers are transported between
π-conjugated units via orbital overlaps and carrier mobilities strongly depend upon
the supramolecular aggregation states of the crystals [1]. In addition, liquid crystals
having extended π-conjugated units also work as semiconductors and applications
to FETs, electroluminescence devices, and solar cells [2].
The theme of this chapter is to create new soft materials to couple of the electronic charge carrier transport with ionic polarization in nanosegregated liquid crystalline (LC) systems [3]. In contrast to molecular crystals, most characteristic feature
of liquid crystals is a soft dynamic structure [4]. In liquid crystal materials, functional domains can be integrated in liquid crystal phases based on nanosegregation
to produce multi-functional systems [5]. Aggregates of electro- and redox-active πconjugated units and hydrophilic mantles are self-assembled in columnar or lamellar
orders to form soft mixed conductors. In mixed conductors, ionic species polarize to
form local electric fields interacting with electronic charge carriers by the application of the external DC bias, resulting in electro- and redox-active functions of the
materials [6].
For this purpose, ‘side chain engineering’ is indispensable [7]. Electro- and redoxactive materials have extended π-conjugated systems, which indicate strong π –π
stacking interaction. In order to maintain the solubility and flexibility of thin films,
rigid active units should be connected to flexible side chains. Moreover, the side
chains also promote nanosegregation to form flexible nanostructures to induce electronic and electrochemical functions. The author found that oligosiloxane moieties
do not only increase solubilities of LC materials but also promote formation of
nanostructures to enhance electronic and electrochemical functions [8].
In the electrochromism, the electrical double layers formed at the cathode and
anode promote hole or electron injection in the active layer, resulting in reduction
or oxidation of the redox-active units [9]. Thin films of crystals have no nanopores
through which ionic species penetrate and polarization based on the formation of
electrical double layers therefore does not occur. In contrast, thin films of the nanostructured LC mixed conductors have nanochannels, in which ionic species are mobile.
By the DC bias application in the presence of electrolytes, electrical double layers
are immediately formed.
In the doping of nanostructured π-conjugated liquid crystals, ionic reductants
diffuse through the ion channels of the LC thin films and reduce the electron deficient
π-conjugated cores to increase the electron density. Dopant ions are separated from
the electron-transporting π-stacks, and this polarized structure is categorized as an
interstitial doping [10]. This is quite different from the substitution doping in the
organic molecular crystals in which dopant molecules occupy the lattice points of
the host crystals [11].