Synthesis and optical properties of π-conjugated polymers and oligomers bearing hexaphenylbenzene and tetraphenyl ethene units

π-Conjugated polymers (CPs) and oligomers (COs) bearing hexaphenylbenzene (HPB) and tetraphenyl ethene (TPE) units were synthesized by Sonogashira and Suzuki–Miyaura coupling reactions. The optical properties of the CPs and COs were investigated by UV–vis, photoluminescence (PL), and fluorescence lifetime (τ) measurements. The PL intensities of the solutions of the CPs and COs synthesized by Sonogashira coupling were reduced upon addition of poor solvent like water, which is attributed to aggregation-caused quenching. In contrast, the CP and COs bearing the TPE unit synthesized by Suzuki–Miyaura coupling exhibited aggregation-induced emission enhancement in solution. This difference in fluorescence behavior is discussed herein in terms of the molecular sizes, conformations in solution, and τ values of the CPs and COs.     

Synthesis and photophysical properties of aryl-substituted 2-borylbenzaldimines and their extended π-conjugated congeners

Novel N-aryl-substituted 2-borylbenzaldimines 6 and related systems with extended π-framework 7 based on two borylbenzaldimine units linked by a spacer moiety were synthesized by condensation reactions of 2-(dimesitylboryl)benzaldehyde 3 with various amines 4 and diamines 5. All compounds were completely characterized including X-ray diffraction, especially in view of Lewis acid-base B-O and B-N interactions. The electronic as well as the photophysical properties of bisimines 7 were determined using cyclic voltammetry, UV/vis, and fluorescence spectroscopy and quantum chemistry. These compounds feature large Stokes shifts and reversible reduction waves. Interestingly, UV irradiation experiments unfold enhanced photostability for compounds 7 with an extended π-skeleton. By use of 1,8-diaminonaphthalene we observed the formation of a hitherto unknown BN-heterocyclic compound 9 fused with a perimidine skeleton. Structural and energetic aspects were evaluated by high level quantum chemical methods (DFT and SCS-MP2-calculations).     

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.     

Synthesis of 1,2,5,6- and 1,4,5,8-anthracenetetrone: Building blocks for π-conjugated small molecules and polymers

Reliable reactions for the synthesis of two interesting anthracenetetrones have been identified and optimized. Both syntheses start from dihydroxy-9,10-anthraquinones and were selected for maximized efficiency and minimized workload. Work-up of all reactions can be achieved without column chromatography, which facilitates further scale-up. So far, both target compounds are considerably underexplored despite their promising molecular structure for use in devices and in organic synthesis, especially as building blocks for π-conjugated compounds. The crystal structure of 1,4,5,8-anthracentetrone is reported.     

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.     

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.     

Synthesis and properties of π-conjugated donor–acceptor macrocycles derived from phenanthrylene building blocks

Phenanthrylene-ethynylidene macrocycles combining electron donor and electron acceptor subunits in their shape-persistent fully conjugated core were synthesized. The donor subunits consisted of two 9,10-dialkoxyphenanthrenes linked either with 1,2-ethynylidene or 2,5-thienylene bridge. The acceptors were 9,10-phenanthroquinone and dibenzoquinoxaline and dibenzophenazines derived from it. Solvatochromic photoluminescence from intramolecular-charge-transfer (ICT) excited state was observed mainly in non-polar solvents. In more polar solvents, the excited states favor non-radiative relaxation. DFT calculated HOMO/LUMO energies of the macrocycles correlate well with spectroscopic and electrochemical data. In the series of substituted dibenzophenazine acceptors a good correlation with Hammett substituent constants σ_p^– was found.     

Second-order dispersion interactions in π-conjugated polymers

We calculate the ground state and excited state second-order dispersion interactions between parallel π-conjugated polymers. The unperturbed eigenstates and energies are calculated from the Pariser-Parr-Pople model using CI-singles theory. Based on large-scale calculations using the molecular structure of trans-polyacetylene as a model system and by exploiting dimensional analysis, we find that: (1) For inter-chain separations, R, greater than a few lattice spacings, the ground-state dispersion interaction, ΔE(GS), satisfies, ΔE(GS)～L(2)/R(6) for L ? R and ΔE(GS)～L/R(5) for R ? L, where L is the chain length. The former is the London fluctuating dipole-dipole interaction while the latter is a fluctuating line dipole-line dipole interaction. (2) The excited state screening interaction exhibits a crossover from fluctuating monopole-line dipole interactions to either fluctuating dipole-dipole or fluctuating line dipole-line dipole interactions when R exceeds a threshold R(c), where R(c) is related to the root-mean-square separation of the electron-hole excitation. Specifically, the excited state screening interaction, ΔE(n), satisfies, ΔE(n) ～ L∕R(6) for R(c) < L ? R and ΔE(n) ～ L(0)∕R(5) for R(c) < R ? L. For R < R(c) < L, ΔE(n) ～ R(-ν), where ν ? 3. We also investigate the relative screening of the primary excited states in conjugated polymers, namely the n = 1, 2, and 3 excitons. We find that a larger value of n corresponds to a larger value of ΔE(n). For example, for poly(para-phenylene), ΔE(n = 1) ? 0.1 eV, ΔE(n = 2) ? 0.6 eV, and ΔE(n = 3) ? 1.2 eV (where n = 1 is the 1(1)B(1) state, n = 2 is the m(1)A state, and n = 3 is the n(1)B(1) state). Finally, we find that the strong dependence of ΔE(n) on inter-chain separation implies a strong dependency of ΔE(n) on density fluctuations. In particular, a 10% density fluctuation implies a fluctuation of 13 meV, 66 meV, and 120 meV for the 1(1)B(1), m(1)A state, and n(1)B(1) states of poly(para-phenylene), respectively. Our results for the ground-state dispersion are applicable to all types of conjugated polymers. However, our excited state results are only applicable to conjugated polymers, such as the phenyl-based class of light emitting polymers, in which the primary excitations are particle-hole (or ionic) states.     

Synthesis and properties of thiophene-functionalized π-extended tetrathiafulvalenes

Two molecular ensembles composed of an array of thiophene-extended tetrathiafulvalene-thiophene were synthesized using Stille coupling and Horner-Wittig reaction as the key steps. Electrochemical redox and electronic absorption properties were investigated by voltammetric and UV-vis spectroscopic analyses.     

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.     

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.     

Synthesis of extended π-conjugated hypercrosslinked polymers via a Friedel–Crafts reaction and an intramolecular Scholl coupling reaction

Long-range conjugated structures are important for increasing the conductivity and broadening the application scope of hypercrosslinked polymers (HCPs). It is, however, difficult to fabricate long-range conjugated structures using conventional HCP synthesis methods. Herein, we report a simple and efficient strategy for the synthesis of HCPs with extended π-conjugated structures through Friedel–Crafts and intramolecular Scholl-coupling reactions from readily available aromatic compounds under mild conditions. Two conjugated HCPs (C-HCPs), namely, C-HCP-ATA and C-HCP-TPB, were prepared from anthracene (ATA) and triphenylbenzene (TPB), respectively, using this method. The proposed strategy not only ensures a high specific surface area of materials but also introduces a long-range conjugated structure into HCPs. The lithium-ion battery applications of the C-HCPs were investigated. As anode materials, C-HCP-ATA and C-HCP-TPB displayed higher lithium storage capacities, better conductivity, and higher stability than their precursors, HCP-ATA and HCP-TPB, which were prepared using the conventional method.     

Electrical properties of σ-conjugated polymers with additives and their applications in sensors

The charge carrier transport in poly[methyl(phenyl)silylene] (PMPSi) proceeds predominantly along the σ-delocalized Si backbone with participation of interchain hopping and polaron formation. The charge carrier mobility increases with increasing electron affinity of acceptor dopands having zero dipole moments. On the other band, the hole drift mobility is influenced by the dipole moment of the dopand. The electrostatic charge-dipole interactions cause a broadening of the energy distribution of transport states, which results in a decrease in the charge carrier mobility. An addition of organic salts leads, under the conditions of increased humidity, to an increase in electrical conductivity and capacitance. This is demonstrated on the layers PMPSi/1,5-dimorpholino-1,5-diphenylpentamethinium perchlorate.     

Synthesis and redox properties of π-conjugated 4,5-diazafluorene derivatives incorporating 9-cyanomethylene moiety as an electron acceptor

We have synthesized π-conjugated acceptor-type molecules 3a-d containing a cyanomethylene unit as the electron acceptor site and a 4,5-diazafluorene ligand for metal complexation. In the crystal, the planar 3a molecules stack along the b axis in the head-to-tail fashion. Compound 3a shows distinctive electrochromism and its three differently colored redox states (dianion (3²⁻), anion radical (3⁻), neutral (3)) exhibit remarkable stability.     

Pyridine-cored V-shaped π-conjugated oligomers: synthesis and optical properties

A new series of V-shaped pyridine-cored π-conjugated oligomers are synthesized utilizing two-fold Heck/Suzuki coupling reactions. Optical properties of these compounds (λ_max=390–449 nm, Φ_fl=79–5%, in solutions) are discussed. They are shown to be thermally stable and soluble in common organic solvents. Stilbenoid oligomers exhibited much higher fluorescence quantum yields than tri- and tetra-phenylethylene substituted oligomers in solutions.     

Synthesis and characterization of porphyrin-terthiophene and oligothiophene π-conjugated copolymers for polymer solar cells

Two novel conjugated polymers with alternating main chain structures of zinc porphyrin-terthiophene (P-PTT) and zinc porphyrin-oligothiophene (P-POT) were synthesized by Stille reaction. The effect of different lengths of thiophene chains on the thermal, optical, electrochemical, and photovoltaic properties of the two copolymers were investigated in detail. P-PTT exhibited higher onset decomposition temperature (392 °C) and glass-transition temperature (152 °C) than those of P-POT. The introduction of thiophene units in the meso-aryl positions of porphyrin resulted in the red shift and broader absorption spectrum compared with zinc porphyrin (P_Zn) monomer both in chloroform solutions and on thin solid films. The electrochemical properties indicated that the energy levels of the polymers were suitable for efficient charge transfer and separation at the interface between the polymer donor and PCBM acceptor. The bulk heterojunction solar cells based on P-PTT and P-POT showed power conversion efficiencies up to 0.32% and 0.18%, respectively.     

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.     

Synthesis, structure, and physical properties of a partial π-stacked phenalenyl-based neutral radical molecular conductor

We report the synthesis, crystallization, and solid-state characterization of the 3,7-ethoxy-substituted spirobiphenalenyl-boron neutral radical 22. The radical is distinguished by its low disproportionation energy and one-dimensional structure. We show that our strategy of substitution of OEt group at the active positions of the phenalenyl units changes the crystal packing from its previously known OMe analogue and the solid-state properties are dictated by the partial π-stack structure and the oxygen atoms at the 3,7-positions and can be best rationalized in terms of the resonating valence bond model. Magnetic susceptibility measurements show that in the solid state the radical remains paramagnetic but there is significant spin-spin interaction between the molecules. Band structure calculations reflect efficient overlap between the molecules along the π stack and show evidence of interactions between the spin-bearing oxygen atoms. The room temperature electrical conductivity (σ(RT)=2.0×10(-2) S cm(-1)) of 22 is higher than that observed in previously known one-dimensional phenalenyl radicals.     

Synthesis and photophysical properties of five-membered ring π-conjugated materials based on bisthiazol-2-yl-amine and their metal complexation studies

Poly(5,5′-(2-hexyldecyl)-bisthiazol-2-yl-amine) (PBTA) is prepared by nickel(0) mediated Yamamoto-type coupling. The photoluminescence (PL) spectrum of PBTA in THF solution displays pure blue emission with a peak centered at 444 nm without any shoulder peaks and the HOMO and LUMO values for PBTA are estimated to be 5.11  and 2.90 eV, respectively. In addition, we have synthesized a novel bisthiazol-2-yl-amine (BTA)-cored donor-acceptor (D-A) chromophore system, namely 5-(4-(diphenylamino)phenyl)-N-(5-(4-(diphenylamino)phenyl)thiazol-2-yl)-N-octylthiazol-2-amine (2-TPA-BTA) in which the electron-donating (D) moiety is triphenylamine group and the electron-withdrawing (A) unit is thiazole group. Furthermore, in this report, we present the complexation studies of both the BTA and 2-TPA-BTA chromophores with Cu(II) and Pd(II), respectively. The crystal structures are established by single-crystal X-ray diffraction analysis. These studies not only provide the general photophysical principles of the materials based on BTA moiety but also encourage progress toward realizing the full potential of its hybrid metal-organic frameworks.     

Synthesis and anticancer activity of new azo compounds containing extended π-conjugated systems

A series of novel azo compounds with extended π-conjugated systems were prepared by azo coupling reaction compounds trans-2-(4′-aminostyryl)-thiophene, 1-(4-aminophenyl)-4-phenyl-1,3-butadiene and 4-amino-4′-methoxystilbene with some phenols. The compounds were evaluated for their cytotoxicity against breast cancer adenocarcinoma (MCF-7), cervix adenocarcinoma (HeLa) and human embryonic kidney (HEK 293) cell lines using the MTT assay. The results showed all derivatives had more toxic effects than tamoxifen. Of all the compounds tested, the azo product obtained from coupling trans-2-(4′-Aminostyryl)-thiophene with 2-naphthol (compound 5b) exhibited the potent in vitro antiproliferative activity with IC₅₀ 27 ± 1 and 18 ± 0 µg/mL against MCF-7 and HeLa cell lines, respectively, while it was devoid of any cytotoxicity against normal HEK 293 cells even at 200 µg/mL.