Mechanochromic polymers are force-sensitive materials that change their color as a response to mechanical stimuli. This visualization of forces can be used to further optimize polymer-based materials by understanding microscopic force transduction or to display fatigue of material. Most mechanochromic systems rely on bond cleavage, so they can only distinguish between “on” and “off” state without any further correlation of the applied force with the optical signal. Although reversibility to the initial state is possible for most of these systems, it often demands time or input of energy making them rather unsuitable for sensing forces in real-time. In this work, the development and application of mechanochromic donor (D)-acceptor (A) torsional springs is presented as a new concept for mechanochromic materials. The mechanically induced planarization of D and A leads to a continuous red-shift of both absorption and emission color. A suitable DA-system is found in ortho-substituted diphenyldiketopyrrolopyrrole (o-DPP) having the large torsional angle required for the equilibrium geometry and therefore showing blue-shifted optical spectra compared to reference compounds with a smaller torsional angle. The covalent incorporation into tough poly(meta,meta,para-phenylene) (PmmpP) by Suzuki polycondensation enables sufficient force transduction to the DA spring during uniaxial elongation of thin-film specimens. The detected mechanochromic response correlates with the applied stress and shows full reversibility upon stress release. Theoretical experiments based on density functional theory (DFT) confirm the experimental results and offer a detailed explanation of the molecular deformations responsible for the optical shift. In addition, the application as stress sensor was tested investigating the molecular force transduction in glassy PmmpP as a function of the number average molecular weight (Mn) by blending o-DPP-PmmpP probe chains of varying Mn with pristine PmmpP. The distinct mechanochromic response for entangled and non-entangled probe chains, respectively, allows the extraction of the critical molar mass (Mc) that is required for entanglements to become effective. The resulting value for the entanglement molar mass Me ≈ 1/2 Mc is in excellent agreement with the value determined by rheology.

Diketopyrrolopyrrole (DPP)-based polymers currently rank among the best performing organic materials for high charge carrier mobility applications due to their high structural planarity and the simple synthetic access. Through chemical modifications on DPP-based polymers, the type of charge carrier transport (p-type, n-type or ambipolar) and the charge carrier mobility can be both modulated. In this thesis, the synthesis of a new n-type dithiazolyldiketopyrrolopyrrole (TzDPPTz)-based copolymer PTzDPPTzF4 with tetrafluorobenzene (F4) as comonomer is reported. PTzDPPTzF4 has a deeper lowest unoccupied molecular orbital (LUMO) energy level compared to the existing dithienyldiketopyrrolopyrrole (ThDPPTh)-based copolymer PThDPPThF4 due to the electron-deficient thiazole flanking units on the bicyclic DPP core. Moreover, the influence of homocoupling (hc) defects and backbone conformation on the properties of PTzDPPTzF4 is systematically investigated. Lastly, in order to further modulate the structural and electrical properties of DPP-based copolymers, polar side chains and comonomers with a different electron-withdrawing ability are introduced to the polymer backbone. In detail, a series of PTzDPPTzF4 polymers with similar molecular weight but varying TzDPPTz hc content from 0.6 – 12.4% is prepared via direct arylation polymerization (DAP) for the investigation of the hc-property relationship. Hc defects are found to red-shift the absorption, decrease the photoluminescence, and lower the LUMO energy level. In contrast, an influence on the film morphology or electron mobility is not observed. In order to study the conformation-property relationship, a structural variation in the DPP monomer is explored, i.e. the replacement of Tz by Th. To this end, a detailed comparative study of the properties between PTzDPPTzF4 and PThDPPThF4, which are prepared via DAP and have both comparable molecular weight and hc content, is presented. It is found that the replacement of Tz flanking units by Th flanking units on the DPP core has significant impact on the backbone conformation due to the occurrence of intramolecular hydrogen bonds, and thus strongly influences the opto-electronic and structural properties of the two polymers. PThDPPThF4 exhibits a stronger aggregation ability, a higher degree of crystallinity, a lower degree of paracrystallinity and an increased long-range order, which finally translates into a 20 times higher field-effect electron mobility. Finally, comonomer and side chain variations of DPP-based polymers are carried out for their potential use in thermoelectric investigations. Through the optimization of the polymerization conditions, a number-average molecular weight of 19.1 kg/mol is achieved for ThDPPTh-based polymers with single-oxygen side chains and F4 as comonomer. In addition, two ThDPPTh-based copolymers with biEDOT as comonomer are synthesized, which contain polar triethylene glycol (TEG) side chains as well as branched aliphatic side chains in different ratios. In summary, the economically efficient and ecologically green DAP method is demonstrated to be an efficient and versatile synthetic tool for copolymerizing TzDPPTz or ThDPPTh monomers bearing either aliphatic or polar side chains with either electron-rich or electron-deficient comonomers.