Tuning the quinoid versus biradicaloid character of thiophene-based heteroquaterphenoquinones by means of functional groups.

A series of quinoidal bithiophenes (QBTs) with controlled variations in steric hindrance and electron activity of the substituents has been synthesized. Evidence of their quinoidal versus biradicaloid ground-state electronic character has been experimentally detected and coherently identified as fingerprints by spectroscopic methods such as NMR, UV-vis, multiwavelength Raman. From this analysis, alkoxy groups have been shown to strongly affect the electronic structure and the ground-state energy and stability of QBTs. Quantum-chemical calculations correctly predict the experimental spectroscopic response, even while changing the alkyl on phenone from a tertiary carbon atom to secondary to primary toward an unsubstituted phenone, further confirming the validity of the approach proposed. A control of the electronic structure accompanied by negligible variations of the optical gap of the molecules has thus been demonstrated, extending the potential use of quinoidal species in fields ranging from photon harvesting to magnetic applications.

Biradicaloid character of thiophene-based heterophenoquinones: the role of electron-phonon coupling.

The quinoidal versus biradicaloid character of the ground state of a series of thiophene-based heterophenoquinones is investigated with quantum-chemical calculations. The role of the ground-state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the R coordinate) maximizing the electron-phonon coupling, and connecting the quinoid and the aromatic biradicaloid resonance structures. The combined experimental and computational investigation supports the biradicaloid nature of the longer oligomers. The modulation of Raman intensities and frequency dispersion, experimentally observed by increasing the length of the chromophore, is shown to be reproduced well by model calculations on a single chromophore as a function of geometry displacements along the R-mode. These results underline the role of electron-phonon coupling in governing the structure-property relationship of highly conjugated organic compounds, underscoring the similarity of thiophene heterophenoquinone systems with other, more classical, oligophenylene and oligothiophene derivatives.

Electronic transport regimes through an alkoxythiolated diphenyl-2,2'-bithiophene-based molecular junction diodes: critical assessment of the thermal dependence.

The detailed understanding of electronic transport through a single molecule or an ensemble of self-assembled molecules embedded between two metallic leads is still a matter of controversy. Multiple factors influence the charge transport in the molecular junction, with particular attention to be given to the band states of the electrodes, molecular orbital energies, bias potential and importantly molecule-electrode electronic coupling. Moreover it is not trivial to disentangle molecular contributions from other possible conduction pathways directly coupling the opposite electrodes. We here investigate the electronic transport properties of an ensemble molecular junction embedding an alkylthiol derivative of a diphenol substituted bithiophene (DPBT) by means of current vs. voltage and temperature dependent measurements. We explored different junction configurations using: micropores (Au//DPBT//Au and Au//DPBT-polymer conductor//Au) and conductive-atomic force microscopy (c-AFM). In all cases, we found a transition voltage V(T) of ∼0.35 V. The consistent presence of a similar V(T) in all the tested configurations is a strong, but not conclusive, indication of a molecular signature in the charge transport, which we assessed and confirmed by temperature dependent measurements. We found a transition from an incoherent resonant tunneling at low biases and close to room temperature, where transport is thermally activated with an activation energy of ∼85 meV, to a coherent tunneling at voltages higher than V(T). Unlike many other molecular junctions reported in the literature, resonant conditions commonly attributed to a hopping transport regime can be found already at room temperature and very low biases for a molecule only ∼1.5 nm long. This paper is the first report to clearly show temperature activated transport through a short and not fully conjugated molecule. Moreover, we could clearly identify a regime at low temperatures and low bias where the transport mechanism is controlled by the thermal conductivity of the metal electrodes rather than the molecule.

Photoactive molecular junctions based on self-assembled monolayers of indoline dyes.

We demonstrate the feasibility of a photodetector based on an ensemble molecular junction, where a self-assembled monolayer of an organic donor-acceptor dye is directly sandwiched between two electrodes. In such a device, upon photoexcitation and generation of a charge-transfer state on the molecule, charges are dissociated and directly collected at the electrodes without the need of transport through a bulk phase, as in usual photodetectors. We show that the device can work in photovoltaic regime and the spectral response can be tuned by varying the light absorbing dye. Therefore, the electro-optical properties of the downscaled device can be unambiguously related to the physical-chemical properties of the molecules, a commonly difficult point to demonstrate in a molecular junction device, because of the uncertainties of the interplay between molecules and electrodes. The proposed device, which relies on a simple self-assembly process, has a strong potentiality for fast responding, downscaled detectors, ultimately limited by charge dissociation dynamics, and can be considered also as a useful tool to investigate fundamental electro-optical processes in molecular monolayers.

Structure-Photoluminescence Correlation for Two Crystalline Polymorphs of a Thiophene-Phenylene Co-Oligomer with Bulky Terminal Substituents.

Two crystal polymorphs of a thiophene-phenylene hexamer with bulky terminal substituents are characterized by different molecular conformations and parallel versus herringbone packing. Irrespective of their similar emissive spectra and common H-aggregate features, evidenced by crystal structure analysis and confirmed by solid-phase and excited-state first-principles calculations, their luminescence is relatively high and, for one form, nearly double than that for the other. Interaromatic packing energy contributions are established by quantum chemical calculations and can be compared quantitatively as the same species in different crystal environments is examined. The different luminescence efficiency of the two phases highlights the crucial role of the interaromatic packing for the luminescence properties of polyaromatic oligomers.

n-Type Semiconducting Polymer Fibers.

Defect-free bicomponent fibers of poly{[N,N'-bis(2-octyl-dodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}/poly(ethyleneoxide) P(NDI2OD-T2)/PEO are fabricated by means of electrospinning and rinsed with a selective solvent to afford pure P(NDI2OD-T2) while maintaining a fibrous morphology. The elongation strength applied on the spun jet by the high electrical field induces a preferential orientation of polymer chains. An electron mobility analogous to the best obtained with a thin film-based device is achieved in single fiber transistors, and the results are unaffected by the dielectric surface treatment.