Pro-aromatic and anti-aromatic π-conjugated molecules: an irresistible wish to be diradicals

Pro-aromatic and anti-aromatic π-conjugated molecules are demonstrated to have an irresistible wish to be diradicals in the ground state.

A pi-stacking terthiophene-based quinodimethane is an n-channel conductor in a thin film transistor

A terthiophene-based quinodimethane, 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face pi-stack, with intermolecular spacings of 3.47 and 3.63 A, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO2(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm2/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm2/Vs. These results indicate that pi-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors.

Aniline degradation by Electro-Fenton and peroxi-coagulation processes using a flow reactor for wastewater treatment

The degradation of 10-30 l of a 1000 ppm aniline solution in 0.050 M Na2SO4 + H2SO4 at pH 3.0 and 40 degrees C by Electro-Fenton and peroxi-coagulation processes at constant current until 20 A has been studied using a pilot flow reactor in recirculation mode with a filter-press cell containing an anode and an oxygen diffusion cathode, both of 100 cm2 area. H2O2 is produced by the two-electron reduction of O2 at the cathode, being accumulated with a current efficiency between 60% and 80% at the first stages of electrolyses performed with a Ti/Pt anode. In the presence of 1 mM Fe2+, less H2O2 is accumulated, but it is not detected using an Fe anode. The Electro-Fenton process with 1 mM Fe2+ and a Ti/Pt or DSA anode yields an insoluble violet polymer, while the soluble total organic carbon (TOC) is gradually removed, reaching 61% degradation after 2 h at 20 A. In this treatment, pollutants are preferentially oxidized by hydroxyl radicals formed in solution from reaction of Fe2+ with H2O2. The peroxi-coagulation process with an Fe anode has higher degradation power, allowing to remove more than 95% of pollutants at 20 A, since some intermediates coagulate with the Fe(OH)3 precipitate formed. Both advanced electrochemical oxidation processes (AEOPs) show moderate energy costs, which increase with increasing electrolysis time and applied current.

Pushing extended p-quinodimethanes to the limit: stable tetracyano-oligo(N-annulated perylene)quinodimethanes with tunable ground states

p-Quinodimethane (p-QDM) is a fundamental building block for the design of π-conjugated systems with low band gap and open-shell biradical character. However, synthesis of extended p-QDMs has usually suffered from their intrinsic high reactivity and poor solubility. In this work, benzannulation together with terminal cyano-substitution was demonstrated to be an efficient approach for the synthesis of a series of soluble and stable tetracyano-oligo(N-annulated perylene)quinodimethanes nPer-CN (n = 1-6), with the longest molecule having 12 para-linked benzenoid rings! The geometry and electronic structures of these oligomers were investigated by steady-state and transient absorption spectroscopy, nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and FT Raman spectroscopy assisted by density functional theory calculations. They showed tunable ground states, varying from a closed-shell quinoidal structure for monomer, to a singlet biradical for dimer, trimer, and tetramer, and to a triplet biradical for pentamer and hexamer. Large two-photon absorption cross-section values were observed in the near-infrared range, which also exhibited a clear chain-length dependence.

Lemniscular [16]Cycloparaphenylene: A Radially Conjugated Figure-Eight Aromatic Molecule

A cycloparaphenylene-based molecular lemniscate (CPPL) was obtained in a short synthesis involving masked p-phenylene equivalents. The strained figure-eight geometry of CPPL is sustained by the incorporated 9,9'-bicarbazole subunit, which also acts as a stereogenic element. The shape of the distorted [16]cycloparaphenylene nanohoop embedded in CPPL is accurately approximated with a Booth lemniscate. The structure of CPPL, investigated using NMR and Raman spectroscopic methods, revealed strain-dependent features, consistent with the variable curvature of the ring. The electronic and optical properties of CPPL combine features more characteristic of smaller cycloparaphenylenes, such as a reduced optical bandgap and red-shifted fluorescence. CPPL was resolved into enantiomers, which are configurationally stable and provide strong chiroptical responses, including circularly polarized luminescence.

International validation of novel pyrogen tests based on human monocytoid cells

It is a requirement that parenteral medicines be tested for pyrogens (fever causing agents) using one of two animal-based tests: the rabbit pyrogen test and the bacterial endotoxin test. Understanding the human fever reaction has led to novel non-animal alternative tests based on in vitro activation of human monocytoid cells in response to pyrogens. Using 13 prototypic drugs, clean or contaminated with pyrogens, we have validated blindly six novel pyrogen tests in ten laboratories. Compared with the rabbit test, the new tests have a lower limit of detection and are more accurate as well as cost and time efficient. In contrast to the bacterial endotoxin test, all tests are able to detect Gram-positive pyrogens. The validation process showed that at least four of the tests meet quality criteria for pyrogen detection. These validated in vitro pyrogen tests overcome several shortcomings of animal-based pyrogen tests. Our data suggest that animal testing could be completely replaced by these evidence-based pyrogen tests and highlight their potential to further improve drug safety.

Quinonoid oligothiophenes as electron-donor and electron-acceptor materials. A spectroelectrochemical and theoretical study

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.

Nitro-functionalized oligothiophenes as a novel type of electroactive molecular material: spectroscopic, electrochemical, and computational study

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end alpha-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT(3)NO(2) and BrT(3)NO(2) behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO(2)T(3)NO(2) displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO(2)T(3)NO(2). Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT(3)NO(2) and BrT(3)NO(2). Dianions, not radical anions, and trianions are obtained for NO(2)T(3)NO(2). Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.

Tuning First Molecular Hyperpolarizabilities through the Use of Proaromatic Spacers

In this paper, we describe the second-order nonlinear optical properties of a series of 1,3-dithiole-based electron donor-acceptor systems incorporating proaromatic donor and spacer groups. Modification of the proaromaticity of the quinoid spacer gives rise to NLO-phores with mubeta values ranging from -2000 x 10(-)(48) esu to +3000 x 10(-)(48) esu. Quite surprisingly, compounds with a p-benzoquinoid spacer and a strong acceptor group show negative mubeta values, usually associated to zwitterionic ground states, and yet they are largely quinoid, as evidenced by crystallographic data and theoretical calculations. Progressive benzoannulation of the spacer and introduction of alkylsulfanyl substituents on the dithiole donor unit result in a shift to more positive mubeta values. DFT and ab initio calculations verify these empirical trends.

Tuning the supramolecular chirality of one- and two-dimensional aggregates with the number of stereogenic centers in the component porphyrins

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.

Microwave-assisted sidewall functionalization of single-wall carbon nanotubes by Diels-Alder cycloaddition

The first Diels-Alder cycloaddition of o-quinodimethane to SWNT has been performed under microwave irradiation.

High Yield Ultrafast Intramolecular Singlet Exciton Fission in a Quinoidal Bithiophene

We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet-triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.

Structure−Property Relationships in Push−Pull Amino/Cyanovinyl End-Capped Oligothiophenes: Quantum Chemical and Experimental Studies

A series of push-pull chromophores built around thiophene-based pi-conjugating spacers and bearing various types of amino donors and cyanovinyl acceptors have been analyzed by means of UV-vis-NIR, IR, and Raman spectroscopic measurements in the solid state as well as in solution. The intramolecular charge transfer (ICT) of these pi-conjugated systems has also been tested by analyzing the ability of the solute molecules to undergo shifts in their fluorescence emission maxima with increasing solvent polarity. These push-pull oligomers also display an attractive electrochemical behavior since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the electron-rich aminooligothienyl half-part of the molecule and leads to the formation of stable cations. On the other hand, reduction to radical anions and dianions is mainly cyanovinyl-centered but also affects the pi-conjugated electron relay. Density functional theory (DFT) calculations have been carried out to help the assignment of the most relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO-phores.