Roberto Termine

High charge mobility in discotic liquid-crystalline triindoles: just a core business?

This work has been supported by CTQ2007-65683/BQU, (CTQ2009- 09030), S2009/MAT-1756/CAM, MiUR through the PRIN 2007 (2007WJMF2W) project, Gobierno de Aragon, 200960I086 PIE.

Video-rate compatible photorefractive polymers with stable dynamic properties under continuous operation

We report on photorefractive polymer composites that exhibit stable dynamic properties under continuous operation. These materials are based on a bis-triarylamine side-chain polymer matrix with a low ionization potential. The evolution of the response time for exposures up to 4kJ∕cm2 was studied and compared with that obtained in poly(n-vinylcarbazole) (PVK) based composites. In the composites, operational stability is combined with video-rate compatible dynamics, large dynamic range at moderate fields, and long shelf lifetimes.

Supramolecular dendrimers based on the self-assembly of carbazole-derived dendrons and triazine rings: liquid crystal, photophysical and electrochemical properties

Two novel families of dendrimers have been synthesized by hydrogen bonding between a triazine moiety (M), as an electron-transporting central core, and three peripheral dendrons of generations 1 and 2 derived from bis(hydroxymethyl)propionic acid (bis-MPA). The dendrons contain both carbazole-based groups as hole-transporting moieties and promesogenic units derived from 5-[4-(4-butoxybenzoyloxy)phenyloxy]pentanoic acid (family A) or cholesteryl hemisuccinate (family B) (in a 1 : 3 ratio). The formation of the complexes was confirmed by IR and NMR data. All synthesized complexes displayed mesogenic properties and their liquid crystalline properties have been investigated by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffractometry (XRD). Complexes derived from CzAn dendrons (family A) exhibit nematic mesomorphism whereas complexes derived from CzBn dendrons (family B) exhibit a smectic A phase. The electrochemical behavior of the dendritic complexes was investigated by cyclic voltammetry (CV) and the dendrimers show irreversible or quasi-reversible electrochemical behavior in the range 0.0–2.0 V at a scan rate of 100 mV s−1. The UV-vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers have also been investigated.

UV/Vis to NIR photoconduction in cyclopalladated complexes.

The incorporation of a rigid core, formed by a cyclopalladated azobenzene fragment bonded to an ancillary Schiff base ligand, into molecules with 12 or 11 peripheral alkyl chains has been successfully achieved. These new complexes, 1 and 2, respectively, are columnar liquid crystals between room temperature and about 50 degrees C. Both cyclometallated and ancillary ligands have been polyalkylated through either aryl ester (electron-withdrawing group) or aryl ether (electron-releasing group) linkages, in order to tune the HOMO/LUMO energy levels. The photoconductive properties of 1 and 2 have been studied as a function of their absorption properties before and after annealing, from the UV/Vis to NIR region. Compared with the reference compounds, tris-alkynyl benzene discotics, these new materials gave similar performances (sigma/I approximately 8x10(-13) S cm W(-1) with E = 10 V microm(-1) at lambda = 370 nm). Moreover, complex 2 shows a normalized photoconductivity sigma/I = 8.5x10(-13) S cm W(-1) at lambda = 760 nm. Organic photoconductors in such a high wavelength spectral range are not common and are usually assembled by mixing dyes with organic semiconductors.

Photorefractive Properties of Undoped Chiral Smectic C Phases of Cyclopalladated Complexes

ABSTRACT The photorefractive properties of chiral smectic C phases formed by cyclopalladated complexes were studied. The complexes were not doped with photosensitizers. A gain coefficient higher than 100 cm−1 at an applied field of 5.4 V/µm was measured. Measurements of phase shift between interference fringes and index grating were also performed. By using a previously developed model, the amplitude of the space-charge field was estimated and shown to be much smalled than what predicted by the commonly used model for photorefractive inorganic crystals.

H-Bonded Donor–Acceptor Units Segregated in Coaxial Columnar Assemblies: Toward High Mobility Ambipolar Organic Semiconductors

A novel approach to ambipolar semiconductors based on hydrogen-bonded complexes between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acids is described. The formation of 1:3 supramolecular complexes was evidenced by different techniques. Mesogenic driving forces played a decisive role in the formation of the hydrogen-bonded complexes in the bulk. All of the complexes formed by nonmesogenic components gave rise to hexagonal columnar (Colh) liquid crystal phases, which are stable at room temperature. In all cases, X-ray diffraction experiments supported by electron density distribution maps confirmed triphenylene/tris(triazolyl)triazine segregation into hexagonal sublattices and lattices, respectively, as well as remarkable intracolumnar order. These highly ordered nanostructures, obtained by the combined supramolecular H-bond/columnar liquid crystal approach, yielded donor/acceptor coaxial organization that is promising for the formation of ambipolar organic semiconductors with high mobilities, as indicated by charge transport measurements.

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.

Liaisons between photoconductivity and molecular frame in organometallic Pd(II) and Pt(II) complexes

The synthesis and a comprehensive experimental study of molecular structure, electrochemical properties, photoconductivity, absorption and emission properties of two homologous series of square planar cyclometallated Pd(II) and Pt(II) complexes are described. In these complexes, a five member (C,N) metallacycle is formed by either azobenzene, 2-phenylpyridine or benzo[h]quinoline, while the ancillary (O,N) ligand required to complete the metal coordination sphere is the Schiff base resulting from the condensation of 4-n-hexylaniline with 2-hydroxy-4-n-hexyloxybenzaldehyde. The study is complemented by Density Functional Theory calculations of molecular structure in the ground state, frontier orbitals distribution and energies. Absorption energies, intensities for the transitions and composition of the excited states are characterized by Time Dependent Density Functional Theory computations. Results show that for these complexes the HOMO is located on the Schiff base and the LUMO on the cyclometalated ligand. As also indicated by spectroscopic and photoconductivity results, the photogeneration of charge carriers might be associated with the spatial separation of HOMO and LUMO, while an additional contribution could derive from a conformational variation in the excited state. The separation of the frontier orbitals on different ligands provides simple synthetic routes towards band-gap tuning and energy matching with other molecular species and electrodes, both important features for several applications of molecular semiconductors. The results of the present investigation suggest that these square planar cyclometalated Pd(II) and Pt(II) complexes actually form a new class of photoconductors.

Electropolymerized highly photoconductive thin films of cyclopalladated and cycloplatinated complexes.

The complete characterization of novel electropolymerizable organometallic complexes is presented. These are newly synthesized cyclometalated complexes of general formula (PPy)M(O ∧ N)(n) (H(PPy) = 2-phenylpyridine, M = Pd(II) or Pt(II), H(O ∧ N)(n) = Schiff base). Polymeric thin films have been obtained from these complexes by electropolymerization of the triphenylamino group grafted onto the H(O ∧ N)(n) ancillary ligand. The redox behavior and the photoconductivity of both of the monomers (PPy)M(O ∧ N)(n) and the electropolymerized species have been investigated. The polymeric films of (PPy)M(O ∧ N)(n) have shown a very significant enhancement of photoconductivity when compared to their monomeric amorphous counterparts. The high stability of the obtained films strongly suggests that electropolymerization of cyclometalated complexes represents a viable deposition technique of quality thin films with improved photoconduction properties, hence opening the door to a new class of materials with suitable properties for optoelectronic applications.

Open circuit voltage tuning through molecular design in hydrazone end capped donors for bulk heterojunction solar cells

The design, synthesis and characterization of five diphenylhydrazone end capped molecules as donors for Bulk Heterojunction (BHJ) solar cells are described. The use of the hydrazone donor instead of the more common amine one is advantageous in terms of synthetic access and overall stability. We show that the variation of both the nature and the substitution pattern of the electron deficient conjugated bridge enables the tuning of the optical energy gap as well as of the HOMO and LUMO. In particular, our data show that the low energy shift of the HOMO level along the series of hydrazone compounds, one of the critical parameters affecting the performance of BHJ cells, is as high as 0.42 eV. We tested all derivatives as donors in a BHJ configuration, using the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. We found that the trend we observed in the progressive stabilization of the chromophore HOMO level upon increasing the electron accepting capabilities of the conjugated bridge is reflected in the progressive increase of the cell open circuit voltage (Voc). Our preliminary results show that it is possible to obtain a Voc as high as 0.74 V, along with a power conversion efficiency of 1.2%.