Intercalation and Ordering of F6TCNNQ and F4TCNQ Dopants in Regioregular Poly(3-hexylthiophene) Crystals: Impact on Anisotropic Thermoelectric Properties of Oriented Thin Films

Spontaneous Modulation Doping in Semi-Crystalline Conjugated Polymers Leads to High Conductivity at Low Doping Concentration

The possibility to control the charge carrier density through doping is one of the defining properties of semiconductors. For organic semiconductors, the doping process is known to come with several problems associated with the dopant compromising the charge carrier mobility by deteriorating the host morphology and/or introducing Coulomb traps. While for inorganic semiconductors these factors can be mitigated through (top-down) modulation doping, this concept has not been employed in organics. Here, this work shows that properly chosen host/dopant combinations can give rise to spontaneous, bottom-up modulation doping, in which the dopants preferentially sit in an amorphous phase, while the actual charge transport occurs predominantly in a crystalline phase with an unaltered microstructure, spatially separating dopants and mobile charges. Combining experiments and numerical simulations, this work shows that this leads to exceptionally high conductivities at relatively low dopant concentrations.

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications.

Reversible Multielectron Release from Redox-Active Three-Dimensional Molecular Barrels with Ruthenium-Alkenyl Moieties

Three-dimensional molecular barrels Ru₆-4 and Ru₆-5 were synthesized in high yields from dinuclear ruthenium-vinyl clamps and tritopic triphenylamine-derived carboxylate linkers and characterized by multinuclear NMR spectroscopy including ¹H-¹H COSY and ¹H DOSY measurements, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography. The metal frameworks of the cages adopt the shape of twisted trigonal prisms, and they crystallize as racemic mixtures of interdigitating Δ- and Λ-enantiomers with a tight columnar packing in Ru₆-4. Electrochemical studies and redox titrations revealed that the cages are able to release up to 11 electrons on the voltammetric timescale and that their cage structures persist up to the hexacation level. IR and UV-vis-near-infrared spectroelectrochemical studies confirm substituent-dependent intramolecular electronic communication within the π-conjugated 1,3-divinylphenylene backbone in the tricationic states, where all three divinylphenylene-bridged diruthenium clamps are present in mixed-valent radical cation states. The formation of 1:3 charge-transfer salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as the electron acceptor is also demonstrated.

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4 TCNQ Charge-Transfer Salts

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(Pi Pr3 )2 (L)} entities (L=Cl- in complexes Ru2 -3 and Ru2 -7; L=acetylacetonate (acac- ) in complexes Ru2 -4 and Ru2 -8) and with π-conjugated 2,7-divinylphenanthrenediyl (Ru2 -3, Ru2 -4) or 5,8-divinylquinoxalinediyl (Ru2 -7, Ru2 -8) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene (Ru2 -7, Ru2 -8) or a 6,7-benzoquinoxaline (Ru2 -3, Ru2 -4) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru2 -7, Ru2 -8 as intrinsically delocalized mixed-valent species, and those of complexes Ru2 -3 and Ru2 -4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac- congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F4 TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F4 TCNQ.- radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.