Light-induced reorientation and birefringence in polymeric dispersions of nano-sized crystals

Nanocrystals (50-250 nm) of a Palladium complex within a polyisobutylmethacrylate matrix were prepared by a phase separation method. In these dispersions, a light-induced birefringence with Deltan approximately 10(-3) was induced, without the application of an electric field. This effect was related to the photoconducting properties of the dispersion. Evidence for charge photogeneration without any applied field was obtained. The photorefractive behaviour of the material confirmed that the nanocrystals reorientation is a consequence of photoconducting properties. A light-generated electric field approximaely E 3 V/microm was estimated. These results illustrate the potential of materials with a nano-crystalline dispersion morphology as light-responsive media

Side-Chain Multifunctional Photoresponsive Polymeric Materials

The contribute reviews the recent literature concerning the state-of-the-art of the research on amorphous polymeric derivatives bearing side-chain photoactive moieties such as the azo-aromatic and the carbazole chromophore as functional groups, in addition to the presence of structural or chemical features suitable to also provide the macromolecules of chiral properties

Multifunctional supramolecular dendrimers with an s-triazine ring as the central core: Liquid crystalline, fluorescence and photoconductive properties

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s-triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X-ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This work was supported by the seventh FP THE PEOPLE PROGRAMME, The Marie Curie Actions, ITN, no. 215884-2, the MINECO, Spain, (under Projects: CTQ2012-35692 and MAT2012-38538-CO3-01), which included FEDER funding, and the Aragón Government-FSE (Project E04). A.G. and R.T. acknowledge support from the European Community’s Seventh Framework Program (FP7 2007–2013) through the MATERIA Project (PONa3_00370) and from MIUR through the PRIN 2012JHFYMC project. M.B. acknowledges support from the EU through an ESR fellowship.Peer Reviewe

Striking Increase in Hole Mobility upon Metal Coordination to Triphenylene Schiff Base Semiconducting Multicolumnar Mesophases

Producción CientíficaThis paper reports the synthesis, liquid-crystal behavior, and charge-transport properties in the mesophase of triphenylene Schiff bases and their copper(II), nickel(II), and oxovanadium(IV) complexes. The thermal and electronic properties of the Schiff bases are modulated by coordination to the corresponding metal moieties, which have the ability to self-assemble into linear structures and help the alignment of the triphenylene columns. This produces two kinds of electronically nonconnected columnar regions, one purely organic and one more inorganic. The most remarkable effect is a striking charge mobility enhancement in the metal-containing mesophases, due to the contribution of the more inorganic columns: in comparison to values of hole mobility along the columnar stacking for the purely organic columnar mesophases, on the order of 10–7 cm2 V–1 s–1, these values jump to 1–10 cm2 V–1 s–1 in these hybrid inorganic/organic columnar materials.Ministerio de Ciencia, Innovación y Universidades (Project CTQ2017-89217-P)Junta de Castilla y León (Project VA038G18)Universidad del País Vasco / EHU (ProjectGIU18/146

High One-Dimensional Charge Mobility in Semiconducting Columnar Mesophases of Isocyano-Triphenylene Metal Complexes

Producción CientíficaThis paper reports the synthesis, liquid crystal behavior, and charge-transport properties in the mesophase of isocyano-triphenylene gold, copper, palladium, and platinum complexes [MX(CNR)] (CNR = 2-(6-(4-isocyanophenoxy)hexyloxy)-3,6,7,10,11-pentakisdodecyloxytriphenylene; M = Au, X = Cl, C6F5, C6F4OC10H21, CN; M = Cu, X = Cl), [(μ-4,4′-C6F4C6F4){Au(CNR)}2], [(μ-Cl2){Cu(CNR)2}2], and [MX2(CNR)2] (M = Pd, Pt; X = Cl, Br, I, and M = Pt, X = CN). The thermal and electronic properties of these materials are modulated by the metal fragment. The complexes that display columnar mesomorphism are those that support more than one triphenylene per molecule or those that produce a similar effect by dipole–dipole interactions between the metal groups. These circumstances improve the balance of favorable enthalpic interactions versus unfavorable entropic contributions into a columnar stacking. Hybrid inorganic/organic dual columnar mesophases with high SCLC hole mobility along the columnar stacking, above 1 cm2 V–1 s–1, have been found. It is worth noting that the dicyanoplatinum complex displays mesophase phosphorescence based on Pt···Pt interactions.Ministerio de Ciencia e Innovación (Proyect CTQ2014-52796-P

Semiconducting pi-extended discotic liquid-crystalline triindoles: studying their FET vs. SCLC mobilities

The field of organic electronics has experienced a vast development in the last few years, having the first generation of devices based in this technology already reached the market. Advances achieved in this field have been associated with the research of organic semiconductors able to transport charge carriers with a high mobility. However, finding organic semiconductors with the right balance between mobility and processability is still a challenge in the area. In this context, discotic liquid crystals, constituted by an aromatic central core surrounded by flexible alkyl tails, are among the most promising new candidates. Triindoles have been widely studied as a π-conjugated platforms in the construction of high mobility semiconducting liquid crystals. Three-fold oxidative cyclodehydrogenation of hexaphenyltriindole renders the significantly enlarged aromatic core, with structural characteristics of both triphenylene moieties and triindole. In this π-extended discotic core, the attachment of three flexible alkyl chains to the nitrogen atoms is sufficient to induce mesomorphism. The ratio of conducting versus isolating fraction is impressively enhanced when comparing to triindole liquid crystals, not only by enlarging the size of the central core but also by reducing the amount of isolating peripheral chains. In this presentation the electrical properties of the aforementioned semiconductor have been analyzed by applying two different methods: space-charge limited current (SCLC) measurements in a diode-like structure and field effect mobility measurements in a thin film transistor device. The mobility found on a diode type device is higher than that determined on thin film transistors, which can be understood by the high tendency of large π-conjugated molecules to deposit on surfaces with their extended core parallel to the substrate. This is demonstrated with a full analysis of the active layer of the thin film transistor, in order to analyze the molecular orientation in the semiconductor-dielectric interface, via Surface Enhanced Raman Spectroscopy (SERS). The observation of field effect behavior in a discotic liquid crystal processed by simple drop-casing suggests an increased dimensionality of charge transport by facilitating hopping between neighboring columns as a result of the large conducting/isolating ratio found in this discotic platform. Density Functional Theory (DFT) calculations have been performed in order to enlighten with more detail the charge-transport parameters at a molecular level.Universidad de Málaga, Campus de Excelencia Internacional Andalucía Tec

Photoconductive properties and electronic structure in 3,5-disubstituted 2-(2′-pyridyl)pyrroles coordinated to a Pd(II) salicylideneiminate synthon

The synthesis and the electrochemical, photophysical, structural, and photoconductive properties of three new heteroleptic Pd(II) complexes with various 3′,5′- disubstituted-2-(2′-pyridil) pyrroles H(N^N) as coordinated ligands are reported. The coordination of the metal center was completed by a functionalized Schiff base H(O^N) used as an ancillary ligand. The [(N^N)Pd(O^N)] complexes showed highly interesting photoconductive properties which have been correlated to their electronic and molecular structures. Theoretical density functional theory (DFT) and time-dependent DFT calculations were performed, and the results were confronted with the organization in crystalline phase, allowing to point out that the photoconductive properties are mainly a consequence of an efficient intramolecular ligand-to-metal charge transfer, combined to the proximity between the central metal and the donor moieties in the solid-state molecular stacks. The reported results confirm that these new Pd(II) complexes form a novel class of organometallic photoconductors with intrinsic characteristics suitable for molecular semiconductors applications.Supported by Ministero dell’Istruzione, dell’Universitàe della Ricerca by the ELIOTROPO.Peer reviewe

Charge Mobility in Discotic Liquid Crystals

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals

Charge Mobility in Discotic Liquid Crystals

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals