Pluripotent Features of Doubly Thiophene-Fused Benzodiphospholes as Organic Functional Materials

Linear ladder-type π-conjugated molecules have attracted much interest because of their intriguing physicochemical properties. To modulate their electronic structures, an effective strategy is to incorporate main-group elements into ladder-type π-conjugated molecules. In line with this strategy, a variety of ladder-type π-conjugated molecules with main-group elements have been synthesized to explore their potential utility as organic functional materials. In this context, phosphole-based π-conjugated molecules are highly attractive, owing to their unique optical and electrochemical properties, which arise from the phosphorus atom. Herein, the synthesis and physicochemical properties of doubly thiophene-fused benzodiphospholes, as a new class of phosphole-based ladder-type π-conjugated molecule, are reported. Systematic investigations into the physicochemical properties of doubly thiophene-fused benzodiphospholes revealed their pluripotent features: intense near-infrared fluorescence, excellent two-photon absorption property, and remarkably high electron-transporting ability. This study demonstrates the potential utility of doubly thiophene-fused benzodiphospholes as organic functional materials for biological imaging, nonlinear optics, organic transistors, and organic photovoltaics.     

Four-coordinate organoboron compounds with a π-conjugated chelate ligand for optoelectronic applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.     

Insulated π-conjugated metallopolymers

Recently, there has been a progressive development of insulated π-conjugated metallopolymers with accumulated features of π-conjugated bridging units, transition metal complexes, and encapsulating moieties, as higher-order functionalized materials. A number of insulated conjugated metallopolymers have been successfully synthesized and their fascinating properties have been reported. In addition to the conventional features derived from π-conjugation and transition metals, their insulated structures can compensate for solubility, a disadvantage in conventional metallopolymers, and enhance their functionalities, such as sensing, luminescence, and conduction. In this review, we summarize the synthetic methodologies, structural characteristics, and functionalities of one-dimensional insulated π-conjugated metallopolymers, while focusing on the effect of transition metals and insulation on their properties.     

Anthracene-Fused Oligo-BODIPYs: A New Class of π-Extended NIR-Absorbing Materials

Large π-conjugated systems are key in the area of molecular materials. Herein, we prepare via Au^I-catalyzed cyclization a series of fully π-conjugated anthracene-fused oligo-BODIPYs. Their structural and optoelectronic properties were studied by several techniques, ranging from X-ray, UV/Vis, and cyclic voltammetry to transient absorption spectroscopy. As a complement, their electronic structures were explored by means of Density Functional Theory (DFT) calculations. Depending on the size and shape of the π-conjugated skeleton, unique features—such as face-to-face supramolecular organization, NIR absorption and fluorescence as well as strong electron accepting character—were noted. All in all, the aforementioned features render them valuable for technological applications.     

Phosphole modified pentathienoacene: synthesis, electronic properties and self-assembly

A series of new ladder π-conjugated materials, phosphole modified pentathienoacene (PO-PTA), are synthesized and characterized. Single-crystal X-ray results demonstrate that methyl-disubstituted PO-PTA forms a face-to-face dimer structure driven by π-π interactions. The investigations of optical properties showed that the oxidized phosphole moiety in this ladder system can effectively narrow the band gap. PO-PTA is a promising building block in π-conjugated polymers and oligomers for optoelectronic applications. The derivative of PO-PTA, obtained by introducing four long alkyl chains, can self-assemble into one-dimensional (1D) fibers based on intermolecular π-π interactions, dipole-dipole interactions and van der Waals interactions. Interestingly, the uniform and well-ordered monolayers were also obtained for PO-PTA derivative on a HOPG (highly oriented pyrolytic graphite) surface.     

Structure–Property Relationships in Bithiophenes with Hydrogen-Bonded Substituents

The use of crystal engineering to control the supramolecular arrangement of π-conjugated molecules in the solid-state is of considerable interest for the development of novel organic electronic materials. In this study, we investigated the effect of combining of two types of supramolecular interaction with different geometric requirements, amide hydrogen bonding and π-interactions, on the π-overlap between calamitic π-conjugated cores. To this end, we prepared two series of bithiophene diesters and diamides with methylene, ethylene, or propylene spacers between the bithiophene core and the functional groups in their terminal substituents. The hydrogen-bonded bithiophene diamides showed significantly denser packing of the bithiophene cores than the diesters and other known α,ω-disubstituted bithiophenes. The bithiophene packing density reach a maximum in the bithiophene diamide with an ethylene spacer, which had the smallest longitudinal bithiophene displacement and infinite 1D arrays of electronically conjugated, parallel, and almost linear N−H⋅⋅⋅O=C hydrogen bonds. The synergistic hydrogen bonding and π-interactions were attributed to the favorable conformation mechanics of the ethylene spacer and resulted in H-type spectroscopic aggregates in solid-state absorption spectroscopy. These results demonstrate that the optoelectronic properties of π-conjugated materials in the solid-state may be tailored systematically by side-chain engineering, and hence that this approach has significant potential for the design of organic and polymer semiconductors.     

Photophysics of π-conjugated oligomers and polymers that contain transition metal complexes

There has been a surge of interest concerning the synthesis, optical and electronic properties of π-conjugated polymers that contain transition metal complexes. The integration of transition metal chromophores that feature metal to ligand charge transfer (MLCT) excited states into a π-conjugated polymer permits easy variation of the material’s optical and electronic properties. In this review, we survey a number of recent photophysical studies that examine π-conjugated oligomer or polymer/transition metal complex hybrids. The effects of the types of π-conjugated backbone, oligomer and polymer structure, the conjugation length and coordination to a variety of metal chromophores on the photophysics of the organic-metal hybrids are discussed. The degree of interaction between the polymer (or oligomer) and metal complex based excited states dramatically modulates the observed photophysics.     

Extended benzodifuran–furan derivatives as example of π-conjugated materials obtained from sustainable approach

The synthesis of extended benzodifuran–furan systems are described as an example of π-conjugated materials prepared by following a green approach with only water being produced as waste and using furan derivatives from renewable sources. Investigation of their optical and electrochemical properties shows that the new compounds present electronic properties compatible for application in organic electronics.     

Electro-functional octupolar π-conjugated columnar liquid crystals

A series of propeller-shaped π-conjugated molecules based on 2,4,6-tris(thiophene-2-yl)-1,3,5-triazines has been designed and synthesized to obtain ambipolar charge-transporting liquid-crystalline materials. The 3-fold electron-donating aromatic units are attached to the electron-accepting triazine core, which forms electro-functional octupolar π-conjugated structures. These octupolar molecules self-organize into one-dimensional columnar nanostructures and exhibit ambipolar carrier transport behavior, which has been revealed by time-of-flight measurements. In this approach, electron-donor and acceptor electro-active segments are assembled individually in each column to give one-dimensional nanostructured materials with precisely tuned electronic properties. Their desirable electronic structures responsible for both hole and electron conductions have also been examined by cyclic voltammetry and theoretical calculations. The present results provide a new guideline and versatile approach to the design of ambipolar conductive nanostructured liquid-crystalline materials.     

Design of the Ionic Organic Nonlinear Optical Material NH4[LiC3H(CH3)O4] with Ultrawide Band Gap and Moderate Birefringence

Ionic organic crystals containing organic planar π-conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π-conjugated [C₃H(CH₃)O₄]²⁻ unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO-favourable layered design, a new ionic organic material, NH₄[LiC₃H(CH₃)O₄], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π-conjugated NLO-active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.     

π-Conjugated Macrocycles Bearing Angle-Strained Alkynes

Angle-strained alkyne-containing π-conjugated macrocycles are attractive compounds both in functional materials chemistry and biochemistry. Their interesting reactivity as well as photophysical and supramolecular properties have been revealed in the past three decades. This review highlights the recent advances in angle-strained alkyne-containing π-conjugated macrocycles, especially their synthetic methods, the bond angles of alkynes (∠_sp at C≡C−C), and their functions. The theoretical and experimental research on cyclo[n]carbons and para-cyclophynes consisting of ethynylenes and para-phenylenes are mainly summarized. Related macrocycles bearing other linkers, such as ortho-phenylenes, meta-phenylenes, heteroaromatics, biphenyls, extended aromatics, are also overviewed. Bond angles of strained alkynes in π-conjugated macrocycles, which are generable, detectable, and isolable, are summarized at the end of this review.     

Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications

Organic and polymeric molecules based on π-conjugated units represent an important class of components for optical and optoelectronic functionalized soft materials. Inspired by the innovative molecular design made by synthetic chemists, new functions and applications of π-conjugated molecules are continuously emerging. However, a challenge that remains is to soften these molecules. Alkylation is a commonly employed synthetic strategy to achieve functionalization in order to improve processability, i.e., solubility in volatile solvents, for better utilization in the rapidly-developing field of organic electronics. In addition it is recognized as a powerful strategy to tune the interaction among the π-conjugated moieties. In a different interpretation of alkylation, alkylated-π compounds can be viewed as a class of hydrophobic amphiphiles, since the rigid π-conjugated moiety and flexible alkyl chains are intrinsically immiscible. Recent studies have shown that such compounds can form a variety of self-organized solid and thermotropic liquid crystalline structures as well as nonassembled liquid forms depending upon the position, number and kinds of attached alkyl chains. Here, we present a brief overview of recent developments of alkylated-π chemistry, with an emphasis on the relationships between molecular design, self-assembly behavior and applications in optical and optoelectronic devices. We hope this review can serve as a guide and reference for people working in different research areas, including self-assembly and colloid sciences, synthetic and materials chemistry was well as organic electronics.     

Electron-Deficient Conjugated Materials via p–π* Conjugation with Boron: Extending Monomers to Oligomers,Macrocycles, and Polymers

The extension of conjugated organoboranes from monomeric species to oligomers, macrocycles, and polymers offers access to a plethora of fascinating new materials. The p–π* conjugation between empty orbitals on boron and the conjugated linkers not only affects the electronic structure and optical properties, but also enables mutual interactions between electron-deficient boron centers. The unique properties of these electron-deficient π-conjugated systems are exploited in highly luminescent materials, organic optoelectronic devices, and sensing applications.     

Intramolecular Energy Transfer in Dithienogermole Derivatives

Dithienogermole (DTG) has been applied as a useful building unit of optical/semiconducting materials for organic optoelectronic devices because of its extended conjugation, high chemical stability, and good emissive properties. Although DTG has two substituents on the Ge atom, the substituents have been limited to simple alkyl and aryl groups in previous work. In this work, to further uncover the new functionalities of this useful building unit, various π-conjugated groups were introduced on Ge of DTG. It was expected that the introduction of π-conjugated groups would give rise to efficient energy transfer between the substituents and the DTG core, which are in proximity and linked by a Ge atom. The thus-prepared DTG compounds with fluorene, terthiophene, and pyrene units on Ge possessed well-separated frontier orbitals on the substituents and the DTG core, as proved by the absorption spectra and DFT calculations. The substituted DTG derivatives showed clear emission only from the energy acceptor even though the energy donor was photoexcited. This indicated the highly efficient energy transfer in these compounds. We also prepared more π-extended compound DTGFl2-Ph with phenyl groups on the DTG thiophene rings. DTGFl2-Ph showed strong emission in the visible region with efficient energy transfer properties. These results clearly indicate the potential application of the present DTG system as optical functional materials.     

Branched charge-transfer chromophores featuring a 4,5-dicyanoimidazole unit

Six branched and stable push-pull chromophores featuring 4,5-dicyanoimidazole as an acceptor moiety, an N,N-dimethylamino group as a donor and various π-conjugated linkers are reported. Systematic extension of the π-linker revealed that the optical and electrochemical properties of A-π-D chromophores are mainly affected by the nature of the π-conjugated backbone (length and planarity) as well as by the number of appended donors.     

New synthetic methods of π-conjugated inclusion complexes with high conductivity

A new method for the synthesis of an insulated π-conjugated molecule was developed via the sequential self-inclusion of π-conjugated guest branched permethylated α-cyclodextrin followed by the elongation of the π-conjugated unit. Covering a single π-conjugated wire by an α-cyclodextrin derivatives can suppress conductance fluctuation. The insulated π-conjugated molecules were utilized in the synthesis of highly conductive zigzag- and functionalized-insulated molecular wires.     

Organophosphorus π-conjugated materials: the rise of a new field

The synthesis and electronic properties of new linear organic π-conjugated systems incorporating phosphole rings are described. Well defined α,α′-(phosphole-thiophene) oligomers possess low HOMO-LUMO gaps and their optical and electrochemical properties can be tuned via chemical modifications of the P-atoms. The physical properties of these compounds make them valuable materials for OLED’s. The coordination ability of phosphole-based dipoles has been exploited for the synthesis of efficient multipolar NLO-phores. Lastly, phospholes have been used for the synthesis of assemblies exhibiting through-bond interaction between two π-systems via P-P σ-skeletons.     

Isomers of B←N-Fused Dibenzo-azaacenes: How B←N Affects Opto-electronic Properties and Device Behaviors?

The B←N unit has a large dipole and it is isoelectronic to C−C moiety with no dipole. Incorporating B←N units into π-conjugated system is a powerful strategy to design organic small molecules and polymers with intriguing opto-electronic properties and excellent opto-electronic device performance. However, it is unclear how the B←N unit affects electronic structures and opto-electronic properties of large π-conjugated molecules. In this work, to address this question, we developed three dibenzo-azaacene molecules in which two B←N units were introduced at different positions. Although the dibenzo-azaacene skeleton is fully π-conjugated, the effect of B←N unit on the electronic structures of the adjacent rings is much stronger than that of the distant rings. As a result, the three molecules with isomerized B←N incorporation patterns possess different electronic structures and exhibit tunable opto-electronic properties. Among the three molecules, the centrosymmetrical molecule exhibits higher LUMO/HOMO energy levels than those of the two axisymmetrical molecules. When used as the active layer in organic field-effect transistors (OFETs), while the two axisymmetrical molecules show unipolar electron transporting property, the centrosymmetrical molecule exhibits ambipolar hole and electron transporting behavior. This work not only deepens our understanding on organoboron π-conjugated molecules, but also indicates a new strategy to tune opto-electronic properties of organic semiconductors for excellent device performance.     

Conjugated macrocycles: concepts and applications

One of the most important objectives in materials, chemical, and physical sciences is the creation of large conjugated macrocycles with well-defined shapes, since such molecules are not only theoretically and experimentally interesting but also have potential applications in nanotechnology. Fully unsaturated macrocycles are regarded as models for infinitely conjugated π systems with inner cavities, and exhibit unusual optical and magnetic behavior. Macrocycles have interior and exterior sites, and site-specific substitution at both or either site can afford attractive structures, such as 1D, 2D, and 3D supramolecular nanostructures. These nanostructures could be controlled through the use of π-extended large macrocycles by a bottom-up strategy. Numerous shape-persistent π-conjugated macrocycles have been synthesized, but only a few are on the nanoscale. This Review focuses on nanosized π-conjugated macrocycles (>1 nm diameter) and giant macrocycles (>2 nm diameter), and summarizes their syntheses and properties.     

Ladder-type P,S-bridged trans-stilbenes

Phosphole-containing π-systems have emerged as building blocks with enormous potential as electronic materials because of the tunability of the phosphorus center. Among these, asymmetric P-bridged trans-stilbenes are still rare, and here an elegant and efficient synthesis toward such fluorescent molecular frameworks is described. Fine-tuning of the photophysical properties is attempted by enforcing the planarization of the phosphorus tripod and thus increasing the interaction between the phosphorus lone pair and the π-system. The electronic structure of the π-conjugated frameworks is analyzed with NMR, UV-vis and fluorescence spectroscopy, and time-dependent density functional theory (TD-DFT) calculations.