Twisted but Conjugated: Building Blocks for Low Bandgap Polymers

Bis(tricyclic) Aromatic Enes That Exhibit Efficient Fluorescence in the Solid State

We report herein that bis(tricyclic) aromatic enes (BAEs) consisting of 6-6-6-membered frameworks such as acridine, xanthene, thioxanthene, and thioxanthene-S,S-dioxide act as a new class of organic luminophores that exhibit blue-to-green fluorescence in the solid state and in polymer film with good to excellent quantum yields. The BAEs were prepared by the palladium-catalyzed double cross-coupling reaction of phenazastannines or 10,10-dimethyl-10H-phenothiastannin with 9-(dibromomethylene)xanthene, 9-(dibromomethylene)thioxanthene, or 9-(dibromomethylene)-9H-thioxanthene-10,10-dioxide. Microcrystals or powder samples of the BAEs exhibited brilliant fluorescence with good to high quantum yields (Φ = 0.45-0.88). Furthermore, more efficient emission of blue-to-green light (Φ = 0.59-0.91) was observed for the BAEs dispersed in the poly(methyl methacrylate) (PMMA) films. Density functional theory (DFT) calculations suggest that the photo-absorption of the (thio)xanthene moiety-containing BAEs proceeds via π-π* transitions, whereas the optical excitation of 10,10-dioxido-9H-thioxanthene moiety-containing BAEs involves an intramolecular charge transfer from the acridine/thioxanthene part to the electron-accepting 10,10-dioxido-9H-thioxanthene moiety.

Twisted diphenoquinones fused with thiophene rings: thiophene analogs of bianthrone

The twisted conformer of bistricyclic aromatic enes (BAEs) has a small HOMO-LUMO gap owing to the twisted double bond. In this study, we synthesized diphenoquinones fused with thiophene rings as a new twisted conformer-predominant BAE. They exhibited deep LUMO energy levels and apparent n-type semiconductor properties.

Synthesis of Fused-Ring Derivatives Containing Bifluorenylidene Units via Pd-Catalyzed Tandem Multistep Suzuki Coupling/Heck Cyclization Reaction

A series of bifluorenylidene derivatives has been facilely synthesized via a palladium-catalyzed tandem multistep Suzuki cross-coupling and Heck cyclization in one pot. The tandem reaction involves two sequential Suzuki couplings and a subsequent Heck cyclization. The target products are obtained in good yields up to 94%. Even in an extended conjugated substrate, the anticipated products can be prepared.

Introducing an orthogonally polarized electron-rich alkene: synthesis of a zwitterionic boron-containing π-conjugated system

The synthesis of an alkene is reported which is concurrently twisted (twist angle = 86.6(8)°), push-pull (dipole moment = 7.48 D), and electron-rich (E1/2 = -1.45 V and -0.52 V vs. Fc/Fc+) in nature, comprising a unique trinity combination for the alkene class of compounds. Subsequently, this newly synthesized alkene-motif was used as a donor for the synthesis of a zwitterionic boron-containing π-conjugated compound (dipole moment = 12.17 D) through an intramolecular charge transfer process exploiting the π-conjugated donor-acceptor system.

Twisted Push-Pull Alkenes Bearing Geminal Cyclicdiamino and Difluoroaryl Substituents

The systematic combination of N-heterocyclic olefins (NHOs) with fluoroarenes resulted in twisted push-pull alkenes. These alkenes carry electron-donating cyclicdiamino substituents and two electron-withdrawing fluoroaryl substituents in the geminal positions. The synthetic method can be extended to a variety of substituted push-pull alkenes by varying the NHO as well as the fluoroarenes. Solid-state molecular structures of these molecules reveal a notable elongation of the central C-C bond and a twisted geometry in the alkene motif. Absorption properties were investigated with UV-vis spectroscopy. The redox properties of the twisted push-pull alkenes were probed with electrochemistry as well as UV-vis/NIR and EPR spectroelectrochemistry, while the electronic structures were computationally evaluated and validated.

Access to Highly Luminescent N-Doped Diazaborepins with Penta-, Hexa-, and Heptagon Substructures

Two diazaborepins (BNN1 and BNN2) have been accomplished via a highly efficient pathway in the step-economic transformation. Oxidative dimerization of carbazole derivatives followed by borylation reaction gave heterocyclic diazaborepin as the key building block. Replacement of carbon moiety in traditional borepins with N-N functionality led to a significant red shift of their emissions up to 585 nm with strong red color as solids. This work offered an approach to diazaborepin-based materials for applications in light-emitting field.

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non-Planar Conjugated Organic Molecules

Non-planar conjugated organic molecules (NPCOMs) contain π-conjugation across their length and also exhibit asymmetry in their conformation. In other words, certain molecular fragments in NPCOMs are either twisted or curved out of planarity. This conformational asymmetry in NPCOMs leads to non-uniform charge-distribution across the molecule, with important photophysical and electronic consequences such as altered thermodynamic stability, chemical reactivity, as well as materials properties. Majorly, NPCOMs can be classified as having either Fused or Rotatable architectures. NPCOMs have been the focus of significant scientific attention in the recent past due to their exciting photophysical behavior that includes intramolecular charge-transfer (ICT), thermally activated delayed fluorescence (TADF) and long-lived charge-separated states. In addition, they also have many useful materials characteristics such as biradical character, semi-conductivity, dynamic conformations, and mechanochromism. As a result, rational design of NPCOMs and mapping their structure-property correlations has become imperative. Researchers have executed conformational changes in NPCOMs through a variety of external stimuli such as pH, temperature, anions-cations, solvent, electric potential, and mechanical force in order to tailor their photophysical, optoelectronic and magnetic properties. Converging to these points, this review highlights the lucrative electronic features, photophysical traits and upcoming applications of NPCOMs by a selective survey of the recent scientific literature.

Ring-expansion approach towards extended asymmetric benzopentafulvalenes: overcrowded olefinic structure and chain length-dependent properties

A strain energy-induced ring-expansion is described to construct novel asymmetric benzopentafulvalenes precisely, which show chain length-dependent physical properties and air-stable ambipolar carrier transport.

The synthesis and property investigation of π-bridged 9,9′-bifluorenylidene ladder as an electron acceptor

We synthesized a ladder-type 9,9′-bifluorenylidene dimer with good electron acceptability and demonstrated the π-bridged insertion exhibited significant effects on photophysical and electrochemical properties compared with its macrocyclic counterpart.

Cobalt-Nitrogen-Doped Helical Carbonaceous Nanotubes as a Class of Efficient Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is of significant importance in the development of fuel cells. Now, cobalt-nitrogen-doped chiral carbonaceous nanotubes (l/d-CCNTs-Co) are presented as efficient electrocatalysts for ORR. The chiral template, N-stearyl-l/d-glutamic acid, induces the self-assembly of well-arranged polypyrrole and the formation of ordered graphene carbon with helical structures at the molecular level after the pyrolysis process. Co was subsequently introduced through the post-synthesis method. The obtained l/d-CCNTs-Co exhibits superior ORR performance, including long-term stability and better methanol tolerance compared to achiral Co-doped carbon materials and commercial Pt/C. DFT calculations demonstrate that the charges on the twisted surface of l/d-CCNTs are widely separated; as a result the Co atoms are more exposed on the chiral CCNTs. This work gives us a new understanding of the effects of helical structures in electrocatalysis.

On the Electrochromic Properties of Borepins: A Computational Prediction

The spectroelectrochemical features of some recently synthesized borepins have been predicted herein using the methods based on density functional theory. The computed electronic spectra of neutral, radical anion, and dianion species clearly suggest that these molecules can be used as new electrochromic materials. The excellent agreement with the available structural and absorption experimental data for the neutral systems made us confident for the results obtained for charged species and suggests their potential use as electrochromic materials.

Electrochemical amphotericity and NIR absorption induced via the step-wise protonation of fused quinoxaline-tetrathiafulvalene-pyrroles

We describe an effective approach to producing electrochemical amphoteric character and tuning optical properties.

A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C-H borylation

Heteroatom doping into polyaromatic hydrocarbons (PAHs) is a powerful approach for modifying key physical properties, however, there are extremely few modular routes that enable facile formation of B-, B2- and B,N-(specifically not containing direct B-N bonds) doped PAHs despite the growing importance of these materials. Sequential, one pot borylative cyclisation/intramolecular electrophilic C-H borylation of naphthyl-alkynes provides a simple new route to access novel B-, B,N- and B2-doped (PAHs). The initial products, dihydronaphthalene/dihydroquinoline B-mesityl PAHs, were reacted with [Ph3C][BF4]/pyridyl base to form the oxidised B-, and B,N-doped PAHs. However, for B-triisopropylphenyl (Trip) PAH congeners oxidation has to be performed prior to Trip installation due to preferential oxidation of an isopropylaryl moiety to the styrene. This alternative sequence enables access to Trip-B-PAHs and to structurally constrained B and B2-PAHs. Analysis of the solid state structures and optoelectronic properties of these PAHs confirm that frontier orbital energies, extended packing structures, Stokes shift and quantum yields all can be rationally modified using this methodology. The simplicity of this synthetic approach makes it a powerful tool for rapidly generating novel bench stable boron doped PAHs, which is important for facilitating further structure-property relationship studies and the wider utilisation of these materials in optoelectronic applications.

A fluorenylidene-acridane that becomes dark in color upon grinding - ground state mechanochromism by conformational change

We report mechanochromic color change controlled by conformational change (folded and twisted conformers) of fluorenylidene-acridanes (FAs).

We report mechanochromic color change controlled by conformational change (between folded and twisted conformers) of fluorenylidene-acridanes (FAs). FAs with four N-alkyl groups (methyl, ethyl, n-butyl and n-octyl) were synthesized via the Barton–Kellogg reaction of diazofluorene and electrophilic N-tert-butoxy carbonyl thioacridone, deprotection of the tert-butoxy carbonyl group gives fluorenylacridine, and alkylation on the nitrogen atom is done using alkyl tosylate or triflate. FAs were characterized by NMR, UV-vis absorption and photoluminescence spectroscopy, theoretical calculation, cyclic voltammetry, and powder and single-crystal X-ray analyses. The color and folded/twisted conformation of the FAs were changed by the choice of substituent on the nitrogen atom, physical state (solution or solid), and morphology (crystalline or amorphous). Grinding of N-methyl FA solids, using an agate mortar, caused the morphology to change from a crystalline to amorphous state, which induced a conformational change from the folded to the twisted conformer, and a mechanochromic color change from yellow to dark green. The reverse color change, along with a morphological and conformational change to the folded conformer, was performed by solvent vapor exposure (chloroform). The twisted and folded conformers showed ambipolar (hole/electron) and hole-only transport properties, respectively.

A reversible single-molecule switch based on activated antiaromaticity

Single-molecule electronic devices provide researchers with an unprecedented ability to relate novel physical phenomena to molecular chemical structures. Typically, conjugated aromatic molecular backbones are relied upon to create electronic devices, where the aromaticity of the building blocks is used to enhance conductivity. We capitalize on the classical physical organic chemistry concept of Hückel antiaromaticity by demonstrating a single-molecule switch that exhibits low conductance in the neutral state and, upon electrochemical oxidation, reversibly switches to an antiaromatic high-conducting structure. We form single-molecule devices using the scanning tunneling microscope-based break-junction technique and observe an on/off ratio of ~70 for a thiophenylidene derivative that switches to an antiaromatic state with 6-4-6-π electrons. Through supporting nuclear magnetic resonance measurements, we show that the doubly oxidized core has antiaromatic character and we use density functional theory calculations to rationalize the origin of the high-conductance state for the oxidized single-molecule junction. Together, our work demonstrates how the concept of antiaromaticity can be exploited to create single-molecule devices that are highly conducting.

Protonation tuning of quantum interference in azulene-type single-molecule junctions

The protonation of azulene cores offers significant conductance tuning in single-molecule junctions with quantum interference.

The protonation of azulene derivatives with quantum interference effects is studied by the conductance measurements of single-molecule junctions. Three azulene derivatives with different connectivities are synthesized and reacted with trifluoroacetic acid to form the protonated states. It is found that the protonated azulene molecular junctions produce more than one order of magnitude higher conductance than the neutral states, while the molecules with destructive interference show more significant changes. These experimental observations are supported by our recently-developed parameter free theory of connectivity, which suggests that the largest conductance change occurs when destructive interference near the Fermi energy in the neutral state is alleviated by protonation.

The effect of a highly twisted CC double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

A highly twisted CC double bond elicits changes in the physicochemical properties of π-systems.

Enhancing electron affinity and tuning band gap in donor-acceptor organic semiconductors by benzothiadiazole directed C-H borylation

Electrophilic borylation using BCl3 and benzothiadiazole to direct the C-H functionalisation of an adjacent aromatic unit produces fused boracyclic materials with minimally changed HOMO energy levels but significantly reduced LUMO energy levels. In situ alkylation and arylation at boron using Al(alkyl)3 or Zn(aryl)2 is facile and affords boracycles that possess excellent stability towards protic solvents, including water, and display large bathochromic shifts leading to far red/NIR emission in the solid state with quantum yields of up to 34%. Solution fabricated OLEDs with far red/NIR electroluminescence are reported with EQEs > 0.4%.

Modulating structure and properties in organic chromophores: influence of azulene as a building block

The properties of isomeric azulene derivatives, substituted through the 5-membered ring, were examined using a combination of experimentation and theoretical calculations for a series of well-defined electroactive oligomers.