Π-conjugated Main Chain Research Articles

Alkoxy-Functionalized Bithiophene/thiazoles: Versatile Building Blocks for High-Performance Organic and Polymeric Semiconductors

ConspectusOrganic electronics has experienced substantial advances in the past decade, driven by the development of high-performance organic semiconductors (OSCs) in combination with device engineering. While the pursuit of new aromatic building blocks has been a central topic in OSC innovation, the installation of novel side chains is also of significance for accessing high-performance solution-processable OSCs due to their great impact on (macro)molecular conformation/configuration, energy levels, intra/intermolecular interaction, and packing motifs, as well as film morphology of the materials.Compared to tuning the length, branching point, anchoring position, and terminal group of alkyl side chains, alkoxy functionalization can afford multifaceted advantages by modulating the properties of both π-conjugated main chains and side chain substituents. For instance, the oxygen atom in alkoxy chains not only greatly decreases the steric hindrance between adjacent aromatic rings due to its reduced van der Waal radius (∼1.4 Å) versus that of a CH2 moiety (∼2.0 Å) but also induces intramolecular noncovalent interaction for improving backbone coplanarity and charge transport properties. The highly electron-donating alkoxy chains can also greatly facilitate the intramolecular charge transfer (ICT), and hence the resulting semiconductors can yield absorption beyond the infrared region, which is essential for full coverage of solar absorption in photovoltaic devices. Additionally, the high polarity of oligo(ethylene glycol)-type alkoxy side chains can improve the miscibility of the OSCs with molecular dopants and ions, thus enabling them as a key part of the OSCs for emerging applications such as organic thermoelectrics and organic electrochemical transistors.In this Account, we summarize our pioneering and systematic efforts on the rational design, synthesis, and applications of novel alkoxy-functionalized head-to-head (HH)-linked bithiophene/thiazole-based building blocks and related organic/polymeric semiconductors. First, starting with a brief retrospective to the long pursuit of regioregular polythiophenes to avoid HH linkages for accessing highly planar polymers with high mobility, we introduce the basic design guidelines for developing alkoxy-functionalized bithiophene/thiazole-based building blocks via various molecular engineering strategies, including noncovalent interaction incorporation, symmetry-breaking, fluorination, cyanation, esterification, etc. In this part, the merits of HH-linked alkoxy-functionalized bithiophene/thiazoles in constructing high-performance OSCs will be elaborated. Then the principles of designing organic and polymeric semiconductors based on these building blocks toward applications in different optoelectronic devices will be further discussed. Afterward, we present recent examples of alkoxy-functionalized bithiophene/thiazoles-based materials which have delivered state-of-the-art performance in various optoelectronic devices, showing how the judicious structure tailoring of these building blocks can optimize the materials properties and device performance. Finally, we offer our insights into the further development of alkoxy-functionalized bithiophene/thiazoles-based derivatives and semiconductors for high-performance optoelectronic devices.

Precise synthesis of α,ω-chain-end-functionalized poly(dimethylsiloxane) with bromoaryl groups for incorporation in naphthalene-diimide-based N-type semiconducting polymers

To date, poly(dimethylsiloxane) (PDMS)-based materials have been developed to utilize their unique properties, such as backbone flexibility and good solubility in organic solvents, in many applications. In the present study, we successfully synthesized multiblock copolymers consisting of PDMS, naphthalene-diimide (NDI), and bithiophene units by statistical terpolymerization. First, α,ω-chain-end-functionalized PDMS with 4-bromophenyl groups (BrPh-PDMS-PhBr) was obtained by metal-free controlled ring-opening polymerization of cyclotrisiloxane using a strong organo-base catalyst, followed by an end-capping reaction with (4-bromophenyl)chlorodimethylsilane. BrPh-PDMS-PhBr was found to be a key material in the statistical terpolymerization of BrPh-PDMS-PhBr, NDI, and bithiophene monomers based on the Migita-Kosugi-Stille coupling reaction. Indeed, three PDMS-containing multiblock copolymers were successfully synthesized with different PDMS compositions of 10, 16, and 36 wt%. In practice, the multiblock copolymer with 10 wt% PDMS content showed a typical n-type semiconducting character (μe = 1.5 × 10−3 cm2 V−1 s−1), according to the results of organic field-effect transistor (OFET) fabrication and evaluation, despite the incorporation of insulating PDMS segments along the π-conjugated main chains.

Solid-State Low-Temperature Thermoresponsive and Reversible Color Changes of Conjugated Polymer in Layered Structure: Beyond Infrared Thermography.

Emergence of thermoresponsive and reversible color changes at low temperature is a challenging target. In general, it is not easy to induce sufficient dynamic motion of rigid molecules including chromophore at a lower temperature. The present work shows unusually low-temperature color-change properties originating from the dynamic motion of rigid conjugated polymer in solid state. The layered composites of polydiacetylene (PDA) and guest l-arginine (L-Arg) (PDA-(L-Arg)) exhibit temperature-responsive gradual color changes with reversibility in the range of 123-333 K in solid crystalline state. The dynamic properties are induced by gradual and reversible distortion of the π-conjugated main chain in response to temperature. The tuned flexibility of the layered structure facilitates motion of the rigid π-conjugated molecule at low temperature. The PDA-(L-Arg)-coated substrates are applied to visualization and quantification of 2D and 3D temperature distributions generated by cooling with liquid nitrogen. These thermographic devices afford to image lower temperature range than typical infrared thermography. The present work indicates potentials of layered architectures with tunable flexibility for emergence of dynamic properties.

Effects of the Isomerized Thiophene-Fused Ending Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells.

Recently, benefiting from the merits of small-molecule acceptors (NFAs), polymer solar cells (PSCs) have achieved tremendous advances. From the perspective of the structural characteristics of the π-conjugated acceptor-donor-acceptor (A-D-A) type of organic molecules, the backbone's planarity and the terminal groups and their substituents have strong influences on the performances of the constructed NFAs. Through enlarging the dihedral angle of the conjugated main chain of NFAs, a certain degree of enhancement of photovoltaic parameters has been achieved. To further probe the influences of ending groups on the performances of nonplanar NFAs, we synthesized two new NFAs i-cc23 and i-cc34 with isomerized thiophene-fused ending groups and a twisted π-conjugated main chain. Compared to i-cc23 containing the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene)malononitrile ending group, the acceptor i-cc34 containing 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile has a relatively higher molar extinction coefficient, bathochromic-shifted absorption spectrum, and deepened energy levels. When mixed with PBDB-T in solar cells, the i-cc23-based device achieved an excellent open-circuit voltage (VOC) of 1.10 V and a moderate power conversion efficiency of 7.34%. Although the VOC of the i-cc34-related device was decreased to 0.96 V, the short-circuit current density and fill factor were improved, giving rise to an enhanced efficiency of 9.51%. Apart from the distinct photovoltaic performances, the two isomer-based devices exhibit a high radiative efficiency of 8 × 10-4, leading to a very small nonradiative loss of 0.19 V. Our results emphasize the importance of the isomerized thiophene-fused ending groups on the performances of nonplanar NFA-based PSCs.

Benzoquinone-Based Polyimide Derivatives as High-Capacity and Stable Organic Cathodes for Lithium-Ion Batteries.

Multicarbonyl polyimide derivatives were synthesized by a facial condensation polymerization of dianhydrides with a new diamine monomer containing a benzoquinone unit that was prepared according to the Michael addition reaction. The ingenious combination of dedicated carbonyl groups from the benzoquinone and dianhydride with an aniline structure linkage not only provided stable polymeric chains with a high number of carbonyl groups per unit but also guaranteed their large π-conjugated main chains, which is favorable to their long cycle life and fast kinetics. When explored as cathode materials for lithium-ion batteries, the polyimide derivatives based on naphthalic dianhydride delivered a reversible specific capacity of 145 mAh/g at 0.1 C, a high rate performance with a capacity of 108 mAh/g at 1 C, and an ultralong stable cyclic performance with a capacity retention of 80.3% after 1000 cycles at 0.5 C. Based on the theoretical calculations and the exploration of the electrochemical behaviors, sensible predictions for the reversible ion-insertion reaction of the as-prepared sample were proposed to deeply understand the charge storage mechanisms. Moreover, a stable solid electrolyte interphase film formed in the ether-based electrolyte was confirmed to improve the electrochemical properties.

Intermolecular Arrangement of Fullerene Acceptors Proximal to Semiconducting Polymers in Mixed Bulk Heterojunctions.

Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite the correlation between structural characteristics and efficiency in organic photovoltaics (OPVs). This study reveals that the substitution patterns of linear and branched alkyl side chains on electron-donating/-accepting alternating copolymers can control the positions of an acceptor molecule (C60 ) around the π-conjugated main chains in mixed BHJs. Two-dimensional solid-state NMR demonstrates a marked difference in the location of C60 in the blend films. A copolymer with an electron-accepting unit positioned in close proximity to C60 demonstrated higher OPV performance in combination with various fullerene derivatives. This molecular design offers precise control over the interfacial molecular structure, thereby paving the way for overcoming the current limitations of OPVs comprising mixed BHJs.

π-Conjugated Main Chain Polymers Containing Bis(bithiophenyl dithienothiophene)-based Repeating Group and their Application to Polymer Solar Cells

New dithienothiophene (DTT)-containing conjugated polymers, such as poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-yl)dithieno[3,2-b;2′,3′-d]thiophene, 1 and poly(2,6-bis(2-thiophenyl-4-dodecylthiophene-2-yl)dithieno[3,2-b;2′,3′-d]thiophene, 2 have been successfully synthesized via Stille coupling reactions using dodecyl-substituted thiophene-based monomer, bistributyltin dithienothiophene, and bistributyltin bithiophene. The main difference between two polymers is the substitution position of dodecyl side chains in a repeating group. The semiconducting properties of the two polymers have been evaluated in organic photovoltaics (OPVs). Two conjugated polymers are also well mixed with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) for fabricating bulk heterojunction PV devices. As a result, the polymer solar cell devices made of polymers 1 and 2 had the best preliminary results with an open-circuit voltage of 0.43–0.44 V, a short-circuit current density of 5.95–5.96 mA/cm2, and a fill factor of 0.34, offering an overall power conversion efficiency (PCE) of 0.9%.

π-Conjugated Poly(aryleneethynylene)s Consisting of Salophen and Ni−Salophen Units in the π-Conjugated Main Chain: Preparation and Chemical Properties

π-Conjugated poly(aryleneethynylene)s consisting of N,N′-bis(salicylidene)-1,2-phenylenediamine (salophen) units (Poly-1−Poly-3) and salophen−nickel complex units (Poly-1−Ni) were prepared in high yields by the palladium-catalyzed polycondensation of H−≡−Ar−≡−H (Ar = fluorene or 2,5-dialkoxy-p-phenylene) with dibromo compounds of salophen (Br2−Salophen) and its nickel complex (Br2−Salophen−Ni), respectively. The Ni-free polymers (Poly-1−Poly-3) showed good solubility in CHCl3 and THF, had high thermal stability, and exhibited number-average molecular weights (Mn) of 9200−14000 in GPC analysis. The UV−vis spectra of the polymers showed π−π* transition peaks at about 420 nm, which was comparable to those of poly(p-phenyleneethynylene)s. The UV−vis spectrum of Poly-1−Ni exhibited additional intermediate and small peaks at about 480 and 600 nm, which were assigned to a charge-transfer (CT) electronic transition and a d−d transition, respectively. The complexation of Poly-1 with Ni2+ proceeded clearly and quan...

Synthesis of Organoboron Quinoline-8-thiolate and Quinoline-8-selenolate Complexes and Their Incorporation into the π-Conjugated Polymer Main-Chain

Low-molecular-mass organoboron quinoline-8-thiolate and -selenolate complexes as model compounds, and organoboron polymers incorporated their complex structures into the poly(p-phenylene-ethynylene) main chain were prepared. Tetracoordination states of boron atoms in the obtained compounds were confirmed by 11B NMR spectroscopy, and the detailed structures of the model compounds were determined by single-crystal X-ray diffraction analysis. The polymers were synthesized by Sonogashira-Hagihara coupling reaction of organoboron quinolate-based monomers having diiodo groups with 1,4-diethynylbenzene derivatives bearing electron-donating or -withdrawing groups in moderate yields. Their optical properties were studied by UV−vis absorption and photoluminescence spectroscopies. Increasing atomic number of the 16 Group atom adjacent to the boron atom caused emission shift to longer wavelength and decreasing of absolute quantum yields for both the model compounds and the polymers. There are no differences between the polymers with the donating π-conjugated linker and with the accepting one in the photoluminescence property, resulting from efficient energy transfer from π-conjugated main chain to Q ligand. Furthermore, the obtained polymers showed high refractive indices (nd > 1.66).

Synthesis and through-bond spin interaction of stable 1,3-phenylene-linked polyradical carrying aminoxyls in the π-conjugated main chain

A stable polyradical carrying aminoxyls in the π-conjugated main chain was synthesized and its through-bond spin interaction was investigated. The polyradical is constructed with 1,3-phenylene as a spin coupler and 9,9-di-n-propyl-9,10-dihydroacridin-10-yloxy as a stable spin source. The synthetic method of the spin source moiety was modified by using conventional organic reactions from a previously reported method. The polymeric structure was synthesized by Suzuki–Miyaura coupling. The degree of polymerization and the spin concentration of the polyradical were ca. 11 and 0.71 spins per repeating unit, respectively, indicating that there were ca. 8 spins per molecule. The average S value of the polyradical was 3/2, therefore, 3 spins out of 8 spins were aligned in the polyradical. This showed that the through-bond spin interaction occurs more effectively than in our previous polyradical carrying aminoxyls in the side chain.

Intermolecular interaction effects on second hyperpolarizabilities of clusters including charged species

We investigate the intermolecular-interaction effects on the longitudinal second hyperpolarizabilities (γ) for two types of π−π stacking clusters: C$_{5}$H$_{7}^{p}$ dimer and π-conjugated main chains (C$_{n}$H$_{np2}$, 6 l n l 16) interacting in both end regions with two allyl cations (C$_{3}$H$_{5}^{p})$. The γ values are calculated by the finite-field approach using several ab initio molecular orbital and density functional methods. It is found that in the first model the π− π interaction between the upper and lower π-orbitals significantly reduces the magnitude of effective γ per monomer, while the second model exhibits a remarkable enhancement of γ values as compared to those of isolated main chains. The analysis using the hyperpolarizability density and orbital interaction elucidates that the reduction of longitudinal γ value in the first model originates in the reduction of longitudinal π-conjugated electron distributions due to the perpendicular π−π orbital interactions, whereas the enhancement of longitudinal γ in the second model is caused by the intermolecular charge transfer between both end perturbing molecules via the main chain. On the basis of these results, we discuss a novel guideline of controlling third-order nonlinear optical properties of clusters.

Intramolecular spin interaction of 1,3-phenylene linked polyradicals bearing nitroxides in the π-conjugated polymeric main and side chains

Stable 1,3-phenylene linked polyradicals bearing nitroxides in the π-conjugated main and side chains were synthesized and the intramolecular spin interaction was investigated. The S values numerically calculated based on the degree of polymerization and the spin concentration of polyradicals was in good agreement with the experimental values. This showed that the spin defect intercepts the intramolecular spin interaction in the polyradicals bearing nitroxides only in the π-conjugated main chain, whereas, in the polyradical bearing another nitroxide in the side chain, there is the intramolecular spin interaction across the spin defect.

Synthesis and Luminescent Properties of Novel Twisted Copolymers Containing Dithienophosphole Oxide/Dithienosilole and Spirobifluorene

Abstract Two new series of twisted polymers, poly[fluorenedithieno[3,2-b:2′,3′-d]phosphole oxide-alt-2,2′-spirobifluorelene] (PFDTPO-SBF) and poly[fluorenedithieno[3,2-b:2′,3′-d]silole-alt-2,2′-spirobifluorelene (PFDTS-SBF), in which the backbone was sufficiently twisted and the effective conjugation length was controlled, were synthesized through Suzuki coupling reaction. The introduction of DTPO or DTS into π-conjugated main chains apparently turned the emission wavelength, and possessed the low-lying LUMO energy levels.

Transfer of Photoenergy in π-Conjugated Polymers. Two Types of Photoluminescence that Involve Energy Transfer along a Polymer Chain

Two types of energy transfer in pi-conjugated polymers have been investigated using time-resolved photoluminescence (PL) techniques: type i, perpendicular-type energy transfer from the 2,3-di(p-tolyl)quinoxaline unit to the pi-conjugated main chain of poly[2,3-di(p-tolyl)quinoxaline-5,8-diyl], and type ii, parallel-type energy transfer from the oligo(pyridine-2,5-diyl) (O-Py) unit to the oligo(selenophene-2,5-diyl) (O-Se) unit in a block-type copolymer of O-Py and O-Se. Both types of energy transfer were very fast with a time constant shorter than approximately 0.1 ns; in particular, the type ii energy transfer took place with a time constant of approximately 5 ps. Both pi-conjugated polymers were considered to contain segments with various effective pi-conjugation lengths, and the energy transfer to the segment with a larger effective pi-conjugation length and a smaller pi-pi* transition energy required a longer transition time. A polarizing film was obtained by utilizing the perpendicular-type energy transfer.

New soluble unsaturated polyketone derived from diarylidenecycloalketone: synthesis and optical and electrochemical properties of π-conjugated poly(diarylidenecyclohexanone) with long side chains

Abstract A new type of unsaturated polyketone having cyclohexanone moiety in a π-conjugated main chain was prepared by polycondensation between 2,6-bis(4-bromobenzylidene)cyclohexanone and 2,5-dihexyloxy- p -phenylene diboric ester in the presence of Pd(PPh 3 ) 4 . The polymer had good solubility in common organic solvents. Analysis by gel permeation chromatography (GPC; polystyrene standards) showed that the polymer had M n and M w values of 7800 and 18 200, respectively. The polymer exhibited a [ η ] value of 0.70 dl g −1 in benzene at 30 °C. The chloroform solution of the polymer showed an UV–Vis peak at 392 nm, and the PL spectrum gave a peak at 533 nm. DSC exhibited that the polymer had a T g of 85 °C. The DSC data, observation with a polarizing microscope, X-ray diffraction data and UV–Vis data of the obtained polymer showed a phase transition above 200 °C. TGA showed that the polymer had good thermal stability with 5 wt% loss temperature of 407 °C under N 2 . Electrochemical oxidation (or p-doping) of the polymer started at about 0.7 V vs. Ag/AgNO 3 and gave a peak at 1.06 V vs. Ag/AgNO 3 with a color change of the film from yellow to deep red. The color change was followed by UV–Vis spectroscopy. The corresponding p-dedoping peak appeared at 0.58 V vs. Ag/AgNO 3 .

New Soluble, Coplanar Poly(naphthalene-2,6-diyl)-Type π-Conjugated Polymer, Poly(pyrimido[5,4-d]pyrimidine-2,6-diyl), with Nitrogen Atoms at All of the o-Positions. Synthesis, Solid Structure, Optical Properties, Self-Assembling Phenomena, and Redox Behavior

New n-type π-conjugated poly(pyrimido[5,4-d]pyrimidine-2,6-diyl)s with a poly(naphthalene-2,6-diyl)-type structure and alkylamino substituents were prepared by dehalogenative polycondensation using a zerovalent nickel complex in 90−98% yield. The polymer is considered to have a coplanar π-conjugated main chain and a strong tendency to form a π-stacked structure, as judged from its CPK molecular model, UV−vis data, photoluminescence data, and XRD data. The obtained polymers were soluble in organic solvents and a polymer with N-octylamino substituents showed a number-average molecular weight, Mn, of about 8000 in GPC analysis. Other polymers showed analogous Mn values. The UV−vis spectra of the polymers gave a main absorption peak at about 450 nm, which was shifted by about 130 nm to a longer wavelength compared to those of corresponding monomeric compounds. The degree of the red shift was larger than those observed with other analogous poly(naphthalene-2,6-diyl)-type π-conjugated polymers, and the large red shift was attributed to the coplanar structure of the polymer. UV−vis and photoluminescence data revealed a strong tendency for the polymer to form a self-assembled structure. Powder X-ray diffraction analysis indicated that the polymers took an end-to-end packing mode. Electrochemical reduction (or n-doping) of the polymers proceeded with an Epc of about −2.3 V vs Ag+/Ag.

New Soluble π -Conjugated Tetrathiafulvalene (TTF) Polymers Bearing Long Alkyl Side Chains: Preparation by Organometallic Polycondensation, Structure and Chemical Properties of the Polymers

New soluble poly(arylene) polymers containing TTF units in a π-conjugated main chain were obtained by organometallic polycondensation. The polymers assume an ordered structure in the solid and are electrochemically active to be oxidized at about 0.3 V vs. Ag + /Ag in cast films.

Single molecule of a π-conjugated polymer slowly twinkles in solution at room temperature

A phenylacetylene monomer, 4-(decyloxycarbonyl)phenylacetylene (DecCPA), was synthesized and polymerized using the combined catalyst system, i.e. [Rh(norbornadiene)Cl]2 and triethylamine, to give a yellow solid π-conjugated polymer [poly(DecCPA)] of high molecular weight and cis-transoidal stereo-regular main chain. Poly(DecCPA) was measured using the total internal reflection fluorescence microscopy, which is a single molecule detection technique that is carried out in a tetrahydrofuran solution at room temperature with the light-twinkling of a single molecule being observed. This twinkling is a dynamic photon emission phenomenon of single poly(DecCPA) molecule that we mention specifically as a resultant from the time scale — changing very slowly in the time scale of milliseconds and seconds — with the changed fluorescent intensity being associated with the changing single polymer molecule conformation caused by the micro-Brownian motion. Although molecular dynamics was of the picosecond time scale, generally, this poly(DecCPA) molecule had the mode of the molecular motion of π-conjugated main chain. This was very slow in the second time scale as was shown.

A Polymer with Two Different Redox Centers in the π-Conjugated Main Chain: Alternate Combinations of Ferrocene and Dithiafulvene

A redox-active alternating π-conjugated copolymer of ferrocene with dithiafulvene was synthesized by cycloaddition polymerization of aldothioketene derived from 1,1‘-bis(trimethylsilylethynyl)ferrocene (1). The obtained polymer (2) was soluble in common organic solvents such as THF, CH2Cl2, CHCl3, DMF, and DMSO. The structure of 2 was confirmed by IR, 1H NMR, and 13C NMR spectra compared with those of 2,6-bis(ferrocenyl)-1,4-dithiafulvene as a model compound (3). The cyclic voltammogram of 3 showed three oxidation peaks, which were originated from the two ferrocene units and the dithiafulvene unit. The polymer 2 showed only single-broad oxidation peak at 0.58 V vs Ag/Ag+. This result was caused by an effective interaction between two different donors in 2. Compounds 2 and 3 did not undergo oxidative dimerization to form extended tetrathiafulvalene during cyclic voltammetry. The polymer 2 formed a soluble charge-transfer (CT) complex with TCNQ in DMSO. The ratio of the dithiafulvene unit in 2 to TCNQ in th...

Preparation of poly(aryleneethynylene) type π-conjugated polymers constituted of 2-alkylbenzimidazole-4,7-diyl units by using palladium catalyzed cross coupling reaction

Palladium(0)-catalyzed polycondensations between 4,7-dibromo-2-alkylbenzimidazoles (alkyl= n-C 7H 15, n-C 12H 25 and n-C 18H 37) and 1,4-diethynyl-2,5-disubstituted-benzenes (substituents: O- n-C 6H 13, O- n-C 12H 25, n-C 12H 25 and O- n-C 18H 37) give poly(aryleneethynylene) (PAE) type polymers including the benzimidazole unit in the π-conjugated main chain. Pd(PPh 3) 4 was effective for the polycondensation. Yields of these polymers were in the range of 70–90%. IR and NMR spectra agree with the structure of the polymers. The polymers are soluble in general organic solvents such as chloroform, tetrahydrofuran and toluene. UV–Vis spectra of PAEs show π–π* absorption peaks at a longer wavelength than those of the corresponding monomers due to the expansion of the π-conjugated system.