Cove-Edged Chiral Graphene Nanoribbons with Chirality-Dependent Bandgap and Carrier Mobility.

Graphene nanoribbons (GNRs) have garnered significant interest due to their highly customizable physicochemical properties and potential utility in nanoelectronics. Besides controlling widths and edge structures, the inclusion of chirality in GNRs brings another dimension for fine-tuning their optoelectronic properties, but related studies remain elusive owing to the absence of feasible synthetic strategies. Here, we demonstrate a novel class of cove-edged chiral GNRs (CcGNRs) with a tunable chiral vector (n,m). Notably, the bandgap and effective mass of (n,2)-CcGNR show a distinct positive correlation with the increasing value of n, as indicated by theory. Within this GNR family, two representative members, namely, (4,2)-CcGNR and (6,2)-CcGNR, are successfully synthesized. Both CcGNRs exhibit prominently curved geometries arising from the incorporated [4]helicene motifs along their peripheries, as also evidenced by the single-crystal structures of the two respective model compounds (1 and 2). The chemical identities and optoelectronic properties of (4,2)- and (6,2)-CcGNRs are comprehensively investigated via a combination of IR, Raman, solid-state NMR, UV-vis, and THz spectroscopies as well as theoretical calculations. In line with theoretical expectation, the obtained (6,2)-CcGNR possesses a low optical bandgap of 1.37 eV along with charge carrier mobility of ∼8 cm2 V-1 s-1, whereas (4,2)-CcGNR exhibits a narrower bandgap of 1.26 eV with increased mobility of ∼14 cm2 V-1 s-1. This work opens up a new avenue to precisely engineer the bandgap and carrier mobility of GNRs by manipulating their chiral vector.

Enhancing Chiroptical Responses in Helical Nanographenes via Geometric Engineering of Double [7]Helicenes.

Helical nanographenes with high quantum yields and strong chiroptical responses are pivotal for developing circularly polarized luminescence (CPL) materials. Here, we present the successful synthesis of novel π-extended double [7]helicenes (ED7Hs) where two helicene units are fused at the meta- or para-position of the middle benzene ring, respectively, as the structural isomers of the reported ortho-fused ED7H. The structural geometry of these ED7Hs is clearly characterized by single-crystal X-ray analysis. Notably, this class of ED7Hs exhibits bright luminescence with high quantum yields exceeding 40 %. Through geometric regulation of two embedded [7]helicene units from ortho-, meta- to para-position, these ED7Hs display exceptional amplification in chiroptical responses. This enhancement is evident in a remarkable approximate fivefold increase in the absorbance and luminescence dissymmetry factors (gabs and glum), respectively, along with a boosted CPL brightness up to 176 M-1 cm-1, surpassing the performance of most helicene-based chiral NGs. Furthermore, DFT calculations elucidate that the geometric adjustment of two [7]helicene units allows the precise alignment of electric and magnetic transition dipole moments, leading to the observed enhancement of their chiroptical responses. This study offers an effective strategy for magnifying the CPL performance in chiral NGs, promoting their expanded application as CPL emitters.

Evolution of Length-Dependent Properties of Discrete n-Type Oligomers Prepared via Scalable Direct Arylation.

Efficient organic electronic devices are fabricated from both small molecules and disperse polymers, but materials with characteristics in between remain largely unexplored. Here, we present a gram-scale synthesis for a series of discrete n-type oligomers comprising alternating naphthalene diimide (NDI) and bithiophene (T2). Using C-H activation, discrete oligomers of type T2-(NDI-T2)n (n ≤ 7) and persistence lengths up to ∼10 nm are made. The absence of protection/deprotection reactions and the mechanistic nature of Pd-catalyzed C-H activation allow one to produce symmetrically terminated species almost exclusively, which is key to the fast preparation, high yields, and the general success of the reaction pathway. The reaction scope includes different thiophene-based monomers, end-capping to yield NDI-(T2-NDI)n (n ≤ 8), and branching at T2 units by nonselective C-H activation under certain conditions. We show how the optical, electronic, thermal, and structural properties depend on oligomer length along with a comparison to the disperse, polymeric analogue PNDIT2. From theory and experiments, we find that the molecular energy levels are not affected by chain length resulting from the strong donor-acceptor system. Absorption maxima saturate for n = 4 in vacuum and for n = 8 in solution. Linear oligomers T2-(NDI-T2)n are highly crystalline with large melting enthalpies up to 33 J/g; NDI-terminated oligomers show reduced crystallinity, stronger supercooling, and more phase transitions. Branched oligomers and those with bulky thiophene comonomers are amorphous. Large oligomers exhibit similar packing characteristics compared to PNDIT2, making these oligomers ideal models to study length-structure-function relationships at constant energy levels.

π-Extended Helical Multilayer Nanographenes with Layer-Dependent Chiroptical Properties.

Helical nanographenes (NGs) have attracted increasing attention recently because of their intrinsic chirality and exotic chiroptical properties. However, the efficient synthesis of extended helical NGs featuring a multilayer topology is still underdeveloped, and their layer-dependent chiroptical properties remain elusive. In this study, we demonstrate a modular synthetic strategy to construct a series of novel helical NGs (1-3) with a multilayer topology through a consecutive Diels-Alder reaction and regioselective cyclodehydrogenation from the readily accessible phenanthrene-based precursors bearing ethynyl groups. The resultant NGs exhibit bilayer, trilayer, and tetralayer structures with elongated π extension and rigid helical backbones, as unambiguously confirmed by single-crystal X-ray or electron diffraction analysis. We find that the photophysical properties of these helical NGs are notably influenced by the degree of π extension, which varies with the number of layers, leading to obvious redshifted absorption, a fast rising molar extinction coefficient (ε), and markedly boosted fluorescence quantum yield (Φf). Moreover, the embedded [7]helicene subunits in these NGs result in stable chirality, enabling both chiral resolution and exploration of their layer-dependent chiroptical properties. Profiting from the good alignment of electric and magnetic dipole moments determined by the multilayer structure, the resultant NGs exhibit excellent circular dichroism and circularly polarized luminescence response with unprecedented high CPL brightness up to 168 M-1 cm-1, rendering them promising candidates for CPL emitters.

Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons.

The precision synthesis of helical bilayer nanographenes (NGs) with new topology is of substantial interest because of their exotic physicochemical properties. However, helical bilayer NGs bearing non-hexagonal rings remain synthetically challenging. Here we present the efficient synthesis of the first helical bilayer nonbenzenoid nanographene (HBNG1) from a tailor-made azulene-embedded precursor, which contains a novel [10]helicene backbone with two embedded heptagons. Single-crystal X-ray analysis reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2 Šamong the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramolecular through-space conjugation as revealed by in situ spectroelectrochemistry studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by circular dichroism and circularly polarized luminescence.

Benzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States.

Open-shell non-alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene-based diradicaloids (1-3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (Eg opt =0.52-0.69 eV) and persistent stability under ambient conditions (t1/2 =11.7-33.3 h). More importantly, this new type of diradicaloids possess a low-lying triplet state with an extremely small singlet-triplet energy gap, as low as 0.002 kcal mol-1 , with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non-alternant open-shell PHs with high-spin ground states, and opens up novel possibilities and insights into understanding the structure-property relationships.

Defective Nanographenes Containing Seven-Five-Seven (7-5-7)-Membered Rings.

Defects have been observed in graphene and are expected to play a key role in its optical, electronic, and magnetic properties. However, because most of the studies focused on the structural characterization, the implications of topological defects on the physicochemical properties of graphene remain poorly understood. Here, we demonstrate a bottom-up synthesis of three novel nanographenes (1-3) with well-defined defects in which seven-five-seven (7-5-7)-membered rings were introduced to their sp2 carbon frameworks. From the X-ray crystallographic analysis, compound 1 adopts a nearly planar structure. Compound 2, with an additional five-membered ring compared to 1, possesses a slightly saddle-shaped geometry. Compound 3, which can be regarded as the "head-to-head" fusion of 1 with two bonds, features two saddles connected together. The resultant defective nanographenes 1-3 were well-investigated by UV-vis absorption, cyclic voltammetry, and time-resolved absorption spectra and further corroborated by density functional theory (DFT) calculations. Detailed experimental and theoretical investigations elucidate that these three nanographenes 1-3 exhibit an anti-aromatic character in their ground states and display a high stability under ambient conditions, which contrast with the reported unstable biradicaloid nanographenes that contain heptagons. Our work reported herein offers insights into the understanding of structure-related properties and enables the control of the electronic structures of expanded nanographenes with atomically precise defects.

Persistent peri-Heptacene: Synthesis and In Situ Characterization.

n-peri-Acenes (n-PAs) have gained interest as model systems of zigzag-edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n-PAs larger than peri-tetracene remains challenging because of their intrinsic open-shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n-PA, that is, peri-heptacene (7-PA), in which the reactive zigzag edges are kinetically protected with eight 4-tBu-C6 H4 groups. The formation of 7-PA is validated by high-resolution mass spectrometry and in situ FT-Raman spectroscopy. 7-PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t1/2 ≈25 min) under inert conditions. Moreover, electron-spin resonance measurements and theoretical studies reveal that 7-PA exhibits an open-shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next-generation (3 N+1)-PAs (where N≥3).

A Curved Graphene Nanoribbon with Multi-Edge Structure and High Intrinsic Charge Carrier Mobility.

Structurally well-defined graphene nanoribbons (GNRs) have emerged as highly promising materials for the next-generation nanoelectronics. The electronic properties of GNRs critically depend on their edge topologies. Here, we demonstrate the efficient synthesis of a curved GNR (cGNR) with a combined cove, zigzag, and armchair edge structure, through bottom-up synthesis. The curvature of the cGNR is elucidated by the corresponding model compounds tetrabenzo[a,cd,j,lm]perylene (1) and diphenanthrene-fused tetrabenzo[a,cd,j,lm]perylene (2), the structures of which are unambiguously confirmed by the X-ray single-crystal analysis. The resultant multi-edged cGNR exhibits a well-resolved absorption at the near-infrared (NIR) region with a maximum peak at 850 nm, corresponding to a narrow optical energy gap of ∼1.22 eV. Employing THz spectroscopy, we disclose a long scattering time of ∼60 fs, corresponding to a record intrinsic charge carrier mobility of ∼600 cm2 V-1 s-1 for photogenerated charge carriers in cGNR.

Tailoring Magnetic Features in Zigzag-Edged Nanographenes by Controlled Diels-Alder Reactions.

Nanographenes (NGs) with tunable electronic and magnetic properties have attracted enormous attention in the realm of carbon-based nanoelectronics. In particular, NGs with biradical character at the ground state are promising building units for molecular spintronics. However, most of the biradicaloids are susceptible to oxidation under ambient conditions and photolytic degradation, which hamper their further applications. Herein, we demonstrated the feasibility of tuning the magnetic properties of zigzag-edged NGs in order to enhance their stability via the controlled Diels-Alder reactions of peri-tetracene (4-PA). The unstable 4-PA (y0 =0.72; half-life, t1/2 =3 h) was transformed into the unprecedented benzo-peri-tetracenes (BPTs) by a one-side Diels-Alder reaction, which featured a biradical character at the ground state (y0 =0.60) and exhibited remarkable stability under ambient conditions for several months. In addition, the fully zigzag-edged circumanthracenes (CAs) were achieved by two-fold or stepwise Diels-Alder reactions of 4-PA, in which the magnetic properties could be controlled by employing the corresponding dienophiles. Our work reported herein opens avenues for the synthesis of novel zigzag-edged NGs with tailor-made magnetic properties.

Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties.

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X-ray crystallographic analysis. The embedded azulene unit in 2 possesses a record-high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis-spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in-situ EPR/Vis-NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties.

Open-Shell Nonbenzenoid Nanographenes Containing Two Pairs of Pentagonal and Heptagonal Rings.

Nonbenzenoid carbocyclic rings are postulated to serve as important structural elements toward tuning the chemical and electronic properties of extended polycyclic aromatic hydrocarbons (PAHs, or namely nanographenes), necessitating a rational and atomically precise synthetic approach toward their fabrication. Here, using a combined bottom-up in-solution and on-surface synthetic approach, we report the synthesis of nonbenzenoid open-shell nanographenes containing two pairs of embedded pentagonal and heptagonal rings. Extensive characterization of the resultant nanographene in solution shows a low optical gap, and an open-shell singlet ground state with a low singlet-triplet gap. Employing ultra-high-resolution scanning tunneling microscopy and spectroscopy, we conduct atomic-scale structural and electronic studies on a cyclopenta-fused derivative on a Au(111) surface. The resultant five to seven rings embedded nanographene displays an extremely narrow energy gap of 0.27 eV and exhibits a pronounced open-shell biradical character close to 1 (y0 = 0.92). Our experimental results are supported by mean-field and multiconfigurational quantum chemical calculations. Access to large nanographenes with a combination of nonbenzenoid topologies and open-shell character should have wide implications in harnessing new functionalities toward the realization of future organic electronic and spintronic devices.

On-Surface Synthesis of a Nonplanar Porous Nanographene.

On-surface synthesis provides an effective approach toward the formation of graphene nanostructures that are difficult to achieve via traditional solution chemistry. Here, we report on the design and synthesis of a nonplanar porous nanographene with 78 sp 2 carbon atoms, namely C78. Through a highly selective oxidative cyclodehydrogenation of 2,3,6,7,10,11-hexa(naphthalen-1-yl)triphenylene (2), propeller nanographene precursor 1 was synthesized in solution. Interestingly, although 1 could not be cyclized further in solution, porous nanographene C78 was successfully achieved from 1 by on-surface assisted cyclodehydrogenation on Au(111). The structure and electronic properties of C78 have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy, and scanning tunneling spectroscopy, complemented by computational investigations. Our results provide perspectives for the on-surface synthesis of porous graphene nanostructures, offering a promising strategy for the engineering of graphene materials with tailor-made properties.

Toward Full Zigzag-Edged Nanographenes: peri-Tetracene and Its Corresponding Circumanthracene.

Zigzag-edged nanographene with two rows of fused linear acenes, called as n- peri-acene (n-PA), is considered as a potential building unit in the arena of organic electronics. n-PAs with four ( peri-tetracene, 4-PA), five ( peri-pentacene, 5-PA) or more benzene rings in a row have been predicted to show open-shell character, which would be attractive for the development of unprecedented molecular spintronics. However, solution-based synthesis of open-shell n-PA has thus far not been successful because of the poor chemical stability. Herein we demonstrated the synthesis and characterization of the hitherto unknown 4-PA by a rational strategy in which steric protection of the zigzag edges playing a pivotal role. The obtained 4-PA possesses a singlet biradical character ( y0 = 72%) and exhibits remarkable persistent stability with a half-life time ( t1/2) of ∼3 h under ambient conditions. UV-vis-NIR and electrochemical measurements reveal a narrow optical/electrochemical energy gap (1.11 eV) for 4-PA. Moreover, the bay regions of 4-PA enable the efficient 2-fold Diels-Alder reaction, yielding a novel full zigzag-edged circumanthracene.

π-Extended and Curved Antiaromatic Polycyclic Hydrocarbons.

Synthesis of antiaromatic polycyclic hydrocarbons (PHs) is challenging because the high energy of their highest occupied molecular orbital and low energy of their lowest unoccupied molecular orbital cause them to be reactive and unstable. In this work, two large antiaromatic acene analogues, namely, cyclopenta[pqr]indeno[2,1,7-ijk]tetraphene (CIT, 1a) and cyclopenta[pqr]indeno[7,1,2-cde]picene (CIP, 1b), as well as a curved antiaromatic molecule with 48 π-electrons, dibenzo[a,c]diindeno[7,1,2-fgh:7',1',2'-mno]phenanthro[9,10-k]tetraphene (DPT, 1c), are synthesized on the basis of the corona of indeno[1,2-b]fluorene. These three antiaromatic PHs possess a narrow energy gap down to 1.55 eV and exhibit high kinetic stability under ambient conditions. Moreover, these compounds display reversible electron transfer processes in both the cathodic and anodic regimes. Their cation and anion radicals are characterized by in situ vis-NIR absorption and electron paramagnetic resonance spectroelectrochemistry. The X-ray crystallographic analysis confirms that while CIP and CIT manifest planar structures, DPT shows a curved π-conjugated carbon skeleton. The synthetic strategy starting from ortho-substituted benzene units to construct five-membered rings in this work provides a unique entry to novel pentagon-embedding or curved antiaromatic polycyclic hydrocarbons. In addition, besides the detailed chemical and physical investigations, microscale single-crystal fiber field-effect transistors were also fabricated.

A Stable Saddle-Shaped Polycyclic Hydrocarbon with an Open-Shell Singlet Ground State.

Diindeno-fused bischrysene, a new diindeno-based polycyclic hydrocarbon (PH), was synthesized and characterized. It was elucidated in detailed experimental and theoretical studies that this cyclopenta-fused PH possesses an open-shell singlet biradical structure in the ground state and exhibits high stability under ambient conditions (t1/2 =39 days). The crystal structure unambiguously shows a novel saddle-shaped π-conjugated carbon skeleton due to the steric hindrance of the central cove-edged bischrysene unit. UV/Vis spectral measurements revealed that the title molecule has a very narrow optical energy gap of 0.92 eV, which is consistent with the electrochemical analysis and further supported by density functional theory (DFT) calculations.

Sphere-Like Protein-Glycopolymer Nanostructures Tailored by Polyassociation.

Key parameters allow a reproducible polyassociation between avidin and biotinylated glycopolymers in order to fabricate defined supramolecular nanostructures for future (bio)medical and biotechnological applications. Thus, the polymerization efficiency of biotinylated glycopolymers in the fabrication of biohybrid structures (BHS) was investigated with regard to the influence of (i) the degree of biotinylation of the dendritic glycoarchitectures, (ii) two biotin linkers, (iii) the dendritic scaffold (perfectly branched vs hyperbranched), and (iv) the ligand-receptor stoichiometry. The adjustment of all these parameters opens the way to fabricate defined sizes of the final biohybrid structures as a multifunctional platform ready for their use in different applications. Various analytical techniques, including purification of BHS, were used to gain fundamental insights into the structural properties of the resulting protein-glycopolymer BHS. Finally, the elucidation of pivotal conformational properties of isolated BHS with defined sizes by asymmetrical flow field flow fractionation study revealed that they mainly possess spherical-/star-like properties. From this study, the fundamental knowledge can be likely transferred to other assemblies formed by molecular recognition processes (e.g., adamantane-ß-cyclodextrin).

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation.

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C-H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C-H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at Mn ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities µsat up to 3 cm(2)/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls.

C­H arylation of unsubstituted furan and thiophene with acceptor bromides: access to donor­acceptor­donor-type building blocks for organic electronics.

Pd-catalyzed direct arylation (DA) reaction conditions have been established for unsubstituted furan (Fu) and thiophene (Th) with three popular acceptor building blocks to be used in materials for organic electronics, namely 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2), N,N'-dialkylated 2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) (NDIBr2), and 1,4-dibromotetrafluorobenzene (F4Br2). Reactions with BTBr2, F4Br2, and NDIBr2 require different solvents to obtain high yields. The use of dimethylacetamide (DMAc) is essential for the successful coupling of BTBr2 and F4Br2, but detrimental for NDIBr2, as the electron-deficient NDI core is prone to nucleophilic core substitution in DMAc as solvent but not in toluene. NDIFu2 is much more planar compared to NDITh2, resulting in an enhanced charge-transfer character, which makes it an interesting building block for conjugated systems designed for organic electronics. This study highlights direct arylation as a simple and inexpensive method to construct a series of important donor­acceptor­donor building blocks to be further used for the preparation of a variety of conjugated materials.

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).