Synthesis and Photocyclization of Conjugated Diselenophene Pyrrole-2,5-dione Based Monomers for Optoelectronics

Structural-Functional Properties of Asymmetric Fluoro-Alkoxy Substituted Benzothiadiazole Homopolymers with Flanked Chalcogen-Based Heterocycles

The synthesis and characterization of asymmetric alkoxy- are reported, fluoro-benzothiadiazole (BT) acceptor core derivatized with a series of six different heterocycles (selenophene, thiophene, furan, 5-thiazole, 2-thiazole and 2-oxazole). The effect of the flanked-heterocycles containing different chalcogen atoms of the six homopolymers (HPX) is studied using optical, thermal, electrochemical, and computational analysis. Computational calculations indicate a strong relationship between the most stable conformation for each homopolymer and their bearing heterocycle, thus homopolymers HPSe', HPTp', HPFu', and HPTzC5, adopted the syn-syn and syn-anti conformations due to their noncovalent interactions with shorter distances, while HPTzC2' and HPOx' demonstrate preference for the anti-anti conformation. Optical property studies of the homopolymers reveal a strong red-shift in solution and film upon exchanging the chalcogen atom from Oxygen < Sulfur < Selenium in HPFu, HPTp, and HPSe, respectively. In addition, deeper highest occupied molecular orbital (HOMO) energy levels are observed when the donor-acceptor moieties (HPSe, HPTp, and HPFu) are substituted for the acceptor-acceptor systems such as HPTzC5, HPTzC2, and HPOx. Improved packing and morphology are exhibited for the donor-acceptor homopolymers. Thus, having a flanked heterocycle containing different chalcogen-atoms in polymeric systems is one way of tuning the physicochemical properties of conjugated materials for optoelectronic applications.

Truncated conjugation in fused heterocycle-based conducting polymers: when greater planarity does not enhance conjugation

One of the main assumptions in the design of new conjugated polymer materials for their use in organic electronics is that higher coplanarity leads to greater conjugation along the polymer backbone. Conventionally, a more planar monomer structure induces a larger backbone coplanarity, thus leading to a greater overlap of the carbon π-orbitals and therefore a higher degree of π-electron delocalisation. However, here we present a case that counters the validity of this assumption. Different diselenophene-based polymers were studied where one polymer possesses two selenophene rings fused together to create a more rigid, planar structure. The effects of this greater polymer coplanarity were examined using Raman spectroscopy and theoretical calculations. Raman spectra showed a large difference between the vibrational modes of the fused and unfused polymers, indicating very different electronic structures. Resonance Raman spectroscopy confirmed the rigidity of the fused selenophene polymer and also revealed, by studying the excitation profiles of the different bands, the presence of two shorter, uncoupled conjugation pathways. Supported by Density Functional Theory (DFT) calculations, we have demonstrated that the reason for this lack of conjugation is a distortion of the selenophene rings due to the induced planarity, forming a new truncated conjugation pathway through the selenophene β-position and bypassing the beneficial α-position. This effect was studied using DFT in an ample range of derivatives, where substitution of the selenium atom with other heteroatoms still maintained the same unconventional conjugation-planarity relationship, confirming the generality of this phenomenon. This work establishes an important structure-property relationship for conjugated polymers that will help rational design of more efficient organic electronics materials.

n-Type Polymer Semiconductors Based on Dithienylpyrazinediimide

The development of n-type organic semiconductors critically relies on the design and synthesis of highly electron-deficient building blocks with good solubility and small steric hindrance. We report here a strongly electron-deficient dithienylpyrazinediimide (TPDI) and its n-type semiconducting polymers. The pyrazine substitution leads to the resulting polymers with much lower-lying lowest unoccupied molecular orbital (LUMO) levels and improved backbone planarity compared to the reported dithienylbenzodiimide (TBDI)- and fluorinated dithienylbenzodiimide (TFBDI)-based polymer analogues, thus yielding n-type transport character with an electron mobility up to 0.44 cm² V^-1 s^-1 in organic thin-film transistors. These results demonstrate that dithienylpyrazinediimide is a highly promising electron-deficient building block for constructing high-performance n-type polymers and the incorporation of pyrazine into imide-functionalized (hetero)arenes is an effective strategy to develop n-type polymers with deep-lying frontier molecular orbital (FMO) levels for organic optoelectronic devices.

Design, synthesis and characterization of fused bithiazole- and dithiophene-based low bandgap thienylenevinylene copolymers

The electron-deficient thiazole moiety has high planarity and effective π–π stacking, which leads to the reduction in the energy levels, exhibiting promising charge carrier mobilities.