Multi-Functional 2D Covalent Organic Frameworks with Diketopyrrolopyrrole as Electron Acceptor.

2D covalent organic framework (COF) materials with extended conjugated structure and periodic columnar π-arrays exhibit promising applications in organic optoelectronics. However, there is a scarcity of reports on optoelectronic COFs, mainly due to the lack of suitable π-skeletons. Here, two multi-functional optoelectronic 2D COFs DPP-TPP-COF and DPP-TBB-COF are constructed with diketopyrrolopyrrole as electron acceptor (A), and 1,3,6,8-tetraphenylpyrene and 1,3,5-triphenylbenzene as electron donor (D) through imine bonds. Both 2D COFs showed good crystallinities and AA stacking with a rhombic framework for DPP-TPP-COF and hexagonal one for DPP-TBB-COF, respectively. The electron D-A and ordered intermolecular packing structures endow the COFs with broad UV-vis absorptions and narrow bandgaps along with suitable HOMO/LUMO energy levels, resulting in multi-functional optoelectronic properties, including photothermal conversion, supercapacitor property, and ambipolar semiconducting behavior. Among them, DPP-TPP-COF exhibits a high photothermal conversion efficiency of 47% under 660 nm laser irradiation, while DPP-TBB-COF exhibits superior specific capacitance of 384 F g-1. Moreover, P-type doping and N-type doping are achieved by iodine and tetrakis(dimethylamino)ethylene on a single host COF, resulting in ambipolar semiconducting behavior. These results provide a paradigm for the application of multi-functional optoelectronic COF materials.

Radical Cation Salts of Hetera-Buckybowls: Polar Crystals, Negative Thermal Expansion and Phase Transition.

Radical cation salts of π-conjugated polycycles are rich in physical properties. Herein, two kinds of hetera-buckybowls, ethoxy-substituted trithiasumanene (3SEt) and triselenasumanene (3SeEt), are synthesized as electron donors. Galvanostatic oxidation of them affords radical cation salts (3SEt)5 (TTFMPB)3 , (3SeEt)5 (TTFMPB)3 , (3SEt)4 PMA, and (3SeEt)4 PMA, where PMA is Keggin-type phosphomolybdate and TTFMPB is tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate. In these salts, 3SEt/3SeEt are partially charged and show distinct conformation change with the site charge and counter anions. In TTFMPB salts, (TTFMPB)- forms hexagonal channels that accommodate the packing columns of 3SEt/3SeEt. In particular, (3SEt)5 (TTFMPB)3 adopts the R3c space group and is a polar crystal with the columns of 3SEt all in the up-bowl direction. The PMA salts of 3SEt/3SeEt are polar crystals (C2 space group) with 3SEt/3SeEt being planar and forming columnar stacks. (3SeEt)4 PMA shows a structural modulation below 200 K, namely, negative thermal expansion (NTE) of the unit cell volume and enlargement of the intermolecular distances between neighboring 3SeEt molecules. The four salts are semiconductors with an activation energy of 0.18-0.38 eV. The conductivity of (3SeEt)4 PMA shows a reversible transition upon cooling and heating, in accordance to the NTE structural modulation. This work paves the way toward conducting materials based on hetera-buckybowls.

Transforming Hetera-Buckybowls into Chiral Conjugated Polycycles Incorporating Epoxycyclooctadiene: a Two-Step Approach.

Chiral π-conjugated polycycles have garnered increasing attention due to versatile applications in optoelectronic materials and biological sciences. In this study, we report the synthesis of chiral π-conjugated polycycles incorporating a chiral epoxycyclooctadiene moiety. Our synthetic strategy capitalizes on the novel reactions of hetera-buckybowl triselenasumanene (TSS) and is achieved in two-step manner. Firstly, the TSS is regio-selectively transformed into its ortho-quinone form. Subsequently, the nucleophilic addition reactions of TSS ortho-quinone by phenylethynides are metal ion-dependent. When utilizing (phenylethynyl)magnesium bromide as the nucleophile, two phenylethynyls are furnished onto the edged benzene ring of TSS. When the nucleophile is (phenylethynyl)lithium, a cascade of nucleophilic addition, intermolecular electron-transfer, ring-opening, and tetradehydro-Diels-Alder (TDDA) reactions occur sequentially in one-pot, ultimately affording chiral π-conjugated polycycles featuring the epoxycyclooctadiene moiety as an integral part of their backbones. This work represents a step forward in the synthesis of chiral π-conjugated polycycles using TSS as synthon.

Hyperbranched Diketopyrrolopyrrole-based Polymers Constructed via Linear Side-Chains towards Organic Field-Effect Transistors.

Conjugated polymers with high charge mobilities have drawn increasing attention in organic field-effect transistors (OFETs) in recent years. However, OFETs of conjugated polymers with high mobility and good device stability remain a challenge. In this article, we report a hyperbranched polymer approach to improve the charge mobility and device stability. Three hyperbranched diketopyrrolopyrrole-based polymers were designed and synthesized via linear alkyl side-chain linkers. The results show that 2D topological hyperbranched polymers form stable thin film microstructures, and thus improve the device stability, since the conjugated moiety is interconnected by linear alkyl chain. Besides, the incorporation of linear alkyl chain instead of branching alkyl one reduce steric hindrance, and improve the microstructure ordering as well as the charge mobility. Bar-coated OFETs result demonstrates that the devices mobilities and operational stabilities (bias stability and bending resistance) are both improved. All these indicate that hyperbranched polymer is a potential candidate for future application.

Non-symmetric Half-Fused B←N Coordinated Diketopyrrolopyrrole Building Block for n-type Semiconducting Polymers.

The development of conjugated polymers with high semiconducting performance and high reliability is of great significance for flexible electronics. Herein, we developed a new type of electron-accepting building block; i.e., non-symmetric half-fused B←N coordinated diketopyrrolopyrrole (DPP) (HBNDPP), for amorphous conjugated polymers toward flexible electronics. The rigid B←N fusion part of HBNDPP endows the resulting polymers with decent electron transport, while its non-symmetric structure causes the polymer to exhibit multiple conformation isomers with flat torsional potential energies. Thus, it gets packed in an amorphous manner in solid state, ensuring good resistance to bending strain. Combined with hardness and softness, the flexible organic field-effect transistor devices exhibit n-type charge properties with decent mobility, good bending resistance, and good ambient stability. The preliminary study makes this building block a potential candidate for future design of conjugated materials for flexible electronic devices.

Marriage of Heterobuckybowls with Triptycene: Molecular Waterwheels for Separating C60 and C70.

Exploration of π-conjugated polycycles, particularly those have π-frameworks spread over the three-dimensional space, is essential in materials science and synthetic chemistry as these chemical entities possess featured optoelectronic properties and supramolecular assembly. Herein, the bowl-shaped trichalcogenasumanenes are fused onto three branches of triptycene through pyrazine units, affording waterwheel-like three-dimensional polycycles 4 a/4 b. Because the three branches on 4 a/4 b are chemically equal, the molecular orbitals of 4 a/4 b show degenerate feature that results in the strong UV-Vis absorbance at steady state. 4 a/4 b exhibit photo-induced charge-separation and subsequent charge-redistribution at transient state, leading to excited state absorption in NIR-II window (1165-1400 nm). 4 a/4 b are excellent fullerene receptors, and they form 1 : 1 host-guest complexes with C60 /C70 as proved by spectroscopic titrations and single crystal structure analysis. Moreover, 4 a/4 b show much stronger affinity toward C70 than C60 . Consequently, 4 a/4 b are able to separate C60 and C70 from their mixture, giving the purity of C60 up to 99.5 %.

Monoazadichalcogenasumanenes: Synthesis, Structures, and Ring Reconstruction via Atom Transfer under Acidic Conditions.

Buckybowls have unique properties that can be tailored by embedding main-group elements into their π-scaffolds. Herein, a synthetic approach is developed for producing monoazadichalcogenasumanenes (4 a/4 b, 6 a/6 b, 7 a/7 b) derived from sumanene by replacing its three benzylic carbons with one nitrogen and two chalcogen atoms (S for 4 a/4 b, Se for 6 a/6 b, Te for 7 a/7 b). Monoazadichalcogenasumanenes are deeper π-bowls than trichalcogensumanenes as the C-N bond is much shorter than C-X (X=S, Se, Te). The bowl-depth of 4 b (0.95 Å) is greater than that of corannulene (0.85 Å). The nitrogen atom donates electron density to the entire π-system that makes monoazadichalcogenasumanenes electron-rich. They undergo ring reconstruction of chalcogenophene ring via transferring a chalcogen atom from one molecule to another under acidic conditions. The nitrogen and chalcogen atoms play crucial role on this reaction.

Bottom-Up Preparation of Twisted Graphene Nanoribbons by Cu-Catalyzed Deoxygenative Coupling.

Graphene nanoribbons (GNRs) are promising in organic optoelectronic materials, and their properties largely depend on the size, edge, and conformation. Herein, the fully armchair-edged GNRs (AGNRs) with lengths up to 2.65 nm by using a Cu-catalyzed deoxygenative coupling as a key step. The resulting AGNRs (2HBT, 3HBT, and 4HBT) possess highly twisted π-scaffolds, and the torsion angles between the adjacent triphenylene moieties are larger than 32°, as proved by crystallographic analyses. Theoretical and spectroscopic studies show that the butoxy groups endow AGNRs with electron-rich features, the extension of the π-system from 2HBT to 4HBT reinforces S0 →S1 excitation, and the distortion of the π-scaffold enhances the fluorescence quantum yield (ΦF ). In particular, 4HBT has the lowest oxidation potential (Eox 1 =0.55 V vs. SCE) and displays red fluorescence with a ΦF value of 81 %.

Synthesis and derivatization of hetera-buckybowls.

Buckybowls have concave and convex surfaces with distinct π-electron cloud distribution, and consequently they show unique structural and electronic features as compared to planar aromatic polycycles. Doping the π-framework of buckybowls with heteroatoms is an efficient scheme to tailor inherent properties, because the nature of heteroatoms plays a pivotal role in the structural and electronic characteristics of the resulting hetera-buckybowls. The design, synthesis, and derivatization of hetera-buckybowls open an avenue for obtaining fascinating organic entities not only of fundamental importance but also of promising applications in optoelectronics. In this review, we summarize the advances in hetera-buckybowl chemistry, particularly the synthetic strategies toward these scaffolds.

Nonadditive Transport in Multi-Channel Single-Molecule Circuits.

As stated in the classic Kirchhoff's circuit laws, the total conductance of two parallel channels in an electronic circuit is the sum of the individual conductance. However, in molecular circuits, the quantum interference (QI) between the individual channels may lead to apparent invalidity of Kirchhoff's laws. Such an effect can be very significant in single-molecule circuits consisting of partially overlapped multiple transport channels. Herein, an investigation on how the molecular circuit conductance correlates to the individual channels is conducted in the presence of QI. It is found that the conductance of multi-channel circuit consisting of both constructive and destructive QI is significantly smaller than the addition of individual ones due to the interference between channels. In contrast, the circuit consisting of destructive QI channels exhibits an additive transport. These investigations provide a new cognition of transport mechanism and manipulation of transport in multi-channel molecular circuits.

Tetrathiafulvalene-Fused Heterabuckybowl: Protonation-Induced Electron Transfer and Self-Sensitized Photooxidation.

The electron donor tetrathiafulvalene (D1 ) was fused onto the electron-rich heterabuckybowl trichalcogenasumanene (D2 ) through an electron-deficient pyrazine unit (A) to give 1 c, 1 d, 2 c, and 2 d, featuring the D1 -A-D2 structure. Both D1 and D2 play a pivotal role in intramolecular charge-transfer (ICT) transitions, consequently 1 c, 2 d, 2 c, and 2 d show a broad ICT band at 450-720 nm in steady state. They exhibit two charge-separated transient states, CS1 and CS2 , that appear in sequence. CS1 has a short lifetime (542 fs), and the D1 moiety on CS1 is in the radical cation state with an absorption maximum (λmax ) at 889 nm. CS1 then converts into CS2 (λmax , 1105 nm) through an ICT between D1 .+ and D2 , affording D1 (1-δ).+ and D2 δ.+ . Compounds 1 c, 1 d, 2 c, and 2 d show protonation-induced intramolecular electron transfer that leads to absorption at λ=700-1300 nm. Owing to the existence of an electron-rich C=C bond on the D1 moiety and in situ generation of 1 O2 by the pyrazine-fused D2 moiety, compounds 1 c, 1 d, 2 c, and 2 d display self-sensitized photooxidation in 50 s.

Reversible Thermochromism and Strong Ferromagnetism in Two-Dimensional Hybrid Perovskites.

Two-dimensional (2D) hybrid perovskites have shown many attractive properties associated with their soft lattices and multiple quantum well structure. Herein, we report the synthesis and characterization of two new multifunctional 2D hybrid perovskites, (PED)CuCl4 and (BED)2 CuCl6 , which show reversible thermochromic behavior, dramatic temperature-dependent conductivity change, and strong ferromagnetism. Upon temperature change, the (PED)CuCl4 and (BED)2 CuCl6 crystals exhibit a reversible color change between yellow and red-brown. The associated structural changes were monitored by in situ temperature-dependent powder X-ray diffraction (PXRD). The (BED)2 CuCl6 exhibits superior thermal stability, with a thermochromic working temperature up to 443 K. The conductivity of (BED)2 CuCl6 changes over six orders of magnitude upon temperature change. The 2D perovskites exhibit ferromagnetic properties with Curie temperatures around 13 K.

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Doping Sumanene with Both Chalcogens and Phosphorus(V): One-Step Synthesis, Coordination, and Selective Response Toward AgI.

Heterasumanenes 4-6 containing chalcogen (S, Se, and Te) and phosphorus atoms have been synthesized in a one-pot reaction from trichalcogenasumanenes 1-3 by replacing one chalcogen atom with a P=S unit. The P=S unit makes 4-6 almost planar and shrinks the HOMO-LUMO gap as compared to 1-3. The bonding between Ag+ and S atom on P=S brings about a distinct change to the optical properties of 4-6; 4 in particular shows a selective fluorescence response toward Ag+ with LOD of 0.21 µm. Compounds 4-6 form complexes with AgNO3 to be (4)2 ⋅AgNO3 , (5)2 ⋅AgNO3 , and (6)2 ⋅(AgNO3 )3 . In complexes, the coordination between Ag+ and P=S is observed, which leads to shrinkage of C-P and C-X (X=S, Se, Te) bond lengths. As a result, 4, 5, and 6 are all bowl-shaped in complexes with bowl-depths reaching to 0.66 Å, 0.42 Å, and 0.40 Å, respectively. There are Ag-Te dative bonds between Ag+ and Te atom on telluorophene in (6)2 ⋅(AgNO3 )3 .

Distinguishing Diketopyrrolopyrrole Isomers in Single-Molecule Junctions via Reversible Stimuli-Responsive Quantum Interference.

Distinguishing structural isomers at the single-molecule level remains a challenge. We report the single-molecule recognition of two diketopyrrolopyrrole containing isomers (SDPP and SPPO) employing the mechanically controllable break junction technique. The single-molecule conductances of the two isomers are indistinguishable under normal conditions. However, reversible protonation and deprotonation of the SPPO in molecular junction result in more than 1 order of magnitude conductance change, which dramatically enhances the conductance difference between the two isomers. Theoretical study reveals that the dramatic conductance switching is due to reversible quantum interference effect. It is suggested that combination of stimuli-response and quantum interference can be an efficient strategy to enhance isomer recognition and conductance switching in single-molecule junctions.

Dissecting Trichalcogenasumanenes: π-Bowl to Planar, Invertible Curvature, and Chiral Polycycles.

The buckybowl trichalcogenasumanenes show cleavage of flanking benzene ring upon oxidation, which leads their dissection by fusing various amidine moieties onto peripheral region. By gradually increasing the ring size of amidine from five- to six- and seven-membered, the molecule engineering results in the [7-5-6]-, [7-6-6]-, and [7-7-6]-fused polycycles. Three systems are distinct in the molecular geometries, packing motifs, and optoelectronic properties. The [7-5-6]-fused case adopts the flat backbone, displays strong emission with the fluorescence quantum yield up to 52.3 %, and undergoes a two-photon absorption process. The [7-6-6]-fused one is of a curvature with molecular geometry inversion, forms a tight stack of curved π-system, shows broad absorption extended to 700 nm, and exhibits the p-type semiconducting behavior with hole mobility of 4.4×10-3  cm2 V-1 s-1 . The [7-7-6]-fused one possesses the highly twisted skeleton to show stable chirality, and exhibits red emission in both solution and solid state. The surgery on trichalcogenasumanene is a promising approach to create polycycles with diverse functionalities.

Trichalcogenasumanene ortho-Quinones: Synthesis, Properties, and Transformation into Various Heteropolycycles.

The regioselective transformation of heterobuckybowl trichalcogenasumanenes 1 a,b at peripheral butoxy groups afforded trichalcogenasumanene ortho-quinones 2 a,b. Compounds 2 a,b are distinct from 1 a,b in terms of their molecular geometry and electronic state; that is, they have a shallower bowl depth and show absorbance in the NIR region. The reaction of 2 a,b with diamines resulted in a variety of heteropolycycles, including molecular spoon 3 a-6 a, planar π-systems 3 b-6 b, and highly twisted [7-6-6]-fused systems 7 a,b. These new heteropolycycles had different optical/electrical properties: 4 a,b showed hole mobility of approximately 0.002 cm2 V-1 s-1 , 6 a displayed red emission in both solution and the solid state, and 7 a,b formed tight stacks of the curved π-surface.

4,5,9,10-Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two-Photon Excited Emission.

The design and synthesis of high-performance n-type organic semiconductors are important for the development of future organic optoelectronics. Facile synthetic routes to reach the K-region of pyrene and produce 4,5,9,10-pyrene diimide (PyDI) derivatives are reported. The PyDI derivatives exhibited efficient electron transport properties, with the highest electron mobility of up to 3.08 cm2 V-1 s-1 . The tert-butyl-substituted compounds (t-PyDI) also showed good one- and two-photon excited fluorescence properties. The PyDI derivatives are a new family of aromatic diimides that may exhibit both high electron mobility and good light-emitting properties, thus making them excellent candidates for future optoelectronics.

Ring reconstruction on a trichalcogenasumanene buckybowl: a facile approach to donor-acceptor-type [5-6-7] fused planar polyheterocycles.

The transformation of trichalcogenasumanene buckybowls into donor-acceptor-type [5-6-7] fused polyheterocycles is disclosed. The strategy involves a highly efficient ring-opening of the flanking benzene upon oxidation at room temperature, and facile ring closure by functional-group transformation. Crystallographic studies indicate that the resulting [5-6-7] fused polyheterocycles possess a planar conformation owing to the release of ring strain by expansion of one of the six-membered flanking rings to the seven-membered one. Additionally, the [5-6-7] fused polyheterocycles bear electron-withdrawing groups, which reduce the HOMO-LUMO energy gap, and display broad absorption bands extending to λ=590 nm. Consequently, these compounds show strong red emission with fluorescence quantum yields of up to 38 %.

Copper ion salts of arylthiotetrathiafulvalenes: synthesis, structure diversity and magnetic properties.

The combination of CuBr2 and arylthio-substituted tetrathiafulvalene derivatives (1-7) results in a series of charge-transfer (CT) complexes. Crystallographic studies indicate that the anions in the complexes, which are derived from CuBr2, show diverse configurations including linear [Cu(I)Br2](-), tetrahedral [Cu(II)Br4](2-), planar [Cu(II)2Br6](2-), and coexistence of planar [Cu(II)Br4](2-) and tetrahedral [Cu(II)Br3](-) ions. On the other hand, the TTFs show either radical cation or dication states that depend on their redox potentials. The central TTF framework on most of TTFs is nearly planar despite the charge on them, whereas the two dithiole rings on molecule 4 in complex 4·CuBr4 are significantly twisted with a dihedral angle of 38.3°. The magnetic properties of the complexes were elucidated. The temperature-dependent magnetic susceptibility of complex 5·Cu2Br6 shows the singlet-triplet transition with coupling constant J = -248 K, and that of 3·(CuBr4)0.5·CuBr3·THF shows the abrupt change at 270 K caused by the modulation of intermolecular interactions. The thermo variation of magnetic susceptibility for the other complexes follows the Curie-Weiss law, indicating the weak antiferromagnetic interaction at low temperature.