Carbon-Bridged Oligo(phenylenevinylene)s: Stable π‑Systems with High Responsiveness to Doping and Excitation

The high responsiveness of π-conjugated materials to external stimuli, such as electrons and photons, accounts for both their utility in optoelectronic applications and their chemical instability. Extensive studies on heteroatom-stabilized π-conjugated systems notwithstanding, it is still difficult to combine high performance and stability. We report here that carbon-bridged oligo­(<i>p</i>-phenylenevinylene)­s (<b>COPV-</b><i><b>n</b></i>) are not only more responsive to doping and photoexcitation but also more stable than the conventional <i>p</i>-phenylenevinylenes and poly­(3-hexylthiophene), surviving photolysis very well in air, suggesting that they could serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward the doped or photoexcited state and the flat, rigid, and hindered structure endows <b>COPV</b>s with stimuli-responsiveness and stability without recourse to heteroatoms. For example, <b>COPV-6</b> can be doped with an extremely small reorganization energy and form a bipolaron delocalized over the entire π-conjugated system. Applications to bulk and molecular optoelectronic devices are foreseen

Vibrational Circular Dichroism Shows Reversible Helical Handedness Switching in Peptidomimetic l‑Valine Fibrils

We elucidate the supramolecular organization in the form of microsize fibrils of gels formed by a l-Valine peptidomimetic compound. Analysis was based on circular dichroism spectroscopies, vibrational (VCD) and electronic (CD), supported by microscopy (atomic force and scanning electron). We show how the VCD spectra give account of the micrometric structure of the fibrils formed by the helicoidal arrangement of simpler proto-fibrils, which are organized in a lower hierarchical level. This ability is used to monitorize a fully reversible change in the handedness of the helix by modulating different external stimuli as pH or ionic strength, thus providing the first observation by VCD of such a phenomenon in a short peptide

Carbon-Bridged Phenylene-Vinylenes: On the Common Diradicaloid Origin of Their Photonic and Chemical Properties

In this paper, experimental and computational investigations are combined to analyze the impact of the diradical character in the photonic and chemical properties of two tetracyano quinodimethane derivatives of phenylene-vinylenes. The photonic properties are evaluated with high-level quantum-chemical calculations and are rationalized in the context of the four-level model typical of diradical molecules, including one triplet and three singlet states, whereas the diradical extent in the ground electronic state is shown to be the driving force for the dimerization reactions. To this end DFT, CASSCF, and CASPT2//CASSCF calculations are carried out and complemented with experimental UV–vis–NIR absorption and emission measurements, as a function of the temperature along with resonance Raman spectroscopy investigations. The study of diradical molecules with enhanced chemical stability in the framework of organic electronics is an important field of research in organic π-conjugated molecules with direct applications in organic electronics

Delocalization-to-Localization Charge Transition in Diferrocenyl-Oligothienylene-Vinylene Molecular Wires as a Function of the Size by Raman Spectroscopy

In going from short to large size thienylene-vinylene diferrocenyl cations, the transition from a charge delocalized to a localized state is addressed by resonance Raman spectroscopy and supported by theoretical model chemistry. The shorter members, dimer and tetramer, display conjugated structures near the cyanine limit of bond length equalization as a result of the strong interferrocene charge resonance, producing a full charge <i>delocalized</i> mixed valence system. In the longest octamer, charge resonance vanishes and the cation is <i>localized</i> at the bridge center (the mixed valence property disappears). The hexamer is at the <i>delocalized</i>-to-<i>localized</i> turning point. Solvent and variable-temperature Raman measurements highlight this borderline property. A detailed structure–property correlation of bond length alternation data and Raman frequencies is proposed to account for the whole set of spectroscopic properties, with emphasis on the changes observed with the size of the molecular wire

Planarization, Fusion, and Strain of Carbon-Bridged Phenylenevinylene Oligomers Enhance π‑Electron and Charge Conjugation: A Dissectional Vibrational Raman Study

We have used Raman spectroscopy to study the molecular and electronic structures of the radical cations and dications of carbon-bridged oligo­(<i>para</i>-phenylenevinylene)­s (<b>COPV</b><i><b>n</b></i>, <i>n</i> = 1–6) possessing consecutive fused pentagons and hexagons, up to 19, along with COPV derivatives having electron-donating and -withdrawing groups. This study was made possible by the outstanding stability of the charged states of COPVs. We could untangle the effects of π-conjugation in the planar structure on the Raman frequency by distinguishing it from other structural effects, such as strain in the vinylene groups shared by the two pentagons. The analyses showed that the radical cations have benzo-quinoidal structures confined in the center of the molecule, as well as benzo-aromatic rings at the terminal sites. In contrast, dications of <b>COPV</b><i><b>n</b></i> longer than <i>n</i> = 3 exhibit a biradicaloid character because of the recovery of aromaticity in the central rings and quinoidal rings at the terminal positions. These biradicaloids favor a singlet nature in their ground electronic states because of the double spin polarization. The introduction of electron-donating and -withdrawing groups on the termini of a COPV core affords, upon oxidation or reduction, a fully delocalized class III mixed valence system because of the high degree of conjugation of the COPV platform, which favors extensive charge delocalization

Mode Robustness in Raman Optical Activity

By reformulating Raman and ROA invariants we provide ground for the definition of robust modes in ROA spectroscopy. Introduction of two parameters defining robustness helps characterization and assignment of ROA bands. Application and use of robustness parameters to [<i>n</i>]­helicenes and oxirane/thiirane derivatives are presented

Phenyl- and Thienyl-Ended Symmetric Azomethines and Azines as Model Compounds for n‑Channel Organic Field-Effect Transistors: An Electrochemical and Computational Study

The formation energy and stability of radical anions in a series of 12 phenyl- and 2-thienyl-ended linear, symmetric azomethines and azines were investigated by cyclic voltammetry. Replacing 1,4-phenylene with 2,5-thienylene cores and substitution with cyano or methyl moieties have allowed the lowering of lowest unoccupied molecular orbital energy levels even by 1 eV. Methyl capping stabilizes electron carriers (radical anions) toward dimerization, and the mechanism of such radical anion dimerization has been clarified by cyclic voltammetric kinetic analysis. The results have been compared with optical parameters and supported by density functional theory calculations

Novel Thiophene–Phenylene–Thiophene Fused Bislactam-Based Donor–Acceptor Type Conjugate Polymers: Synthesis by Direct Arylation and Properties

Three new donor–acceptor copolymers based on thiophene–phenylene–thiophene fused bislactam and various donors (3,4-dodecylthiophene, 4,4′-didodecyl-2,2′-bithiophene, and ethylenedioxythiophene) were synthesized, characterized, and used in field-effect transistors. Polycondensation was performed using nonactivated thiophene derivatives by employing palladium-catalyzed direct arylation under phosphine-free conditions. This method is superior to traditional cross-coupling polymerization because it requires fewer synthetic operations and does not employ toxic organometallic intermediates. Regioselective polymers can also be generated by using β-substituted thiophene derivatives. The studied polymers were tested in a bottom gate top contact thin film transistor (OTFT) architecture. The best electronic performance was shown by polymer <b>P3</b>, with enhanced π-conjugation due to the appearance of intramolecular attractive interactions

Linear and Nonlinear Optical Properties of Ramified Hexaazatriphenylenes: Charge Transfer Contributions to the Octupolar Response

Four new HAT derivatives with octupolar symmetry have been synthesized. Their linear and nonlinear optical, photophysical, and electrochemical properties have been rationalized in terms of donor–acceptor interactions. We get further insights in the relationship between experimental first hyperpolarizability and two-photon absorption cross section: the δ/β ratio is connected with the electron donor–acceptor strength between the HAT core and the peripheral groups but also with the ethylene/acetylene bridges. This structure–property relationship can be used to design octupoles with more efficient nonlinear responses. A HAT derivative based on the fusion of three HAT cores and without any defined donor–acceptor pattern is studied in comparison with the charge transfer HAT versions. A complete study of the electronic structure of the five molecules in the context of the origin of the relevant optical properties is carried out combining several spectroscopic techniques and quantum chemistry

D−π–A Compounds with Tunable Intramolecular Charge Transfer Achieved by Incorporation of Butenolide Nitriles as Acceptor Moieties

Chromophores where a polyenic spacer separates a 4<i>H</i>-pyranylidene or benzothiazolylidene donor and three different butenolide nitriles have been synthesized and characterized. The role of 2­(5<i>H</i>)-furanones as acceptor units on the polarization and the second-order nonlinear (NLO) properties has been studied. Thus, their incorporation gives rise to moderately polarized structures with NLO responses that compare favorably to those of related compounds featuring more efficient electron-withdrawing moieties. Derivatives of the proaromatic butenolide <b>PhFu</b> show the best nonlinearities. Benzothiazolylidene-containing chromophores present less alternated structures than their pyranylidene analogues, and, unlike most merocyanines, the degree of charge transfer does not decrease on lengthening the π-bridge