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Smectic phases
The smectic phase is characterized by an orientation order and positional order as the molecules are organized in layers [53]. A variety of smectic phases exist, mainly obtained from calamitic molecules. Organization of the mesogens within the layers gives rise to several varieties of the lamellar phases.
In fact, those molecules are, as in the nematic phase, oriented parallel to a preferred direction, but in addition, they are arranged in parallel, equidistant layers, which can be free to slide over each other depending on the nature of the smectic phase.
Generally, when a smectic phase sample is placed between two glass slides, the layers become distorted and can slide over another one in order to adjust to the surface conditions and preserve their thickness. The optical properties (focal-conic texture) of the smectic state arise from these distortions of the layers. Typical optical textures of smectic phase are shown in Figure 1.17.
Moreover, the calamitic dipole director can be aligned in a perpendicular, parallel or tilted manner to the layers responding to special physical characters: such property can be interesting for many applications.
Several smectic phases have been recognized from SmA-K nowadays, but we will introduce the three major conventional A-C, for a better understanding, with simple remarks for other mesophases.
Liquid crystalline semiconductors
It is only in relatively recent years, compared with the discovery of liquid crystals, that the applications of liquid crystals for electronics have attracted significant scientific interest.
In fact, as said before, organic π-conjugated materials offer simple processing technique, excellent charge mobility, and they have been used as many electronic devices, included OFETs. But of course, when discussing charge carrier mobility in organic materials, it is essential to recognize that it is not only depending on intrinsic property of organic molecules but also on the arrangement of the molecules since it requires hopping of charges between the molecules.
Why target liquid crystals for OFET applications?
Compared with organic crystalline semiconducting materials, liquid crystalline semiconductors have exclusive advantages, such as the self-healing ability and various manual processing conveniences from their fluidity. When liquid crystals are applied to electronics, it poses two particular questions: charge transport in mesophase and impurity effects. Charge transport [68, 69].
A typical liquid crystalline molecule consists of a rigid aromatic core with flexible alkyl chains, leading to the crystal-like order and its fluidity respectively. These molecular characters can provide either ionic and/or electronic conduction depending on the applied field.
Generally, less ordered mesophase tends to allow the ionic conduction due to the structural defects prohibiting the charge transport, while a higher ordered mesophase is favorable for the electronic conduction. In fact, experiential results have proved that in nematic phase the electronic mobility is relatively low compared to the observation of high ion mobility, while within a smectic or columnar phase it exhibits a good electronic mobility.
Moreover, for a given class of liquid crystals having the same molecular core, the mobility is enhanced in a step-wise manner from phase to phase according to the increase in the molecular order [67]. In the case of columnar organization formed by discotic compounds, the inter- and intra-columnar ordering give hugely different charge transport. The mobility is enhanced as the molecular order is increased.
For example, the mobility is around 10-3 cm2·V-1·S-1 cm2·V-1·S-1 in a columnar plastic phase and around 10-1 phase in a columnar ordered phase, around 10-2 cm2·V-1·S-1 or higher in a columnar helical In the case of a smectic phase formed by calamitic molecules, in general the typical mobility of each phases is around 10-4 cm2·V-1·S-1 for SmA and SmC phases [70], around 10-3 cm2·V-1·S-1 for SmBhex and SmF phases and around 10-2 cm2·V-1·S-1 for SmBcryst, SmE and SmG phases [71].
To date, there are reports of a series of liquid crystalline compounds which exhibit good charge transport ability, sometimes showing high mobilities ranging from 10-4 cm2·V-1·S-1 up to ~1 cm2·V-1·S-1. These semiconducting compounds include a vast range of materials for example, derivatives of triphenylenes [72, 73 ], phthalocyanines [74], porphirines [75 ], perylenes [76], phenylbenzothiazoles [77 ], phenylnaphthalenes [78], oligothiophenes [79], and benzothienobenzothiophenes [80].
In organic semiconductors, chemicals impurities can work as trap states for holes and electrons. Moreover, in the case where the carriers are trapped in the deep states, the resulting trapped charges are not released from the state in time range of the transit time, and then it affects the charge transport properties of the materials. Therefore, for OEFT applications it requires high purity of materials in order to obtain high mobility [67].
Discotic liquid crystalline semiconductor (small molecules)
The original evidence of disc-like liquid crystals came from the study of S. Chandrasekhar and his colleagues [81] by X-ray diffraction patterns, when they described the mesophase of benzene-hexa-n-alkanoates as » a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. » From that moment, the liquid crystals with discotic shape revealed their diversity.
The topology is rigid core and flexible periphery such as triphenylene and perylene, or it is inverted with flexible core and rigid periphery such as phthalocyanines. The disc-like cores pile up aligning parallel to each other in one column, leading to two-dimensional architecture. In disordered structure, there is no positional orders inter-columns and not exact stacking sequence among discotic units while they have the orientational order (examples in Figure 1.22). Or in ordered structure, the ordered comes from inter-columns or intra-columns, showing as oblique, plastic and helical columnar organizations we have mentioned in advanced.
Because of those stacking of columnar alignment, the overlap between the π-orbitals (HOMOs or LUMOs) of adjacent molecules can be observed, which is beneficial to the charge transport. However, the intermolecular distance in a column is almost constant (around 3.5 °A) irrespective of the nature of the columnar phase. We can consequently say that the nature of materials and the intra-columnar order almost determine the charge transport.
The charge transport of holes (p-type materials) and/or electron (n-type materials) depends on the nature of the materials, thus we will describe both of them separately.
p-type discotic mesogens
The columnar mesophase formed by discotic molecules supplies 1D charge transport. In the columnar organizations, the charge (electron or hole) transport is only possible parallel to the columns along the stacked aromatic mesogens centers, as the exchange of carriers between neighboring columns is strongly hindered due to the insulating aliphatic chains [82]. When using such molecules into device configurations which require the edge-on or homogeneous configurations between the sources and drain electrodes (see in Figure 1.23 b), in this cases the columnar liquid crystals show their advantage with the ability to obtain long-range alignment [83].
Calamitic liquid crystalline semiconductor (small molecules)
Attached to a flexible moiety, the rod-like core can be aromatic, heterocyclic, or alicyclic. As examples, we can notice a rigid core (hard) such as quinquephenyl, a semi-rigid/semi-flexible one like stilbene derivatives, and/or a flexible core [99]. When the calamitic compounds are set on a silane-modified substrate, generally the molecules self-organize as homeotropic alignment to give the in-plane transport between the electrodes: in this context the layer architecture results in the 2D hopping (Figure 1.26 a, b).
Many calamitic liquid crystals with excellent semiconducting performance have been reported. For example (Figure 1.26 c), the [1]benzothieno[3,2-b]benzothiophene (BTBT) with different alkyl chains lengths presented good electric properties in OFET configuration with an electron mobility as high as 2.75 cm2·V-1·S-1 in its SmA phase [100, 101].
Figure 1.26 Schematic illustration of calamitic liquid crystals applied for OFET: a) charge transport in smectic organization; b) smectic phase applied for device configuration; c) some examples of calamitic liquid crystalline semiconductors.
The pyromellitic diimide and its derivatives are as well typical n-type liquid crystal OFET materials, and its smectic structure could supply around 0.07 cm2·V-1·S-1 of electron mobility [102]. As we mentioned it already, the oligothiophene with several smectic phases also presents high electron mobility values, however the layer tilting still exists reducing the π-π interaction hence sometime the mobility is anisotropic, such as in the case of the SmC.
More complex LC materials based on the presence of several mesogens
Since we have introduced the single molecular liquid crystalline structures in the previous section, in this part we focus on more complex thermotropic materials combining several mesogens linked together, such as dimer-, trimer-, tetramer- and polymer-type liquid crystalline materials.
Such high mass materials could indeed present better mesogenic stability and some of them lead to smectic or lamellar mesophases, opening potential applications such as ferroelectric and pyroelectric devices or as displays and sensors [ 113 ]. Moreover, the precise control of the final architecture processing also helps the development and engineering of these complex materials. Concerning the study of the relationship between the molecular structure and mesomorphic property, it has been well known that in most cases, the discotic shape molecules self-assemble to provide columnar intermediate phases while calamitic molecules prefer lamellar (layered) phases. But the self-organization of complex materials owning several mesogens and especially of different geometry (calamitic and discotic ones) has caught the attentions of many physicists and chemists recently. Therefore, we will discuss here this class of new materials, starting from the simplest models i.e. dimers [114] consisting of two mesogens connected by a flexible chain.
Dimers
For a dimer (or dyad) which consists of two mesogenic units, the geometry of the two components involved (A and B in Figure 1.29) is one of key parameters determining the chemical and physical properties. Particularly, the shape of the two entities is essential.
On the one hand, when both mesogen rigid cores are disc-like, such as segregated perylene and hexabenzocoronene (HBC) [115], or discotic triphenylene combined with discotic perylene diimide [102], the dimer (Disc-Disc) is presenting a rigid columnar arrangement, a low HOMO-LUMO gap and numerous efficient optoelectronic characters which are advantageous for organic photovoltaic devices.
On the other hand, when both mesogen rigid cores are rod-like, such as α,ω-bis(4-cyanobiphenyl-49-yloxy)alkanes who presents nematic mesophase [116], we speak about (Cala-Cala) compounds. More precisely, different patterns of dyad have been studied, including linear, H-shape and T-shape as illustrated in the Figure 1.30, and the (Cala-Disc) [119] will be introduced later.
Going into details in the case of the study related to aromatic esters connected by polymethylene spacers, three patterns of dimer liquid crystals with their specific thermal behaviors were compared. It was found that the conventional linear structure and T-shape structure samples had a wider mesophase temperature range than the H-shape structure. Moreover, they tended to be enantiotropic liquid crystalline materials while the H-shape material was monotropic unless the spacer length was fairly long. On the contrary, the conventional linear compounds present highest melting and isotropic temperature. It must be noticed as well that in another report [120], the linear and T-shape structure samples were observed only nematic phase when H-shape compound was observed the smectic phase.
Table of contents :
GLOSSARY
CHAPTER 1 INTRODUCTION
1.1 π-conjugated materials
1.1.1 The concept based on valence bond (VB) and molecular orbital (MO) theories
1.1.2 π-Conjugation pathway and Energy, inducing optical and electronic properties
1.1.3 Potential applications
1.1.3.1 What is an OFET?
1.1.3.2 Towards ambipolar organic transistors
1.2 Liquid Crystals
1.2.1 Short history and definitions
1.2.2 Thermotropic liquid crystals general classifications
1.2.2.1 Classification depending on the shape
1.2.2.2 Classification depending on the mesophase
1.2.3 Discotic versus calamitic thermotropic mesophases
1.2.3.1 Nematic phases
1.2.3.2 Smectic phases
1.2.2.3 Columnar phases
1.3 Liquid crystalline semiconductors
1.3.1 Why target liquid crystals for OFET applications?
1.3.2 Discotic liquid crystalline semiconductor (small molecules)
1.3.2.1 p-type discotic mesogens
1.3.2.2 n-type discotic mesogens
1.3.3 Calamitic liquid crystalline semiconductor (small molecules)
1.3.3.1 p-type calamitic mesogens
1.3.3.2 n-type calamitic mesogens
1.3.4 More complex LC materials based on the presence of several mesogens
1.3.4.1 Dimers
1.3.4.2 Trimer and tetramer
1.3.4.3 Discotic-calamitic combined liquid crystals
1.3.4.4 Supermolecule and polymer liquid crystalline materials
1.3.5 Conclusions
CHAPTER 2 LINEAR DYAD AND TRIAD BASED ON PERYLENE DIIMIDE/TERTHIOPHENE MOIETIES
2.1 Synthesis
2.1.1 Synthesis of precursory building blocks
2.1.1.1 Synthesis of the terthiophene building block
2.1.1.2 Synthesis of the mono-anhydride mono-imide perylene building block
2.1.2 Synthesis of target linear Dyad 1.1 and linear Triad 1.2
2.1.2.1 Synthesis of linear Dyad 1.1
2.1.2.1 Synthesis of linear Triad 1.2
2.2 Optical properties (absorption and emission)
2.2.1 Optical properties of terthiophene and perylene diimide model compounds
2.2.2 Absorption and Emission of Dyad 1.1
2.2.2 Absorption and Emission of Triad 1.2
2.3 Mesomorphic properties
2.3.1 DSC and POM of Dyad 1.1
2.3.2 DSC and POM of Triad 1.2
2.4 Self-organization study (X-ray diffraction and Atomic force microscopy)
2.4.1 X-ray Diffraction (XRD)
2.4.1.1 XRD of Dyad 1.1
2.4.1.2 XRD of Triad 1.2
2.4.2 Atomic force microscopy (AFM)
2.4.2.1 AFM of Dyad 1.1
2.4.2.2 AFM of Triad 1.2
2.5 Conclusions
2.6 Experimental
2.6.1 Synthesis of precursory building blocks (terthiophene and perylene building blocks)
2.6.2 Synthesis of Dyad 1.1
2.6.3 Synthesis of Triad 1.2
CHAPTER 3 LINEAR DYAD AND TRIAD BASED ON PERYLENE DIIMIDE/BTBT MOIETIES
3.1 Synthesis
3.1.1 Synthesis of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) precursor
3.1.2 Synthesis of linear Dyad 2.1
3.1.3 Synthesis of linear Triad 2.2
3.2 Optical properties (absorption and emission)
3.2.1 Absorption and Emission of BTBT and perylene diimide model compounds
3.2.2 Absorption and Emission of Dyad 2.1
3.3 Mesomorphic properties
3.3.1 DSC and POM of Dyad 2.1
3.3.2 DSC and POM of Triad 2.2
3.4 Self-organization study (X-ray diffraction)
3.4.1 XRD of Dyad 2.1
3.4.2 XRD of Triad 2.2
3.5 Conclusions
3.6 Experimental
3.6.1 Synthesis of precursory building blocks (BTBT building blocks)
3.6.2 Synthesis of Dyad 2.1
3.6.3 Synthesis of Triad 2.2
CHAPTER 4 BRANCHED TRIADS BASED ON TRIPHENYLENE/PYROMELLIC DIIMIDE/PERYLENE DIIMIDE/TERTHIOPHENE MOIETIES
4.1 Synthesis
4.1.1 Synthesis of building blocks
4.1.1.1 Synthesis of the triphenylene building block
4.1.1.2 Synthesis of the pyromellitic diimide building block
4.1.2 Synthesis of the four target branched triads
4.1.2.1 Synthesis of branched Triad 3.1
4.1.2.2 Synthesis of branched Triad 3.2
4.1.2.3 Synthesis of branched Triad 3.3
4.1.2.4 Synthesis of branched Triad 3.4
4.2 Optical properties (absorption and emission)
4.2.1 Optical properties of isolated triphenylene, pyromellitic diimide, perylene diimide and terthiophene model compounds
4.2.2 Absorption and Emission of Triad 3.1
4.2.3 Absorption and emission of Triad 3.2
4.2.4 Absorption and emission of Triad 3.3
4.2.5 Absorption and emission of Triad 3.4
4.3 Mesomorphic behavior
4.3.1 DSC and POM of Triad 3.1
4.3.2 DSC and POM of Triad 3.2
4.3.3 DSC and POM of Triad 3.3
4.3.4 DSC and POM of Triad 3.4
4.4 Self-organization study (X-ray diffraction and Atomic force microscopy)
4.4.1 X-ray Diffraction (XRD)
4.4.1.1 XRD of Triad 3.1
4.4.1.2 XRD of Triad 3.2
4.4.1.3 XRD of Triad 3.4
4.4.2 Atomic force microscopy (AFM)
4.5 Preliminary study of charge transport properties (OFET)
4.5 Conclusions
4.6 Experimental
4.6.1 Synthesis of precursory building blocks (triphenylene and pyromellitic building blocks)
4.6.2 Synthesis of Triad 3.1
4.6.3 Synthesis of Triad 3.2
4.6.4 Synthesis of Triad 3.3
4.6.5 Synthesis of Triad 3.4
Supplementary data
CONCLUSIONS AND PERSPECTIVES
References
