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Development of Thiophene Based Conjugated Polymers in Two Dimensions

Periodic Reporting for period 2 - T2DCP (Development of Thiophene Based Conjugated Polymers in Two Dimensions)

Reporting period: 2020-09-01 to 2022-02-28

Our economy and society are in a proceeding process of digitalization. The progressing demand for smart devices in the industrial internet or the internet of things and high-performance electronics requires the development of novel materials. Organic electronics is a disruptive technology featuring low-cost, robust, lightweight, flexible, and affordable devices based on organic small molecules and polymers to meet future requirements. Especially since the discovery and investigations of highly conductive linear trans-polyacetylene in 1977, linear conjugated polymers have played a progressively important role in modern organic electronics. However, the successive dimensional increase from one-dimensional (linear) conjugated polymers to two-dimensional (2D) conjugated polymers is far from being fully explored. Therefore, this EU-funded project aims at developing and investigating thiophene-based 2D conjugated polymers (T2DCPs). The bottom-up synthesis of T2DCPs benefits from precise control over chemical functionality in a highly predictable way, giving a dirigible structure-property relationship to the 2D materials. The electron delocalization in two dimensions makes T2DCPs exciting materials for (opto)electronic applications and are expected to demonstrate superior performance in terms of charge carrier mobility and defect tolerance. Therefore, the project aims to integrate T2DCPs in organic field-effect transistors. In this respect, we aim to establish versatile and reliable synthesis strategies employing thiophene monomers rendering T2DCPs with an entirely C=C/Ar-Ar backbone.
By employing designed monomers and linkage topologies, we tried to accomplish optical and energy gap engineering, control the molecular weight (or crystalline domain size) and conjugation channel densities of the targeted thiophene-based 2D conjugated polymers (T2DCPs) and other 2D polymers.
As crucial achievement can be mentioned, the increased synthetic control over the Knoevenagel reaction for the synthesis of layered 2D conjugated polymers. Moreover, an obtained layered donor-acceptor T2DCP (2D CCP-Th) showed superior performance in photoelectrochemical hydrogen evolution reaction (PEC-HER) with a state-of-the-art H2-evolution photocurrent density for COF materials up to ≈7.9 µA cm−2 at 0 V versus reversible hydrogen electrode. In addition, we investigated the Horner-Wadsworth-Emmons (HWE) reaction of stabilized phosphonate carbanions with aldehydes as an alternative strategy to establish poly(arylene vinylene)s and vinylene-linked T2DCPs. The resulting 2D CP (2D-PPVQ1) revealed a superior conjugation compared to the cyano-vinylene-linked analog (2D-CN-PPQV1).
Furthermore, we synthesized thiophene-based linear polymers (pDTT, etc.) using a copper-mediated Glaser coupling as a proof-of-concept for the metal-templated polymerization approach. The corresponding linear polythiophenes showed superb H2-evolution photocurrent density up to ≈170 μA cm−2 at 0.3 V vs. RHE under solar light irradiation.
Besides, we investigated the surfactant-monolayer assisted interfacial synthesis (SMAIS) as a robust method for obtaining single and few-layer 2D polymers: Obtained 2D polyimide (2DPI) was obtained with high crystallinity, thickness of ~2 nm, and an average crystal domain size of ~3.5 μm2. Similarly to 2DPI, we investigated single-layer and multi-layer 2D polyimine (PI-2DP) films with square and hexagonal lattices. The PI-2DP films reveal polycrystalline multi-layers with tailorable thickness (6 to 200 nm) and large crystalline domains (100–150 nm). Moreover, PI-2DP1 was investigated by time-resolved terahertz spectroscopy showing a p-type semiconductor behavior with hole mobility up to 0.01 cm2 V−1 s−1, superior to previously reported polyimine based materials. Besides the imine and imide linkages, we also synthesized boronate ester-linked 2D polymers: The obtained crystalline boronate ester 2D COFs (2DBECOF-PP) were obtained as free-standing thin films with record-size single-crystalline domains up to ≈60 μm2 and tunable thickness from 6 to 16 nm. Moreover, the developed few-layer 2DBECOF-PP film was integrated into an organic thin-film/ silicon nanowire (SiNW)-based field-effect transistor (FET) to mimic neuronal synapses. This artificial synaptic transistor displayed a learning–erasing–forgetting memory process with a fast response for the saturation of the potentiation. The examples mentioned above prove the SMAIS as an ideal and powerful platform to synthesize single- and few-layer T2DCPs. Moreover, the SMAIS method was demonstrated as general and reliable on-water synthesis toward the preparation of 2D polyimide (2DPI)-graphene (G) van der Waals heterostructures (vdWHs).
We implemented thiophene-based building blocks for the very first time into cyano-vinylene-linked 2D CPs and investigated their superior performance in photoelectrochemical hydrogen evolution reaction (PEC-HER). Moreover, we investigated a novel methodology, the Horner-Wadsworth-Emmons (HWE) reaction, as an alternative strategy to establish vinylene-linked 2D CPs and vinylene-linked T2DCPs. Most importantly, we investigated the controlled synthesis of few-layer two-dimensional polyimide (2DPI) crystals by surfactant-monolayer assisted interfacial synthesis (SMAIS), a significant breakthrough. The investigated SMAIS method gave rise to highly crystalline single or multi-layer 2D polymer films with outstanding properties using boronate ester, imide, and imine linkages. Thus, we provided a robust method to synthesize free-standing 2D polymer films and van der Waals heterostructures, which will be further expanded to synthesize T2DCP and other 2D polymer films using water-soluble and insoluble monomers. The combination of methodology development and material design will give rise to novel T2DCPs with remarkable properties for optoelectronic applications such as organic light-emitting diodes (OLED), organic field-effect transistors (OFET), organic photovoltaics (OPV), organic radio frequency identification tags (ORFID) as the most prominent examples among others.