Charge-transfer complexes of benzanilides with π-acceptors

Spectrophotometric studies of several substituted benzanilides as electron donors with tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) as electron acceptors have given results that are consistent with an interpretation of 1:1 charge-transfer (CT) complexes. The nature of interaction as well as the substituent effects on the CT complexation are discussed.     

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.     

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.     

Spectroscopic probes with changeable π-conjugated systems

Spectroscopic probes have been extensively investigated and used widely in many fields because of their powerful ability to improve analytical sensitivity, and to offer greater temporal and spatial resolution (in some cases a molecule event may be visualized by the naked eye). So far, different photophysical mechanisms, such as charge transfer, photo-induced electron transfer and fluorescent resonance energy transfer, have been employed to develop various spectroscopic probes with superior properties. However, these photophysical mechanisms depend on the energy levels of molecular orbitals, which are usually difficult to accurately determine. This would lead to the poor prediction of analytical performance of the designed probe. Instead, the change of π-conjugated systems induced by chemical reactions is often accompanied by a distinct alteration in spectroscopic signal, which is more predictable and is of high signal/background ratio. This mechanism can serve as an effective measure for developing excellent spectroscopic probes, but to our knowledge, has not been systematically summarized. In this feature article, we review the development of spectroscopic probes with changeable π-conjugated systems, which is catalogued according to the fluorochromes: fluorescein, rhodamine, spiropyran, squaraine, coumarin, cyanine, etc. Two main strategies for constructing these spectroscopic probes, including ring-closing reaction and nucleophilic addition reaction, are summarized, and the merits and limitations of the probes are discussed.     

Molecular complexes of p-benzoquinonechlorimides with aromatic π-donors

The charge transfer spectra of molecular complexes of chloro substituted p-benzoquinone(4)-chlorimides with aromatic π-donors in cyclohexane have been studied. The interaction of 2,6-dichloro p-benzoquinone with the same set of donors has also been studied. The spectroscopic and thermodynamic data indicate that the complexes are very weak in nature. A comparison of the absorption intensities and formation constants for these complexes shows that the contribution of charge transfer forces are not the only dominant stabilising factors.     

Recent advances with π-conjugated salen systems

Schiff base ligands have long been successfully employed as ligands in combination with various metals to give catalysts capable of realizing a variety of synthetic transformations. One of the most widely used Schiff base ligands, the "salen" ligand, has been extensively researched. Recently, there has been increased interest in π-conjugated salen systems, known as "salphen" ligands, as a result of the differences in reactivity of the complexes in catalytic applications compared with the salen analogues. Complexes of salphen ligands display interesting photophysical and supramolecular properties which are not always observed with salen systems as a result of their π-conjugation. This tutorial review therefore describes the most significant advances recently made with salphen and related π-conjugated ligand systems.     

New synthetic routes to cationic arene π-complexes of transition metals

A review is given of the authors' studies on the development of new methods for the synthesis of cationic arene complexes of transition metals. The synthesis in strong protic acids facilitated obtaining -arene cations of Mn, Re, Ru, Os, Co, Rh and Ir. New nonacidic methods allowed the set of available cationic arene -complexes of Mn, Ru, Rh and Ir to be increased considerably. Based on the developed methods a number of previously unknown binuclear organometallic cations are obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1186–1195, July, 1993.     

Noncovalent wrapping of chemically modified graphene with π-conjugated disk-like molecules

A facile and scalable preparation of dispersion of isolated graphene in various organic solvents has been developed by combining between covalent and noncovalent functionalizations of the graphene surface. Covalently functionalized graphene (FRG) was prepared by the reaction of partially reduced graphene oxide with aryl diazonium salts, followed by the graphene oxide being completely reduced with hydrazine. The resulting FRG disperse readily in organic solvents such as N,N′-dimethylformamide (DMF) and N-methyl-2-pyrrolidinone and the functionalization of graphene was characterized by Fourier transform infrared spectroscopy, thermogravimetric thermogram, X-ray photoelectron spectroscopy, and Raman spectroscopy. The hydrophobic surface of FRG was noncovalently wrapped with aromatic hexakis-dodecylhexa-peri-benzocorone (HBC) by simply mixing of dispersion of FRG in DMF with toluene solution of HBC. The complexation of FRG and HBC was monitored by viewing the absorption and fluorescence spectral changes. Atomic force microscopic images confirmed that graphene was covalently and noncovalently functionalized, while keeping a two-dimensional sheet shape.     

Diboron-containing fluorophores with extended ladder-type π-conjugated skeletons

A series of ladder type π-conjugated diboron complexes 1-4 have been designed and synthesized by a very simple synthetic procedure. Single crystals of complex 3 were grown and the molecular structure determined by X-ray diffraction analysis demonstrated that this type of diboron ladder has a rather planar skeleton. All complexes possess very high melting points (275-383 °C) and decomposition temperatures (T(d5): 343-400 °C), indicative of their high thermal stabilities. The electrochemical and photophysical properties as well as theoretical calculations were investigated, suggesting the possibility of these boron complexes as efficient emitters in optoelectronics.     

Electrolytic formation of technetium complexes with π-acceptor ligands

Electrolytic reduction of pertechnetate was performed in aqueous solution containing -acceptor ligands. Cyanide and 1,10-phenanthroline were the selected ligands. In both cases, electrolyses produced a cathodic TcO₂ deposit and soluble Tc complexes. When cyanide was the ligand, the complexes formed were [Tc (CN)₆]^5– and [TcO₂ (CN)₄]^3–. When working with the amine, [Tc (phen)₃]²⁺ and another positively charged species were found after reaction. Results are compared with previous studies with amines, and the usefulness of the electrolytic route to obtain Tc complexes is evaluated.     

Effects of weak interactions on spin crossover properties of iron(II) complexes with extended π-conjugated Schiff-base ligands

The preparation and magnetic properties of two Fe(II) Schiff-base complexes, [Fe(qnal-12)₂]·2C₆H₆ (1) and [Fe(Hqsalc)₂] (2), (Hqnal-12 = N-(8′-quinolyl)-1-hydroxy-2-naphthaldimine, H₂qsalc = 4-hydroxy-3-[(8-quinolinylimino)methyl]benzoic acid) are reported. X-ray single crystal structure analysis of 1 reveals that an Fe(II) ion is coordinated by two qnal-12 ligands in a meridional fashion. Molecular packing of 1 shows that a qnal-12 interacts with neighboring two qnal-12’s through π-π interactions, which results in the formation of one-dimensional chain. Although the magnetic property of 1 shows a high-spin state at all the temperature range measured, the χT-T plot of 2 shows abrupt spin crossover behavior with a wide hysteresis of 21 K, probably due to the hydrogen-bond network originated by carboxyl groups.     

Syntheses, structures, and properties of metal complexes involving π-conjugated tetrathiafulvalene-pyridine ligand

Four metal complexes based on the phenyl-bridged pyridine ligand with tetrathiafulvalene unit (TTF-Ph-Py, L), Ni^II(acac)₂(L)₂ (1, acac = acetylacetonate), M(hfac)₂(L)₂ (M = Ni^II, 2; M = Cu^II, 3; hfac = hexafluoroacetylacetonato) and [Co^II(Tp^Ph2)(OAc)(L)]·H₂O (4, Tp^Ph2 = hydridotri(3,5-diphenylpyrazol-1-yl) borate), have been synthesized and structurally characterized. The absorption spectra and redox behaviors of these new compounds have been studied. Optimized conformation and molecular orbital diagram of L has been calculated with density functional theory (DFT).     

Rhodium-catalyzed hydrothiolation of alkynes with thiols for construction of sulfur-containing π-conjugated systems

To synthesize sulfur-containing π-conjugated polymers, reaction conditions for rhodium-catalyzed hydrothiolation of terminal alkynes with arenethiols are optimized in detail. Under the optimized conditions, rhodium-catalyzed hydrothiolation of terminal alkynes proceeds regio- and stereoselectively to afford the corresponding vinyl sulfides via an anti-Markovnikov and syn-addition process. Then, the rhodium-catalyzed hydrothiolation is applied to polymerization of 2,5-diethynylthiophene with benzene-1,4-dithiol, which successfully provides sulfur-containing π-conjugated polymers with excellent regio- and stereoselectivities.     

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.     

Enhanced nonlinear optical properties of porphyrin with an extended π-conjugated bridge

In this work, a series of molecules with an extended π-conjugated bridge have been theoretically designed based on porphyrin, where -(CH=CH)_n- (n = 1–4, 8, 12) chain is served as an extended π-conjugated bridge. It is found that all molecules exhibit large energy gaps in the range of 3.484–4.151 eV for porphyrin-(CH=CH)_n-NH₂, and 3.624–4.250 for porphyrin-(CH=CH)_n-NO₂. The maximum absorption wavelengths of all molecules show a red shift trend with increasing -(CH=CH)_n- length, which leads to small transition energy. It is observed that long chain brings these molecules the large first hyperpolarizability, which are 1.04 × 10⁵ au for porphyrin-(CH=CH)₁₂-NH₂, 1.26×10⁵ au for porphyrin-(CH=CH)₁₂-NO₂. Moreover, compared with -(CH=CH)_n-NH₂ with the same chain length, -(CH=CH)_n-NO₂ chain can achieve larger nonlinear optical response. It is hoped that the research in this paper can provide a new strategy for the experimental design of nonlinear optical materials.

     

Synthesis and properties of new π-conjugated pyridine ligands with tetrathiafulvalene derivatives and the rhenium(I) tricarbonyl complexes

Three new π-conjugated pyridine ligands with redox-active tetrathiafulvalene (TTF) derivatives, L₁-L₃, have been synthesized and characterized. The Diels-Alder reaction is the key step for this multistep synthetic strategy. The crystal structures of L₁ and L₃ have been studied. The electrochemical and spectroscopic properties of these new ligands, as well as the corresponding tricarbonyl rhenium(I) complexes (ReL₁(CO)₃X, X = Cl, 5; X = Br, 6) have also been investigated.     

Preparation and characterization of inclusion complexes of carvedilol with methyl-β-cyclodextrin

Aim of the present work was to investigate the effect of methyl-β-cyclodextrin (MβCD) on the solubility and dissolution rate of carvedilol (CAR), a drug used orally for the treatment of hypertension. Phase solubility studies showed an A_L-type diagram indicating the formation of inclusion complex in 1:1 molar ratio. Solid binary systems of the drug with MβCD were prepared by various methods. Physicochemical characterizations were performed using Fourier Transformation Infrared Spectroscopy, Differential Scanning Calorimetry and powder X-Ray Diffractometry. It could be concluded that CAR can form inclusion complex with MβCD. The dissolution profiles of inclusion complexes were determined and compared with those of CAR alone and the physical mixture. The dissolution rate of CAR was increased by MβCD inclusion complexation remarkably.     

Fluorescent liquid crystals with rod-shaped π-conjugated hydrocarbon core

Calamitic fluorophores are presented possessing either two phenylethylene or phenylacetylene fragments which are connected via an aromatic entity. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. Grafting acrylate functional groups to the termini of naphthalene centred members of the rod-shaped dyes provides new luminescent reactive mesogens. Uniaxial orientation of mixed systems of the acrylate fluorophores with non-emissive reactive benzoate nematogens was achieved by surface-assisted alignment on polyimide orientation layers. The photocrosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.