Chemical-stimuli-controllable circularly polarized luminescence from anion-responsive π-conjugated molecules

Introduction of a BINOL-boron moiety to dipyrrolyldiketones as precursors of anion-responsive π-conjugated molecules results in the formation of a chiral environment in the form of anion-free receptors and anion-binding complexes. Conformation changes by inversion (flipping) of two pyrrole rings as a result of anion binding can control the chiroptical properties of the anion receptors. In particular, appropriate pyrrole β-substituents induce distorted receptor π-planes and, as a result, give larger circularly polarized luminescence (CPL), which can be tuned by chemical stimuli (anions). This is the first example of chemical-stimuli-responsive CPL properties.     

Syntheses and properties of linear π-conjugated molecules composed of 1-azaazulene and azulene

Two compounds, 6-(1-azaazulen-2-yl)ethynylazulene (8) and 6-(2-azulenyl)ethynylazulene (10), were synthesized using the Sonogashira-Hagihara cross-coupling reaction followed by decarboxylation with concentrated phosphoric acid. Compounds 8 and 10 were characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, ultraviolet–visible (UV–Vis) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. Based on the results, both compounds were confirmed to have π-conjugation throughout their molecular structures. The acidic responsivity of compounds 8 and 10 was evaluated using UV–Vis and ¹H NMR spectroscopy. Compound 8 was found to be highly sensitive to trifluoroacetic acid, with its 1-azaazulenyl moiety acting as a base. Compound 10 generated azulenium cations when mixed with excess amounts of trifluoroacetic acid.     

Quantum-coherence and correlations in π-conjugated molecules and multichromophoric systems

Chemistry is intrinsically founded on quantum mechanical principles and examples of quantum-mechanical phenomena abound on a range of energy and length scales. In this article some examples of quantum-mechanical phenomena that can be probed by optical spectroscopy are discussed. Recent experimental studies of quantumcoherence in electronic energy transfer in π-conjugated polymers are reported as examples of weak correlations. The nature of the electron-hole binding energy for excitons in organic systems is investigated as a case of intermediate correlations. Possible experimental probes of strong correlations involved in chemical reactions are critically examined in the final section of the paper.     

Synthesis of π-conjugated molecules based on 3,4-dioxypyrroles via Pd-mediated decarboxylative cross-coupling

A general scheme for the synthesis of π-conjugated molecules based on 3,4-dioxypyrroles is presented. The π-conjugated molecules were synthesized via Pd-mediated decarboxylative cross-coupling using various 3,4-propylenedioxypyrrole carboxylic acids and aryl bromides, including the base-sensitive electron acceptor 4,7-dibromobenzo[c][1,2,5]thiadiazole (BTD). N-Methylpyrrolidone was used as solvent, Pd(acac)(2) was employed as the palladium source and P(o-tol)(3) as the ligand. The methodology was applied to 3,4-dioxypyrrole monoacids and 3,4-dioxypyrrole diacids to produce multi-ring π-conjugated systems containing phenyl, thiophenyl, BTD, and pyridinyl units. In general, the method has yielded a practical approach for the synthesis of 3,4-dioxypyrrole-based π-conjugated molecules in acceptable to high yields of 44-94%.     

Thermoplasticity of sol–gel-derived titanoxanes chemically modified with benzoylacetone

Titanium tetra-n-butoxide was hydrolyzed in the presence of benzoylacetone (BzAc), and the solution obtained was concentrated and served for spin-coating or dropping on substrates, followed by successive drying at 120, 200 and 250 °C. The dried products were transparent and amorphous, and the infrared absorption and Raman spectroscopic studies showed that BzAc forms chelate rings. Thermomechanical analysis showed that the 120 and 200 °C-dried products showed steep, thermoplastic shrinkage at around 30 and 70–85 °C, respectively, whereas the 250 °C-dried product did not show thermoplasticity. Thus as the drying temperature was increased, the thermoplasticity appeared at a higher temperature and finally disappeared. These changes in thermoplasticity with drying temperature were concluded to result from the progress of condensation between titanoxane polymers and/or clusters, which was evidenced in gel permeation chromatographic analysis.     

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.     

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.     

Comparison of performances of bioanodes modified with graphene oxide and graphene–platinum hybrid nanoparticles

The performances of graphene oxide (GO) and graphene–platinum hybrid nanoparticles (Gr-Pt hybrid NPs) were compared for biofuel cell (BFC) systems. This is the first study that constitutes these nanomaterials in BFC systems. For this purpose, fabricated bioanodes were combined with laccase modified biocathode in a single cell membraneless BFC. Power and current densities of these systems were calculated as 2.40 μW cm^− 2 and 211.90 μA cm^− 2 for GO based BFC and 4.88 μW cm^− 2 and 246.82 μA cm^− 2, for Gr-Pt hybrid NPs based BFC. As a result, a pioneer study which demonstrates the effective performances of combination of graphene with Pt was conducted.     

Chemical sensors based on π-conjugated organic molecules and gold nanoparticles

Scientists have developed techniques for synthesizing and characterizing many new materials including conjugated small molecules, polymers and gold particles protected by conjugated organic chromophores for testing specific sensing properties in the past decade. Still, the design and synthesis or supermolecular systems fabrication of novel materials with controlled sensing properties is a significant and ongoing challenge within nanoscience and nanotechnology. Recently, our group has successfully constructed a series of chemosensors using small organic molecules, conjugated polymers and gold nanoparticles for real-time detection of specific analytes. The chemosensors show high selectivity and sensitivity in the detection of cations and biologic analytes and thus are potentially promising for applications in sensing assay system. In this review, recent sutdies on the design, synthesis and photo-physical properties of novel materials and construct of chemosensors are summarized with an emphasis on the development in our groups in recent years. Supported by the National Natural Science Foundation of China (Grant Nos. 20531060, 20721061 & 20873155), and the National Basic Research 973 Programme of China (Grant No. 2007CB936401)     

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.     

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.     

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.     

Physicochemical properties of π-π complexes of zinctetraphenylporphyrin with aromatic molecules

Features of the solvation of zinctetraphenylporphyrin (ZnTPP) in benzene, toluene,ortho-, meta-, andpara-xylenes were studied by a thermogravimetric method. The temperature ranges of the stability and the compositions of the corresponding specific - complexes were determined from the results of the thermogravimetric investigation of the crystallosolvates of the metalloporphyrin with the solvent molecules, and the energy characteristics of the intermolecular metalloporphyrin—solvent interactions were calculated.Institute of the Chemistry of Nonaqueous Solutions, Russian Academy of Sciences, 153018 Ivanovo. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1545–1548, July, 1992.     

Electronic state and photosensitivity of metal alkoxides chemically modified with β-diketones

Electronic state and optical absorption spectra of metal alkoxides stabilized with β-diketones (with a variety of substituents) were calculated using first-principle molecular orbital methods. The characteristics of the optical absorption and the mechanism of the photodissociation of alkoxides with irradiation of light are discussed. The position of the first peak observed in the near UV region in the theoretical spectra corresponded to that observed in the experimental spectra with a 25 nm shift toward shorter wavelengths. The first absorption peak observed in the visible range originated from electronic transitions to molecular orbitals, including the antibonding components of the C–O bonding in the chelate ring. The results suggested that the bonding of the chelate rings should be important in controlling the photosensitivity of chemically modified titanium alkoxides.     

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.

     

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.     

Ortho-branched ladder-type oligophenylenes with two-dimensionally π-conjugated electronic properties

The synthesis, photochemical and electrochemical properties, and electronic structures of a series of star-shaped ladder-type oligophenylenes Sn (n = 7, 10, 13, 16, 19, and 22), including one multibranched case S19mb, are reported and compared with the linear para-phenylene ladders Rn (n = 2-5 and 8) and the stepladder analogues SFn (n = 10, 16, and 22). The n value refers to the number of π-conjugated phenylene rings. Functionalized isotruxenes are the key synthetic building blocks, and S22 is the largest monodispersed ladder-type oligophenylene known to date. The Sn systems possess the structural rigidity of Rn and the ortho-para phenylene connectivity of SFn. Consequently, Sn represents the first class of branched chromophores with fully two-dimensional conjugation in both ground- and excited-state configurations. Evidences include the excellent linear correlations for the optical 0-0 energies or the first oxidation potentials of Sn and Rn against the reciprocal of their n values, delocalized HOMO and LUMO based on density functional theory calculations, and molecule-like fluorescence anisotropy. The resulting model of effective conjugation plane (ECP) for the two-dimensional π-conjugated systems compliments the concept of effective conjugation length (ECL) for one-dimensional oligomeric systems. Other implications of the observed structure-property relationships are also included.     

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.     

Titanylation and vanadylation of highly π-conjugated porphyrins

We report the efficient synthesis of tetrabicycloporphyrin titanyl and vanadyl without decomposition of the bicyclo[2.2.2]octadiene unit. The complexes were heated under vacuum to give titanyl and vanadyl tetrabenzoporphyrins in 100% yield. We also titanylated and vanadylated tetra[2,3]naphthoporphyrin and tetra[2,3]anthraporphyrin, which have a greater degree of π conjugation than tetrabenzoporphyrin.     

Highly stereoselective synthesis of N-substituted π-conjugated phthalimides

A new regio- and stereoselective synthesis of (E)-N-(2-arylvinyl)phthalimides as well as phthalimide-containing (E,E)-buta-1,3-dienes and (E)-but-1-en-3-ynes has been developed. The one-pot ruthenium-catalyzed silylative coupling/iododesilylation sequence provides (E)-N-(2-iodovinyl)phthalimide 1, which undergoes palladium-catalyzed Suzuki–Miyaura or Sonogashira cross-coupling to afford stereodefined highly π-conjugated phthalimides and functionalized dienimides containing phthalimide groups.