Eva M. García-Frutos

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.

Synthesis and Self-Association Properties of Functionalized C3-Symmetric Hexakis(p-substituted-phenylethynyl)triindoles

A number of differently substituted phenylethynyl triindoles has been synthesized by 6-fold Sonogashira coupling in a key step. This new series of hexaalkynyl triindoles self-associate through arene-arene interactions in solution. The electronic communication of the external substituents with the central electron-rich triindole core has been demonstrated by means of cyclic voltammetry and optical absorption. A study of the effect of the electronic character of peripheral substituents on their self-association behavior is presented in an effort to shed light on the nature of the pi-stacking interactions.

Small organic single-crystalline one-dimensional micro- and nanostructures for miniaturized devices

1D π-conjugated organic micro- or nanostructures keep all the advantages found in organic single crystals, e.g. high crystallinity, but without the tricky growth associated with large-sized crystals. They also present low-cost processing, solution processable materials, and large-area fabrication, and are compatible with flexible substrates. Electronic and optical properties of 1D π-conjugated organic micro- and nanomaterials can be tuned by changing their size at the central core or chemical modification of the aromatic substituents, among other alternatives. Current challenges are to find and develop an accurate control of the morphology, size, degree of uniformity, and alignment of the micro- and nanometer-sized organic single crystals, for future applications in miniaturized devices.

Highly ordered π-extended discotic liquid-crystalline triindoles

New π-extended discotic liquid crystals based on heptacyclic triindole have been synthesized and their mesomorphic behaviour investigated. The synthesis of π-extended compounds was carried out by oxidative cyclodehydrogenation of differently substituted star-shaped hexaaryltriindoles to afford the planar derivatives with an extended core. Extending the conjugation in these materials results in a significant increase in the order and range of the mesophases. The influence of the peripherally attached groups in the mesomorphic and electronic properties offers a great opportunity to further tune the properties of these compounds intended to their incorporation in devices.

Synthesis and Preferred All-syn Conformation of C3-Symmetrical N-(Hetero)arylmethyl Triindoles

A new series of C(3)-symmetrical N-(hetero)arylmethyl triindoles has been synthesized in a straightforward procedure. The structure and conformation in the solid state have been determined for three derivatives (3, 4, and 6) by X-ray crystallographic analysis. In all three cases, the molecules adopt a tripodal conformation with all of the flexible arms directed towards the same side, thereby delimiting an inner cavity. Compound 6 crystallizes and forms C(3)-symmetric dimeric cagelike complexes. Guest molecules of chloroform and water are confined within the resulting cavities with stabilization by different intermolecular interactions; this highlights the potential of these compounds in the construction of supramolecular systems. A computational analysis has been performed to predict the most stable conformers. As a general trend, a preference for a conformation with all branches directed to the same side has been predicted. Comparison between theoretical and experimental results indicates that the computational level selected for the present study, B3LYP/6-31G*, is able to reproduce both the minimum energy conformations and the rotation barriers about the N--CH(2) bond.

7,7′-Diazaisoindigo: a novel building block for organic electronics

In this paper, a new family of 7,7′-diazaisoindigo molecules was synthesized and the electronic properties of these materials were studied by theoretical calculations and photophysical studies. Density functional theory (DFT) calculations at the B3LYP/6-311+G** level of theory demonstrate that the diaza-substitution clearly imposes a higher planarity in these molecules compared to the isoindigo counterparts. This effect is ascribed to an electrostatic attraction between the carbonyl group and the H atom at position 4. The isodensity surfaces and energies of the frontier molecular orbitals (HOMO and LUMO) calculated at the B3LYP/6-311++G(2d,p) level of theory show a stabilization (∼0.35 eV) of both orbitals in the 7,7′-diazaisoindigo derivatives compared to isoindigo and also the intramolecular charge-transfer character for the HOMO → LUMO transition, as it occurs in isoindigo. Photophysical studies were carried out using steady-state and time-resolved picosecond fluorescence techniques. The emission spectra show a red-shift of the peak upon increasing the polarity of the solvents which confirms the charge-transfer character of this transition. Moreover, the intensity of the emission peak decreases with the solvent polarity and increases in viscous solvents, which is tentatively attributed to a non-radiative deactivation pathway connected with a torsion of the central ethylene bond. The time-resolved emission technique shows shorter fluorescence lifetimes with increasing polarity of the solvents, in line with the stronger quenching of the fluorescence signal. The appearance of fluorescence peaks and relatively long fluorescence lifetimes in the 7,7′-diazaisoindigo derivatives compared to the non-fluorescent isoindigo counterpart are ascribed to the fine adjustment of the orbital energies through the aza insertion in the isoindigo core structure. This is a consequence of the slower non-radiative process in 7,7′-diazaisoindigo which may help to promote the use of this building block instead of isoindigo in organic electronics.

Triindole based single crystals and thin films for OLED and OFET applications

The search of new organic molecules with improved properties is of fundamental relevance for hybrid organic-inorganic based devices (OLED, FET, PV, injection layers, flexible large area devices, lasers, etc.). Triindole based materials present extended aromatic cores with disk-like geometry that allow tailoring their electronic properties through chemical functionalization. In this work we present an optical and electronic study of new triindole based single crystals. Pistacking gives rise to highly ordered columnar structures yielding to high mobilities, around 0.4 cm2V-1s-1. External oxidation is found to increase orders of magnitude the conductivity. Slight modifications of this platform modify substantially the crystallization dynamics and therefore the quality of the thin films obtained by spin coating from solutions with different solvents. The morphology, stability and properties of the optimized films are found to be promising for device fabrication. A comparative study of the absorption and emission efficiency of solutions and thin films of the different derivatives is presented. The first tests for OLED and OFET devices are under way.