Key Figures

  • 8 articles in high-impact journals
  • 2 cover pages
  • 10 citations
  • 2 hot topics
  • 13 press releases & highlights
  • 14+ average impact factor
  • 100% collaborative publications

Synthesis of Vinylene‐Linked Two‐Dimensional Conjugated Polymers via the Horner‐Wadsworth‐Emmons Reaction

D. L. Pastoetter, S. Xu, M. Borrelli, M. Addicoat, B. P. Biswal, S. Paasch, A. Dianat, H. Thomas, R. Berger, S. Reineke, E. Brunner, G. Cuniberti, M. Richter, X. Feng

Angew. Chem. Int. Ed., 2020, in press.

Interest in linear conjugated polymers has significantly increased in recent decades due to their semiconducting properties and promising applications in organic optoelectronics. To date, the extension of linear conjugated polymers into two‐dimensional conjugated polymers (2D CPs), which can also be regarded as 2D π‐conjugated covalent organic frameworks (COFs), remains largely unexplored due to limited synthetic methodologies. In this work, we demonstrate the first synthesis of vinylene‐linked 2D CPs, namely, 2D poly(phenylenequinoxalinevinylene)s 2D‐PPQV1 and 2D‐PPQV2, via the Horner‐Wadsworth‐Emmons (HWE) reaction of C2‐symmetric 1,4‐bis(diethylphosphonomethyl)benzene or 4,4'‐bis(diethylphosphonomethyl)biphenyl with C3‐symmetric 2,3,8,9,14,15‐hexa(4‐formylphenyl)diquinoxalino[2,3‐a:2',3'‐c]phenazine as monomers. Density functional theory (DFT) simulations unveil the crucial role of the initial reversible C‐C single bond formation for the synthesis of crystalline 2D CPs. Powder X‐ray diffraction (PXRD) studies and nitrogen adsorption‐desorption measurements demonstrate the formation of proclaimed crystalline, dual‐pore structures with surface areas of up to 440 m2/g. More importantly, the optoelectronic properties of the obtained 2D‐PPQV1 (Eg=2.2 eV) and 2D‐PPQV2 (Eg=2.2 eV) are compared with those of cyano‐vinylene‐linked 2D‐CN‐PPQV1 (Eg=2.4 eV) produced by the Knoevenagel reaction and imine‐linked 2D COF analog (2D‐C=N‐PPQV1, Eg=2.3 eV), unambiguously proving the superior conjugation of the vinylene‐linked 2D CPs using the HWE reaction.

Graphene transistors for real-time monitoring molecular self-assembly dynamics

M. Gobbi, A. Galanti, M.-A. Stoeckel, B. Zyska, S. Bonacchi, S. Hecht, P. Samorì

Nat. Commun., 2020, 11, 4731. Link to article (open access). UNISTRA, ISIS and IRIS Adlershof press releases. Highlighted in Nature Research Device & Materials Engineering Community.

Mastering the dynamics of molecular assembly on surfaces enables the engineering of predictable structural motifs to bestow programmable properties upon target substrates. Yet, monitoring self-assembly in real time on technologically relevant interfaces between a substrate and a solution is challenging, due to experimental complexity of disentangling interfacial from bulk phenomena. Here, we show that graphene devices can be used as highly sensitive detectors to read out the dynamics of molecular self-assembly at the solid/liquid interface in-situ. Irradiation of a photochromic molecule is used to trigger the formation of a metastable self-assembled adlayer on graphene and the dynamics of this process are monitored by tracking the current in the device over time. In perspective, the electrical readout in graphene devices is a diagnostic and highly sensitive means to resolve molecular ensemble dynamics occurring down to the nanosecond time scale, thereby providing a practical and powerful tool to investigate molecular self-organization in 2D.

Luminescent sp2-Carbon-Linked 2D Conjugated Polymers with High Photostability

Y. Li, B. P. Biswal, M. A. Addicoat, S. Paasch, P. Imbrasas, S. Park, H. Shi, E. Brunner, M. Richter, S. Lenk, S. Reineke, X. Feng

Chem. Mater., 2020, 32, 7985–7991. Link to article.

Luminescent organic materials with high photostability are essential in optoelectronics, sensor, and photocatalysis applications. However, small organic molecules are generally sensitive to UV irradiation, giving rise to chemical decompositions. In this work, we demonstrate two novel CN-substituted two-dimensional sp2-carbon-linked conjugated polymers (2D CCPs) containing a chromophore triphenylene unit. The Knoevenagel polymerization between 2,3,6,7,10,11-hexakis(4-formylphenyl)triphenylene (HFPTP) and 1,4-phenylenediacetonitrile (PDAN) or 2,2′-(biphenyl-4,4′-diyl)diacetonitrile (BDAN), provides the crystalline 2D CCP-HFPTP-PDAN (2D CCP-1) and 2D CCP-HFPTP-BDAN (2D CCP-2) with dual pore structures, respectively. 2D CCP-1 and 2D CCP-2 exhibit the photoluminescence quantum yield (PLQY) up to 24.9 and 32.3%, which are the highest values among the reported 2D conjugated polymers and π-conjugated 2D covalent organic frameworks. Furthermore, compared with the well-known emissive small molecule tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), both 2D CCPs show superior photostability under UV irradiation for 2 h, profiting from the twisted and rigid structures of the CN-substituted vinylene linkages. The present work will trigger the further explorations of novel organic emitters embedded in 2D CCPs with high PLQY and photostability, which can be useful for optoelectronic devices.

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza‐Diels–Alder Reaction: Towards High‐Performance Supercapacitor Materials

H. Peng, J. Raya, F. Richard, W. Baaziz, O. Ersen, A. Ciesielski, P. Samorì

Angew. Chem. Int. Ed., 2020, in press. Featured in Functional Porous Materials Chemistry and Hot Topic: Batteries and Supercapacitors.

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza‐Diels–Alder cycloaddition reaction as post‐synthetic modification of MOF@COF‐LZU1, to generate aza‐MOFs@COFs hybrid porous materials with extended π‐delocalization. A a proof‐of‐concept, the obtained aza‐MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm−2 and high volumetric energy density of 1.16 F cm−3. Our approach of post‐synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.

On-surface synthesis of super-heptazethrene

S. Mishra, J. Melidonie, K. Eimre, S. Obermann, O. Gröning, C. A. Pignedoli, P. Ruffieux, X. Feng, R. Fasel

Chem. Commun., 2020, 56, 7467–7470. Link to article and accepted manuscript (open access).

Zethrenes are model diradicaloids with potential applications in spintronics and optoelectronics. Despite a rich chemistry in solution, on-surface synthesis of zethrenes has never been demonstrated. We report the on-surface synthesis of super-heptazethrene on Au(111). Scanning tunneling spectroscopy investigations reveal that super-heptazethrene exhibits an exceedingly low HOMO–LUMO gap of 230 meV and, in contrast to its open-shell singlet ground state in the solution phase and in the solid-state, likely adopts a closed-shell ground state on Au(111).

Photomodulation of Charge Transport in All‐Semiconducting 2D–1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect

Z. Liu, H. Qiu, C. Wang, Z. Chen, B. Zyska, A. Narita, A. Ciesielski, S. Hecht, L. Chi, K. Müllen, P. Samorì

Adv. Mater., 2020, 32, 2001268. Link to article and accepted manuscript (open access). COVER PAGE. Featured in Hot Topic: Carbon, Graphite, and Graphene.

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom‐up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS2 are fabricated. Field‐effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS2, resulting from the interfacial charge transfer process. The MoS2‐GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS2‐GNR VDWHs. The physisorption of photochromic molecules onto the MoS2‐GNR VDWHs enables reversible light‐driven control over charge transport. In particular, the drain current of the MoS2‐GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS2‐GNR VDWHs for multilevel memory devices.

Ultrathin two-dimensional conjugated metal–organic framework single-crystalline nanosheets enabled by surfactant-assisted synthesis

Z. Wang, G. Wang, H. Qi, M. Wang, M. Wang, S. Park, H. Wang, M. Yu, U. Kaiser, A. Fery, S. Zhou, R. Dong, X. Feng

Chem. Sci., 2020, 11, 7665–7671. Link to article (open access).

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) have recently emerged for potential applications in (opto-)electronics, chemiresistive sensing, and energy storage and conversion, due to their excellent electrical conductivity, abundant active sites, and intrinsic porous structures. However, developing ultrathin 2D c-MOF nanosheets (NSs) for facile solution processing and integration into devices remains a great challenge, mostly due to unscalable synthesis, low yield, limited lateral size and low crystallinity. Here, we report a surfactant-assisted solution synthesis toward ultrathin 2D c-MOF NSs, including HHB-Cu (HHB = hexahydroxybenzene), HHB-Ni and HHTP-Cu (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). For the first time, we achieve single-crystalline HHB-Cu(Ni) NSs featured with a thickness of 4–5 nm (∼8–10 layers) and a lateral size of 0.25–0.65 μm2, as well as single-crystalline HHTP-Cu NSs with a thickness of ∼5.1 ± 2.6 nm (∼10 layers) and a lateral size of 0.002–0.02 μm2. Benefiting from the ultrathin feature, the synthetic NSs allow fast ion diffusion and high utilization of active sites. As a proof of concept, when serving as a cathode material for Li-ion storage, HHB-Cu NSs deliver a remarkable rate capability (charge within 3 min) and long-term cycling stability (90% capacity retention after 1000 cycles), superior to the corresponding bulk materials and other reported MOF cathodes.

Demonstration of a Broadband Photodetector Based on a Two‐Dimensional Metal–Organic Framework

H. Arora, R. Dong, T. Venanzi, J. Zscharschuch, H. Schneider, M. Helm, X. Feng, E. Cánovas, A. Erbe

Adv. Mater., 2020, 32, 1907063. Link to article (open access). COVER PAGEcfaed, HZDR, EurekAlert!, NWA,, ScienceDaily, Laser Focus World, Optics & Photonics News and PhotonicsViews press releases.

Metal–organic frameworks (MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Yet, the realization of their proof‐of‐concept devices remains a daunting challenge, attributed to their poor electrical properties. Following recent work on a semiconducting Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11‐triphenylenehexathiol) 2D MOF with record‐high mobility and band‐like charge transport, here, an Fe3(THT)2(NH4)3 MOF‐based photodetector operating in photoconductive mode capable of detecting a broad wavelength range from UV to NIR (400–1575 nm) is demonstrated. The narrow IR bandgap of the active layer (≈0.45 eV) constrains the performance of the photodetector at room temperature by band‐to‐band thermal excitation of charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 108 cm Hz1/2 W−1 are achieved under 785 nm excitation. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are commensurate to those obtained with the first demonstrations of graphene‐ and black‐phosphorus‐based photodetectors. This work demonstrates the feasibility of integrating conjugated MOFs as an active element into broadband photodetectors, thus bridging the gap between materials' synthesis and technological applications.