Side-chain Engineering Research Articles

Highly Processable Covalent Organic Framework Gel Electrolyte Enabled by Side-Chain Engineering for Lithium-Ion Batteries.

Although covalent organic frameworks (COFs) with a graphene-like structure present unique chemical and physical properties, they are essentially insoluble and infusible crystalline powders with poor processability, hindering their further practical applications. How to improve the processability of COF materials is a major challenge in this field. In this contribution, we proposed a general side-chain engineering strategy to construct a gel-state COF with high processability. This method takes advantages of large and soft branched alkyl side chains as internal plasticizers to achieve the gelation of the COF. We systematically studied the influence of the length of the side chain on the COF gel formation. Benefitting from their machinability and flexibility, this novel COF gel can be easily processed into gel-type electrolytes with specific shape and thickness, which were further applied to assemble lithium-ion batteries that exhibited high cycling stability.

Single Ether‐Based Side Chains in Conjugated Polymers: Toward Power Factors of 2.9 mW m−1 K−2

AbstractCombining side chain engineering and controlled alignment of PBTTT is an effective strategy to produce oriented thin films with improved thermoelectric performance when doped sequentially with the acceptor molecule hexafluoro‐tetracyanonaphthoquinodimethane (F6TCNNQ). The substitution of linear alkyl side‐chains (n‐C12) with a chain of identical length including an ether function (n‐C7OC4) leads to a new class of slightly polar PBTTT polymers, preserving the ease of synthesis and air stability of alkylated PBTTTs. This side‐chain modification improves the structural order of PBTTT backbones and the thermo‐mechanical properties of the polymer. It then can be oriented by high temperature rubbing up to 240 °C to reach very high dichroic ratios up to 20 thanks to enhanced cohesive forces within side‐chain layers. The side chain polarity of n‐C7OC4 helps to tune the polymer‐dopant interactions. A multi‐technique approach demonstrates that F6TCNNQ dopants are randomly oriented in the disordered side chain layers of n‐C7OC4 with some evidence of dopant clustering. The combination of improved alignment and random orientation of intercalated F6TCNNQ dopants helps reach a very high charge conductivity σ = 5 ⋅ 104 S cm−1 and a record power factor of 2.9 mW m−1 K−2 in the polymer chain direction.

Donor–Acceptor Type Polymer Bearing Carbazole Side Chain for Efficient Dopant‐Free Perovskite Solar Cells

AbstractIn conventional n–i–p perovskite solar cells (PVSCs), electron donor (D)–acceptor (A) polymers have been found to be potential substitutes for doped spiro‐based small molecule hole‐transporting materials (HTMs) due to their excellent performance in hole mobility, film formability, and stability. Herein, a benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐benzodithiophene‐4,8‐dione (BDD) copolymer PBDB‐Cz is developed by employing carbazole as the conjugated side chain of BDT. PBDB‐O and PBDB‐T with alkoxy and thiophene as the side chain of BDT, respectively, are also synthesized and studied for comparison. The synergistic effect of the carbazole side chain and the BDT‐BDD backbone to promote hole transport properties is found in PBDB‐Cz. The carbazole side chain enhances both coplanarity and interaction of polymer chains, while simultaneously deepening energy levels and improving the hole mobility of the polymeric HTM. Consequently, PBDB‐Cz outperforms two counterparts, exhibiting a promising power conversion efficiency (PCE) of 22.06%. Notably, the PBDB‐Cz also improves the device stability, and the devices can retain more than 90% of their initial PCEs after being stored at ambient conditions for 100 days. To the best of the authors’ knowledge, this is the first report to incorporate carbazole into D–A polymeric HTM by side chain engineering.

Study on the side chain effect of A2-A1-D-A1-A2 type non-fullerene acceptors matched with P3HT

Because of the advantages of low cost, stability and easy synthesis, poly(3-hexylthiophene) (P3HT) was the most classic photovoltaic polymer and promoted the rapid progress of fullerene-based organic solar cells (OSCs) during 2005–2015. In comparison with A-D-A type non-fullerene acceptors (NFAs), A2-A1-D-A1-A2 type molecules can realize higher open-circuit voltage (VOC) and also match well with P3HT. Hence, here we modified the classic A2-A1-D-A1-A2 type NFA of BTA3 by altering the side chains at three different positions, and synthesized five new materials of BTA3b-3f to investigate the structure-properties relationship. By fine-tuning the side chains at middle D unit, BTA3b, BTA3c and BTA3d show totally different power conversion efficiencies (PCEs) of 5.95, 3.80 and 4.74% respectively, which demonstrate the modification of D unit can significantly influence the intermolecular interaction. On the other hand, with the modulation of side chains at A1 and A2 units, BTA3e and BTA3f show relatively high PCEs of 5.48 and 6.15% respectively. Our results indicate the systematical side chain engineering for A2-A1-D-A1-A2 type NFAs play a vital role to improve the photovoltaic performance of P3HT-based OSCs.

Structure and thermoelectric properties of electrochemically doped polythiophene thin films: Effect of side chain density

Intermolecular interaction between a conducting polymer and a dopant molecule is a fundamental factor to determine the structure and electronic properties of doped films, whereas clear guidelines of the molecular design to control such interactions are absent. We compared the structure and transport properties of two typical electrochemically doped polythiophenes with different side chain densities: poly(3-hexylthiophene) and poly[bis(3-dodecyl-2-thienyl)-2,2-dithiophene-5,5-diyl] (PQT). X-ray diffraction patterns show that more conspicuous lattice expansion and crystallinity improvement take place after doping PQT, the low side chain density polymer, although the crystallinity of the pristine film is rather low. The crystallinity improvement results in a large increase in the electrical conductivity, reaching 140 S/cm after doping and indicating the importance of side chain engineering to optimize the electronic properties of the doped state. We further suggest the dynamic motion of the dopant molecules with a temperature gradient in the doped PQT film based on thermoelectromotive force measurements, indicating the weakly bound state of the dopant molecules.

Aldol Polymerization to Construct Half-Fused Semiconducting Polymers

Conventional conjugated polymer synthesis involves costly transition-metal catalysts and even toxic reagents, causing serious environmental challenges around the world. The comonomer linkages of semiconducting polymers are mostly single bonds; charge delocalization and transport have been hindered due to rotation around these single bonds, leading to energetic and conformational disorders. Here, we have developed a novel programmable half-fused polymer library by introducing aldehyde chemistry to promote a very simple and cheap Lewis acid-mediated aldol condensation that affords conjugated polymers with continuous double bonds and single bonds as the linkages between the polymer repeat units, maintaining a coplanar conjugated backbone to eliminate rotation owing to the intramolecular noncovalent interactions. In addition, this innovative concept not only shows an excellent substrate generality but also affords unprecedented polymers with unique properties, allowing us to change the variable conjugated length and side-chain engineering of the backbone to modulate the optoelectronic properties and tune the delocalization of frontier molecular orbitals. These polymers also exhibit very high number-averaged molecular weights (Mn) while showing good solution processability, which makes them applicable as high-performance thin-film transistor materials with reduced conformational disorder. Herein, our results demonstrate that the half-fused design strategy can effectively extend the scope of semiconducting polymers. The low synthetic complexity can be utilized to design organic electronic materials for large-scale production and commercialization.

Effects of the Polarity and Bulkiness of End-Functionalized Side Chains on the Charge Transport of Dicyanovinyl-End-Capped Diketopyrrolopyrrole-Based n-Type Small Molecules.

When designing organic semiconductors, side-chain engineering is as important as modifying the conjugated backbone, which has a significant impact on molecular ordering, morphology, and thus electronic device performance. We have developed three dicyanovinyl-end-capped donor-acceptor diketopyrrolopyrrole-based n-type small molecules (C2C9CN, SiC4CN, and EH4PCN) bearing an identical length of alkyl spacer yet different end-functionalized side chains (i.e., alkyl-, siloxane-, and phosphonate-end pendants). The effects of the end-functionalized side chains on the intrinsic molecular properties, microstructure, and charge transport of the small-molecule series were investigated. In comparison with the alkyl-end side chains, incorporating siloxane-end side chains into the backbone facilitates 2D edge-on oriented high intergrain connectivity/crystallinity and compatibility with the substrate surface, whereas the phosphonate-end analogues have an adverse effect on the film-forming quality due to high polarity. Thereby, an organic field-effect transistor fabricated by SiC4CN shows the best electron mobility up to 1.59 × 10-1 cm2 V-1 s-1 along with a high current on/off ratio >105. This study contributes to our understanding of the role of the end-functionalized side chains (e.g., the effects of polarity and bulkiness of the end groups) for the development of high-performance semiconductors.

Effects of Heteroatom Substitution on the Photovoltaic Performance of Donor Materials in Organic Solar Cells

ConspectusOver the past few years, the innovation of narrow bandgap acceptor combined with wide bandgap donor materials significantly promotes the power conversion efficiencies (PCEs) of organic solar cells (OSCs) to exceed 18%. To build a state-of-the-art OSC, absorption spectra, frontier molecular orbital energy levels, molecular packing and crystallinity, and charge carrier mobilities of the photovoltaic materials should be considered in their molecular design. The donor and acceptor materials are the key components determining the photovoltaic performance of the OSCs. The side chain engineering on the conjugated backbone is a critical strategy to optimize the photovoltaic properties of the donor materials. In this Account, we focus on the topic of heteroatom substitution on the molecular backbone of the donor materials for improving their photovoltaic performance, aiming to provide in-depth understanding of the molecular structure optimization for the design of state-of-the-art photovoltaic materials.First, we highlight the halogen (fluorine and chlorine) atom substitution strategies applied on the conjugated molecular backbone of the organic photovoltaic materials. The strong electronegativity of halogen atoms can downshift the highest occupied molecular orbital (HOMO) energy levels of the donor materials, which could increase open-circuit voltages of the resulting OSCs. In addition, the hydrogen bonding aroused by the halogen atoms is beneficial to improve their charge transport property and crystallinity of the organic semiconductors. On the other hand, flexible side chains are critical components for improving the solubility of the photovoltaic materials. Using the flexible side chains of alkylthio and alkylsilyl is an easy and effective approach to tune the electronic energy levels and absorption spectra of the photovoltaic materials. Due to the formation of pπ(C)–dπ(S) orbital overlap, the empty 3d-orbitals of the sulfur atom in the alkylthio substituents can accept the π-electron of conjugated skeleton to modulate the optical and electrical properties of the photovoltaic materials. Similarly, the silicon atom in the alkylsilyl side chains can stabilize the lowest unoccupied molecular orbital (LUMO) level and downshift the HOMO level of the organic semiconductor, which leads to the improved photovoltaic performance of the organic donor materials. Finally, we briefly discussed the challenges for the photovoltaic materials toward performance optimization and practical application of the OSCs.

Co-delivery of NIR-II semiconducting polymer and pH-sensitive doxorubicin-conjugated prodrug for photothermal/chemotherapy

Semiconducting polymer (SP) is a promising photothermal agent in the antitumor application, but the co-delivery of the second near-infrared window (NIR-II)-based SPs with chemotherapeutic drug (e.g., doxorubicin (DOX)) remains a challenge. Here, SPs were firstly improved via backbone and alkyl side-chain engineering, and afterward, SPs and pH-sensitive prodrug copolymer self-assembled into a nanoparticle for a photoacoustic (PA)-imaging guided combination of photothermal therapy and chemotherapy. SP-encapsulated nanoparticles exhibited a high photothermal conversion efficiency of 45% at a relatively low power level of NIR irradiation (0.3 W/cm2 for 5 min). DOX was rapidly released in response to the acidic lysosomal environment. PA and fluorescence imaging confirmed that the photothermal therapy effectively drove DOX penetration inside tumor tissue, and it resulted in the killing of the surviving tumor cells from hyperthermia. The synergistic effect of SP-based photothermal therapy and DOX-induced chemotherapy was verified in vivo. Overall, the co-delivery of the SP and DOX using pH-sensitive nanoparticles represents a feasible strategy for photothermal therapy with potentially synergistic drug effects. Statement of significanceRecent years have yielded great progress in semiconducting polymers (SPs)-based photothermal therapy for anticancer treatment. However, studies about molecular weight and side-chain of SPs on photothermal conversion efficiency are limited, and investigation of controlled codelivery with chemotherapeutic drug is lacking. Here, we improved the SPs performance via backbone and side-chain engineering, and afterward offered a pH-sensitive DOX-conjugated amphiphilic copolymer to encapsulate SPs. SP-encapsulated nanoparticles exhibited high photothermal conversion efficiency at a clinically feasible power level of NIR irradiation. NIR irradiation-generated hyperthermia not only killed tumor cells but also promoted DOX penetration inside the tumor tissue to ablate the tumor cells that survived hyperthermia. The synergistic effect of SP-based photothermal therapy and DOX-induced chemotherapy was verified in vivo.

Contrasting Effect of Side-Chain Placement on Photovoltaic Performance of Binary and Ternary Blend Organic Solar Cells in Benzodithiophene-Thiazolothiazole Polymers.

π-Conjugated polymers are important materials for organic photovoltaics. While search for new backbone systems is central to the development of π-conjugated polymers, side-chain engineering is also imperative. Here, two benzodithiophene-thiazolothiazole copolymers, PSTz1 and POTz1, were synthesized, for which the side-chain placement was different. Due to less steric hindrance between the side chains, PSTz1 had a more coplanar backbone than POTz1. This led to significant differences in trend of the performance for the binary and ternary blend cells that used a fullerene (PC71 BM) and/or non-fullerene (ITIC) as the acceptor materials. Whereas PSTz1 showed higher photovoltaic performance in the PC71 BM-based cell, POTz1 showed higher performance in the ITIC-based cell. Furthermore, in the ternary blend cell, whereas increase in the PC71 BM content improved the photovoltaic performance for the PSTz1 system, it was detrimental to the performance for the POTz1 system. These results could be a good guideline for maximizing the performance of organic photovoltaics.

Effects of Side-Chain Engineering with the S Atom in Thieno[3,2-b]thiophene-porphyrin to Obtain Small-Molecule Donor Materials for Organic Solar Cells.

To explore the effect of the introduction of heteroatoms on the properties of porphyrin materials, a new porphyrin-based derivative small-molecule donor named as PorTT-T was designed and synthesized based on alkyl-thieno[3,2-b]thiophene(TT)-substituted porphyrins. The linker bridge and end groups of PorTT-T were the same as those of XLP-II small-molecule donor materials, while the side-chain attached to the core of thieno[3,2-b]thiophene(TT)-substituted porphyrin was different. Measurements of intrinsic properties showed that PorTT-T has wide absorption and appropriate energy levels in the UV-visible range. A comparison of the morphologies of the two materials using atomic force microscopy showed that PorTT-T has a better surface morphology with a smaller root-mean-square roughness, and can present closer intermolecular stacking as compared to XLP-II. The device characterization results showed that PorTT-T with the introduced S atom has a higher open circuit voltage of 0.886 eV, a higher short circuit current of 12.03 mAcm−2, a fill factor of 0.499, a high photovoltaic conversion efficiency of 5.32%, better external quantum efficiency in the UV-visible range, and higher hole mobility.

Side chain engineering on D18 polymers yields 18.74% power conversion efficiency

Side chain engineering on D18 polymers yields 18.74% power conversion efficiency

Intrinsically Stretchable n-Type Polymer Semiconductors through Side Chain Engineering

Intrinsically Stretchable <i>n</i>-Type Polymer Semiconductors through Side Chain Engineering

Molecular design strategies for high‐performance organic electrochemical transistors

AbstractOrganic electrochemical transistors (OECTs) utilize ion flow from the electrolyte to modulate the electrical conductivity of the whole bulk organic semiconductor channel. With the characteristic of mixed ionic‐electronic conducting in the entire volume, OECTs exhibit high transconductance and act as good transducers, particularly in bioelectronics. To gain high‐performance OECTs, developing novel high‐performance polymeric semiconductors is important. In this article, operation principles, performance evaluations, and polymerization methods are first discussed. We then analyze the molecular design strategies for high‐performance OECT materials and highlight the characteristics and effects of backbone design and side chain engineering. Finally, we discuss some neglected and unsolved issues and provide an outlook for the OECTs research and development.

Side chain engineering of semiconducting polymers for improved NIR-II fluorescence imaging and photothermal therapy

Development of efficient agents for NIR-II fluorescence imaging (NIR-II FI) offers opportunities to facilitate NIR-II FI in biomedicine and life science. Although semiconducting polymers (SPs) are proved to be excellent candidates for NIR-II FI, their fluorescence quantum yields are generally low mainly because of the accelerated nonradiative decay by the vibrational overlap between the ground state and excited state due to the small energy gap of NIR-II SPs. We herein propose a side chain engineering of SPs for enhanced NIR-II FI and imaging-guided photothermal therapy (PTT). The fluorescence optimization is realized by tuning the conformation and bulk of side chains of SPs, which demonstrates that the bulky branched side chains remarkably contribute to the fluorescence enhancement relative to linear or short branched side chains. Density functional theory (DFT) calculation indicates that bulky branched side chain-attached SP (P3) has the largest dihedral angle between electron donor and acceptor units, which suggests the largest intramolecular steric hindrance that restricts the free rotation in the molecule, thus boosting the radiative decay to generate fluorescence. Water-dispersible nanoparticles with high tumor specificity fabricated from the optimal P3 are employed for in vivo applications, which show excellent NIR-II FI performance and PTT efficacy toward tumor. Our study therefore proposes a feasible molecular guideline to amplify the NIR-II brightness of SPs for improved phototheranostics.

Intermolecular interaction induced spontaneous aggregation enables over 14% efficiency as-cast nonfullerene solar cells

Organic solar cells (OSCs) composed of polymer donors and electron acceptors are facing the challenge to modulate the aggregation-induced morphology in pristine phases and active layers. We herein report our effective chemical modification on thieno[2′',3′':5′,6′]-s-indaceno[2′,1′:4,5]thieno[3,2–b]thieno[2,3–d]pyrrole cored fused-ring electron acceptors (FREAs). By introducing either para-F, -Cl or -methyl onto meta-hexyloxy phenyl side chain, we can finetune the surface electrostatic potentials and aggregation behaviours of the FREAs (IMOF, IMOCl and IMOM) and their intermolecular interactions with polymer donor (PM6). Consequently, F-substituted acceptor IMOF presents higher crystallinity and induces intensified aggregation in blends with PM6. The PM6:IMOF (1:1.2, w/w) as-cast binary OSCs contribute the highest Power conversion efficiency (PCE) of 13.89% and a fill factor of 73.59%. By adding 10 wt% PC71BM, the as-cast ternary devices exhibit an improved PCE of 14.42%. In contrast, the PM6 based binary OSCs with three reference acceptors deliver the maximum PCE of only 12.61%. Such molecular interactions induced aggregation can be further promoted by device optimization via processing additive and thermal annealing. The sidechain engineering to induce intermolecular interactions of FREAs demonstrates as an effective strategy to regulate the molecular aggregation and phase morphology in active layers for high-performance as-cast OSCs.

N-alkyl chain modification in dithienobenzotriazole unit enabled efficient polymer donor for high-performance non-fullerene solar cells

Molecular design of either polymer donors or acceptors is a promising strategy to tune the morphology of the active layer of organic solar cells, enabling a high-performance device. Thereinto, developing novel polymer donors is an alternative method to obtain high photovoltaic performance. Herein, we present a facile side-chain engineering on the dithiophenobenzotriazole (DTBTz) unit of newly-designed polymer donors (named pBDT-DTBTz-EH and pBDT-DTBTz-Me) to boost the performance of non-fullerene solar cells. Compared with pBDT-DTBTz-EH with long N-alkyl side chains, pBDT-DTBTz-Me with a short methyl exhibits stronger molecular aggregation, higher absorption coefficient, and preferred face-on orientation packing. As a consequence, pBDT-DTBTz-Me based devices achieve an optimal power conversion efficiency of 15.31% when donors are paired with the narrow bandgap acceptor Y6, which is superior to that of pBDT-DTBTz-EH based devices (9.17%). Additionally, the pBDT-DTBTz-Me based devices manifest more effective charge separation and transfer than pBDT-DTBTz-EH based devices. These results indicate that fine-tuning side chains of polymer donors provide new insights for the design of high-performance polymer donors in non-fullerene solar cells.

Chlorinated unfused acceptor enabling 13.57% efficiency and 73.39% fill factor organic solar cells via fine-tuning alkoxyl chains on benzene core

Fluorinated noncovalently fused-ring electron acceptors (NFREAs) are more commonly exploited in high-performing organic solar cells (OSCs) than chemically cheaper chlorinated ones. Herein, by modulating central side chains on 2-(4-(thiophen-2-yl)phenyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DBT) asymmetric core, three novel chlorinated NFREAs are synthesized within 4 steps. All three acceptors exhibit similar energy levels and narrow optical band gap (Egopt) below 1.40 eV. The 2-hexyldecyl substituted DBT-HD blends better with PBDB-T for fibrous phase separation and excellent π-π packing, contributing to more efficient exciton dissociation, higher and more balanced charge transport, and lower charge recombination in OSCs. As a result, the champion power conversion efficiency (PCE) of 13.57% and fill factor (FF) of 73.39% are achieved for PBDB-T:DBT-HD based devices, among the highest values for NFREAs based binary OSCs in the literature. The Eloss values of all three devices are found to be as low as 0.52–0.53 eV. Our work manifests that central side chain engineering may be an effective strategy in chlorinated NFREAs for satisfactory device performance, and the DBT core has great potential to explore cost-effective NFREAs.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Benzyl side-chain engineering of non-fullerene acceptors for efficient organic solar cells

Side-chain engineering is one of the most promising ways to develop non-fullerene acceptors (NFAs) for organic solar cells. In this work, we designed and synthesized a novel acceptor IT-BnC6-4F by introducing methylene group between the phenyl group and the sp3 hybrid carbon atoms of central conjugated cores to form benzyl side chains. IT-BnC6-4F shows broader absorption spectrum and higher crystallinity than those of its analogue, IT-4F. When blended with PT2, the IT-BnC6-4F based device achieved a high power conversion efficiency (PCE) of 13.36 %, with an open-circuit voltage (Voc) of 0.87 V and a short-circuit current density (Jsc) of 20.99 mA cm−2.