All-Conjugated, All-Crystalline Donor–Acceptor Block Copolymers P3HT‑<i>b</i>‑PNDIT2 via Direct Arylation Polycondensation

The synthesis and characterization of all-conjugated, all-crystalline donor–acceptor block copolymers (BCPs) containing poly­(3-hexyl­thiophene) (P3HT) and poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyl­dodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (PNDIT2) is presented. Direct arylation polycondensation (DAP) of dibromo­naphthalene­diimide and bithiophene is carried out in the presence of P3HT end-cappers to allow the in situ formation of BCPs P3HT-<i>b</i>-PNDIT2. As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain termini shows nonoptimal reactivity under the DAP conditions used. Therefore, H-P3HT-Br is converted into either H-P3HT-Th (thiophene) or H-P3HT-Mes (mesitylene), giving α,ω-hetero-C–H functionalized P3HT with modulated C–H reactivity. The influence of the different C–H chain termini of P3HT on the ability to act as end-capper and the resulting block structures is investigated in detail using wavelength-dependent size exclusion chromatography (SEC) and NMR spectroscopy. Different C–H reactivities of α,ω-hetero-C–H functionalized P3HT cause different contents of multiblocks, which in turn lead to varied degrees of crystallinity. These results show that careful tuning of C–H reactivity is a promising way to obtain well-defined, all-conjugated block copolymers via DAP

Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C–H Reactivity versus Solvent Quality

The solvent for direct arylation polycondensation (DAP) is of crucial importance. For conjugated polymers exhibiting reduced solubility, the choice of solvent decides on the maximum molecular weight that can be achieved, hence, good aromatic solvents are generally desirable. However, unintentional activation of C–H bonds present in aromatic solvents under DAP conditions leads to in situ solvent termination which competes with step growth. Here we evaluate relative C–H reactivity and solvent quality of seven aromatic solvents for the DAP of defect-free naphthalene diimide (NDI)-based copolymers of different solubility. C–H reactivity is strongly reduced with increasing degree of substitution for both chlorine and methyl substituents. Mesitylene is largely C–H unreactive and, thus, albeit being a moderate solvent, enables very high molecular weights at elevated temperature for NDI copolymers with limited solubility

Expanding the Scope of Electron-Deficient C–H Building Blocks: Direct Arylation of Pyromellitic Acid Diimide

Direct C–H activation of pyromellitic diimide (PMDI) is reported for the first time. The method avoids cumbersome pathways involving bromination usually required for further cross-coupling. Good to excellent yields of mono- and di-substituted PMDI derivatives can be obtained under optimized reaction conditions. The reaction scope was also explored, and the materials were characterized with respect to their thermal, optical, and electronic properties

Conformer Ring Flip Enhances Mechanochromic Performance of <i>ansa</i>-Donor–Acceptor–Donor Mechanochromic Torsional Springs

Mechanochromophores based on conformational changes of donor–acceptor–donor (DAD) springs allow sensing of forces acting on polymer chains by monotonic changes of absorbance or photoluminescence (PL) wavelength. Here, we identify a series of thiophene (D)-flanked quinoxalines (A) as molecular torsional springs for force sensing in bulk polymers at room temperature. The mode of DAD linkage to the polymer matrix and linker rigidity are key parameters that influence the efficacy of force transduction to the DAD spring and thus mechanochromic response, as probed by in situ PL spectroscopy of bulk films during stress–strain experiments. The largest shift of the PL maximum, and thus the highest sensitivity, is obtained from an ansa-DAD spring exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy and density functional theory calculations, we reveal conformer redistribution in the form of a thiophene ring flip as the major part of the overall mechanochromic response. At forces as low as 27 pN at early stages of deformation, the ring flip precedes mechanically induced planarization of the ansa-DAD spring, the latter process producing a PL shift of 21 nm nN–1. Within the stress–strain diagram, the thiophene ring flip and DAD planarization are thus two separated processes that also cause irreversible and reversible mechanochromic responses, respectively, upon sample failure. As the thiophene ring flip requires much smaller forces than planarization of the DAD spring, such micromechanical motion gives access to sensing of tiny forces and expands both sensitivity and the force range of conformational mechanochromophores

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C–H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C–H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at <i>M</i>_n ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μ_sat up to 3 cm²/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls

Direct S₀→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy

A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. (Time-resolved) electron paramagnetic resonance (EPR) is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior resolution compared to optical methods. We present here evidence for a direct S₀→T optical excitation of distinct triplet states in the repeat unit of a conjugated polymer used in organic photovoltaics. These states differ in their electronic structure from those populated via intersystem crossing from excited singlet states. This is an additional and so far unconsidered route to triplet states with potentially high impact on efficiency of organic electronic devices

Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics

Compatibilization of an immiscible binary blend comprising a conjugated electron donor and a conjugated electron acceptor polymer with suitable electronic properties upon addition of a block copolymer (BCP) composed of the same building blocks is demonstrated. Efficient compatibilization during melt-annealing is feasible when the two polymers are immiscible in the melt, i.e. above the melting point of ∼250 °C of the semicrystalline donor polymer P3HT. To generate immiscibility at these high temperatures, the acceptor polymer PCDTBT is equipped with fluorinated side chains leading to an increased Flory–Huggins interaction parameter. Compatibilization in bulk and thin films is demonstrated, showing that the photovoltaic performance of pristine microphase separated and nanostructured BCPs can also be obtained for compatibilized blend films containing low contents of 10–20 wt % BCP. Thermodynamically stable domain sizes range between several tens of microns for pure blends and ∼10 nm for pure block copolymers. In addition to controlling domain size, the amount of block copolymer added dictates the ratio of edge-on and face-on P3HT crystals, with compatibilized films showing an increasing amount of face-on P3HT crystals with increasing amount of compatibilizer. This study demonstrates the prerequisites and benefits of compatibilizing all-conjugated semicrystalline polymer blends for organic photovoltaics

Spectroscopic Signature of Two Distinct H‑Aggregate Species in Poly(3-hexylthiophene)

In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analyzed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there isdependent on temperatureenergy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e., several nanometers in diameter

TBT Entirely Dominates the Electronic Structure of the Conjugated Copolymer PCDTBT: Insights from Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Insight into the electronic structure of conjugated polymers used for organic electronics applications is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons provides access to the electronic structure with molecular resolution. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens our understanding of the structure–function relationship in organic semiconductors. Applying this approach to the copolymer poly­[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo­thiadiazole)] (PCDTBT) known for its efficiency and device stability reveals the electronic structure of the polymer as well as each of the smaller building blocks to be dominated entirely by the TBT moiety. Hence, the usual description of PCDTBT as a carbazole derivative is somewhat misleading. Furthermore, delocalization extends along the backbone, over at least two repeat units, and is consistent for singlet and triplet excitons, quite in contrast to other push–pull systems previously investigated. DFT calculations of the spin density distribution agree well with the experimental results and show the BP86 functional to be superior to B3LYP in the given context. The polymer and all its building blocks show a remarkable homogeneity that by ruling out aggregation phenomena is ascribed to a rather rigid and planar backbone geometry

In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics

A detailed investigation of in situ-synthesized all-conjugated block copolymer (BCP) compatibilized ternary blends containing poly­(3-hexylthiophene) (P3HT) and poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)­naphthalene-1,4,5,8-bis­(dibcarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (PNDIT2) as donor and acceptor polymers, respectively, is presented. Both polymers are incompatible and show strong segregation in blends, which renders compatibilization with their corresponding BCPs promising to enable nanometer-phase-separated structures suitable for excitonic devices. Here, we synthesize a ternary block copolymer/homopolymer blend system and investigate the phase behavior as a function of block copolymer molecular weight and different annealing conditions. The device performance decreases on increasing annealing temperatures. To understand this effect, morphological investigations including atomic force microscopy, high-resolution transmission electron microscopy (HR-TEM), and grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are carried out. On comparing domain sizes of pristine and compatibilized blends obtained from GISAXS, a weak compatibilization effect appears to take place for the in situ-synthesized ternary systems. The effect of thermal annealing is most prevalent for all samples, which, for the highest annealing temperature above the melting point of PNDIT2 (310 °C), ultimately leads to a change from the face-on to edge-on orientation of PNDIT2, as seen in GIWAXS. This effect dominates and decreases all photovoltaic parameters, irrespective of whether a pristine or compatibilized blend is used