The isomerisation behaviour of spiropyrans (SP) depends considerably on their substituents. This work investigates the synthesis, isomerisation behaviour and polymer chemistry of alkylated spiropyrans. While several Kumada- and Suzuki-based protocols failed to alkylate dibromo-spiropyran, classical 9-BBN chemistry is shown to proceed with high yield suitable also for polycondensation. Alkyl substituents strongly influence the acid-induced SP-merocyanine (MC)-protonated MC (MCH+) equilibrium and render the MCH+ form most stable. In agreement with measured isomerisation rates, density functional theory calculations corroborate the enhanced stabilisation of ethyl-substituted MCH+ as compared to phenyl-substituted MCH+. Aliphatic main chain spiropyran copolymers are synthesised from in situ generated bis-9-BBN monomers and dibromo-spiropyrans. The empirical optimisation of stoichiometry yields a molecular weight Mw of ∼15 kg mol−1 after purification. This approach to alkylated spiropyrans opens new opportunities to fine-tune the isomerisation behaviour of small molecules and polymeric SP derivatives for potential use as sensors and smart materials.

One property central to π-conjugated polymers is the ability of polymer backbones to interact intermolecularly through the direct contact of π-orbitals. Poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) is a remarkable exception as the bulky side chains prohibit main chain π–π interactions. However, due to side-chain crystallization of interdigitated n-octyl side chains, PDOPT is semicrystalline, and thus any deviation from a perfect regioregularity can severely affect crystallization. Here, we synthesize PDOPT via direct arylation polycondensation (DAP) for the first time and analyze the effect of various reaction conditions on molecular weight, end groups, defect structures, crystallinity, and morphology. Despite extensive optimization of PDOPT synthesis via DAP including bulk polymerization, MW is limited to Mn,SEC ∼ 10 kg/mol as a result of dehalogenation of chain ends. Extensive NMR spectroscopy is carried out to analyze defect structures present in PDOPT made by DAP and also made by Kumada catalyst transfer polycondensation (KCTP) for comparison. Because of the complex 1H NMR spectra arising from the additional phenyl rings, defect structures are identified using well-defined oligomers and 13C NMR spectroscopy. For the highest MW DAP samples, we find evidence for internal tail-to-tail defects (TT), while PDOPT made by KCTP appears to carry a TT defect at the chain end. The internal TT defect lowers the thermal transitions and enthalpies and leads to smaller and less defined spherulites in isothermally crystallized thin films. These results suggest that internal TT defects, which do not severely affect structure formation in the well-studied poly(3-hexylthiophene) analog, are much more important to be controlled and eliminated in the case of PDOPT, in which ordering occurs exclusively by side-chain crystallization.

Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) is a copolymer composed of alternating thiophene–benzothiadiazole–thiophene (TBT) and carbazole (Cbz) repeat units widely used for stable organic photovoltaics. However, the solubility of PCDTBT is limited, which decreases polymer yield and makes synthesis and purification tedious. Here, we introduce a strategy to increase both solubility and luminescence by the statistical incorporation of additional hexyl side chains at the TBT unit (hex-TBT). An increasing amount of hex-TBT as comonomer from 0 to 100% enhances solubility, leads to backbone torsion, and causes a blue-shift in the absorption and emission spectra. While photovoltaic performance of both PCDTBT:P3HT blends and PCDTBT:PC71BM blends decreases with increasing content of hex-TBT due to weaker and blue-shifted absorption, the luminescence properties can be systematically improved. Both photo- and electroluminescence (PL and EL) quantum efficiencies increase with increasing hex-TBT content. We further demonstrate solution-processed red polymer light-emitting diodes based on fully hexylated PCDTBT showing an EL quantum efficiency enhancement of up to 7 times and 2 orders of magnitude enhancement of brightness compared to standard PCDTBT. Fully hexylated PCDTBT shows a peak external quantum efficiency of 1.1% and a peak brightness of 2500 cd/m2.

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.Keywords: compatibilization; donor−acceptor conjugated block copolymers; organic photovoltaics; P3HT; PCDTBT; phase separation; semifluorinated alkyl side chains; ternary conjugated polymer blends

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 Mn ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μsat 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.

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.

Suzuki–Miyaura polycondensation (SPC) is widely used to prepare a variety of copolymers for a broad range of applications. Although SPC protocols are often used in many instances, the limits of this method and issues of molecular weight reproducibility are not often looked at in detail. By using a spiropyran-based (SP) mechanochromic copolymer, we present an optimized protocol for the microwave-assisted synthesis of a mechanochromic, alternating copolymer P(SP-alt-C10) via SPC that allows the reproduction of molecular weight distributions. Several parameters such as microwave power, temperature, stoichiometry, and ligand are screened, leading to molecular weights up to Mw ∼ 174 kg mol−1. The process of optimization is guided by NMR end group analysis which shows that dehalogenation, oxidative deborylation and SP cleavage are the limiting factors that impede further increase of molar mass, while other classical side reactions such as protiodeborylation are not observed. Embossing films of P(SP-alt-C10) yields the colored merocyanine (MC) copolymer P(MC-alt-C10) that undergoes a thermally facilitated back reaction to P(SP-alt-C10). DFT suggests that the barrier of the SP → MC transition has two contributions, with the first one being related to the color change and the second one to internal bond reorganizations. The barrier height is 1.5 eV, which suggests that the ease of the thermally facilitated back reaction is either due to residual energy stored in the deformed polymer matrix, or arises from an MC isomer that is not in the thermodynamically most stable state.

Pd-catalyzed direct arylation (DA) reaction conditions have been established for unsubstituted furan (Fu) and thiophene (Th) with three popular acceptor building blocks to be used in materials for organic electronics, namely 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2), N,N′-dialkylated 2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) (NDIBr2), and 1,4-dibromotetrafluorobenzene (F4Br2). Reactions with BTBr2, F4Br2, and NDIBr2 require different solvents to obtain high yields. The use of dimethylacetamide (DMAc) is essential for the successful coupling of BTBr2 and F4Br2, but detrimental for NDIBr2, as the electron-deficient NDI core is prone to nucleophilic core substitution in DMAc as solvent but not in toluene. NDIFu2 is much more planar compared to NDITh2, resulting in an enhanced charge-transfer character, which makes it an interesting building block for conjugated systems designed for organic electronics. This study highlights direct arylation as a simple and inexpensive method to construct a series of important donor–acceptor–donor building blocks to be further used for the preparation of a variety of conjugated materials.

Phase separation of all-conjugated donor–acceptor block copolymers is more difficult to achieve compared to classical coil–coil systems owing the intrinsic similarity of the two blocks having both rigid conjugated backbones and alkyl side chains and their generally low degrees of polymerization. Here we demonstrate that side chain fluorination of a poly(carbazole-alt-dithienylbenzothiadiazole) segment (SF-PCDTBT), to be used as electron acceptor block in combination with poly(3-hexylthiophene) P3HT as donor block in all-conjugated donor–acceptor block copolymers of type SF-PCDTBT-b-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT leading to phase separation for already moderate molar masses. Key to the successful synthesis of a new TBT-monomer with semifluorinated side chains is a direct arylation step that elegantly bypasses classical cross-coupling reactions in which the semifluorinated side chain causes low yields. Suzuki polycondensation of the semifluorinated TBT monomer with a suitable carbazole comonomer and in situ termination by P3HT-Br is optimized extensively with respect to the yield of the end-capping efficiency and molar mass control of the PCDTBT segment. When the fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by n-hexyl chains (hex-PCDTBT), the tendency for phase separation with covalently connected P3HT is much reduced as shown by differential scanning calorimetry and grazing incidence small-angle scattering measurements on thin films. Favorably, of all the block copolymers made only SF-PCDTBT-b-P3HT is microphase separated, exhibits face-on orientation of P3HT domains, and additionally displays surface segregation of the SF-PCDTBT segment at the polymer/air interface. All of these properties are beneficial for single layer single component solar cells. SF-PCDTBT-b-P3HT exhibits the best solar cells performance with a high open-circuit voltage of 1.1 V and a power conversion efficiency of ∼1% which largely outperforms devices based on the analogous H-PCDTBT-b-P3HT and hex-PCDTBT-b-P3HT.

Defect structures in high-performance conjugated polymers are generally known but still challenging to characterize on a quantitative basis. Here, we present a detailed analysis of backbone topology of a series of copolymers PDPPTh2F4 having alternating dithienyldiketopyrrolopyrrole (DPPTh2) and tetrafluorobenzene (F4) units made by direct arylation polycondensation (DAP). In contrast to early expectations of unselective C–H activation during the DAP of monomers with multiple C–H bonds, detailed structure analysis by high-temperature 1H NMR spectroscopy reveals well-defined and alternating backbones with a quantifiable amount of 0–12% DPPTh2 homocouplings as the only defect structure in the main chain. Homocoupled −DPPTh2–DPPTh2– structural units are additionally characterized by UV–vis spectroscopy. While −DPPTh2–H end groups are inert to other side reactions, −F4–Br end groups are weakly susceptible to both dehalogenation and reaction with toluene. However, despite the presence of DPPTh2 homocouplings, high field-effect transistor electron mobilities up to ∼0.6 cm2/(V s) are achieved. This study highlights both that DPPTh2 homocouplings pose a prevalent structural defect in DPPTh2-based conjugated polymers made by DAP and that a very simple four-step DAP protocol can yield materials with varying molar mass and excellent n-type transistor performance.

Direct arylation (DA) is emerging as a highly promising method to construct inexpensive conjugated materials for large-area electronics from simple and environmentally benign building blocks. Here, we show that exclusive α-C–H selectivity is feasible in the DA of π-extended monomers having unsubstituted thiophene or furan units, leading to fully linear materials. Two new naphthalene diimide-based conjugated copolymers—P(FuNDIFuF4) and P(ThNDIThF4), composed of naphthalene diimide (NDI), furan (Fu) or thiophene (Th), and tetrafluorobenzene (F4)—are synthesized. Insight into structure–function relationships is given by density functional theory (DFT) calculations and variety of experimental techniques, whereby the effect of the heteroatom on the optical, structural, and electronic properties is investigated. The use of furan (Fu) allows for enhanced solubilities, a smaller dihedral angle between NDI and Fu as a result of the smaller size of Fu, and a smaller π–π-stacking distance in the solid state. P(FuNDIFuF4) also exhibits a more edge-on orientation compared to P(ThNDIThF4). Despite these advantageous properties of P(FuNDIFuF4), P(ThNDIThF4) exhibits the highest electron mobility: ∼1.3 cm2/(V s), which is a factor of ∼3 greater than that of P(FuNDIFuF4). The enhanced OFET performance of P(ThNDIThF4) is explained by reduced orientational disorder and the formation of a terrace-like thin-film morphology.

Naphthalene diimide (NDI) main chain conjugated polymers have seen a high and steadily increasing level of activity during the last five years. It is mainly the intriguing properties of high electron mobilities and tunable absorption up to the near IR region that have driven researchers to design new polymeric structures having main chain NDIs in the backbone. While the field is still in its infancy, many important results have been obtained and the first structure–function relationships can be drawn. By reviewing synthetic aspects (step growth and chain growth polycondensation techniques), polymeric architectures made, structure formation, and applications in OFET devices and organic photovoltaics, the reader is equipped with some of the key aspects of this important class of materials.

A combined experimental and theoretical study on the synthesis and solution isomerization behavior of main chain copolymers with multiple spiropyran incorporation is presented. A series of alternating copolymers P(SP-alt-Cx) of spiropyran (SP) and flexible linkers (Cx, x = 6,8,10) is synthesized by Suzuki polycondensation (SPC). Careful 1H NMR polymer end group analysis is carried out to reveal termination reactions that limit molecular weight. Although methylene indoline and salicyl aldehyde end groups are found arising from SP cleavage during polymerization, acceptable Mn,SEC up to 34 kg mol−1 is achieved. P(SP-alt-Cx) can be transformed into the corresponding protonated form of the red alternating merocyanine (MC) polymer, P(MCH+-alt-Cx), in quantitative yield by direct acidification or pulsed ultrasound. The origin of the latter method lies in the sonochemical degradation of chloroform, which provides a continuous source of hydrochloric acid. Deprotonation of P(MCH+-alt-Cx) occurs upon the addition of a base resulting in the blue form P(MC-alt-Cx), as characterized by UV-vis spectroscopy and confirmed by density functional theory (DFT) calculations of model compounds. DFT further reveals P(MCH+-alt-Cx) to be the thermodynamically most stable form. It is proposed that the para-linkage to the phenyl-based comonomer Cx decreases acidity of protonated merocyanine and hence increases stability of P(MCH+-alt-Cx), which poses a marked difference compared to the commonly employed 6-nitro-substituted analogs.

A highly active nickel catalyst with a hybrid P,N ligand is successfully used for the first time for the polymerization of a thiophene monomer with sterically very demanding side groups. The performance of this catalyst is by far not achievable with commercially available standard catalysts. Polythiophenes with special side chain patterns can thus be made with predetermined molecular weight, low dispersity, and high regioregularity, which enable the preparation of various large crystalline superstructures for the investigation of anisotropic optoelectronic properties in the absence of π–π interactions.

Homocouplings are identified as major side reactions in direct arylation polycondensation (DAP) of 4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (TBT) and 2,7-dibromo-9-(1-octylnonyl)-9H-carbazole (CbzBr2). Using size exclusion chromatography (SEC) and NMR spectroscopy, we demonstrate that both TBT and Cbz homocouplings occur at a considerable extent. TBT homocoupling preferentially occurs under phosphine-free conditions but can be suppressed in the presence of a phosphine ligand. Cbz homocoupling is temperature-dependent and more prevalent at higher temperatures. By contrast, evidence for chain branching as a result of unselective C–H arylation is not found for this monomer combination. These results emphasize that particular attention has to be paid to homocouplings in direct arylation polycondensations as a major source of main-chain defects, especially under phosphine-free conditions.

Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl2 as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ∼70%. For Ni(dppp)Cl2-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.

Fractionated crystallization (FC), a chain-sorting mechanism by length, is identified in well-defined, low molecular weight, and defect-free regioregular poly(3-hexylthiophene) by X-ray scattering and calorimetry of bulk samples. While wide-angle X-ray scattering (WAXS) qualitatively suggests that the degree of crystallinity is similar in all investigated samples, the melting enthalpies are largely different. We ascribe this to intricacies in the integration of the melting and crystallization peaks in calorimetric experiments, which is caused by FC occurring over a large temperature range. The extent of FC decreases with increasing molecular weight and increases with increasing polydispersity. The temperature-dependent investigation of the long period LP and the (100)-WAXS reflection of a sample in which FC is absent allows to disentangle effects from main-chain and side-chain crystallization.

We present the synthesis, purification, and characterization of all-conjugated block copolymers comprising poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PF8TBT) and poly(3-hexylthiophene) (P3HT). Suzuki step-growth polycondensation is used for the synthesis of PF8TBT, which is subsequently terminated via the addition of narrow-distributed, monobrominated P3HT-Br. Purification via preparative GPC is carried out to reduce polydispersity and to remove excess P3HT. Wavelength-dependent GPC and careful NMR end group analysis, assisted by model compounds, reveal pure diblock copolymers of PF8TBT-b-P3HT. Insight into structure formation is given by temperature-dependent UV–vis absorption, DSC, and X-ray scattering. These indicate that PF8TBT-b-P3HT does not microphase-separate within the investigated range of composition and molecular weight. The critical role of introducing sufficient dissimilarity between the segments in all-conjugated block copolymers in order to induce phase separation is discussed, with the conclusion that careful tuning of side chains is crucial for achieving self-organization.

Naphthalene diimide (NDI) main chain conjugated polymers have seen a high and steadily increasing level of activity during the last five years. It is mainly the intriguing properties of high electron mobilities and tunable absorption up to the near IR region that have driven researchers to design new polymeric structures having main chain NDIs in the backbone. While the field is still in its infancy, many important results have been obtained and the first structure–function relationships can be drawn. By reviewing synthetic aspects (step growth and chain growth polycondensation techniques), polymeric architectures made, structure formation, and applications in OFET devices and organic photovoltaics, the reader is equipped with some of the key aspects of this important class of materials.

To develop greener protocols toward the sustainable production of conjugated polymers, we combine the advantages of atom-economic direct arylation polycondensation (DAP) with those of the green solvent 2-methyltetrahydrofuran (MeTHF). The n-type copolymer PNDIT2 is synthesized from unsubstituted bithiophene (T2) and 2,6-dibromonapthalene diimide (NDIBr2) under simple DAP conditions in MeTHF. Extensive optimization is required to suppress nucleophilic substitution of NDIBr end groups, which severely limits molar mass. Different carboxylic acids, bases, palladium precursors and ligands are successfully screened to enable quantitative yield and satisfyingly high molar masses up to Mn,SEC ∼ 20 kDa. In contrast to PNDIT2 made via DAP in toluene with tolyl-chain termini, nucleophilic substitution of NDIBr chain ends in MeTHF finally leads to NDI-OH termination. The influence of different chain termini on the optical, thermal, structural and electronic properties of PNDIT2 is investigated. For samples with identical molecular weight, OH-termination leads to slightly reduced aggregation in solution and bulk crystallinity, a decreased degree of alignment in directionally deposited films, and a consequently reduced, but not compromised, electron mobility with promising values still close to 0.9 cm2 V−1 s−1.