Π-conjugated Units Research Articles

Regulation of peptide-π-peptide nanostructure bundling: the impact of ‘cruciform’ π-electron segments

The self-assembly and photophysical properties of peptide-π-peptide molecules with ‘cruciform’ presentation of the π-electron core were studied. Derivatives of 1,4-bis(phenylethynyl)benzene were incorporated into a peptide backbone via the alkynylation of a central phenyl biscarboxamide core to yield peptide-π-peptide trimers wherein the π-conjugated unit was oblique to the peptide backbone. These molecules exhibited pH triggered self-assembly and yielded 1-D nanostructures with controlled diameters closely corresponding to their molecular lengths. We postulate that this size control is due to the oblique oligophenyleneethynylene (OPE) chromophores effectively blocking the hierarchical associations among the peptide stacks initially formed after self-assembly. Although the molecular design appeared to frustrate the higher order assembly, regardless of the peptide or OPE composition, significant variations in the photophysical properties were observed. The molecular design principles described here can be used to develop fundamental understanding of properties of self-assembled peptide based nanostructures of interest in biotechnological applications.

Aggregation-Induced Delayed Fluorescence Based on Donor/Acceptor-Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs.

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic-inorganic conjugated systems based on o-carboranes directly bonded to electron-donating and electron-accepting π-conjugated units were elaborated as novel optoelectronic materials. These o-carborane derivatives simultaneously possessed aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow-to-red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light-emitting diodes utilizing these o-carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.

Synthesis of Polyphthalaldehyde-Based Block Copolymers: Utilization of a Thermo-Sacrificial Segment for an Easy Access to Fine-Tuned Poly(3-hexylthiophene) Nanostructured Films

This work deals with the synthesis and characterization of new diblock copolymers based on π-conjugated and depolymerizable units. These diblock copolymers are based on a regioregular poly(3-hexylthiophene) sequence associated with a sacrificial block, namely polyphthalaldehyde. The conjugated polymer was obtained by Grignard metathesis polymerization and end-capped by an alkynyl group while the depolymerizable segment was synthesized by an anionic cyclopolymerization from an azide moiety. Diblock copolymers with different molecular weights were then successfully synthesized via an alkyne–azide coupling reaction. Under specific conditions, these copolymers self-assemble into fibrillar nanostructures in thin films. The elimination of polyphthalaldehyde was carried out by thermal treatment, generating nanoporous poly(3-hexylthiophene) films. The use of a dry treatment to remove the polyphthalaldehyde block strongly reduces the morphological damages that would occur with a “wet” processing route. These nanop...

Exfoliating and Dispersing Few-Layered Graphene in Low-Boiling-Point Organic Solvents towards Solution-Processed Optoelectronic Device Applications.

With normal organic surfactants, graphene can only be dispersed in water and cannot be dispersed in low-boiling-point organic solvents, which hampers its application in solution-processed organic optoelectronic devices. Herein, we report the exfoliation of graphite into graphene in low-boiling-point organic solvents, for example, methanol and acetone, by using edge-carboxylated graphene quantum dots (ECGQD) as the surfactant. The great capability of ECGQD for graphene dispersion is due to its ultralarge π-conjugated unit that allows tight adhesion on the graphene surface through strong π-π interactions, its edge-carboxylated structure that diminishes the steric effects of the oxygen-containing functional groups on the basal plane of ECGQD, and its abundance of carboxylic acid groups for solubility. The graphene dispersion in methanol enables the application of graphene:ECGQD as a cathode interlayer in polymer solar cells (PSCs). Moreover, the PSC device performance of graphene:ECGQD is better than that of Ca, the state-of-the-art cathode interlayer material.

Structural Effects of the Donor Moiety on Reduction Kinetics of Oxidized Dye in Dye-Sensitized Solar Cells

One of the major factors influencing the regeneration rate of the oxidized dye in dye-sensitized solar cells (DSSCs) is the energy difference (ΔG) between the levels of the dye’s HOMO and redox couple in the electrolyte. To investigate other factors that influence this process, we examined the effect of structural differences of donor moieties on the reduction rate of the oxidized dye of organic dyes that is composed of an acceptor unit, a π-conjugated linker unit, and a donor unit, including carbazole dye (MK-1), triphenylamine dye (MK-88), and coumarin dye (MK-31). The DSSCs using MK-88 showed the fastest regeneration rate even though the ΔG was not the largest among the dye structures evaluated. The regeneration rates of all of the dyes were enhanced by reducing the number of adsorbed dyes. On the basis of the results, we attribute the fast regeneration of MK-88 to the large collision cross section of the oxidized dye, that is, the increased reduction rate to the larger exposure of the HOMO of the dyes...

Cracking perylene diimide backbone for fullerene-free polymer solar cells

Through cracking perylene diimide by inserting a single bond, small π-conjugation unit thiophene and large π-conjugation unit indaceno[1,2-b:5,6-b′]dithiophene, three molecules were synthesized and used as electron acceptors for solution-processed polymer solar cells. These three compounds showed excellent thermal stability with decomposition temperatures over 370 °C. As the size of inserted conjugation units increased, the extinction coefficient increased from 2.9 × 104 to 6.5 × 104 M−1 cm−1, the absorption wavelength maximum in thin film red shifted from 360 to 488 nm, the optical band gap decreased from 2.98 to 2.25 eV, the highest occupied molecular orbital energy level was upshifted from −6.18 to −5.67 eV, the lowest unoccupied molecular orbital energy level was downshifted from −3.59 to −3.68 eV, power conversion efficiencies of solar cells based on blends of poly(3-hexylthiophene) donor and these acceptors increased from 0.28% to 2.36%.

Effect of π-conjugate units on the ferrocene-based complexes: Switchable second order nonlinear optics controlled by redox stimuli

The control of rotational motion, using an external impulse (such as redox), has been intensively pursued as nonlinear optics switch. Four redox responsive ferrocene-based actuators with varied π-conjugated units have been systematically investigated by the density functional theory. The open structures of these complexes with antiparallel orientation of both π-conjugated units are due to the large charge repulsion effect, while the closed forms are stabilized by the intramolecular π–π stacking interactions.The rotational motion can be triggered by reduction process preferentially centered on the π-conjugated units. The polarizabilities and first hyperpolarizabilities of complexes [3]closed2+ and [4]closed2+ are significant enhanced, which are respectively related to the electronic spatial extent and intramolecular interlayer charge transfer occurred between two respective π-conjugated units. Overall, the present work shows that one can design the molecular nonlinear optical switches characterized by a large contrast varying from zero to a large value along the reversible transformations.

New Fluorescence Domain "Excited Multimer" Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π-Orbitals in Crystals.

The crystal-packing structures of seven derivatives of diaroylmethanatoboron difluoride (1 a-gBF2 ) are characterized by no overlap of the π-conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π-orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π-orbitals of adjacent molecules (type III). The crystal-packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer-like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π-orbitals generates new FL domains, referred to as "excited multimers", which possess allowed S0 -S1 electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π-orbitals in the type III crystals leads to "excited multimer" domains with forbidden S0 -S1 electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.

Extreme electron polaron spatial delocalization in π-conjugated materials

The electron polaron, a spin-1/2 excitation, is the fundamental negative charge carrier in π-conjugated organic materials. Large polaron spatial dimensions result from weak electron-lattice coupling and thus identify materials with unusually low barriers for the charge transfer reactions that are central to electronic device applications. Here we demonstrate electron polarons in π-conjugated multiporphyrin arrays that feature vast areal delocalization. This finding is evidenced by concurrent optical and electron spin resonance measurements, coupled with electronic structure calculations that suggest atypically small reorganization energies for one-electron reduction of these materials. Because the electron polaron dimension can be linked to key performance metrics in organic photovoltaics, light-emitting diodes, and a host of other devices, these findings identify conjugated materials with exceptional optical, electronic, and spintronic properties.

Novel Carbazole-Based Hole-Transporting Materials with Star-Shaped Chemical Structures for Perovskite-Sensitized Solar Cells.

Novel carbazole-based hole-transporting materials (HTMs), including extended π-conjugated central core units such as 1,4-phenyl, 4,4'-biphenyl, or 1,3,5-trisphenylbenzene for promoting effective π-π stacking as well as the hexyloxy flexible group for enhancing solubility in organic solvent, have been synthesized as HTM of perovskite-sensitized solar cells. A HTM with 1,3,5-trisphenylbenzene core, coded as SGT-411, exhibited the highest charge conductivity caused by its intrinsic property to form crystallized structure. The perovskite-sensitized solar cells with SGT-411 exhibited the highest PCE of 13.00%, which is 94% of that of the device derived from spiro-OMeTAD (13.76%). Time-resolved photoluminescence spectra indicate that SGT-411 shows the shortest decay time constant, which is in agreement with the trends of conductivity data, indicating it having fastest charge regeneration. In this regard, a carbazole-based HTM with star-shaped chemical structure is considered to be a promising candidate HTM.

Synthesis, crystal structures and properties of lead phosphite compounds

Here, we report the preparation and characterization of two lead(II) phosphites, namely, Pb2(HPO3)2 and Pb2(HPO3)(NO3)2 through hydrothermal reaction or simple solution synthesis, respectively. A new lead phosphite, namely, Pb2(HPO3)2, crystallizes in the noncentrosymmetric space group Cmc21 (no. 36), which features 3D framework formed by the interconnection of 2D layer of lead(II) phosphites and 1D chain of [Pb(HPO3)5]∞. The nonlinear optical properties of Pb2(HPO3)(NO3)2 have been studied for the first time. The synergistic effect of the stereo-active lone-pairs on Pb2+ cations and π-conjugated NO3 units in Pb2(HPO3)(NO3)2 produces a moderate second harmonic generation (SHG) response of ∼1.8×KDP (KH2PO4), which is phase matchable (type I). IR, UV–vis spectra and thermogravimetric analysis (TGA) for the two compounds were also measured.

ChemInform Abstract: Peptide Nanostructures with π‐Ways: Photophysical Consequences of Peptide/π‐Electron Molecular Self‐Assembly

AbstractReview: 26 refs.

Effect of donor-donor-π-acceptor architecture of triphenylamine-based organic sensitizers over TiO2 photocatalysts for visible-light-driven hydrogen production

In this paper, we report a series of triphenylamine (TPA) donor based dyes in sensitized hydrogen production over TiO2–Pt catalysts under visible light. Among the selected dyes, 4-(diphenylamino) phenylcyanoacrylic acid (DN-F01), 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (DN-F02), 3-(5-(4-(diphenylamino)styryl) thiophen-2-yl)-2-cyanoacrylic acid (DN-F03), (E)-3-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl) thiophen-2-yl)-2-cyanoacrylic acid (DN-F04) and 3-(6-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl)-4,4-dihexyl-4H-cyclopenta[1,2-b:5,4-b′] dithiophen-2-yl)-2-cyanoacrylic acid (DN-F05) were adsorbed on Pt–TiO2 and tested for photocatalytic hydrogen production under visible-light in presence of triethanolamine (TEOA) as sacrificial electron donor (SED). The enhanced light absorption and effective interfacial charge transfer from excited dye to TiO2 is significant in DN-F05-TiO2. The DN-F05 sensitized Pt–TiO2 (DN5@T) photocatalyst exhibited higher turnover number (TON 1864) and apparent quantum efficiency (AQE ∼44%) when compared to their corresponding simple architecture dye molecules. Moreover, the additional donating group di-butoxyphenyl and dihexyl-cyclopentyl π-conjugated bridge units provide an excellent surface protection through steric hindrance leading to a good photocatalytic performance. Electrochemical and computational studies suggest the better photocatalytic activity of DN5@T. The photocatalyst shows long term stability and reproducibility at neutral pH.

A Facile Synthetic Route to a New SHG Material with Two Types of Parallel π‐Conjugated Planar Triangular Units

AbstractA new SHG material, namely, Pb2(BO3)(NO3), which contains parallel π‐conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO3)2 and Mg(BO2)2⋅H2O as starting materials. Its structure contains honeycomb [Pb2(BO3)]∞ layers with noncoordination [NO3]− anions located at the interlayer space. Pb2(BO3)(NO3) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase‐matchable. The large SHG coefficient of Pb2(BO3)(NO3) arises from the synergistic effect of the stereoactive lone pairs on Pb2+ cations and parallel alignment of π‐conjugated BO3 and NO3 units. Based on its unique properties, Pb2(BO3)(NO3) may have great potential as a high performance NLO material in photonic applications.

A facile synthetic route to a new SHG material with two types of parallel π-conjugated planar triangular units.

A new SHG material, namely, Pb2(BO3)(NO3), which contains parallel π-conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO3)2 and Mg(BO2)2⋅H2O as starting materials. Its structure contains honeycomb [Pb2(BO3)]∞ layers with noncoordination [NO3](-) anions located at the interlayer space. Pb2(BO3)(NO3) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase-matchable. The large SHG coefficient of Pb2(BO3)(NO3) arises from the synergistic effect of the stereoactive lone pairs on Pb(2+) cations and parallel alignment of π-conjugated BO3 and NO3 units. Based on its unique properties, Pb2(BO3)(NO3) may have great potential as a high performance NLO material in photonic applications.

Supramolecular Assembly of Functional Hybrid Fibrils with Peptide-π-System-Peptide Monomers Near Silver-Nanoparticles

Abstract Oligopeptides containing internal π-system units are shown to self-assemble in water near nanoparticles into fibrillar nanostructures. We have investigated the biopolymer hybrid monomer of the overall structure `peptide-π-system-peptide' with the sequence VDFAG – perylene diimide (PDI) – GAFDV in the presence of Ag-nanoparticles. Unique to these self-assembling monomers is the fact that the π-conjugated unit has been directly embedded within the peptide backbone by way of a special synthetic coupling. The symmetric peptide-based molecular design enforces intimate π-π communication within the aggregates after self-assembly, making these nano structures attractive for optical or electronic applications. The assembly process can in principle be followed by the change in optical spectra of the monomer and ordered monomer π-systems. Here we show that the Ag-nanoparticles act as seeds for the fibrillation, which makes the fibrils grow in a defined fashion and the supramolecular assembly can be followed via the optical spectrum of the plasmon resonances of the silver nanoparticles, which depend sensitively upon the local environment, making the optical plasmon resonance spectra a nice probe for the assembly process, as well as the interaction of monomers and fibrillar structures in contact with the nanoparticles. While it has been known that these systems may aggregate under certain conditions towards nano fibrils the new aspect here is the controlled and accelerated fibrillation near silver nanoparticles, which results in fibrils of 15–150 nm in diameter and up to 10 μm long, partially incorporating the seed nanoparticles. This approach provides a novel avenue to synthesize bioorganic-hybrid systems (in high yields) in contact with Ag metal nanoparticles and possibly enables us to control the location of growth for the materials in future applications.

Synthesis, Electronic and Photophysical Characterization of π-Conjugated meso-Ferrocenyl-porphyrin Fluorescent Redox Switches

A series of meso-ferrocenyl-porphyrin dyads linked by four different π-conjugated bridging units (directly bound, vinyl, ethynyl, and phenyl) have been synthesized to investigate the influence of the conjugated linker on both the electronic and photochemical properties of the porphyrin chromophore. The basic structure consists of 5-(Fc)-15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II), where Fc = ferrocene, vinylferrocene, ethynylferrocene, or phenylferrocene. Upon introduction of the various electron-donating ferrocenyl moieties at the meso-position of the porphyrin ring, Soret and Q-band electronic transitions of the resultant dyads are red-shifted compared with those of the nonferrocenyl reference porphyrin system 15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II). The electronic properties of these systems have been investigated by electrochemical (cyclic voltammetry) and computational (DFT/TDDFT) methods, while UV/vis absorption and fluorescence emission spectroscopic analysis is also prese...

Patternable Conjugated Polymers with Latent Hydrogen-Bonding on the Main Chain

Conjugated polymers with latent hydrogen-bonding on the main chain were synthesized using Suzuki coupling reaction. The resulting polymers with latent hydrogen-bonding can be converted to the actual hydrogen-bonded polymers by thermal annealing or UV irradiation. As the hydrogen-bonding sites are fused with π-conjugated units on the polymer backbone, the intermolecular interactions between the polymer chains will be strongly enhanced when the hydrogen-bonds are formed. By removing the protection group and forming hydrogen-bonding, the polymers exhibited a bathochromic shift over those with latent hydrogen-bonding, indicating a hydrogen-bonding-mediated enhancement of π–π stacking. In addition, the fused hydrogen-bond sites and π-conjugated units led to closely packed polymer chains, resulting in insoluble pigment-like polymers. This drastic solubility change from polymers with latent hydrogen-bonding to hydrogen-bonded polymers can be used to pattern conjugated polymers directly. The photolithography of the conjugated polymer with latent hydrogen-bonding was demonstrated, and the patterned electrochromic devices were fabricated and tested.

Polymers containing nickel(II) complexes of Goedken's macrocycle: optimized synthesis and electrochemical characterization.

The synthesis and characterization of a new class of nickel-containing polymers is described. The optimized copolymerization of alkyne-bearing nickel(II) complexes of Goedken's macrocycle (4,11-dihydro-5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine) and brominated 9,9-dihexylfluorene produced polymers with potential application as functional redox-active materials. The title polymers exhibit electrochemically reversible, ligand-centered oxidation events at 0.24 and 0.73 V versus the ferrocene/ferrocenium redox couple. They also display exceptional thermal stability and interesting absorption properties due to the presence of the macrocyclic nickel(II) complexes and π-conjugated units incorporated in their backbones.

ChemInform Abstract: Insulated π‐Conjugated Metallopolymers

AbstractReview: [39 refs.