Π-conjugated Backbone Research Articles

Ordered π-conjugated polymer backbone in amorphous blend for high efficiency nonfullerene organic photovoltaics

In π-conjugated polymers, the amorphous region absent from π–π stacking order typically limits polymer functions compared to the crystalline region with high π–π stacking order. Here, we show that a benzodithiophene–thiazolothiazole copolymer containing tripropylsilyl groups (PSTz2) has a greater coplanar backbone structure when the π–π stacking order is absent, such as in solution and in a film blended with a nonfullerene acceptor, than when it is present in a neat film. The excitons and charge carriers generated in PSTz2 are more highly delocalized in the blend film than in the neat film, presumably through the backbone. The unconventional structural feature of PSTz2 shows higher photovoltaic performance in nonfullerene-based cells compared to its alkyl-functionalized counterpart. Our results show that it is possible to develop π-conjugated polymers that perform well in amorphous blends due to the ordered backbone structure.

Effect of Surface Chemistry on Macrocycle and Conducting Polymer Based-Carbon Composites

Redox-active carbon composite-based electrodes, especially nitrogen-based redox active materials, can provide high power and energy storage in electrochemical capacitors. These materials include porphyrin macrocycles which are found in nature and possess unique electronic and redox-active properties from their large π-conjugated systems [1], [2]. When compositing with carbon-based materials such as carbon nanotubes (CNTs), they have shown increased stability and long-term performance. Previous work using macrocyclic tetraphenyl porphyrin sulfonate (TPPS) in CNT-based composites demonstrated improved capacitive profiles, better interfacial kinetics, and charge retention introduced in capacitive electrodes [3], [4]. On the other hand, conducting polymers with π-conjugated backbones also enabled high charge storage when used in composites [5]. To further advance these capabilities, the effects and mechanisms of surface functionalities of carbon substrates need to be investigated. For example, the presence of carboxyl groups on CNTs has been shown to improve charge storage through favourable interfacial interactions in some conducting polymers [6], [7]. The challenge as highlighted in Figure 1 lies in understanding which redox-active species, e.g. macrocycle or conducting polymer, will favour certain surface functionalities based on the nature of their interactions. Thus, a systematic investigation to design and create desirable composites and interactions for high energy and power densities, and to extend to low-cost carbon sources including waste biomass-based activated carbons.In this work, we conducted a systematic study comparing the effect of several surface functionalities like carboxyl groups on TPPS macrocycles and on conducting polymer-based carbon composites to answer the question on why certain surface functionalities are favored for each species. Preliminary studies have shown presence of TPPS has increased the capacitive profiles, reaction kinetics and rate capabilities of CNT composites, exceeding the conducting polymer counterparts. But this capacitive increase can be further improved through surface modification. This study leveraged layer-by-layer deposition approaches to fabricate redox-active carbon composites, followed by electrochemical characterizations using cyclic voltammetry and electrochemical impedance spectroscopy. Surface morphology was studied using electron microscopy while surface functional groups were investigated using x-ray photoelectron spectroscopy. The findings from this study can be used to implement systematic surface modification for redox-active carbon composites to improve energy storage, towards a more sustainable future.

Intrinsically Stretchable And Efficient Fully Π-Conjugated Polymer via Internal Plasticization for Flexible Deep-blue Polymer Light-emitting Diodes with CIEy = 0.08.

Intrinsically stretchable polymeric semiconductors are essential to flexible polymer light-emitting diodes (PLEDs) owing to their excellent strain tolerance capacity under long-time deformation operation. Obtaining intrinsic stretchability, robust emission properties, and excellent charge-transport behavior simultaneously from fully π-conjugated polymers (FCPs) is difficult, particularly for applications in deep-blue PLEDs. Herein, wepropose an internal plasticization strategy to introduce a phenyl-ester plasticizer into polyfluorenes (PF-MC4, PF-MC6, and PF-MC8) for narrowband deep-blue flexible PLEDs. Compared with controlled poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPFs) (2.5%), the freestanding PF-MC8 thin film shows a fracture strain of > 25%. The three stretchable films exhibit stable and efficient deep-blue emission (PLQY > 50%) because of the encapsulation of π-conjugated backbone via pendant phenyl-ester plasticizers. The PF-MC8-based PLEDs show deep-blue emission, which corresponds to CIE and EQE values of (0.16, 0.10) and 1.06%, respectively. Finally, the narrowband deep-blue electroluminescence (FWHM of ∼25nm; CIE coordinates: (0.15, 0.08)) and performance of the transferred PLEDs based on the PF-MC8 stretchable film are independent of the tensile ratio (up to 45%); however, they show a maximum brightness of 1976cd m-2 at a ratio of 35%. Therefore, internal plasticization is a promising approach for designing intrinsically stretchable FCPs for flexible electronics. This article is protected by copyright. All rights reserved.

A redox-active polymer material for superior capacitive storage in ionic liquid electrolyte

Supercapacitors based on ionic liquid (IL) electrolytes offer realistic possibilities for high-performance and sustainable energy storage systems with their unique electrochemical properties, especially wide operation voltage and high usage security. However, there is still a lack of suitable capacitive-storage electrode materials in IL electrolytes. Herein, we have synthesized a highly redox-active polymer (PDPPZ) material with extended conjugated imine-based backbones for superior capacitive storage. Theoretical calculations, such as localized orbital locator-π (LOL-π) and molecular orbital techniques, reveal extraordinary electron delocalization and excellent electron affinity throughout the highly extended π-conjugated polymeric backbones. As a result, the PDPPZ polymer as an organic electrode possesses a high capacitive storage of 135.6 F g−1 and long-term stability with as low as 0.003% decline per cycle in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) IL electrolyte, evidencing its shining in the electrochemical energy storage field.

An In-Situ Cyanidation Strategy to Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10 cm2 V-1 s-1.

A family of novel highly π-extended tetracyano-substituted acene diimides, named as tetracyanodiacenaphthoanthracene diimides (TCDADIs), are synthesized using a facile four-fold Knoevenagel condensation strategy. Unlike conventional cyano substitution reactions, our approach enables access to a large π-conjugated backbone with the in-situ formation of four cyano substitutes at room temperature while avoiding extra cyano-functionalization reactions. TCDADIs decorated with different N-alkyl substituents present good solubility, near-coplanar backbones, good crystallinity, and low-lying lowest unoccupied molecular orbital energies of -4.33 eV, all of which contribute to desirable electron-transport performance when applied to organic field-effect transistors (OFET). The highest electron mobility of OFET based on 2-hexyldecyl-substituted TCDADIs single crystal reaches 12.6 cm2 V-1 s-1, which is not only among the highest values for the reported n-type organic semiconductor materials (OSMs) but also exceeds that of most n-type OSMs decorated with imide units.

High field-effect mobility in oriented thin films of D-A type semiconducting polymers by engineering stable interfacial system

Conjugated polymers (CPs) have emerged as excellent organic semiconducting materials for flexible electronic applications. CPs exhibit a notably fast carrier transport along their main π-conjugated backbone followed by inter-chain hopping compelling the need to form large and ordered domains in thin films. Therefore, the formation of larger domains, along with their orientation, is critical in controlling the electrical properties of CP-based organic electronic devices. However, controlling the molecular ordering along with the formation of large aggregates is a challenge. In this work, the unidirectional floating film transfer method (UFTM) has been utilized to prepare oriented thin films of CPs, which is a highly facile, cost-effective and material utilization efficient technique. We have demonstrated that extent and direction of molecular orientation in the fabricated thin films can be tuned along with the higher degree of aggregation. The rheological aspects, especially viscosity, play a significant role in determining molecular ordering along with the formation of these large aggregates. Active control of viscosity to achieve a stable interfacial viscous system is paramount to achieving this, resulting in a very high mobility of 12.4 cm2V−1s−1.

Metal-free molecular catalysts for electrochemical CO2 reduction

In a recent Journal of the American Chemical Society paper, Choudhury et al. report several pyridinic molecules, heterohelicenes, as homogeneous and metal-free catalysts for electrochemical CO2 reduction. The imidazolium-based cations and π-conjugated backbones of the heterohelicenes can stabilize the radical species during catalytic process and eventually boost the catalytic activity. In a recent Journal of the American Chemical Society paper, Choudhury et al. report several pyridinic molecules, heterohelicenes, as homogeneous and metal-free catalysts for electrochemical CO2 reduction. The imidazolium-based cations and π-conjugated backbones of the heterohelicenes can stabilize the radical species during catalytic process and eventually boost the catalytic activity.

Detrimental Effects of “Universal” Singlet Photocrosslinkers in Organic Photovoltaics

The use of singlet photocrosslinkers based on diazirine or azide has been investigated for organic photovoltaics (OPVs) to stabilize the morphology of mixed bulk heterojunction (BHJ) films. The photocrosslinking reactions in BHJs have major detrimental effects on the performance of OPVs. This is in striking contrast to previous reports on organic field-effect transistors, where these singlet crosslinkers can solidify the films without affecting the charge transport properties. Based on the OPV results with the layer-by-layer-deposited films and the transferred films, we found that the crosslinking reactions had greater effects on the non-fullerene acceptor than on the donor polymer. The photoluminescence measurements suggested that the crosslinkers could react with the π-conjugated backbone of the non-fullerene acceptor to form trace amounts of byproducts that cause severe exciton quenching. These results indicate that the use of the “universal” singlet photocrosslinkers in OPVs requires careful selection of the organic semiconducting materials and crosslinkers.

Photoluminescent Properties and Mechanism of Novel Cyanine-Borondifluoride Curcuminoid Hybrids as Red-to-Near Infrared and Endoplasmic Reticulum-Targeting Dyes.

Synthesis-oriented design led us to the discovery of a series of novel cyanine-borondifluoride curcuminoid hybrids called Nanchang Red (NCR) dyes that overcome the intrinsic low synthetic yields of symmetrical cyanine-difluoroboronate (BF2 )-hybridized NIR dyes. The hybridization endows NCR dyes with high molar extinction coefficients, efficient red-to-NIR emission, and enlarged Stokes shifts. Quantum chemical calculations revealed that the asymmetrical layout of the three key electron-withdrawing and electron-donating fragments results in a special pattern of partial charge separation and inconsistent degrees of charge delocalization on their π-conjugated backbones. While the nature of the hemicyanine fragment exerts significant influence on the excitation modes of NCR dyes, the borondifluoride hemicurcuminoid fragment is the major contributor to the enlarged Stokes shifts. Cell imaging experiments illustrated that a subtle change in the N-heterocycle of the hemicyanine fragment has a remarkable effect on the subcellular localization of NCR dyes. Unlike other previously reported cyanine-BF2 hybridized dyes, which mainly target mitochondria, the benzothiazole and indole-based NCR dyes accumulate in both the endoplasmic reticulum (ER) and lipid droplets of HeLa cells, whereas the benzoxazole and quinoline-based NCR dyes stain the ER specifically.

Orbital Nature of Carboionic Monoradicals Made from Diradicals.

The electronic, optical, and solid state properties of a series of monoradicals, anions and cations obtained from starting neutral diradicals have been studied. Diradicals based on s-indacene and indenoacenes, with benzothiophenes fused and in different orientations, feature a varying degree of diradical character in the neutral state, which is here related with the properties of the radical redox forms. The analysis of their optical features in the polymethine monoradicals has been carried out in the framework of the molecular orbital and valence bond theories. Electronic UV-Vis-NIR absorption, X-ray solid-state diffraction and quantum chemical calculations have been carried out. Studies of the different positive-/negative-charged species, both residing in the same skeletal π-conjugated backbone, are rare for organic molecules. The key factor for the dual stabilization is the presence of the starting diradical character that enables to indistinctively accommodate a pseudo-hole and a pseudo-electron defect with certainly small reorganization energies for ambipolar charge transport.

1,4-Divinylphenylene-bridged diruthenium complexes with 2-hydroxypyridine- and 2- or 8-hydroxyquinoline-olate ligands

Abstract Four divinylphenylene-bridged diruthenium complexes [{Ru(CO)(P i Pr3)2(L-κ 2 [N,O] – )}2(μ-CH=CH-C6H4-CH=CH-1,4)] (2a–2d) with N,O-chelating 2-hydroxypyridine and 2-hydroxy- or 8-hydroxyquinoline ligands are presented. They were studied by NMR spectroscopy, electrochemical methods and, in their neutral and oxidized states, by IR, UV/Vis/NIR and, if applicable, by EPR spectroscopy. The experimental studies are complimented by (TD-)DFT calculations. Our results indicate that the pyridine-olate complexes 2a,b exist as three isomers with a ratio of about 78:20:2 that differ with respect to the orientation of the N and O donors relative to the CO and alkenyl ligands in the equatorial coordination plane. Only the isomer with both imine N donors trans to the alkenyl ligand is observed for complexes 2c,d with quinolinato ligands. All complexes undergo two consecutive, chemically and electrochemically reversible one-electron oxidations at low potentials. Our results indicate strong contributions of the divinylphenylene bridge to the redox processes and an even delocalization of the electron hole and the unpaired spin density over the entire π-conjugated divinylphenylene diruthenium backbone with only minor involvement of the peripherally attached κ 2 [N,O] – donor ligands.

C3-symmetric trimeric imidazole naphthoquinone derivative with dual redox-active sites for high-performance cathodic lithium storage

Organic lithium-ion batteries (LIBs) are among the most promising energy storage systems owing to their high energy density, low-cost and excellent sustainability. Although many organic materials with preferable redox centers and π-conjugated structure have been utilized as the cathodes, they often have unsatisfactory properties, such as low practical capacity, poor rate capability and short cycle life as well as high production complexity, which hindered the large-scale development of organic LIBs. Herein, a novel trimeric imidazole naphthoquinone derivative (2,2′,2′'-(benzene-1,3,5-triyl)tris(1H-naphtho [2,3-d]imidazole-4,9-dione, BTNID) with a highly π-extended conjugation backbone has been achieved by directly introducing quinone moieties from initial reactants and forming imidazole unit in-situ via Schiff base condensation. Exploited as organic cathode materials for LIBs, the BTNID delivers a specific capacity of up to 483 mAh g−1 at 0.2 A g−1 with an average discharge potential of ∼ 2.0 V, thus realizing an impressive energy density of ∼ 966 Wh kg−1. Moreover, the BTNID cathode shows remarkable rate capability (245 mAh g−1 at 10 A g−1) and excellent cycling stability (74.8% capacity retention after 3000 cycles at 10 A g−1), much higher than most of the so far reported results, which are mainly attributed to unique trigonal geometrical shape and extended π-conjugated backbone of BTNID molecule that allows electrons of redox-active quinone and imidazole moieties to delocalize through the π-π orbital interactions.

Redox Switching Behavior in Resistive Memory Device Designed Using a Solution-Processable Phenalenyl-Based Co(II) Complex: Experimental and DFT Studies.

We herein report a novel square-planar complex [CoIIL], which was synthesized using the electronically interesting phenalenyl-derived ligand LH2 = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one). The molecular structure of the complex is confirmed with the help of the single-crystal X-ray diffraction technique. [CoIIL] is a mononuclear complex where the Co(II) ion is present in the square-planar geometry coordinated by the chelating bis-phenalenone ligand. The solid-state packing of [CoIIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [CoIIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [CoIIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/CoIIL/Al, and characterized using the write-read-erase-read cycle. The device has interestingly shown a stable and reproducible switching between two different resistance states for more than 2000 s. Observed bistable resistive states of the device have been explained by corroborating the electrochemical characterizations and density functional theory studies, where the role of the CoII metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed.

Decoupling Planarizing and Steric Energetics to Accurately Model the Rigidity of π-Conjugated Polymers.

The π-conjugated backbone of semiconducting polymers gives rise to both their electronic properties and structural rigidity. However, current computational methods for understanding the rigidity of polymer chains fail in one crucial way. Namely, standard torsional scan (TS) methods do not satisfactorily capture the behavior of polymers exhibiting a high degree of steric hindrance. This deficiency in part stems from the method by which torsional scans decouple energy related to electron delocalization from that related to nonbonded interactions. These methods do so by applying classical corrections of the nonbonded energy to the quantum mechanical (QM) torsional profile for polymers that are highly sterically hindered. These large corrections to the energy from nonbonded interactions can substantially skew the calculated QM energies related to torsion, resulting in an inaccurate or imprecise estimation of the rigidity of a polymer. As a consequence, simulations of the morphology of a highly sterically hindered polymer using the TS method can be highly inaccurate. Here, we describe an alternative, generalizable method by which the delocalization energy can be decoupled from the energy associated with nonbonded interactions─the "isolation of delocalization energy" (DE) method. From torsional energy calculations, we find that the relative accuracy of the DE method is similar to the TS method (within 1 kJ/mol) for two model polymers (P3HT, PTB7) when compared to quantum mechanical calculations. However, the DE method significantly increased the relative accuracy for simulations of PNDI-T, a highly sterically hindered polymer (8.16 kJ/mol). Likewise, we show that comparison of the planarization energy (i.e., backbone rigidity) from torsional parameters is significantly more precise for both PTB7 and PNDI-T when using the DE method as opposed to the TS method. These differences affect the simulated morphology, with the DE method predicting a significantly more planar configuration of PNDI-T.

Bay-Functionalized Perylene Diimide Derivative Cathode Interfacial Layer for High-Performance Organic Solar Cells.

The field of organic solar cells (OSCs) has acquired rapid progress with the development of nonfullerene acceptors. Interfacial engineering is also significant for the enhancement of the power conversion efficiency (PCE) in OSCs. Among the cathode interfacial materials (CIMs), perylene diimide (PDI) small molecules are promising owing to the excellent electron affinity and electron mobility. Although the well-known PDINN molecule has excellent properties, it has a high planarity formed by an extensive rigid π-conjugated backbone. Because the PDI molecular backbone has a strong tendency to aggregate, it causes the problem of excessive molecular aggregation and stacking, which directly leads to excessive crystallinity. Proper accumulation is beneficial for charge transport, but oversized crystals formed by overaggregation will hinder charge transport, ultimately affecting the film morphology and charge transport efficiency. Modifying the bay position of PDINN is an effective strategy to reduce the planarity, modulate the molecular aggregation, optimize the morphology, and enhance the charge-collecting efficiency. Therefore, PDINN-S was synthesized from PDINN by substituting the hydrogen with thiophene. The optimal PCE in the PM6:Y6 active layer was 16.18% and remained at 80% of the initial value after 720 h in a glovebox. This provides some guidance for exploring CIMs and preparing large-scale OSCs in the future.

The role of branched alkylthio side chain on dispersion and thermoelectric properties of regioregular polythiophene/carbon nanotubes nanocomposites

Herein, poly(3-alkylthiophene)s (P3ATs) and poly[(3-alkylthio)thiophene]s (P3ATTs) are respectively synthesized using poly(thiophene)s with linear/branched alkyl groups and branched alkylthio moieties with short/long side chain lengths for dispersing single-walled carbon nanotubes (SWCNTs), and the resulting composites are solution-processed for use as p-type thin film thermoelectric devices. With an additional sulfur atom on the side chain, a well-dispersed P3ATTs/SWCNTs nanocomposite is obtained by polymer-wrapping at the SWCNTs surfaces driven by S − π and π − π interactions between the alkylthio side chain substituents and π-conjugated backbones of the P3ATTs and SWCNTs, thereby contributing to greatly improved thermoelectric performance relative to the P3ATs/SWCNTs nanocomposite without the added sulfur. The poly[3-(2-hexyldecylthio)thiophene] (P3HDTT) nanocomposite with the longest alkylthio side chain exhibits the strongest interaction with the SWCNTs, yielding the highest power factor (PF) of 307.7 μW m−1 K−2. The ability of this spray-coated nanocomposite film to generate power is demonstrated with a prototype flexible thermoelectric generator (TEG) that produces 888.1 μW m−2. Thus, the introduction of debundled SWCNTs networks wrapped by P3ATTs with alkylthio side chains suggests a feasible strategy for the design of high-performance thermoelectric materials and opens up new opportunities for capturing waste heat in wearable electronics.

A tetrathienopyrrole-based ladder-type donor polymer for high-performance organic near-infrared cavity detectors.

Organic semiconductors can afford detection at wavelengths beyond commercial silicon photodetectors. However, for each targeted near-infrared wavelength range, this requires individually optimized materials, which adds to the complexity and costs. Moreover, finding molecules with strong absorption beyond 1 μm that perform well in organic photodetectors remains a challenge. In microcavity devices, the detection window can be extended to wavelengths inaccessible for silicon without the need for new materials by adopting an intelligent design. Previous work has demonstrated the applicability of a dithienopyrrole-based donor polymer (PDTPQx) in such a cavity photodetector device, with a photoresponse up to 1200 nm. In this work, the π-conjugated backbone of the polymer is extended, affording higher hole mobility and better donor:acceptor intermixing. This leads to enhanced peak external quantum efficiencies up to 1450 nm. The (thermal noise limited) detectivities achieved with the PTTPQx polymer (1.07 × 1012 to 1.82 × 1010 Jones) are among the very best in the 900-1400 nm wavelength regime.

Shape-responsive adsorption of an oval macrocycle co-deposited with guest molecules

Shape persistent macrocycles (SPMs) M1 consist of an oval π-conjugated backbone and flexible side chains. The shape of the M1 changed from an oval shape to a roundish shape when triphenylene molecules were immobilized in the cavities.

Stretchable π-conjugated polymer electrets for mechanoelectric generators

Electrets are materials that retain quasi-permanent electric charges and are attracting attention as key components of batteryless micropower supplies. A chemical structure that facilitates ionization and that can stabilize these charges, such as a π-conjugated system, is expected to increase the charge density compared with that of conventional insulating polymers. Here, we report a mechanoelectric generator (MEG) (vibrational energy harvester) that uses alkylated π-conjugated polymers (Alk-CPs), which can be monopolarized either into positive or negative mode electrets. With the attachment of insulating, bulky, yet flexible alkyl side chains to the π-conjugated backbone, the poled Alk-CPs showed long charge lifetime suitable for MEGs. The elastic modulus of the electret was adjusted to approximately match that of the stretchable polyurethane substrate by blending two miscible Alk-CPs with different elastic moduli, producing a laminated film that could be stretched up to 300%. The MEG presented showed conformability when applied to a deformable object.

Cover Feature: Light‐Responsive Oligothiophenes Incorporating Photochromic Torsional Switches (Chem. Eur. J. 65/2022)

A novel class of light-responsive π-conjugated compounds incorporates photochromic torsional switches (PTS). These compounds are able to tune the planarization of their π-conjugated backbone, and thus their optical and electronic properties, by using light as an external stimulus. The PTS unit is based on a bithiophene fragment linked to a photochromic azobenzene moiety. When this PTS-containing oligothiophene is not exposed to light, the azobenzene moiety assumes its extended trans conformation, which forces the oligothiophene backbone to twist out of coplanarity. Exposure to UV light results in isomerization to the cis conformation, which allows the bithiophene fragment to assume a planar, π-conjugated conformation. More information can be found in the Research Article by D. Fazzi, E. Orentas, G. Sforazzini, and co-workers (DOI: 10.1002/chem.202202698).