Tuning Packing and Solubility of Donor (D)–Acceptor (A) Polymers by <i>cis</i>–<i>trans</i> Isomerization within Alkenyl Side Chains

The impact of alkenyl substituents on the behavior of cyclopentadithiophene–benzothiadiazole (CDT–BTZ) donor (D)–acceptor (A) polymers in organic field-effect transistors (OFETs) and on the supramolecular organization was investigated. Linear <i>cis</i>- and <i>trans-</i>alkenes were attached to the donor unit of CDT–BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between <i>cis-</i> and <i>trans-</i>alkenes. While <i>trans-</i>alkenes exhibit zigzag structures that are beneficial for close packing, <i>cis</i>-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with <i>trans-</i>alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the <i>cis</i>-substituents toward their <i>trans-</i>conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies

Structural, Thermo-Optical, and Photophysical Properties of Highly Oriented Thin Films of Quino­xalino­phenanthro­phenazine Derivative

The structural, thermo-optical, and anisotropic photophysical properties of highly oriented thin solid films of 2,11-bis­(1,1-di­methyl­ethyl)-6,7,15,16-tetrakis­(do­decyloxy)­quinoxalino­[2′,3′:9,10]­phenanthro­[4,5-<i>abc</i>]­phenazine (TQPP-OC₁₂) prepared by a zone-casting method are discussed. The films were obtained on glass substrate by solution processing. The in-plane alignment of TQPP-OC₁₂ molecules in the formed layers was studied by X-ray diffraction methods, optical polarized microscopy combined with thermo-optical analysis, atomic force microscopy (AFM), and UV–vis absorption and fluorescence spectroscopy both with polarized light methods. The high molecular order of zone-casted TQPP-OC₁₂ was studied in conjunction with the observed abundance of phase transitions as a function of thermal conditions to assess the material’s suitability for optoelectronic device applications. The molecular disk planes in the as-cast samples are parallel to the casting direction and orient almost perpendicularly (∼96°) to the substrate. Continuous films with a thickness of 200–300 nm are formed from lamellas arranged parallel to the surface. All observed phase transitions have crystal–crystal character; however, molecular primary arrangement remains basically identical for all processing conditions. Sample annealing destroys the multiple polymorphs observed in the as-cast sample and leads to an increase of molecular ordering

Free-Standing Monolayer Two-Dimensional Supramolecular Organic Framework with Good Internal Order

Utilizing dynamic self-assembly and self-sorting to obtain large-area, molecularly precise monolayered structures represents a promising approach toward two-dimensional supramolecular organic frameworks (2D SOF) or 2D supramolecular polymers. So far, related approaches suffer from small domain sizes, fragility and weak long-range internal order. Here we report on the self-assembly of a host–guest enhanced donor–acceptor interaction, consisting of a tris­(methoxynaphthyl)-substituted truxene spacer, and a naphthalene diimide substituted with <i>N</i>-methyl viologenyl moieties as donor and acceptor monomers, respectively, in combination with cucurbit[8]­uril as host monomer toward monolayers of an unprecedented 2D SOF. Featuring orthogonal solubility, the participating molecules self-assemble at a liquid–liquid interface, yielding exceptionally large-area, insoluble films, which were analyzed by transmission electron microscopy, atomic force microscopy and optical microscopy to be monolayers with a thickness of 1.8 nm, homogeneously covering areas up to 0.25 cm², and featuring the ability to be free-standing over holes of 10 μm². Characterization with ultraviolet–visible absorption spectroscopy, solid-state nuclear magnetic resonance spectroscopy, infrared spectroscopy, and grazing incidence wide-angle X-ray scattering allowed for confirmation of a successful complexation of all three monomers toward an internal long-range order and gave indications to an expected hexagonal superstructure. Our results extend the existing variety of two-dimensional soft nanomaterials by a versatile supramolecular approach, whereas the possibility of varying the functional monomers is supposed to open adaptability to different applications like membranes, sensors, molecular sieves, and optoelectronics

Ambipolar Charge Transport in Isoindigo-Based Donor–Acceptor Polymers

A series of donor–acceptor isoindigo (iI)-based copolymers synthesized with increasing numbers of thiophene rings in the repeat unit (from zero to three thiophene rings, including silole and germole-bridged fused bithiophene units) is applied toward solution-processed OFET devices. Differential pulse voltammetry on thin films of the polymers recorded LUMO energy levels confined within a 0.1 eV range around 3.9 eV, while their bandgaps are estimated at 1.5 to 1.7 eV. The interchain π-stacking distance of each sample was evaluated from the 2D-WAXS diffraction patterns of annealed extruded filaments and the GIWAXS patterns of thin films, and were found to be all in the same range, between 3.65 and 3.75 Å for the thin films. Both p-type and n-type charge transport in thin film bottom gate, bottom contact transistor devices were recorded. In particular, the copolymer P­(T-iI) containing one thiophene ring afforded well-balanced ambipolar p-type and n-type mobilities of 0.04 cm²/(V s) and 0.1 cm²/(V s), respectively. Under our processing conditions, the charge transport properties evolved from exclusively n-type to solely p-type as the number of thiophene rings within the repeat unit is increased to three rings in the case of P­(T3-iI). This was observed despite all polymers displaying similar LUMO energy levels, interchain π-stacking distances, and microscopic thin film morphology (all face-on arrangement on the dielectric surface). This prompted a molecular-scale morphological analysis of P­(T-iI) and P­(T3-iI) in particular, using solid-state NMR spectroscopy in order to further investigate the stark difference in n-type mobilities between these two polymers. Using the complete assignment of solution 2D-NMR spectra of a thiophene-iI-thiophene model compound as guideline, the analysis of proton–carbon correlations in the solid-state 2D ¹³C­{¹H} FSLG-HETCOR NMR spectra of P­(T-iI) and P­(T3-iI) revealed differences in the molecular environment surrounding each iI unit. The latter suggests a stronger correlation of neighboring iI units in P­(T-iI), whereas a stronger intermixing of iI and thiophenes prevails in P­(T3-iI). We conclude that, in this study, the choice of the donor unit length within the primary structure of the D–A polymer can be responsible for hindering its n-type character

Structure–Property Relationships Directing Transport and Charge Separation in Isoindigo Polymers

Since being introduced to the open literature in 2010, the isoindigo heterocycle has been extensively studied as a novel electron-deficient building block for organic electronic materials in conjugated polymers, discrete length oligomers, and molecular systems, particularly targeting high charge mobility values and ambipolar transport in organic field effect transistors, along with high power conversion efficiencies in organic photovoltaic devices. This article introduces results obtained on copolymers of isoindigo with thiophene and alkylated terthiophenes to highlight fundamental characteristics in isoindigo-based polymers and the resulting organic field-effect transistors and photovoltaic devices. By comparing and contrasting the optoelectronic properties, thin film morphology, organic field-effect transistor (OFET) mobilities, and organic photovoltaic (OPV) performance to previously reported polymers, structure–processing–property relationships were uncovered. In particular, isoindigo-containing polymers with more rigid backbones and higher coherence lengths in thin films lead to increased charge mobility in OFET devices. In OPV devices, efficiencies over 6% can be obtained by balancing high ionization potentials typically dictating the open-circuit voltage and the charge transfer energy, and blend morphology impacting short-circuit currents. Furthermore, the impact of polymer structure on solubility and on phase separation in blends with PC₇₁M is discussed, with isoindigo-based polymers exhibiting lower solubility possibly leading to more fiber-like morphologies stemming either from polymer dissolution in the casting solvent or from polymer self-assembly during film formation. This fiber-like polymer morphology remains unaffected by the presence of processing additives, such as 1,8-diiodooctane. These structure–property relationships developed for isoindigo-based polymers can also be discussed in the broader context of diketo­pyrrolopyrrole (DPP) and thieno­isoindigo (TiI) as electron-deficient moieties that can also be doubly substituted on their amide functionality