Π-conjugated Materials Research Articles

Multi-stimuli-responsive fluorescence of axially chiral 4-ene-β-Diketones

A unique series of simple, smart, and chiral binaphthalene-substituted 4-ene-β-diketones molecules has been designed and prepared. Their optical properties, charge contribution, and transition process highly depend on their chemical structures. These π-conjugated materials are highly emissive in both solution and solid (emission quantum yield up to 68%), owing to the inhibition of enol-keto tautomerization and the effect of steric hindrance from binaphthalene. Through ethylenic bond hydrolysis, they can be used for not only cation/anion sensing but also chiral amino acids recognition. Moreover, at low concentrations, they have little cytotoxicity to living cells and can stain cytoplasm. Therefore, they afford a new platform in the design of multi-stimuli-responsive, smart, and chiral materials.

Reductive methylation of triphthaloylbenzene: Isolation and characterization of hexamethoxy-trinaphthylene and two unexpected trinaphthylene derivatives

With the aim of developing new π-conjugated materials for application in electronics, the reductive methylation of triphthaloylbenzene is presented. Several new products are obtained: 5,6,11,12,17,18-hexamethoxy-trinaphthylene (1a) and two unexpected compounds, i.e., the corresponding monohydroxy-pentamethoxy-trinaphthylene (1b) and a peculiar trinaphtylene derivative (2) possessing hydroxy, methoxy, carbonyl, and ketal groups and a tetrahydrofuran moiety fused on the core. X-ray crystallographic analysis of 1b and 2 reveals that both compounds exhibit deformed frameworks. The investigation of their optical properties shows that the lowest-energy absorption bands for 1a and 1b are a shoulder band around 350 nm, whereas 2 gives rise to a one broad and weak band at 573 nm. All the molecules have negligible fluorescence. Addition of 1,8-diazabicyclo[5.4.0]undec-7-ene into a solution of 2 causes a color change from purple to olive brown, which renders it suitable as a pH indicator.

Sustainable Access to π-Conjugated Molecular Materials via Direct (Hetero)Arylation Reactions in Water and under Air.

Direct (hetero)arylation (DHA) is playing a key role in improving the efficiency and atom economy of C–C cross coupling reactions, so has impacts in pharmaceutical and materials chemistry. Current research focuses on further improving the generality, efficiency and selectivity of the method through careful tuning of the reaction conditions and the catalytic system. Comparatively fewer studies are dedicated to the replacement of the high-boiling-point organic solvents dominating the field and affecting the overall sustainability of the method. We show herein that the use of a 9:1 v/v emulsion of an aqueous Kolliphor 2 wt% solution while having toluene as the reaction medium enables the preparation of relevant examples of thiophene-containing π-conjugated building blocks in high yield and purity.

Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

Upon photon absorption, π-conjugated organics are apt to undergo ultrafast structural reorganization via electron-vibrational coupling during non-adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self-trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited-state symmetry-dependent electron-vibrational coupling. By accessing two high-lying excited states, one-photon and two-photon allowed states, a clear discrepancy in the initial time-resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited-state symmetry on ultrafast structural reorganization and exciton self-trapping in the emerging class of π-conjugated materials is provided.

Impact of Chirality on Hydrogen-Bonded Supramolecular Assemblies and Photoconductivity of Diketopyrrolopyrrole Derivatives.

Hydrogen bonds can efficiently guide the self-assembly of organic materials, enabling to tune the properties of the aggregation processes. In the case of π-conjugated materials, several parameters such as temperature, concentration and solvent can be used to modify the aggregation state while tuning the optoelectronic properties. Chirality can be included within the impacting parameters due to the differences in molecular packing. Here, chiral and achiral thiophene-capped diketopyrrolopyrrole derivatives were designed and synthesized containing amide bonds, with the aim to study the interplay between chiral assemblies and their stabilization through hydrogen-bonding. Differences in aggregation properties were observed with spectroscopy and microscopy, and a contactless microwave-based technique was used to study their intrinsic charge carrier mobility. The positive role of hydrogen-bonding has been highlighted and the differences between chiral and achiral compounds have been elucidated.

Recent Advances in π-Conjugated N^C-Chelate Organoboron Materials.

Boron-containing π-conjugated materials are archetypical candidates for a variety of molecular scale applications. The incorporation of boron into the π-conjugated frameworks significantly modifies the nature of the parent π-conjugated systems. Several novel boron-bridged π-conjugated materials with intriguing structural, photo-physical and electrochemical properties have been reported over the last few years. In this paper, we review the properties and multi-dimensional applications of the boron-bridged fused-ring π-conjugated systems. We critically highlight the properties of π-conjugated N^C-chelate organoboron materials. This is followed by a discussion on the potential applications of the new materials in opto-electronics (O-E) and other areas. Finally, attempts will be made to predict the future direction/outlook for this class of materials.

Side Chain Regioisomers that Dictate Optical Properties and Mechanofluorochromism through Crystal Packing

Designing strong interactions into π-conjugated materials can direct their typically uncontrolled molecular packing and is an important strategy toward tuning their technologically relevant propert...

Synthesis of mesoscale ordered two-dimensional π-conjugated polymers with semiconducting properties.

Two-dimensional materials with high charge carrier mobility and tunable band gaps have attracted intense research effort for their potential use in nanoelectronics. Two-dimensional π-conjugated polymers constitute a promising subclass because the band structure can be manipulated by varying the molecular building blocks while preserving key features such as Dirac cones and high charge mobility. The major barriers to the application of two-dimensional π-conjugated polymers have been the small domain size and high defect density attained in the syntheses explored so far. Here, we demonstrate the fabrication of mesoscale ordered two-dimensional π-conjugated polymer kagome lattices with semiconducting properties, Dirac cone structures and flat bands on Au(111). This material has been obtained by combining a rigid azatriangulene precursor and a hot dosing approach, which favours molecular diffusion and eliminates voids in the network. These results open opportunities for the synthesis of two-dimensional π-conjugated polymer Dirac cone materials and their integration into devices.

Efficient Spin Selectivity in Self-Assembled Superhelical Conducting Polymer Microfibers.

Chiral materials, natural or synthetic, have been widely studied since Pasteur's separation of enantiomers over a century ago. The connection between electron transmission and chirality was, however, established recently where one spin was preferably selected by the chiral molecules, displaying a typical chirality-induced spin selectivity (CISS) effect. Currently, this CISS effect was mainly demonstrated in the molecular-scale devices. Herein, we explored this effect in a microscale device where an efficient spin selectivity was found in the self-assembled superhelical conducting polyaniline (PANI) microfibers. A spin-selective efficiency up to 80% (not magnetoresistance) was achieved when spins traversed the ca. 2-6 μm-long helical channels at room temperature. Importantly, the long-range ordering of chiral PANI molecules is crucial to observe this efficient spin selectivity, whereas no selective transmission was found in the "amorphous" chiral PANIs. This efficient spin selectivity was subsequently rationalized by using an extended Su-Schrieffer-Heeger model where the Rashba spin-orbit coupling was considered. We expect these results could inspire the research of organic spintronics by using molecularly ordered, self-assembled, and chiral π-conjugated materials.

Guest-oriented non-fullerene acceptors for ternary organic solar cells with over 16.0% and 22.7% efficiencies under one-sun and indoor light

Abstract Given that the ring-fusion strategy can effectively tail the electrical, optical, and structural properties of π-conjugated materials, we have herein developed Y-series non-fullerene acceptors with a dual acceptor–donor–dual acceptor-type structure, bearing non-fused bare bithiophene (Y–Th2), hexagonal ring-fused bithiophene (Y-BDT), or pentagonal ring-fused bithiophene (Y-CDT) central donor units. Several characterization techniques were applied to perform a comparative investigation of their optical and physical properties and frontier energy levels with respect to the different ring-fusion of the central units. Moreover, for the cascade energy level alignment, the synthesized acceptors were employed as a third component in the PM6:Y6 that served as a binary host platform. The introduction of the optimal amount of Y–Th2 or Y-BDT into the host system improved the device performance. Specifically, Y–Th2 exhibited the best power conversion efficiency (PCE) (16.01%) along with improved photovoltaic parameters, whereas the addition of Y-CDT impaired the PCE. Moreover, the optimized Y–Th2-based ternary organic solar cell achieved a PCE of 22.72% under indoor illumination at 1000 lux. Thus, the in-depth structural, morphological, and electrical analyses not only established a structure–property correlation, but also provided better design criteria for the guest-oriented non-fullerene acceptors for ternary photovoltaic applications, especially for Y6-containing systems.

Dopant-free hole transporting materials with supramolecular interactions and reverse diffusion for efficient and modular p-i-n perovskite solar cells

The rational design of dopant-free organic hole-transporting layer (HTL) materials is still a challenge for realizing high-efficient and stable p-i-n planar perovskite solar cells (pero-SCs). Here, we synthesized two π-conjugated small-molecule HTL materials through tailoring the backbone and conjugated side chain to carefully control molecular conformation. The resultant BDT-TPA- s Th containing a planar fused benzo[1,2- b :4,5- b′ ]dithiophene (BDT) core and a conjugated thiophene side chain showed the planar conformation. X-ray crystallography showed a favorable stacking model in solid states under the parallel-displaced π-π and additional S-π weak-bond supramolecular interactions, thus achieving an obviously increased hole mobility without dopants. As an HTL material in p-i-n planar pero-SCs, the marginal solubility of BDT-TPA- s Th enabled inverse diffusion into the perovskite precursor solution for assisting the subsequent perovskite film growth and passivating the uncoordinated Pb2+ ion defects. As a result, the planar p-i-n pero-SCs exhibited a champion power conversion efficiency (PCE) of 20.5% and enhanced moisture stability. Importantly, the BDT-TPA- s Th HTL material also showed weak thickness-photovoltaic dependence, and the pero-SCs with blade-coated BDT-TPA- s Th as a HTL achieved a 15.30% PCE for the 1-cm2 modularized device. This HTL material design strategy is expected to pave the way toward high-performance, dopant-free and printing large-area planar p-i-n pero-SCs.

Enforced Planar FOX-7-like Molecules: A Strategy for Thermally Stable and Insensitive π-Conjugated Energetic Materials.

Exploring new energetic derivatives of 1,1-diamino-2,2-dinitroethylene (FOX-7) is still a key aspect in the field of energetic materials. However, so far most of the attention has been focused on modification of FOX-7 via different reaction strategies. Now we report the design of three new FOX-7-like compounds (3-5) where one nitro group in FOX-7 is replaced by a nitrogen-rich heterocyclic ring. Each of them is characterized by single-crystal X-ray crystallography. Electronic structures are studied through computational methods in comparison with FOX-7. In addition, the chemical reactivity of 3 was also investigated. Its hydroxylammonium (7), hydrazinium (8), and ammonium (9) salts were prepared, and the nitrate product (10) was also isolated. Compound 10 has a C-N bond length of 1.577 Å that is one of the longest values found for the C-NO2 bond. It was found that the incorporation of a tetrazole or triazole ring into the backbone of a conjugated nitroenamine does lead to a planar structure, which not only enhances the thermal stability but also improves the sensitivity of the product.

Room-temperature Pd/Ag direct arylation enabled by a radical pathway.

Direct arylation is an appealing method for preparing π-conjugated materials, avoiding the prefunctionalization required for traditional cross-coupling methods. A major effort in organic electronic materials development is improving the environmental and economic impact of production; direct arylation polymerization (DArP) is an effective method to achieve these goals. Room-temperature polymerization would further improve the cost and energy efficiencies required to prepare these materials. Reported herein is new mechanistic work studying the underlying mechanism of room temperature direct arylation between iodobenzene and indole. Results indicate that room-temperature, Pd/Ag-catalyzed direct arylation systems are radical-mediated. This is in contrast to the commonly proposed two-electron mechanisms for direct arylation and appears to extend to other substrates such as benzo[b]thiophene and pentafluorobenzene.

Clar Rules the Electronic Properties of 2D π-Conjugated Frameworks: Mind the Gap.

The key electronic properties of a family of 2D frameworks structurally convergent with holey graphenes were studied. The bandgap of these materials decreases monotonically with size, showing a common trend with anthracenes and kekulenes. This was rationalized by Clar's sextet rule, which reveals a direct relationship between the molecular systems and the 2D frameworks. In addition, a detailed benchmark against experimental data showcased the high quality of the models, which reproduce accurately available electronic properties. Overall, it was shown that DFT can be used to screen and understand the intrinsic bandgaps and electrochemistry potentials for technological applications prior to the synthesis of π-conjugated porous materials.

Structure‐Independent Conductance of Thiophene‐Based Single‐Stacking Junctions

The experimental investigation of intermolecular charge transport in π-conjugated materials is challenging. Herein, we describe the investigation of charge transport through intermolecular and intramolecular paths in single-molecule and single-stacking thiophene junctions by the mechanically controllable break junction (MCBJ) technique. We found that the ability for intermolecular charge transport through different single-stacking junctions was approximately independent of the molecular structure, which contrasts with the strong length dependence of conductance in single-molecule junctions with the same building blocks, and the dominant charge-transport path of molecules with two anchors transited from an intramolecular to an intermolecular path when the degree of conjugation increased. An increase in conjugation further led to higher binding probability owing to the variation in binding energies, as supported by DFT calculations.

Highly Efficient and Tunable Filtering of Electrons' Spin by Supramolecular Chirality of Nanofiber-Based Materials.

Organic semiconductors and organic-inorganic hybrids are promising materials for spintronic-based memory devices. Recently, an alternative route to organic spintronic based on chiral-induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical π-conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin-filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both "up" and "down" orientations of filtered spins can be obtained in a single enantiopure system via the temperature-dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure-property relationship for spintronic applications.

Fred Wudl. A giant in π-conjugated materials

Qichun Zhang, Dmitrii F. Perepichka, Hong Meng and Yongsheng Chen introduce the Materials Chemistry Frontiers themed issue celebrating Professor Fred Wudl's 80th birthday.

Direct C—H Arylation-Derived π-Conjugated Functional Materials for Device Applications

Direct C—H Arylation-Derived π-Conjugated Functional Materials for Device Applications

Recent advances in dithiafulvenyl-functionalized organic conjugated materials

This review highlights the recent studies of advanced organic π-conjugated materials that contain 1,4-dithiafulvene (DTF) as a redox-active component.

Heteroatom-bridged heterofluorenes: a theoretical study on molecular structures and optoelectronic properties

Heteroatom incorporation is highly effective in tuning the molecular structures and optoelectronic properties of conjugated organic molecules. Here, we performed systematic theoretical studies on heteroatom-bridged heterofluorenes (BXFs) constructed by double heteroatom bridges of biphenyl to reveal the effects of heavily incorporated heteroatoms on molecular architecture and π-conjugation for different optoelectronic properties. Nine novel BXFs in three series were investigated and all of them exhibit promising potential optoelectronic properties owing to their highly fused molecular structure with heavy π-conjugation, although the introduction of different types and numbers of heteroatoms will lead to varied properties. Moreover, spiropolymers of BSiF and BGeF polymerized at the bridging position were also designed for the first time and found to have attractive optoelectronic properties of poly(BXF)s inherited from their monomers, demonstrating further the effectiveness of the bis-heteroatom introduction strategy in the construction of high-performance optoelectronic polymers. This heteroatom introduction strategy in constructing highly rigid π-conjugated materials could be applicable to other systems, representing a new concept advance to design novel conjugated small molecules and polymers for high-performance optoelectronic applications.