Alternated π‐conjugated polymers based on a 1,2‐diiminocyclohexane chiral unit for nitroaromatics sensing

The detection of 2,4‐dinitrotoluene (DNT) by fluorescence quenching of a new class of polyimines consisting in π‐conjugated segments regularly alternated with chiral C₂ symmetry units has been studied for solutions and thin films. Their photophysical properties and their sensitivity towards DNT detection has been compared to those of a small model molecule incorporating the same π‐conjugated segment. In solution, all the compounds exhibit the same photo‐physical properties and sensitivity towards DNT detection. In contrast, for thin films, better performances are observed in static conditions for this new class of polyimines compared to the small model molecule. It seems that C₂ symmetry units prevent from the stacking of the π‐conjugated segments and provide in addition to high fluorescence signal an improved diffusion of the analyte inside the films. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4141–4149, 2009     

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.     

Influence of Polymer Electronics on Selective Dispersion of Single‐Walled Carbon Nanotubes

The separation and isolation of semiconducting and metallic single‐walled carbon nanotubes (SWNTs) on a large scale remains a barrier to many commercial applications. Selective extraction of semiconducting SWNTs by wrapping and dispersion with conjugated polymers has been demonstrated to be effective, but the structural parameters of conjugated polymers that dictate selectivity are poorly understood. Here, we report nanotube dispersions with a poly(fluorene‐co‐pyridine) copolymer and its cationic methylated derivative, and show that electron‐deficient conjugated π‐systems bias the dispersion selectivity toward metallic SWNTs. Differentiation of semiconducting and metallic SWNT populations was carried out by a combination of UV/Vis‐NIR absorption spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and electrical conductivity measurements. These results provide new insight into the rational design of conjugated polymers for the selective dispersion of metallic SWNTs.     

Dissymmetrical U‐Shaped π‐Stacked Supramolecular Assemblies by Using a Dinuclear CuI Clip with Organophosphorus Ligands and Monotopic Fully π‐Conjugated Ligands

Reactions between the U‐shaped binuclear Cu^I complex A that bears short metal–metal distances and the cyano‐capped monotopic π‐conjugated ligands 1 – 5 that carry gradually bulkier polyaromatic terminal fragments lead to the formation of π‐stacked supramolecular assemblies 6 – 10 , respectively, in yields of 50–80 %. These derivatives have been characterized by multinuclear NMR spectroscopic analysis and X‐ray diffraction studies. Their solid‐state structures show the selective formation of U‐shaped supramolecular assemblies in which two monotopic π‐conjugated systems present large ( 6 , 7 , and 9 ) or medium ( 8 and 10 ) intramolecular π overlap, thus revealing π–π interactions. These assemblies self‐organize into head‐to‐tail π‐stacked dimers that in turn self‐assemble to afford infinite columnar π stacks. The nature, extent, and complexity of the intermolecular contacts within the head‐to‐tail π‐stacked dimer depend on the nature of the terminal polyaromatic fragment carried by the cyano‐capped monotopic ligand, but it does not alter the result of the self‐assembling process. These results demonstrate that the dinuclear molecular clip A that bears short metal–metal distances allows selective supramolecular assembly processes driven by the formation of intra‐ and intermolecular short π–π interactions in the resulting self‐assembled structures; thus, demonstrating that their shape is not only dictated by the symmetry of the building blocks. This approach opens perspectives toward the formation of extended π‐stacked columns based on dissymmetrical and functional π‐conjugated systems.     

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.     

Silicon‐Containing Formal 4π‐Electron Four‐Membered Ring Systems: Antiaromatic,Aromatic, or Nonaromatic?

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π‐electron systems of 2,4‐disila‐1,3‐diphosphacyclobutadiene (compound 1 ) and the tetrasilacyclobutadiene dication (compound 2 ). The calculated nucleus‐independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π‐electron four‐membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π‐electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π‐orbital splitting. This ligand‐induced π‐orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system.     

From Open‐Shell Singlet Diradicaloid to Closed‐Shell Global Antiaromatic Macrocycles

A dithieno[a,h]‐s‐indacene‐ (DTI‐) based diradicaloid DTI‐2Br was synthesized and its open‐shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI‐MC3 and tetramer DTI‐MC4 turned out to be closed‐shell compounds with global antiaromaticity, which was supported by X‐ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D‐ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin–spin coupling along the π‐conjugated macrocycles. The dications of DTI‐MC3 and DTI‐MC4 turned out to be open‐shell singlet diradical dications, with a singlet–triplet energy gap of ?2.90 and ?2.60 kcal mol^?1, respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin–spin interactions play important roles on electronic properties of π‐conjugated macrocycles.     

Robust and high‐resolution simulations of nonlinear electrokinetic processes in variable cross‐section channels

We present a model and an associated numerical scheme to simulate complex electrokinetic processes in channels with nonuniform cross‐sectional area. We develop a quasi‐1D model based on local cross‐sectional area averaging of the equations describing unsteady, multispecies, electromigration‐diffusion transport. Our approach uses techniques of lubrication theory to approximate electrokinetic flows in channels with arbitrary variations in cross‐section; and we include chemical equilibrium calculations for weak electrolytes, Taylor–Aris type dispersion due of nonuniform bulk flow, and the effects of ionic strength on species mobility and on acid–base equilibrium constants. To solve the quasi‐1D governing equations, we provide a dissipative finite volume scheme that adds numerical dissipation at selective locations to ensure both unconditional stability and high accuracy. We couple the numerical scheme with a novel adaptive grid refinement algorithm that further improves the accuracy of simulations by minimizing numerical dissipation. We benchmark our numerical scheme with existing numerical schemes by simulating nonlinear electrokinetic problems, including ITP and electromigration dispersion in CZE. Simulation results show that our approach yields fast, stable, and high‐resolution solutions using an order of magnitude less grid points compared to the existing dissipative schemes. To highlight our model's capabilities, we demonstrate simulations that predict increase in detection sensitivity of ITP in converging cross‐sectional area channels. We also show that our simulations of ITP in variable cross‐sectional area channels have very good quantitative agreement with published experimental data.     

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.     

Versatile Self‐Complexing Compounds Based on Covalently Linked Donor–Acceptor Cyclophanes

A range of covalently linked donor–acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5‐dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the π‐donor, and 4) cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the π‐accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The π‐donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self‐complexing/decomplexing equilibria were shown to be highly temperature dependent when the π‐donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable‐temperature ¹H NMR spectroscopy. The negative ΔH° and ΔS° values account for the fact that the “uncomplexed” conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor–acceptor interactions. The arm's in‐and‐out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self‐complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF‐containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self‐complexing compounds could find applications as thermo‐ and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.     

L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations Stabilized by Cyclic (alkyl)(amino)carbenes

Alkynes usually oligomerize to give rings with a conjugated π‐electron system. In contrast, phosphaalkynes, R?C≡P, frequently give compounds with polycyclic structures, which are thermodynamically more stable than the corresponding π‐conjugated isomers. The syntheses of the first C₃P₃ tricyclic compounds are reported with either radical or cationic ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono‐radicals CAAC‐CP^.. The mechanism for the formation of these tricarbontriphosphide radicals has been rationalized by a combination of experiments and DFT calculations.     

Conformational Switching of π‐Conjugated Junctions from Merocyanine to Cyanine States by Solvent Polarity

Directed by the solvent polarity, the prevalent conformation of a polymethine dye bearing a branched π‐conjugated junction can be switched from a heptamethine donor–acceptor (DA) merocyanine‐type π‐conjugated system to a nonamethine DAD cyanine‐type π‐conjugated scaffold. Concomitantly the absorption maximum shifts from 585 nm in dichloromethane to 748 nm in methanol solution.     

Thieno[3,4‐c]phosphole‐4,6‐dione: A Versatile Building Block for Phosphorus‐Containing Functional π‐Conjugated Systems

A versatile phosphorus‐containing π‐conjugated building block, thieno[3,4‐c]phosphole‐4,6‐dione (TPHODO), has been developed. The utility of this simple but hitherto unknown building block has been demonstrated by preparing novel functional organophosphorus compounds and bandgap‐tunable conjugated polymers.     

Synthesis of π‐Functional Molecules through Oxidation of Aromatic Amines

In this review, we focus on the synthesis of π‐conjugated functional molecules by the oxidation of aromatic amines, which is one of the most effective methods for the construction of C?C, C?N, and N?N bonds between two π‐conjugated molecular units, and consider their characteristics and applications. Polyanilines are the most common products of the oxidation of aromatic amines; however, azobenzenes, phenazines, and 1,1′‐binaphthyl‐2,2′‐diamines may be produced in this manner also, depending on the reaction conditions. Recent advances in the methodology of aniline oxidation have led to the development of high‐regioselectivity industrial‐scale syntheses of optically or electroactive π‐functional dyes containing nitrogen atoms. In particular, the regioselective fusion of π‐extended aromatic amines can be used to prepare distorted π‐conjugated molecules under mild reaction conditions, allowing the construction of unprecedented curved nitrogen‐containing π‐conjugated molecules.     

A Dicyanomethylene‐Substituted Triangulene: Effects of Molecular‐Symmetry Reduction and Electron‐Accepting Substituents on a Fused Polycyclic Neutral π‐Radical System

A triangulene‐based C₂‐symmetric 33 π‐conjugated stable neutral π‐radical, 2^. , which possesses two dicyanomethylene groups and one oxo group, has been designed, synthesized, and isolated as an analogue of tris(dicyanomethylene) derivative 1^. and trioxo derivative TOT^. with C₃ symmetry. Effects of molecular‐symmetry reduction and electron‐accepting substituents on this fused polycyclic neutral π‐radical system were studied in terms of their molecular structure, electronic‐spin structure, and electrochemical and optical properties with the help of theoretical calculations. Interestingly, this system ( 2^. ) has a four‐stage redox ability, like TOT^. , as well as low frontier energy levels and a small SOMO–LUMO gap, similar to 1^. , in spite of the loss of the degenerate LUMOs in symmetry‐lowered 2^. , which is associated with the attachment of the weaker electron‐accepting oxo group instead of the dicyanomethylene group in 1^. . These prominent results are attributable to the structural and electronic properties in the triangulene‐based highly delocalized fused polycyclic neutral π‐radical system.     

Side Methyl Groups Control the Conformation and Contribute to Symmetry Breaking of Isoprenoid Chromophores

Ab initio DFT computations reveal that the essential structural and photophysical features of the conjugated π‐electron system of retinal and carotenoids are dictated by “innocent” methyl substituents. These methyl groups shape the conformation and symmetry of the isoprenoid chromophores by causing a sigmoidal distortion of the polyene skeleton and increasing its flexibility, which facilitates fitting to their binding pockets in proteins. Comparison of in vacuo conformations of the chromophores with their native (protein‐bound) conformations showed, surprisingly, that the peripheral groups and interactions with the protein environment are much less significant than the methyl side groups in tuning their structural features. The methyl side groups also contribute to a loss of symmetry elements specific to linear polyenes. In effect, the symmetry‐imposed restrictions on the chromophore electronic properties are disabled, which is of tremendous relevance to their photophysics. This is evidenced by their non‐negligible permanent dipole moments and by the simulated and experimentally measured circular dichroism spectra, which necessarily reflect the chirality of the conjugated π‐electron system.     

Conjugated circuits currents in hexabenzocoronene and its derivatives formed by joining proximal carbons

We report on calculated CC bond currents for a dozen derivatives of hexabenzocoroenene in which one or more proximal carbon atoms at the molecular periphery have been bridged. The approach that we use is graph‐theoretical in nature, following our outline of this method in 2003, which is based on finding all conjugated circuits in all Kekulé valence structures of these molecules. To the π‐electrons having 4n + 2 π‐electrons are assigned anticlockwise π‐electron currents and to conjugated circuits having 4n π‐electrons are assigned π‐electron currents. One may summarize the results reported in this work by stating that CC bond currents in the compounds considered decrease on going from peripheral rings to the central ring of the molecule, and also that CC bond currents decrease by insertion of bridges to proximal peripheral benzenoid rings. © 2012 Wiley Periodicals, Inc.     

Controlling the Emergence and Shift Direction of Mechanochromic Luminescence Color of a Pyridine‐Terminated Compound

In this study, mechanochromic luminescence was induced in a complex of mechano‐inactive compounds. Dye/acid complexes containing the same π‐conjugated backbones were prepared. While the luminophore showed blue and red shifts in photoluminescence spectra when combined with different acids by grinding, it exhibited slight mechanoresponsiveness itself. Also, compounds with similar molecular backbones to the dye/acid complex were synthesized to clarify the color change mechanism. The compounds showed both blue and red shifts in photoluminescence and diffuse reflectance spectra upon grinding, indicating that mechanochromic luminescence in the hydrogen‐bonded complex is like its monomeric analogue and that aggregation structure plays an important role in mechanoresponsive behavior rather than the π‐conjugated structure. It was shown that a color change can be mechanically induced by imitating the solid‐state aggregation structure of other mechanoresponsive compounds without synthetic modification.     

Influence of the Delocalization Error and Applicability of Optimal Functional Tuning in Density Functional Calculations of Nonlinear Optical Properties of Organic Donor–Acceptor Chromophores

Nonempirically tuned hybrid density functionals with range‐separated exchange are applied to calculations of the first hyperpolarizability (β_∥) and charge‐transfer (CT) excitations of linear “push–pull” donor–acceptor‐substituted organic molecules with extended π‐conjugated bridges. An unphysical delocalization with increasing chain length in density functional calculations can be reduced significantly by enforcing an asymptotically correct exchange‐correlation potential adjusted to give frontier orbital energies representing ionization potentials. The delocalization error for a number of donor–acceptor systems is quantified by calculations with fractional electron numbers and from orbital localizations. Optimally tuned hybrid variants of the PBE functional incorporating range‐separated exchange can produce similar magnitudes for β_∥ as Møller–Plesset second‐order perturbation (MP2) correlated calculations. Improvements are also found for CT excitation energies, with results similar to an approximate coupled‐cluster model (CC2).     

Naphthalene‐Functionalized,Photoluminescent Room Temperature Ionic Liquids Bearing Small Counterions

Obtaining π‐conjugated room temperature ionic liquids (RTILs) is difficult because of the relatively strong π–π interaction among the π‐moieties. Existing strategies by using bulky counterions greatly hindered further property optimization and potential applications of these intriguing functional fluids through simple ion exchange. Herein, four naphthalene‐functionalized, π‐conjugated RTILs with small counterions (Br^?) have been facilely synthesized with high yields. Our strategy is to attach branched alkyl chains to the cationic backbone of the target compounds ( 2 a – d ), which effectively tune inter‐ and intramolecular interactions. Compounds 2 a – d have satisfactory thermal stability (up to 300 °C) and low melting points (<?19 °C). Rheological measurements revealed the fluid character of 2 a – d , whose viscosity decrease with the increase of the alkyl chain length and temperature. The presence of the π‐conjugated naphthalene moiety imparts 2 a – d photoluminescent properties in bulk solutions. Moreover, the absence of strong π–π stacking among the naphthalene units in solvent‐free states enables them to be used as a new generation of photoluminescent inks.