Nanographenes and their Assemblies

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Nanographenes and their Assemblies Wojciech Pisula, Xinliang Feng, Klaus Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, E- mail: [email protected] Improved performance of organic electronics is highly dependent on the order of π-conjugated semiconductors which determine the transport of charge carriers over the macroscopic scale. 1 Amongst various new materials for flexible electronics, conjugated liquid crystals (LCs) are currently considered as a new generation of organic semiconductors. 2 Defined molecular order and dynamics are characteristic for LCs which classifies the intermediate state between the isotropic melt and the crystal. As an essential advantage, the order of LCs can be controlled in the bulk and on the surfaces at all length scales ranging from the molecular to the macroscopic level. 3 Self-healing of structural defects such as grain boundaries, which act typically as traps of charge carriers and limit the performance of the semiconductor in the device, occurs due to the pronounced molecular dynamics of LC. 4 Discotic LCs (DLCs) typically consist of a flat, rigid aromatic core decorated with flexible aliphatic chains which self-assemble into columnar superstructures serving as one-dimensional pathways for charge carriers. 5 The most prominent DLCs are phthalocyanines, 6 triphenylenes, 7 and hexa-peri-hexabenzocoronenes. 8 The charge migration in these materials is expected to be quasi one-dimensional due to insulating alkyl chains around the conducting aromatic pathway . 9 It has been observed that the conductivity along the columnar axis is several orders of magnitude higher than in the perpendicular direction. 10 A local charge carrier mobility of 1.1 cm 2 V -1 s -1 has been determined for crystalline HBCs. 11 Larger aromatic cores lead to discotic LCs with enhanced columnar stability and high supramolecular order and thus to semiconductors with improved charge carrier transport due to a more extended π-orbital overlap. 12 Hydrogen bonds Functional groups can establish multiple non-covalent interactions such as dipole, ionic, and hydrophobic, as well as hydrogen bonds. 13 In comparison to π-interactions, the strength of hydrogen bonds can reach up to 120 kJmol -1 . Therefore, these non-covalent interactions can serve as an additional efficient instrument to tune the molecular packing in or between the columns by the introduction of functional groups at specific locations on disc-shaped molecules. 14 Hydrogen bonds in HBCs can be either formed at the aromatic core between building blocks within the columnar stacks or between columns by spacing them from the disc by a flexible linker (Fig. 1). For intracolumnar hydrogen bonds, groups like amido at the core lead to especially strong intracolumnar hydrogen bonds and to irreversible phase transitions of the LC phase (Fig. 1a). 15 These intermolecular hydrogen bonds between side chains fix an intracolumnar non-tilted packing after cooling to the initial crystalline phase which typically reveals a herringbone structure. Such behavior occurs only for hydrogen bonds located close to the aromatic core. The formation of intercolumnar hydrogen bonds requires functional units, for instance carboxylic acid or alcohol groups, which are spaced from the π-stacking aromatic core by long flexible alkyls (Fig. 1b). 16 The spacer length is crucial for supramolecular organization and results for all derivatives in non-tilted stacking in a pseudocrystalline phase with an unusually high mesophase transition temperature. Especially short alkyl spacers with reduced freedom

Transcript of Nanographenes and their Assemblies

Nanographenes and their Assemblies Wojciech Pisula, Xinliang Feng, Klaus Müllen

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, E-mail: [email protected] Improved performance of organic electronics is highly dependent on the order of π-conjugated semiconductors which determine the transport of charge carriers over the macroscopic scale.1 Amongst various new materials for flexible electronics, conjugated liquid crystals (LCs) are currently considered as a new generation of organic semiconductors.2 Defined molecular order and dynamics are characteristic for LCs which classifies the intermediate state between the isotropic melt and the crystal. As an essential advantage, the order of LCs can be controlled in the bulk and on the surfaces at all length scales ranging from the molecular to the macroscopic level.3 Self-healing of structural defects such as grain boundaries, which act typically as traps of charge carriers and limit the performance of the semiconductor in the device, occurs due to the pronounced molecular dynamics of LC.4 Discotic LCs (DLCs) typically consist of a flat, rigid aromatic core decorated with flexible aliphatic chains which self-assemble into columnar superstructures serving as one-dimensional pathways for charge carriers.5 The most prominent DLCs are phthalocyanines,6 triphenylenes,7 and hexa-peri-hexabenzocoronenes.8 The charge migration in these materials is expected to be quasi one-dimensional due to insulating alkyl chains around the conducting aromatic pathway .9 It has been observed that the conductivity along the columnar axis is several orders of magnitude higher than in the perpendicular direction.10 A local charge carrier mobility of 1.1 cm2V-1s-1 has been determined for crystalline HBCs.11 Larger aromatic cores lead to discotic LCs with enhanced columnar stability and high supramolecular order and thus to semiconductors with improved charge carrier transport due to a more extended π-orbital overlap.12 Hydrogen bonds Functional groups can establish multiple non-covalent interactions such as dipole, ionic, and hydrophobic, as well as hydrogen bonds.13 In comparison to π-interactions, the strength of hydrogen bonds can reach up to 120 kJmol-1. Therefore, these non-covalent interactions can serve as an additional efficient instrument to tune the molecular packing in or between the columns by the introduction of functional groups at specific locations on disc-shaped molecules.14 Hydrogen bonds in HBCs can be either formed at the aromatic core between building blocks within the columnar stacks or between columns by spacing them from the disc by a flexible linker (Fig. 1). For intracolumnar hydrogen bonds, groups like amido at the core lead to especially strong intracolumnar hydrogen bonds and to irreversible phase transitions of the LC phase (Fig. 1a).15 These intermolecular hydrogen bonds between side chains fix an intracolumnar non-tilted packing after cooling to the initial crystalline phase which typically reveals a herringbone structure. Such behavior occurs only for hydrogen bonds located close to the aromatic core. The formation of intercolumnar hydrogen bonds requires functional units, for instance carboxylic acid or alcohol groups, which are spaced from the π-stacking aromatic core by long flexible alkyls (Fig. 1b).16 The spacer length is crucial for supramolecular organization and results for all derivatives in non-tilted stacking in a pseudocrystalline phase with an unusually high mesophase transition temperature. Especially short alkyl spacers with reduced freedom

show robust hydrogen-bonds which increase the order. At low temperatures these hydrogen bonds distort the preferred tilted arrangement towards a non-tilted packing.

Fig. 1. HBCs with functional units capable for hydrogen bonds and corresponding schematic illustrations of a) intracolumnar and b) intercolumnar hydrogen bonds. Red arrows indicate interactions of hydrogen bonds between functional groups represented as blue units. Dipole functionalization Similar to hydrogen bonds, dipole interactions are applied as an effective non-covalent force to control the molecular motions and intracolumnar packing. These supramolecular forces are introduced within the HBC columnar stacks to improve the molecular packing. After cooling the HBC derivative (Fig. 2a) with three alternating methoxy and alkyl substituents from its LC state a highly ordered helical packing with a large intracolumnar correlation was obtained (Fig. 2b).17 Identically to the effect of hydrogen bonds, the dipole forces positioned close to PAH core change the intracolumnar packing mode from a tilted to a non-tilted arrangement after thermal annealing. The X-ray scattering results indicated that the helical organization was induced by local dipole moments between the C-O bonds of individual neighboring molecules resulting in a relatively small molecular rotation angle and tight packing.

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