Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Tuning the Electronic Properties and Acid‐Response Behavior of N‐Heteroacene‐Based π‐Conjugated Liquids by Changing the Number of π‐Conjugated Substituents

We have designed and synthesized two room‐temperature‐fluorescent π‐conjugated liquids based on the N‐heteroacene framework ( 1 and 2 ). These two π‐conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single‐crystal X‐ray analysis of dithiophene‐appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene‐appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pK_a values in compounds 1 and 2 , these slight structural differences gave rise to marked differences in their acid‐response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pK_a values was dissolved in compounds 1 and 2 , phase transition occurred from an isotropic liquid state to a self‐organized liquid‐crystalline phase.     

Synthesis and properties of through‐space conjugated polymers based on π–π stacked 1,3‐biarylpropane tethering units

Novel skipped‐π polymers in which the π‐components are connected with 2‐substituted trimethylene tethering units exhibit bathochromically shifted, broadened ultraviolet absorption with a unique lower‐energy absorption band and a largely red‐shifted fluorescent emission. These results suggest that through‐space π–π interactions owing to a stair‐like stacking substructure in these polymers extend the π‐conjugation of the components in the ground and excited states. As the photophysical properties of the polymers observed both in a solution and in a dried film are similar to those of the J‐aggregates of π‐molecules, these polymers may be considered as pseudo J‐stacking (or J‐like‐stacking) polymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3412–3419     

Diboryldiborenes: π‐Conjugated B4 Chains Isoelectronic to the Butadiene Dication

B(sp²)–B(sp³) diborane species based on bis(catecholato)diboron and N‐heterocyclic carbenes (NHCs) underwent catechol/bromide exchange selectively at the sp³‐hybridized boron atom. The reduction of the resulting 1,1‐dibromodiborane adducts led to reductive coupling and isolation of doubly NHC‐stabilized 1,2‐diboryldiborenes. These compounds are the first examples of molecules exhibiting π‐electron delocalization over an all‐boron chain.     

Calculated geometries of dications of bis odd‐membered π‐ring systems containing a NCN fragment and related π‐systems. An opposite out‐of‐plane rotation of the 4n π‐ring subsystems

Quantum chemical calculations have been carried out on dications of bis odd‐membered π‐ring systems containing a NCN fragment and related π‐systems. An opposite out‐of‐plane rotation of both subsystems was found if these systems contain 4n π‐electrons (antiaromatic). A planar situation was found for 4n+2 π‐electrons (aromatic). The geometric representations could be compared with X‐ray crystallographic three‐dimensional structures of related compounds. Calculations at different levels clearly show that separation of the σ‐ and π‐electron contribution is an effective way to elucidate the origin of the geometrical changes. We also give attention to some fundamental aspects of the subsystems related to the 1,3‐azolium cations because of their biochemical relevance such as fast C₂? H proton exchange. We postulate that at least two molecules of water are involved in this process. The significance of a trigonal pyramidal (TP) geometry has been emphasized. © 2001 Wiley Periodicals, Inc. Int J Quantum Chem, 2001     

Synthesis, π‐Face‐Selective Aggregation,and π‐Face Chiral Recognition of Configurationally Stable C3‐Symmetric Propeller‐Chiral Molecules with a π‐Core

The C₃‐symmetric propeller‐chiral compounds (P,P,P)‐ 1 and (M,M,M)‐ 1 with planar π‐cores perpendicular to the C₃‐axis were synthesized in optically pure states. (P,P,P)‐ 1 possesses two distinguishable propeller‐chiral π‐faces with rims of different heights named the (P/L)‐face and (P/H)‐face. Each face is configurationally stable because of the rigid structure of the helicenes contained in the π‐core. (P,P,P)‐ 1 formed dimeric aggregates in organic solutions as indicated by the results of ¹H NMR, CD, and UV/Vis spectroscopy and vapor pressure osmometry analyses. The (P/L)/(P/L) interactions were observed in the solid state by single‐crystal X‐ray analysis, and they were also predominant over the (P/H)/(P/H) and (P/L)/(P/H) interactions in solution, as indicated by the results of ¹H and 2D NMR spectroscopy analyses. The dimerization constant was obtained for a racemic mixture, which showed that the heterochiral (P,P,P)‐ 1 /(M,M,M)‐ 1 interactions were much weaker than the homochiral (P,P,P)‐ 1 /(P,P,P)‐ 1 interactions. The results indicated that the propeller‐chiral (P/L)‐face interacts with the (P/L)‐face more strongly than with the (P/H)‐face, (M/L)‐face, and (M/H)‐face. The study showed the π‐face‐selective aggregation and π‐face chiral recognition of the configurationally stable propeller‐chiral molecules.     

Amphiphilic Hybrid π‐Conjugated Polymers Containing Polyhedral Oligomeric Silsesquioxanes

Amphiphilic hybrid π‐conjugated polymers that have polyhedral oligomeric silsesquioxanes on their side chains have been successfully synthesized by the Sonogashira–Hagihara polycondensation reaction. The obtained polymers were studied with ultraviolet‐visible absorption and photoluminescence spectra. In these polymers, the π‐conjugation length was extended along the poly(p‐phenylene‐ethynylene) backbone. Furthermore, the content of the POSS substituents can influence the aggregation behavior of the polymers and subsequent luminescent properties.

     

Unconventional hydrogen bonding and π‐stacking in two substituted pyridine carboxamides

The crystal structures of two para‐substituted aryl derivatives of pyridine‐2‐carboxamide, namely N‐(4‐fluorophenyl)pyridine‐2‐carboxamide, C₁₂H₉FN₂O, (I), and N‐(4‐nitrophenyl)pyridine‐2‐carboxamide, C₁₂H₉N₃O₃, (II), have been studied. Compound (I) exhibits unconventional aryl–carbonyl C—H...O and pyridine–fluorine C—H...F hydrogen bonding in two dimensions and well defined π‐stacking involving pyridine rings in the third dimension. The conformation of (II) is more nearly planar than that of (I) and the intermolecular interactions comprise one‐dimensional aryl–carbonyl C—H...O hydrogen bonds leading to a stepped or staircase‐like progression of loosely π‐stacked molecules. The close‐packed layers of planar π‐stacked molecules are related by inversion symmetry. Two alternating interplanar separations of 3.439 (1) and 3.476 (1) Å are observed in the crystal lattice and are consistent with a repetitive packing sequence, ABA′B′AB…, for the π‐stacked inversion pairs of (II).     

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

MP2/aug‐cc‐pVTZ calculations were performed on complexes of boron and aluminum trihydrides and trihalides with hydrogen cyanide (ZH₃‐NCH and ZX₃‐NCH; Z=B, Al; X=F, Cl). The complexes are linked through the B???N and Al???N interactions, which are named as triel bonds and which are classified as π‐hole bonds. It was found that they possess numerous characteristics of typical covalent bonds, since they are ruled mainly by processes of the electron charge shift from the Lewis base to the Lewis acid unit. Other configurations of the ZH₃‐NCH and ZX₃‐NCH complexes linked by the dihydrogen, hydrogen, and halogen bonds were found. However, these interactions are much weaker than the corresponding π‐hole bonds. The quantum theory of atoms in molecules and the natural bond orbital approaches were applied to characterize the complexes and interactions analyzed. The crystal structures of triel trihydrides and triel trihalides were also analyzed for comparison with the results of calculations.     

A Highly Efficient Near‐Infrared‐Emissive Copolymer with a N=N Double‐Bond π‐Conjugated System Based on a Fused Azobenzene–Boron Complex

Fused azobenzene–boron complexes (BAzs) show highly efficient near‐infrared (NIR) emission from the nitrogen–nitrogen double bond (N=N) containing π‐conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron–nitrogen (B?N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor–acceptor (D–A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λ_PL=751 nm, Φ_PL=0.25) and in the film (λ_PL=821 nm, Φ_PL=0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation‐induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π‐conjugated materials.     

Arsole‐Containing π‐Conjugated Polymer by the Post‐Element‐Transformation Technique

A synthetic method to obtain an arsole‐containing π‐conjugated polymer by the post‐transformation of the organotitanium polymer titanacyclopentadiene‐2,5‐diyl unit with an arsenic‐containing building block is described. The UV/Vis absorption maximum and onset of the polymer were observed at 517 nm and 612 nm, respectively. The polymer exhibits orange photoluminescence with an emission maximum (E_max) of 600 nm and the quantum yield (Φ) of 0.05. The polymer proved to exhibit a quasi‐reversible redox behavior in its cyclic voltammetric (CV) analysis. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were estimated to be ?5.43 and ?3.24 eV, respectively, from the onsets for oxidation and reduction signals in the CV analysis. Further chemical modification of the arsole unit in the π‐conjugated polymer by complexation of gold(I) chloride occurred smoothly resulting in the bathochromic shift of the UV/Vis absorption and lowering of the LUMO energy level.     

Synthesis,Characterization and Optical Properties of a Novel Si‐Containing σ‐π‐Conjugated Hyperbranched Polymer

The polymerization of bis(4‐ethynylphenyl)methylsilane catalyzed by RhI(PPh₃)₃ afforded a regio‐ and stereoregular hyperbranched polymer, hb‐poly[(methylsilylene)bis(1,4‐phenylene‐trans‐vinylene)] (poly( 1 )), containing 95% trans‐vinylene moieties. The weight loss of this polymer at 900°C in N₂ was 9%. Poly( 1 ) displayed an absorption due to π‐π* transition around 275 nm as a shoulder and a weak absorption around 330 nm due to π‐to‐σ charge transfer, which was hardly seen in the corresponding linear polymer.     

Through‐Space Conjugated Molecular Wire Comprising Three π‐Electron Systems

A [2.2]paracyclophane‐based through‐space conjugated oligomer comprising three π‐electron systems was designed and synthesized. The arrangement of three π‐conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Förster mechanism. The energy transfer efficiency and rate constants were estimated to be >0.999 and >10¹² s^?1, respectively. The key point for the efficient energy transfer is the orientation of the transition dipole moments. The time‐dependent density functional theory (TD‐DFT) studies revealed the transition dipole moments of each stacked π‐electron system; each dipole moment was located on the long axis of each stacked π‐electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).     

Advances in Star‐Shaped π‐Conjugated Systems: Properties and Applications

Until recently π‐conjugated organic materials are based mainly on linear systems. Recent years, however, have brought about increasing interest in molecules boasting a dendritic, branched, or star‐shaped architecture. This tendency is a direct result of the ongoing search for materials with progressively better properties. Such compounds, featuring novel, 3D architectures, exhibit a multitude of interesting qualities, making them stand out from well‐known materials. The direction of star‐shaped compound application is determined by whether they are able to form aggregates, π‐stacks. This feature is a source of some astounding properties, coveted in numerous applications. Among this class of compounds high charge mobility, high fluorescence efficiency, and good charge separation are all found. Depending on the structure of the core, the molecule may adopt various types of symmetry. Similarly, the conjugation of orbitals may extend over the whole structure or be interrupted at chosen segments. The number of papers pertaining to star‐shaped oligomers and polymers is ascending with each year, evidencing a growing interest in them. Consequently, this Review focuses particularly on the most recent reports concerning modification of the structure and properties of the aforementioned type of compounds, as well as on the development of devices based on them.

     

Jumping Crystals of Pyrene Tweezers: Crystal‐to‐Crystal Transition Involving π/π‐to‐CH/π Assembly Mode Switching

Crystals of pyrene tweezers 1 with interdigitating pyrenyl blades jump vigorously at around 160 °C. Single‐crystal X‐ray diffraction analysis before jumping revealed the presence of a “pyrene tetrad” in the crystal lattice, where four pyrenyl blades are π ‐stacked on top of each other. Upon heating the crystal to induce the jumping event, inner two pyrenyl blades in the “pyrene tetrad” probably rotate to switch off their π ‐stacking interaction with the neighboring outer pyrenyl blades and form new CH−π bonds. Different from reported salient crystals, our crystal jumps with the release of CHCl₃ as inclusion solvent.     

On‐Surface Self‐Assembly of a C3‐Symmetric π‐Conjugated Molecule Family Studied by STM: Two‐Dimensional Nanoporous Frameworks

The on‐surface self‐assembled behavior of four C ₃‐symmetric π‐conjugated planar molecules ( Tp , T12 , T18 , and Ex ) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen‐bonded 2D on‐surface self‐assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self‐assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.     

Unexpected Luminescence Behavior of Coinage Metal π‐Diborene Complexes

A number of unprecedented photophysical phenomena were observed in the study of luminescent π‐diborene complexes of Cu and Ag. These observations included unusually high fluorescence quantum yields (up to 100%) in solution for complexes of these metals. This result indicates that very little or no intersystem crossing between S₁ and T_n occurs in the complexes, despite the strong spin–orbit coupling of the metal atoms. The replacement of carbon with boron thus yields luminescent isolobal analogues of otherwise non‐emissive olefin complexes of Cu and Ag.     

Effect of Extended π‐Conjugation Structure of Donor–Acceptor Conjugated Copolymers on the Photoelectronic Properties

New donor–acceptor conjugated copolymers based on alkylthienylbenzodithiophene (BDTT) and alkoxynaphthodithiophene (NDT) have been synthesized and compared with their benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based analogues to investigate the effect of the extended π conjugation of the polymer main chain on the physicochemical properties of the polymers. A systematic investigation into the optical properties, energy levels, field‐effect transistor characteristics, and photovoltaic characteristics of these polymers was conducted. Both polymers demonstrated enhanced photovoltaic performance and increased hole mobility compared with the BDT‐based analogue. However, the BDTT‐based polymer (with π‐conjugation extension perpendicular to main chain) gave the highest power conversion efficiency of 5.07 % for the single‐junction polymer solar cell, whereas the NDT‐based polymer (with π‐conjugation extension along the main chain) achieved the highest hole mobility of approximately 0.1 cm² V^?1 s^?1 based on the field‐effect transistor; this indicated that extending the π conjugation in different orientations would have a significant influence on the properties of the resulting polymers.     

Boron(III) Carbazosubphthalocyanines: Core‐Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues

Condensation of 1,8‐diamino‐3,6‐dichlorocarbazole with a series of disubstituted 1,3‐diiminoisoindolines, followed by treatment with BF₃?OEt₂ led to the formation of the corresponding core‐expanded boron(III) subphthalocyanine analogues. These air‐stable π‐conjugated boron(III) carbazosubphthalocyanines possess two boron‐containing seven‐membered‐ring units and a 16 π‐electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single‐crystal X‐ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone‐shaped structure, the π skeleton of this compound is almost planar.     

Synthesis of π‐Conjugated Benzocyclotrimers

Polycyclic aromatic hydrocarbons (PAHs), especially three branchphene benzocyclotrimers represent a series of molecules with intriguing physical and chemical properties. Benzocyclotrimers are also important precursors to construct fullerenes and graphenes. In this article, we review the recent progress in the preparation methods of π‐conjugated benzocyclotrimers. In particular, cyclotrimerization reactions to construct varying shaped and edged benzocyclotrimers are illustrated. Various typical characterization methods for these materials, such as variable‐temperature ¹H‐NMR, single crystal X‐ray analysis, density functional theory (DFT) calculations and atomic force microscope (AFM) measurements are included for discussion.