Π-conjugated Oligomers Research Articles

Rational design of diketopyrrolopyrrole-based oligomers with deep HOMO level and tunable liquid crystal behavior by modulating the sequence and strength of the donor moiety

A family of narrow-band gap π-conjugated oligomers and isomers based on diketopyrrolopyrrole and difluorobenzothiadiazole coupled with an oligothiophene or thiazole ring have been successfully synthesized.

Syntheses and properties of π-conjugated oligomers containing furan-fused and thiophene-fused aromatic units

π-conjugated oligomers 1a–b and 2a–b, containing furan-fused and thiophene-fused aromatic units, were systematically designed and successfully synthesized. Their physicochemical properties were thoroughly investigated. Experimental results showed that different furan/thiophene ratios and fused-aromatic skeletons strongly affected the HOMO energy levels, fluorescent quantum yields and aggregation behaviors of the oligomers. Single crystal structures revealed compound 1b adopted typical herringbone structure with strong C–H⋯π, S⋯π and S⋯S interactions in the crystals. 2a formed slipped π-π stacking in the crystals, and strong π⋯π and O⋯O interactions were observed.

Solar Cells: π‐Extended Narrow‐Bandgap Diketopyrrolopyrrole‐Based Oligomers for Solution‐Processed Inverted Organic Solar Cells (Adv. Energy Mater. 17/2014)

High-efficiency solution-processed organic solar cells based on narrow-bandgap π-conjugated oligomers are demonstrated by Takuma Yasuda, Chihaya Adachi, and co-workers in article number 1400879. Well-organized donor/acceptor bulk-heterojunction domains with pronounced mesoscopic phase segregation are spontaneously formed in a photoactive binary blend layer. A high power conversion efficiency of 5.9% is achieved for small-molecule organic solar cells.

Supramolecularly Engineered Functional π-Assemblies Based on Complementary Hydrogen-Bonding Interactions

Abstract As details of exciting photonic and electronic properties arising from self-assembled functional dyes and related π-conjugated molecules become clearer, strategies for control of short and long-range chromophore–chromophore orientations, regulation of π–π stacking arrangements, construction of self-organized nano-to-microscale architectures, diversification of materials properties, analysis of self-assembly pathways, and impartment of stimuli-responsive properties, are all becoming important issues in the research area of organic functional materials. This article focuses on the supramolecular assemblies of dyes and related π-conjugated systems whose organization processes are rationally controlled by complementary multiple hydrogen bonding (CMHB) interactions between melamine and barbituric acid/cyanurate functional units. The article includes an overview of the construction principle of such systems, but special attention is focused on the work carried out in the authors’s group (since 2002) using azobenzene, diarylethene, merocyanine, perylene bisimide dyes, and π-conjugated oligomers including oligo(p-phenylenevinylene), oligo(p-phenyleneethynylene), and oligothiophenes. The design principles discussed in this account could open up new avenues to novel functional aspect and application of functional dyes and small-molecular π-conjugated systems.

Photophysics and non-linear absorption of Au(I) and Pt(II) acetylide complexes of a thienyl-carbazole chromophore.

In order to understand the photophysics and non-linear optical properties of carbazole containing π-conjugated oligomers of the type ET-Cbz-TE (E = ethynylene, T = 2,5-thienylene, Cbz = 3,6-carbazole), a detailed investigation was carried out on a series of oligomers that feature Au(i) or Pt(ii) acetylide "end groups", as well as a Pt(ii)-acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt). These organometallic chromophores were characterized by UV-visible absorption and variable temperature photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, open aperture nanosecond z-scan and two photon absorption (2PA) spectroscopy. The Au(i) and Pt(ii) oligomers and polymer exhibit weak fluorescence in fluid solution at room temperature. Efficient phosphorescence is observed from the Pt(ii) systems below 150 K in a solvent glass; however, the Au(i) oligomer exhibits only weak phosphorescence at 77 K. Taken together, the emission results indicate that the intersystem crossing efficiency for the Pt(ii) chromophores is greater than for the Au(i) oligomer. Nonetheless, nanosecond transient absorption indicates that direct excitation affords moderately long-lived triplet states for all of the chromophores. Open aperture z-scan measurement shows effective optical attenuation can be achieved by using these materials. The 2PA cross section in the degenerate S0→S1 transition region was in the range 10-100 GM, and increased monotonically toward shorter wavelengths, reaching 800-1000 GM at 550 nm.

Benchmark Torsional Potentials of Building Blocks for Conjugated Materials: Bifuran, Bithiophene, and Biselenophene.

The utility of π-conjugated oligomers and polymers continues to grow, and oligofurans, oligothiophenes, and oligoselenophenes have shown great promise in the context of organic electronic materials. Vital to the performance of these materials is the maintenance of planarity along the conjugated backbone. Consequently, there has been a great deal of work modeling the torsional behavior of these prototypical components of conjugated organic materials both in the gas and condensed phases. Such simulations generally rely on classical molecular mechanics force fields or density functional theory (DFT) potentials. Unfortunately, there is a lack of benchmark quality, converged ab initio torsional potentials for bifuran, bithiophene, and biselenophene against which these lower level theoretical methods can be calibrated. To remedy this absence, we present highly accurate torsional potentials for these three molecules based on focal point analyses. These potentials will enable the benchmarking and parametrization of DFT functionals and classical molecular mechanics force fields. Here, we provide an initial assessment of the performance of common DFT functional and basis set combinations, to identify methods that provide robust descriptions of the torsional behavior of these prototypical building blocks for conjugated systems.

The Synthesis and Characterization of -3,4-Ethylenedioxythiophene Derivatives with Electroactive Features

We report the synthesis of conjugated oligomers based on electron withdrawing 3,4-ethylenedioxythiophene (EDOT) in the case of which band gaps are tuned by the insertion of alkylated diphenylamine or phenoxazine/phenothiazine units. The structures and properties of synthesized compounds were characterized by 1H NMR, 13C NMR, UV-Vis absorption, and fluorescence spectroscopy as well as electrochemical measurements. The luminescence spectra demonstrate that copolymers are good chromophores. Also the electrical properties confirm the applicability of these novel aryl-based π-conjugated oligomers for the development of various electrical and electrochemical devices.

1,2-dihydrophosphete: a platform for the molecular engineering of electroluminescent phosphorus materials for light-emitting devices.

The discovery and molecular engineering of novel electroluminescent materials is still a challenge in optoelectronics. In this work, the development of new π-conjugated oligomers incorporating a dihydrophosphete skeleton is reported. Variation of the substitution pattern of 1,2-dihydrophosphete derivatives and chemical modification of their P atoms afford thermally stable derivatives, which are suitable emitters to construct organic light-emitting diodes (OLEDs). The optical and electrochemical properties of these new P-based oligomers have been investigated in detail and are supported by DFT calculations. The OLED devices exhibit good performance and current-independent CIE coordinates.

Photo-switching effect in stilbene organic field effect transistors

In this paper, we have investigated experimentally the photo-response of the organic field effect transistors (OFETs) based on π-conjugated stilbene oligomers with mutual optical and electrical properties. The cis-trans photoisomerization of these oligomers in the active layer of the OFETs disturbs the intermolecular interactions. This perturbation disrupts the charge transport in the conduction channel and turns-off the transistor. We used this phenomenon to modulate the drain current of the OFETs optically. The photo-response activity of the transistors is temperature dependent, in agreement with the isomerization of the stilbenes. Photoluminescent evaluations verify the isomerization of the oligomers in the thin film.

Electronic Energy Gaps for π-Conjugated Oligomers and Polymers Calculated with Density Functional Theory.

In varying contexts, the terms "energy gap" (energy difference) or "band gap" may refer to different experimentally observable quantities or to calculated values that may or may not represent observable quantities. This work discusses various issues related to calculations of electronic energy gaps for organic π-conjugated oligomers and linear polymers by density functional theory (DFT). Numerical examples are provided, juxtaposing systematic versus fortuitous agreement of orbital energy gaps with observable fundamental (ionization vs electron attachment) or optical (electronic excitation) energy gaps. Successful applications of DFT using nonempirically tuned hybrid density functionals with range-separated exchange (RSE) for calculations of optical gaps, fundamental gaps, and electron attachment/detachment energies are demonstrated. The extent of "charge-transfer like" character in the longest-wavelength singlet electronic excitations is investigated.

Charge transfer (CT) mechanochromism: dramatic CT absorption change of crystalline π-conjugated oligomers containing TCNQ upon mechanical grinding.

New π-conjugated oligomers with high crystallinity were prepared from the simple solvothermal reaction of squaric acid and diaminopyrenes. The oligomers were bonded at the 1,3- and 1,6-positions of the pyrene units, oligo(sq-alt-1,3py) and oligo(sq-alt-1,6py), which greatly affected their planar configuration and resulting mechanochromic properties. Oligomers containing a charge transfer (CT) complex were selectively synthesized in one step. Upon mechanical grinding in the solid state, the color changed from orange to deep metallic green.

Synthesis and 2D self-assembly at the liquid–solid interface of novel H-bonding linear π-conjugated oligomers terminated by uracil and melamine units

2D self-assembly of a novel π-conjugated oligomer terminated by two uracil units is solvent dependent and involves multiple intermolecular H-bonds.

Estimating the shape and size of supramolecular assemblies by variable temperature diffusion ordered spectroscopy.

Reported is characterization of the self-assembly of π-conjugated oligomers, molecules studied recently in photovoltaic devices, using variable temperature diffusion ordered spectroscopy (VT-DOSY). Iterative fitting of diffusion coefficient versus temperature data to a modified Stokes-Einstein equation, molecular modelling, and comparison to non-assembling model compounds, has allowed estimation of assembly size, shape, and molecularity.

A direct arylation-derived DPP-based small molecule for solution-processed organic solar cells

A diketo-pyrrolo-pyrrole (DPP) oligomer containing three DPP cores (Ph4Th4(DPP)3) was synthesized via direct arylation of C–H bonds (DACH). Ph4Th4(DPP)3 has good solubility in many organic solvents, and shows a broad absorption band from the visible to near-infrared region as well as a field-effect hole mobility as high as 0.006 cm2 V−1 s−1. Solution-processed bulk heterojunction organic solar cells based on blends of Ph4Th4(DPP)3 as electron donor and fullerene derivative as electron acceptor were fabricated. An optimized power conversion efficiency of 3.76% with a high open-circuit voltage of 0.85 V was achieved after finely tuning the morphology by changing the blend ratio and by adding additives. These results indicate that DACH is an effective way to produce π-conjugated oligomers for organic solar cells.

Novel liquid crystalline oligomer with thiazolothiazole-acceptor for efficient BHJ small molecule organic solar cells

A Novel and efficient π-conjugated, liquid crystalline semiconducting oligomer comprised of thiazolothiazole (acceptor) and two different donor units as – terminal alkyl-bithiophene (D1) and triphenyl amine (D2) was strategically designed and synthesized in four steps synthetic route via-Suzuki cross-coupling reactions. The presence of terminal alkyl unit might induce good solubility as well as liquid-crystal (LC) property of oligomer. Differential scanning calorimetry reveals the presence of different LC phases of oligomer which affects the morphology of devices during solution-processed fabrication process by acquiring self-assembly behaviour. As photoactive materials, π-conjugated oligomer based organic solar cell showed significantly high photocurrent density of ∼12.05mA/cm2 with reasonably good power conversion efficiency of ∼2.43%. The high performance might due to the good absorption with improvement in the thin film morphology and good intermolecular interaction behaviour of the oligomer blending with fullerene (PC60BM) acceptor.

Impact of Donor–Acceptor Functionalization on the Properties of Linearly π-Conjugated Oligomers: Establishing Quantitative Relationships for the Substituent and Substituent Cooperative Effect Based on Quantum Chemical Calculations

To understand better the impact of donor-acceptor substitution on the properties of linearly π-conjugated compounds, we performed a computational study on a series of variably substituted trans-polyacetylenes, polyynes, and polythiophenes. The focus of this work is on how rapidly the impact of a given substituent or a given combination of substituents vanishes along the π-conjugated chain. The response of the structural (bond-length alternation, rotational barrier) and molecular properties ((hyper)polarizability, chemical shift) to substitution is analyzed using different protocols, including a superposition model for the evaluation of the cooperative effect of substituents in homo- and heterosubstituted oligomers. With the exception of the (hyper)polarizability, the impact of donor-acceptor substitution is found to vanish following an exponential. The rate of decay of the substituent impact is found to be characteristic for each backbone, whereas the choice of substituent determines the absolute value of the respective property. The combination of substituents is shown to determine whether the substituent cooperative effect on a property is of an enhancing or damping nature. The rate of decay of the cooperative effect on most properties, including the (hyper)polarizability, is also found to follow an exponential law.

Nanomaterials: Polymers Containing Highly Polarizable Conjugated Side Chains as High‐Performance All‐Organic Nanodielectric Materials (Adv. Funct. Mater. 45/2013)

High-performance nanodielectric materials are built by C. Tang and co-workers on page 5638 based on nano-dipolar π-conjugated oligomer thiophenecontaining polymers that exhibit high permittivity and low dielectric loss over a wide range of frequencies (100 Hz to about 10 MHz). Highly polarizable and fast-responsive nano-dipoles from nanoscale crystalline domains from π–π stacked oligomer thiophenes are believed to dictate the performance. These materials could be used to construct high energy-density capacitors. The cover image is courtesy of Xiaodong Yin and Mitra Ganewatta.

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.

Self-assembly, Physicochemical, and Field-effect Transistor Properties of Solution-crystallized Organic Semiconductors Based on π-Extended Dithieno[3,2-b:2′,3′-d]thiophenes

Abstract Thienoacene-based π-conjugated oligomers, DTT-NP8 and DTT-BT8, were designed and synthesized. Their strong intermolecular interactions originating from S···S contacts enable the fabrication of single-crystal microribbons via solution processing. Organic single-crystal transistors exhibited field-effect mobilities as high as 0.54 cm2 V−1 s−1 and on/off current ratio of ca. 106.

Assembly of peptide–thiophene conjugates: the influence of peptide content and location

Biomolecule-directed self-assembly of π-conjugated oligomers has attracted great attention in the past decade. In this contribution, two conjugates composed of quaterthiophene and tetrapeptide (Gly-Val-Gly-Val) were synthesised, namely peptide–thiophene–peptide (PTP) and thiophene–peptide–thiophene (TPT), to investigate the influence of peptide content ratio and its location in the molecular structures on the nanostructures and properties of the assemblies. Both conjugates formed organogels consisting of left-handed twisted nanostructures; however, anti-parallel β-sheets were observed in PTP while parallel β-sheets were obtained for TPT, although in both cases oligothiophenes adopted an H-like stacking mode. Obvious solvent-induced supramolecular chirality inversion from the oligothiophene segment was observed for PTP while such phenomenon was not clear for TPT. PTP and TPT gels also showed different stabilities towards temperature increase, as evidenced by variable-temperature circular dichroism study. From the data, it is suggested that the rational design of the location and ratio of peptide plays a key role in constructing materials with determined properties based on peptide–thiophene conjugates.