Marta Reig

Molecular order of air-stable p-type organic thin-film transistors by tuning the extension of the π-conjugated core: the cases of indolo[3,2-b]carbazole and triindole semiconductors

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10−5 to 10−2 cm2 V−1 s−1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics.

Fastest molecular photochromic switches based on nanosecond isomerizing benzothiazolium azophenolic salts

In search of strategies to improve the switching speeds and fatigue resistances of photochromic molecular switches, we have analysed the kinetic behaviour of a series of photoactive azoderivatives which combine the strong electron withdrawing character of both thiazolium and benzothiazolium salts with the ability of azophenols to establish keto–enolic equilibrium. The excitation of these molecules with a green laser at 532 nm, a wavelength that is compatible with biological applications, induces trans-to-cis photoisomerisation in less than 5 ns. The photogenerated cis isomer reverts to the thermodynamically stable trans form with lifetimes as short as 55 ns in ethanol at room temperature. Thus, a full switching cycle can be completed on a nanosecond timescale. To the best of our knowledge, these are the fastest molecular photochromic switches found heretofore. Furthermore, all of the switches reported herein are able to tolerate thousands of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, which is convenient for further technological applications. In addition, these molecules can be adsorbed onto microcrystalline cellulose thereby exhibiting a thermal isomerisation rate as fast as that observed in alcoholic solutions. This feature affords novel solid switchable materials which operate under similar conditions with nanosecond switching speeds.

Charge Transfer States in Stable Neutral and Oxidized Radical Adducts from Carbazole Derivatives

In this paper we report the spectral properties of the stable radical adducts 1(•)-3(•), which are formed by an electron donor moiety, the carbazole ring, and an electron acceptor moiety, the polychlorotriphenylmethyl radical. The molecular structure of radical adduct 1(•) in the crystalline state shows a torsion angle of approximately 90° between the phenyl and the carbazole rings due to steric interactions. They exhibit a charge transfer band in the visible range of the electronic spectrum. All of them are chemically oxidized with copper(II) perchlorate to the respective cation species, which show a strong charge transfer band into the near-infrared region of the spectrum. Radical adducts 1(•)-3(•) and the corresponding stable oxidized species 1(+)-3(+) are real organic mixed-valence compounds due to the open-shell nature of their electronic structure. Charge transfer bands of the cation species are stronger and are bathochromically shifted with respect to those of the neutral species due to the greater acceptor ability of the positively charged central carbon atom of the triphenylmethyl moiety. The cationic species 1(+)-3(+) are diamagnetic, as shown by the absence of a signal in the EPR spectrum in acetonitrile solution at room temperature, but they show an intense and unique band in frozen solutions (183 K).

Molecular photo-oscillators based on highly accelerated heterocyclic azo dyes in nematic liquid crystals

Benzothiazole-pyrrole-based azo dyes greatly enhance their thermal isomerisation rate by up to 160 times when they are under the influence of the nematic mean field yielding the LC-based photochromic oscillators with the highest oscillation frequencies reported so far (2.6 kHz at 298 K).

New solution-processable carbazole derivatives as deep blue emitters for organic light-emitting diodes

Two new compounds based on three carbazole units connected by triple bonds as π-spacers have been developed as deep blue emitters for organic light-emitting diodes (OLEDs). Their optical and electrochemical properties were examined and their charge carrier transport properties were investigated by means of the xerographic time-of-flight (XTOF) technique. The prepared diodes demonstrate the feasibility of the new molecules as effective emitters in the deep blue region yielding devices with low turn-on voltages.

Stable all organic radicals with ambipolar charge transport

A series of neutral long-lived purely organic radicals based on the stable [4-(N-carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methyl radical adduct (Cbz-TTM) is reported herein. All compounds exhibit ambipolar charge-transport properties under ambient conditions owing to their radical character. High electron and hole mobilities up to 10-2 and 10-3  cm2  V-1  s-1 , respectively, were achieved. Xerographic single-layered photoreceptors were fabricated from the radicals studied herein, exhibiting good xerographic photosensitivity across the visible spectrum.

Twisted intramolecular charge transfer in a carbazole-based chromophore: the stable [(4-N-carbazolyl)-2,3,5,6-tetrachlorophenyl]bis(2,3,5,6-tetrachlorophenyl)methyl radical

A neutral stable organic radical adduct, 1, composed of a donor–acceptor dyad is reported. The electron-donor part is the carbazolyl ring directly linked to the electron-acceptor polychlorotriphenylmethyl radical through the para position of a phenyl ring. In the synthetic procedure a C(sp2)–H bond is transformed into a C(sp2)–N bond through the radical–radical cross-coupling process. Theoretical calculations predict that the tetrachlorophenyl bridge moiety lies perpendicular to the carbazolyl group to minimize the repulsion with the chlorine atoms in the ortho position. The electron paramagnetic resonance (epr) spectrum of 1 exhibits a small coupling of the electronic spin with the carbazolyl nitrogen (0.32 G), the spin density being mainly located in the central sp2 carbon atom (30.22 G). The radical adduct presents a charge transfer band (λ = 598–640 nm) showing a hypsochromic shift with solvent polarity. In DMF solution, 1 exhibits a new weak band (λ = 493 nm) which is tentatively attributed to a zwitterionic structure of the molecule resulting from a net electron transfer from the nitrogen to the central sp2 carbon atom. Cyclic voltammetry of 1 confirms the amphoteric character of the molecule. Computed values of ionization potential (IP) and electron affinity (EA) are in good agreement with the experimental values.

Spatially Close Azo Dyes with Sub‐Nanosecond Switching Speeds and Exceptional Temporal Resolution

Photoswitchable bis-azo dyes with an outstanding temporal resolution of 10(15) times between the thermal relaxation rates of their two constituting photochromes are reported. Remarkably, the close spatial proximity of both azo photochromes in these molecular assemblies translates in an unprecedented 10(3) -fold acceleration of the thermal isomerization rate of their faster azo unit compared to the one displayed by the isolated counterpart. Indeed, the relaxation time of the fast-isomerizing platform of the herein reported bis-azobenzenes is as low as 200 ps under ambient conditions. In the wake of these results, the bis-azo dyes described herein are invaluable chromophores for the design of multifunctional light-addressable materials in which simultaneous switching in two very different timescales might be essential.

A study of the properties, reactivity and anticancer activity of novel N-methylated-3-thiazolyl or 3-thienyl carbazoles and their Pd(II) and Pt(II) complexes

Abstract The synthesis and characterization of two hybrid N-methylated carbazole derivatives containing a thiazolyl or a thienyl ring is reported. The thiazolyl derivative has been also characterised by X-ray diffraction analysis. The study of its reactivity in front of [MCl2(dmso)2] (M = Pd or Pt) or Na2[PdCl4] in methanol has allowed us to isolate and characterize its complexes. However, for the thienyl analogue, the formation of any Pd(II) or Pt(II) complex was not detected, indicating that it is less prone to bind to the M(II) ions than its thiazolyl analogue. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations have also been carried out in order to rationalize the influence of the nature of the thiazolyl or thienyl group on the electronic delocalization. Molecular mechanics calculations show that the free rotation of the thiazolyl in relation to the carbazole requires a greater energy income than for its thienyl analogue. Studies of the cytotoxic activity of the new compounds on colon (HCT116) and breast (MDA-MB231 and MCF7) cancer cell lines show that the thiazolyl carbazole ligand and its Pt(II) complex are the most active agents of the series and in the MCF7 line their potency is higher than that of cisplatin. In the non-tumoral human skin fibroblast BJ cell line, all the compounds were less toxic than cisplatin. Their potential ability to modify the electrophoretic mobility of pBluescript SK+ plasmid DNA and to act as inhibitors of Topoisomerases I and IIα or cathepsin B has also been investigated.