Dolores Velasco

Recent advances towards azobenzene-based light-driven real-time information-transmitting materials

Summary Photochromic switches that are able to transmit information in a quick fashion have attracted a growing interest within materials science during the last few decades. Although very fast photochromic switching materials working within hundreds of nanoseconds based on other chromophores, such as spiropyranes, have been successfully achieved, reaching such fast relaxation times for azobenzene-based photochromic molecular switches is still a challenge. This review focuses on the most recent achievements on azobenzene-based light-driven real-time information-transmitting systems. Besides, the main relationships between the structural features of the azo-chromophore and the thermal cis-to-trans isomerisation, the kinetics and mechanism are also discussed as a key point for reaching azoderivatives endowed with fast thermal back-isomerisation kinetics.

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.

Influence of the photo-active azo cross-linker spacer on the opto-mechanics of polysiloxane elastomer actuators

Nematic liquid single crystal elastomers (LSCEs) that contain azobenzenes as cross-linkers have a clear application in the field of light-controlled actuators. They are able to modify their macroscopic dimensions upon irradiation with UV light and furthermore relax back thermally or by irradiation with visible light. Many efforts have been put forward to improve the photo-mechanical effect. However, there are no previous studies about the role of the length of the photo-active cross-linker spacer on the opto-mechanics of light-controlled actuators. In this way, we have prepared several photo-active side-chain nematic polysiloxane LSCEs that contain 4,4′-dialkoxyazobenzenes as cross-linkers, which bear spacers of different lengths. The opto-mechanical behaviour of the different LSCEs prepared has been analyzed. The use of photo-active cross-linkers with long spacers yields actuators with higher mechanical responses under irradiation with UV light. Neither the irradiation nor the thermal relaxation time of the actuator depends on the spacer length.

Azophenol-based liquid-crystalline elastomers for light-driven actuators.

Para-substituted azophenols exhibit a fast thermal cis-to-trans isomerization rate in ethanol, which can be transferred to the solid state by obtaining liquid-crystalline elastomeric systems. The absence of protic solvent is compensated by the establishment of hydrogen bonding between azophenol monomers that are close to each other. Opto-mechanical experiments reveal that azophenol-containing liquid single-crystal elastomers are valuable materials for light-controlled actuators exhibiting relaxation times of 1 s at room temperature.

A short route to multiply substituted fluorenones

Abstract We report a two-step synthesis of polysubstituted fluorenones by condensation of malononitrile with aromatic aldehydes and methylketones or with β-aryl-α,β-unsaturated carbonyl compounds, aldehydes or ketones. The yields of the fluorenone systems depend on the nature of the substitution on the aromatic rings.

Increasing the Isomerisation Kinetics of Azo Dyes by Chemical Bonding to Liquid‐Crystalline Polymers

It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.

Taking advantage of the radical character of tris(2,4,6-trichlorophenyl)methyl to synthesize new paramagnetic glassy molecular materials.

This paper describes the synthesis of the novel bis[4-(N-carbazolyl)-2,6-dichlorophenyl](2,4,6-trichlorophenyl)methyl radical (2*) and tris[4-(N-carbazolyl)-2,6-dichlorophenyl]methyl radical (3*). A Friedel-Crafts reaction on [4-(N-carbazolyl)-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl]methyl radical (1*), 2*, and 3* leads to the introduction of acyl chains in the 3- and 6-positions of the carbazolyl moiety without impairment of the radical character of the molecule to give radicals 5*, 6*, and 7*. All of these novel radical adducts are thermally stable, 5* and 6* being amorphous solids by differential scanning calorimetry. Electron paramagnetic resonance spectra of them show a multiplet at low temperature due to the electron-coupling with six aromatic hydrogens. They show electrochemical amphotericity being reduced and oxidized to their corresponding stable anionic and cationic species, respectively. These radical adducts have luminescent properties covering the red spectral band of the emission with high intensities.

All-Organic Discotic Radical with a Spin-Carrying Rigid-Core Showing Intracolumnar Interactions and Multifunctional Properties**

The ability of liquid crystals (LCs) to rearrange and thus modify their physical properties as a response to an external stimulus, such as light incidence, temperature changes, or the application of an electrical or magnetic field, is a useful feature for technological applications. Discotics are especially interesting from the point of view of the development of semiconductor devices because of their ability to form onedimensional superstructures that allow free electric charges to migrate. This feature has been recognized as an attractive property for the construction of light-emitting diodes, photovoltaic cells, and field-effect transistors. In particular, carbazole derivatives play an important role as organic materials for electronic applications. However, not many carbazole-containing discotic LCs have been reported to date. 3, 4] The development of spin systems with LC properties is particularly interesting in the field of magnetism because of the possibility of altering the magnetic properties through the phase transition, or the modulation of the magnetic response by external fields. Some calamitic all-organic radical LCs containing 2,2,6,6-tetramethylpiperidine-N-oxide and 2,2,5,5tetramethyl-1-pyrrolidine-N-oxide as spin sources, 21,22] which can be ordered into a variety of LC nematic or layered phases, can be found in the literature. However, there is only one example of a disk-like all-organic radical that has the spin-supporting unit introduced into the disk-like triphenylene core as a lateral chain-bonded moiety. With this in mind, we have developed a new discotic allorganic LC with a radical rigid core based on the tris[4-(Ncarbazolyl)-2,6-dichlorophenyl]methyl aromatic system. Free radicals of the tris(2,4,6-trichlorophenyl)methyl (TTM) series are very persistent species both in the solid state and in solution owing to the presence of the very bulky polychlorophenyl substituents around the trivalent carbon atom. In particular, radical adducts based on carbazolyl TTM radical exhibit electrochemical amphotericity and remarkable luminescent properties into the visible range of the electronic spectrum. 26, 27] To the best of our knowledge, the radical adduct presented herein is the first disk-like LC with the magnetic spin source placed in the center of the rigid discotic core.

Enantiomeric resolution and determination of the absolute configuration of dibenzophosphole 5-oxides

Abstract The resolution of asymmetrically substituted dibenzophospholes via chromatographic separation of their cyclopalladium derivatives is described for the first time. The absolute configuration of the phosphorus atom was determined by X-ray crystallographic analysis in one case. A general NMR criterion for the complete determination of the configuration of this group of products is proposed. The enantiomeric excess of the resolved compounds was determined by NMR Eu(III) chemical displacement experiments and by HPLC using a chiral stationary phase.

Photo-driven optical oscillators in the kHz range based on push–pull hydroxyazopyridines

Push-pull azophenols are valuable target molecules for stable photo-driven optical oscillators. Hydroxyazopyridinium methyl iodide salts show oscillation frequencies up to 10 kHz with no signs of fatigue upon continuous work.