Florian Lombeck

Simple Synthesis of P(Cbz- alt -TBT) and PCDTBT by Combining Direct Arylation with Suzuki Polycondensation of Heteroaryl Chlorides

Direct arylation (DA) of 2-chlorothiophene and 2-chloro-3-hexylthiophene with 4,7-dibromo-2,1,3-benzothiadiazole is used to synthesize 4,7-bis(5-chloro-2-thienyl)-2,1,3-benzothiadiazole (TBTCl2) and 4,7-bis(5-chloro-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DH-TBTCl2) in one step. Suitable conditions of the Suzuki polycondensations (SPC) of TBTCl2 and DH-TBTCl2 with the carbazole comonomer CbzPBE2 are established, furnishing PCDTBT and P(Cbz-alt-TBT) with high molecular weight and yield. Compared with control samples made from the corresponding dibromides, high-temperature NMR and UV-vis spectroscopy indicate similar properties for PCDTBT but an increased content of Cbz-Cbz homocouplings for P(Cbz-alt-TBT).

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.

High molecular weight mechanochromic spiropyran main chain copolymers via reproducible microwave-assisted Suzuki polycondensation

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.

Soluble and stable alternating main-chain merocyanine copolymers through quantitative spiropyran–merocyanine conversion

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.

To branch or not to branch: C–H selectivity of thiophene-based donor–acceptor–donor monomers in direct arylation polycondensation exemplified by PCDTBT

Debate remains about potentially occuring non-selective C–H activation during the direct arylation polycondensation of monomers with more than two C–H bonds. Non-selective reactions with dihalides lead to undesirable kinking, branching and cross-linking of conjugated polymer chains, and thus to severe deviations of all opto-electronic properties. Using thiophene–benzothiadiazole–thiophene (TBT) having four different C–H bonds and its popular copolymer with dibromocarbazole, PCDTBT, as an example, we demonstrate that unselective C–H activation, often referred to as “β-arylation”, does not occur under typical polycondensation reactions. However, using extreme stoichiometries, i.e. a large excess of halide, model reactions reveal that in addition to the typical α-C–H activation of TBT, γ-arylation takes place. This reaction is also seen in polymer analogous reactions of PCDTBT. If γ-arylation of PCDTBT is mimicked using Suzuki polycondensation with tribrominated TBT, the reaction yield drops significantly, the charge transfer absorption band is blued-shifted and solar cell performance of the corresponding PCBM blends is reduced drastically through both a reduction in short circuit current and fill factor. While many reports have meanwhile shown that direct arylation polymerization can be employed to make well-defined conjugated polymers, this paper demonstrates that (i) unselective arylation of thiophene-based monomers is very unlikely for stoichiometry reasons, (ii) how branching can be identified in donor–acceptor copolymers on a spectroscopic basis and (iii) how the optoelectronic properties change if branching still takes place.

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.

Different routes towards triplet states in organic semiconductors: direct S0→T excitation probed by time-resolved EPR spectroscopy

ABSTRACT Understanding the electronic structure of semiconducting polymers and their respective building blocks is an essential prerequisite to develop efficient materials for organic electronics. Due to its molecular resolution and capability to directly probe and assign triplet states, (time-resolved) electron paramagnetic resonance spectroscopy is particularly suited. We present here evidence for a direct optical excitation of distinct triplet states in the small molecule TBT, often used as building block for semiconducting polymers. These states are subtly different in their electronic structure from those populated via intersystem crossing from excited singlet states. With these results, we extend our previous investigations of CbzTBT, the repeat unit of the polymer PCDTBT comprising the TBT and an additional carbazole moiety. Hence, the strong acceptor unit TBT seems responsible for this additional triplet route. Due to the widespread use of TBT as an acceptor unit in many different co-polymers, this has potentially high impact on the efficiency of organic electronic devices. GRAPHICAL ABSTRACT

Mixed side-chain geometries for aggregation control of poly(fluorene-alt-bithiophene) and their effects on photophysics and charge transport

Abstract In organic optoelectronics, order of conjugated molecules is required for good charge transport, but strong aggregation behavior may generate grain boundaries and trapping, opposing those benefits. Side chains on a polymer’s backbone are major reason for and also tool to modify its morphological characteristics. In this report, we show on the example poly(9,9-dioctylfluorenyl- co -bithiophene) (F8T2) that by a combination of two types of side-chains on the backbone of equal number of carbons, one promoting crystallization, another hindering it, organization of the main chains can be controlled, without changing its major properties. We compare the traditional F8T2 derivative with octyl substituent with two modified species, one containing solely 2-ethylhexyl side-chains and another with both types randomly distributed. Thermal characteristics, photophysics and morphology are compared and effects on film formation and charge transport in bulk-heterojunction blends demonstrated on photovoltaic devices utilizing F8T2s as donor and the fullerene derivative ICBA as acceptor material.