2,2']paracyclophane-based π-conjugated molecular wires reveal molecular-junction behavior

The electronic coupling as well as the attenuation factor (β), which depends primarily on the nature of the molecular bridge and is used as a benchmark to test the molecular wire behavior, have been determined in a systematic study carried out on a series of ZnP/C(60) conjugates connected through a [2,2']paracyclophane-oligophenylenevinylene (pCp-oPPV). The convergent synthesis involves a series of Horner-Emmons olefination reactions or double palladium-catalized Heck-type reactions. ZnP-pCp-C(60) conjugates were finally obtained by the 1,3-dipolar cycloaddition reaction of the in situ-generated azomethyne ylide containing the ZnP-pCp moiety to the [60]fullerene using Prato conditions. Experimental (UV-vis, fluorescence, transient absorption spectroscopy, and solution electrochemistry) and theoretical studies revealed that the pCps act as molecular junctions. If hole transfer is assumed to be the dominant charge transfer (CT) mechanism, CT is facilitated in one direction (from C(60) to ZnP via pCp) but disfavored in the other direction (from ZnP to C(60) via pCp).     

2,2':6',2"-联三吡啶配体的合成

近年来,2,2':6',2"-联三吡啶作为典型的三齿有机杂环配体在超分子化学与材料化学领域得到广泛应用.通过取代基团对配体的结构修饰或改变金属离子的类型可以调控联三吡啶金属络合物的光物理和电化学性能,从而得到多种不同用途的功能化材料及超分子组装体.本文对单核联三吡啶配体近年来发展的典型合成方法加以阐述,并简单介绍联三吡啶配体在超分子化学中的应用.     

Survey of recent advances of in the field of π-conjugated heterocyclic azomethines as materials with tuneable properties

This account gives an overview of our recent work in the area of conjugated azomethines derived from 2-aminothiophenes.It will be presented that mild reaction conditions can be used to selectively prepare symmetric and unsymmetric conjugated azomethines.It further will be demonstrated that azomethines consisting of various 5-membered aryl heterocycles lead to chemically,reductively,hydrolytically,and oxidatively robust compounds.The optical and electrochemical properties of these materials can be tuned contingent on the degree of conjugation,type of aryl heterocycle,and by including various electronic groups.The end result is materials having colors spanning 250 nm across the visible spectrum.These colors further can be tuned via electrochemical or chemical doping.The resulting doped states have high color contrasts from their corresponding neutral states.The collective opto-electronic properties and the means to readily tune them,make thiophenoazomethine derivatives interesting materials for potential use in a gamut of applications.     

Towards catenanes using π-stacking interactions and their influence on the spin-state of a bis(2,2':6',2"terpyridine)iron(II) domain

The ligand 6,6"bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2"terpyridine (2) has been prepared and characterized; deprotection using pyridinium chloride leads to the formation of 6,6"bis(4-hydroxyphenyl)-4'-phenyl-2,2':6',2"terpyridinium chloride ([H3]Cl). Treatment of the latter with 3-(2-(2-bromoethoxy)ethoxy)prop-1-ene under basic conditions yields ligand 4 containing pendant, alkene-terminated chains. Whereas direct complexation of 4 with ruthenium(II) proved problematical, the homoleptic complexes [Fe(2)(2)][PF(6)](2) and [Ru(2)(2)][PF(6)](2) were prepared in good to moderate yields. In the solid state, both complexes exhibit multiple face-to-face π-stacking of arene and pyridine rings which influences the coordination geometry about the metal ion. Consequential weakening of the ligand field results in [Fe(2)(2)][PF(6)](2) being high-spin. Variable temperature solution (1)H NMR spectroscopic studies confirm the iron(ii) centre remains high-spin between 200 and 295 K. The paramagnetically shifted (1)H NMR spectrum exhibits signals in the range δ 109.7 to -66.5 ppm and has been fully assigned. Paramagnetic relaxation enhancement (PRE) has been used to correlate the observed proton line-widths to the distances of the protons from the metal centre and these are in good agreement with the Fe···H separations observed in the solid state. The [Fe(2)(2)](2+) ion undergoes two dynamic processes (i) rotation of the pendant phenyl rings which is fast on the NMR timescale at 200 K, and (ii) twisting and sliding of the aromatic rings of the tpy and anisyl units which interconverts the two enantiomers of [Fe(2)(2)](2+) at 295 K.     

Synthesis of extended, π-conjugated isoindolin-1-ones

The synthesis and characterization of extended, conjugated molecules containing isoindolinone units was explored. Nucleophilic cyclizations between an amide and an alkyne were found to be an efficient method of producing the desired isoindolin-1-ones in high yields. A variety of derivatives were synthesized, demonstrating that a number of structural alterations could be made while maintaining good regio- and stereospecificity in the cyclized product.     

First-principles approach to the understanding of π-conjugated organic semiconductors

We show how ab initio calculations based on density functional theory contribute to the understanding of the electronic and optical properties of organic semiconducting materials, which form the active layers in many opto-electronic applications. As a textbook example, we present the electronic structure and the optical properties of the oligo-phenylenes as evolving from their benzene-constituents. Thereby we discuss the dependence on the molecular length and introduce the modifications in the opto-electronic properties due to intermolecular interactions which are inherently present in the bulk phase.     

Synthesis,chemical properties,and CO2 electroreduction of π-conjugated polymers bearing imidazolium and imidazolinium rings in the polymer backbone

π-Conjugated polymers (CPs) bearing imidazolium and imidazolinium rings in the polymer backbone were prepared from a previously reported monomer, 1,3-bis(2,3-dimethyl-4-bromophenyl)imidazolium chloride, and a newly synthesized monomer, 1,3-bis(2,3-dimethyl-4-bromophenyl)imidazolinium chloride, via Sonogashira and Suzuki–Miyaura coupling reactions. The corresponding model compounds were also synthesized via the same reactions and monomers. The absorption maxima and onset position of the UV–vis spectra of the polymers were observed at longer wavelengths than those of the model compounds, thus revealing the presence of extended π-conjugation along the polymer backbone chain. Both polymers and model compounds were photoluminescent in solution and exhibited solvatochromism. The cast films of the polymers on the Pt and Ag electrodes were demonstrated to be effective catalysts for CO₂ electroreduction.     

Reviewing extrapolation procedures of the electronic properties on the π-conjugated polymer limit

In this article, the extrapolation procedures of π-π* electronic transition energy on π-conjugated oligomers are reexamined. Different models, including the simplest coupled oscillator, the free electron, the Hückel approach, the molecular exciton model, and some specific fitting-functions, are compared using the transition energies derived from theoretical calculations on three thiophene-based oligomer series. Specifically, oligomers of up to 30 repeating units have been considered to include the saturation effects as a function of chain length. The coupled oscillator model of W. Kuhn and the fitting-function of Hirayama are the models that present the better suit on the transition energy interpolation as a function of chain length. Using only the first four oligomers of the series (n = 2 up to 8) yields an estimation of the transition energy on the polymer limit with an average error of ～1.5%. The vertical and adiabatic ionization potential present a better fit with the Hückel model approach. Finally, implications of the environmental polarity on the electronic properties, molecular geometry, charge distribution, and aromaticity are shortly discussed.     

Features of charge carrier concentration and mobility in π-conjugated polymers

We show the possibilities of experimental investigation of charge carrier mobility and concentration features by extraction methods of equilibrium, photoexcited and injected charge carriers in π-conjugated polymers, where, due to relatively high conductivity, the classic time-of-flight method is inappropriate.     

Further evidences of the quality of double-hybrid energy functionals for π-conjugated systems

Despite numerous interesting efforts along decades to improve the accuracy of density functionals with broad applicability, such as B3LYP, there are still large sets of molecular systems where improvements are badly needed. We select π-conjugated systems as an example of the latter due to the subtle interplay between some physical effects affecting possibly most of the calculations: self-interaction or delocalization error, medium-range correlation signatures, dispersive-like weak interactions, and static correlation effects. We further assess a recently proposed modification of the B2-PLYP double-hybrid functional, called B2π-PLYP, that is expected to yield substantial progress for this kind of systems. Generally speaking, when compared with other more popular and older density functionals, double hybrids behave particularly accurate for π-conjugated systems without suffering the large errors that are common in former yet conventional methods.     

Intramolecular charge transfer in π-conjugated oligomers: a theoretical study on the effect of temperature and oxidation state

Intramolecular charge transfer (ICT) of gaseous π-conjugated oligo-phenyleneethynylenes (OPE) induced by a homogeneous applied electric field has been theoretically investigated using a combined approach integrating molecular dynamics (MD) simulations and Perturbed Matrix Method calculations. In line with recent investigations, our results indicate the peculiar role of conformational transitions on OPE electronic properties which reflects on a strong temperature effect on ICT. Electron transfer reactions inducing chemical alteration on OPE, also taken into account in this study, revealed extremely important for explaining non-linear ICT effects and probably plays a central role in the mechanisms underlying molecular transport junctions. Our study further points out the necessity of using MD-based approach for modelling molecular electronics, even when relatively rigid molecular systems are concerned.     

Conformational and electronic consequences in crafting extended, π-conjugated, light-harvesting macrocycles

The synthesis of a new series of free‐base, Ni^II and Zn^II 2,3,12,13‐tetra(ethynyl)‐5,10,15,20‐tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3‐diethynyl‐5,20‐diphenylpiceno[10,11,12,13,14,15‐jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free‐base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully‐cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15‐fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc‐pVTZ(‐f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol^?1 higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin–enediynes with ethene‐ and H‐functionality substitutions at the meso‐phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol^?1 in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.     

Topography of scalar fields: molecular clusters and π-conjugated systems

The pioneering works due to Bader and co-workers have generated widespread interest in the study of the topography of molecular scalar fields, the first step of which is the identification and characterization of the corresponding critical points (CPs). The topography of a molecular system becomes successively richer in going from the bare nuclear potential (BNP) to the molecular electrostatic potential (MESP) through the molecular electron density (MED). The present work clearly demonstrates, through the study of some π-conjugated test molecules as well as molecular clusters, that the CPs could be economically located by following this path within ab initio level theory. Further, the topography mapping of large molecules, especially at a higher level of theory, is known to be a demanding task. However, it is rendered possible by following the above sequential mapping assisted by a divide-and-conquer-type method termed as the molecular tailoring approach (MTA). This is demonstrated with the topography mapping of β-carotene and benzene nonamer at MP2 and a (H(2)O)(32) cluster at the HF level of theory, which are rather challenging problems with contemporary off-the-shelf computer hardware.     

Study of the aggregation behavior of a π-conjugated dendrimer with a twisted core

A dendrimer having phenylene vinylene and phenylene ethynylene moieties, a twisted core, and eighteen chiral centers on the periphery has been prepared in high yield by using Sonogashira and Horner–Wadsworth–Emmons reactions. UV–visible and fluorescence spectra and circular dichroism measurements have been envisaged to study the aggregation behavior.     

Solid-phase Pd-catalysed cross-coupling methods for the construction of π-conjugated peptide nanomaterials

We describe the development of two procedures for the synthesis of peptides that are embedded with a variety of π-conjugated semi-conducting oligomers. These procedures utilise solid-phase variants of classical palladium-catalysed cross-couplings commonly used to prepare π-conjugated oligomers. The resulting peptide–π–electron hybrids are soluble in aqueous media and self-assemble to produce 1D nanostructures, simultaneously forming networks of π-stacked conduits. The procedures have allowed for the inclusion of complex chromophores including mixed aryl units, ethynylene linkers and sexithiophenes where the latter peptide's nanostructures demonstrated substantial conductivity when employed as an active layer in a field-effect transistor.     

Electron correlation effects on the first hyperpolarizability of push-pull π-conjugated systems

The first hyperpolarizability (β) of representative push-pull π-conjugated compounds has been calculated at several levels of approximation to assess the effects of electron correlation. First, the 6-31+G(d) basis set has been shown to give the best balance between accuracy and computational resources for a polyene linker whereas for polyyne linker, the 6-31G(d) basis set is already an optimal choice. As a result of cancellations between higher order contributions, the MP2 method turns out to be the method of choice to predict β of push-pull π-conjugated systems since it closely reproduces the values obtained with the reference CCSD(T) scheme. Moreover, the SDQ-MP4 and CCSD approaches provide rarely improved estimates over MP2 while the MP4 method does not represent an improvement over MP4-SDQ or the SCS-MP2 method, over MP2. Among density functional theory exchange-correlation functionals, LC-BLYP is reliable when characterizing the changes of first hyperpolarizability upon enlarging the π-conjugated linker or upon changing the polyyne linker into a polyene segment. Nevertheless, its reliability is very similar to what can be achieved with the Hartree-Fock method and the MP2 scheme is by far more accurate. On the other hand, the BLYP, B3LYP, and BHandHLYP functionals perform quantitatively better in a number of cases but the trends are poorly described. This is also the case of the B2-PLYP and mPW2-PLYP functionals, which are often the most accurate, though they underestimate the increase of β when going from polyyne to polyene linkers and overestimate the enhancement of β with chain length.     

Redox-active π-conjugated polymer nanotubes with viologen for encapsulation and release of fluorescent dye in the nanospace

We report the encapsulation of negatively charged Au nanoparticles and anionic fluorescent (FL) dye in the inner cavity of redox-active cationic polymer nanotubes with viologen via electrostatic interaction and the release of the FL dye from the FL dye confined in the cavity of the polymer nanotubes by electrochemical and chemical reduction.     

New π-conjugated cyanostilbene derivatives:Synthesis,characterization and aggregation-induced emission

Two novel π-conjugated cyanostilbene derivatives,FLU-CNPH and TPE-CNPH,were designed and synthesized by introducing the strong electron donor 1,4-dihydropyrro[3,2-b]indole and AIE electron donor tetraphenylethylene(TPE)to the(3',5'-bis(trifluoromethyl)-biphenyl-4-yl)-acetonitrile,respectively.Both of them were fully characterized and their AIE behaviors were investigated using fluorescence spectroscopy and FE-SEM images.Their optimized structures and frontier molecular orbitals were calculated with the DFT by using Materials Studio 7.0 software to study the relationship between the structure and properties.     

Comparison of the spectroscopic properties of π-conjugated, fused salphen triads embedded with Zn-homo-, Ni-homo-, and Ni/Zn-heteronuclei

Stepwise condensation reactions of 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde and a phenylenediamine with concomitant binding of metal ions afforded a trinuclear complex of a fully π-conjugated, fused salphen ligand. By changing the synthetic pathway, we obtained a series of homo- and heteronuclear complexes containing selected combinations of nickel(II) and zinc(II) ions. Comparison of the trinuclear complexes' spectroscopic features with those of analogous dinuclear complexes revealed that the absorption spectrum of each trinuclear complex is composed of a salphen-centered absorption at 400 nm and a naphthalene-centered absorption around 500-600 nm, suggesting that the π-conjugated system is divided into several compartments, each of which independently undergoes electronic excitation. Molecular orbital calculations revealed that the formal fusion of the salphen moieties increases the highest occupied molecular orbital (HOMO) level by ～0.4 eV, which in turn causes the low-energy absorption observed in the spectra. In contrast, interorbital interactions mediated by the N(2)O(2) metal coordination site are small, even though this site is bridged by an o-phenylene linkage. These results suggest that the coordination site effectively breaks electronic communication between the compartments, which in turn affect various spectroscopic properties of the π-conjugated metallo-polysalphens.