Π-conjugated Backbone Research Articles

Alkyl-Substituted N,S-Embedded Heterocycloarenes with a Planar Aromatic Configuration for Hosting Fullerenes and Organic Field-Effect Transistors.

Cycloarenes and heterocycloarenes display unique physical structures and hold great potential as organic semiconductors. However, the synthesis of (hetero)cycloarenes remains a big challenge, and there are limited reports on their applications. Herein, a series of nitrogen- and sulfur-codoped cycloarenes NS-Octulene-n (n = 2, 3, 4) with branched alkyl substituents containing linear spacer groups from C2 to C4 have been conveniently synthesized. Compared with their isoelectronic analogues Octulene and S-Octulene, both having a saddle-shaped configuration, the coincorporation of two nitrogen atoms and two sulfur atoms leads to a fully coplanar aromatic backbone structure. Each of these three planar heterocycloarenes acts as a supramolecular host for encapsulation of both fullerenes C60 and C70 with a stronger donor-acceptor interaction for the complexation between the heterocycloarene and C70 due to the unique molecular geometry and defined cavity. Meanwhile, the electron-rich nitrogen atoms also slightly increase the energies of both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in this series of planar heterocycloarenes, indicating that they can be used as p-type semiconductors. Most importantly, benefitting from the planar π-conjugated backbone structure accompanied by excellent crystallinity and ordered molecular packing, as well as upon the engineering of the alkyl chain branching position, thin-film field-effect transistors of NS-Octulene-3 with moderate alkyl branching point exhibit the maximum hole mobility of 0.86 cm2 V-1 s-1, which is the highest for (hetero)cycloarene-based organic semiconductors. This study will shed new light on designing novel high-performance macrocyclic polycyclic aromatic hydrocarbon (PAH) semiconductors.

Bright short-wavelength infrared organic light-emitting devices

Organic LEDs that emit light in the short-wavelength infrared (SWIR) region, which spans the 1–2 μm region, are attractive for applications in biosensors, biomedical imaging and spectroscopy, and surveillance. However, fabrication of such devices with high radiance has not yet been achieved owing to an intrinsic limitation imposed by the energy-gap law, which leads to extremely low emission efficiencies. Here, we report that acceptor–donor–acceptor-type molecules with high coplanarity, rigid π-conjugated backbones, an extremely small reorganization energy and an electron–phonon coupling factor are capable of simultaneously providing a strongly suppressed non-radiative recombination rate and a high operation stability at high current density. We achieve electrically driven SWIR organic LEDs with an irradiance of up to 3.9 mW cm−2 (corresponding to 7% of direct sunlight infrared irradiance). These findings should open a wide avenue to a new class of organic SWIR light sources for a broad range of applications.

Synthesis, properties, and material hybridization of bare aromatic polymers enabled by dendrimer support

Aromatic polymers are the first-choice platform for current organic materials due to their distinct optical, electronic, and mechanical properties as well as their biocompatibility. However, bare aromatic polymer backbones tend to strongly aggregate, rendering them essentially insoluble in organic solvent. While the typical solution is to install many solubilizing substituents on the backbones, this often provokes undesired property changes. Herein, we report the synthesis of bare aromatic polymers enabled by a dendrimer support. An initiator arene containing a diterpenoid-based dendrimer undergoes Pd-catalyzed polymerization with monomers bearing no solubilizing substituents to furnish bare aromatic polymers such as polythiophenes and poly(para-phenylene)s. The high solubility of dendrimer-ligated polymers allows not only the unveiling of the properties of unsubstituted π-conjugated backbone, but also mild release of dendrimer-free aromatic polymers and even transfer of aromatic polymers to other materials, such as silica gel and protein, which may accelerate the creation of hybrid materials nowadays challenging to access.

The role of crystal packing on the optical response of trialkyltetrelethynyl acenes.

The electronic and optical responses of an organic semiconductor (OSC) are dictated by the chemistries of the molecular or polymer building blocks and how these chromophores pack in the solid state. Understanding the physicochemical nature of these responses is not only critical for determining the OSC performance for a particular application, but the UV/visible optical response may also be of potential use to determine aspects of the molecular-scale solid-state packing for crystal polymorphs or thin-film morphologies that are difficult to determine otherwise. To probe these relationships, we report the quantum-chemical investigation of a series of trialkyltetrelethynyl acenes (tetrel = silicon or germanium) that adopt the brickwork, slip-stack, or herringbone (HB) packing configurations; the π-conjugated backbones considered here are pentacene and anthradithiophene. For comparison, HB-packed (unsubstituted) pentacene is also included. Density functional theory and G0W0 (single-shot Green's function G and/or screened Coulomb function W) electronic band structures, G0W0-Bethe-Salpeter equation-derived optical spectra, polarized ϵ2 spectra, and distributions of both singlet and triplet exciton wave functions are reported. Configurational disorder is also considered. Furthermore, we evaluate the probability of singlet fission in these materials through energy conservation relationships.

Porous Supramolecular Assembly of Pentiptycene-Containing Gold(I) Complexes: Persistent Excited-State Aurophilicity and Inclusion-Induced Emission Enhancement.

We report herein a porous supramolecular framework formed by a linear mononuclear Au(I) complex (1) via the tongue-and-groove-like joinery between the pentiptycene U-cavities (grooves) and the rod-shaped π-conjugated backbone and alkyl chains (tongues) with the assistance of C-H···π and aurophilic interactions. The framework contains distorted tetrahedral Au4 units, which undergo stepwise and persistent photoinduced Au(I)-Au(I) bond shortening (excited-state aurophilicity), leading to multicolored luminescence photochromism. The one-dimensional pore channels could accommodate different solvates and guests, and the guest inclusion-induced luminescence enhancement (up to 300%) and/or vapochromism are characterized. A correlation between the aurophilic bonding and the luminescence activity is uncovered by TDDFT calculations. Isostructural derivatives 2 and 3 corroborate both the robustness of the porous supramolecular assembly and the mechanisms of the stimulation-induced luminescence properties of 1. This work demonstrates the cooperation of aurophilicity and structural porosity and adaptability in achieving novel supramolecular photochemical properties.

Optimizing Interfacial Energetics for Conjugated Polyelectrolyte Electron Injection Layers in High Efficiency and Fast Responding Polymer Light Emitting Diodes.

Modification of the π-conjugated backbone structure of conjugated polyelectrolytes (CPEs) for use as electron injection layers (EILs) in polymer light emitting diodes (PLEDs) has previously brought conflicted results in the literature in terms of device efficiency and turn-on response time. Herein, we determine the energetics at the CPE and the light emitting polymer (LEP) interface as a key factor for PLED device performance. By varying the conjugated backbone structure of both the LEP and CPE, we control the nature of the CPE/LEP interface in terms of optical energy gap offset, interfacial energy level offset, and location of the electron–hole recombination zone. We use a wide gap CPE with a shallow LUMO (F8im-Br) and one with a smaller gap and deeper LUMO (F8imBT-Br), in combination with three different LEPs. We find that the formation of a type II heterojunction at the CPE/LEP interfaces causes interfacial luminance quenching, which is responsible for poor efficiency in PLED devices. The effect is exacerbated with increased energy level offset from ionic rearrangement and hole accumulation occurring near the CPE/LEP interface. However, a deep CPE LUMO is found to be beneficial for fast current and luminance turn-on times of devices. This work provides important CPE molecular design rules for EIL use, offering progress toward a universal PLED-compatible CPE that can simultaneously deliver high efficiency and fast response times. In particular, engineering the LUMO position to be deep enough for fast device turn-on while avoiding the creation of a large energy level offset at the CPE/LEP interface is shown to be highly desirable.

High performance ambipolar semiconductor of pyridine-capped diketopyrrolopyrrole-porphyrin oligomers: Roles of thiophen substitution and oligomer length

Series of seven donor-acceptor pyridine-capped diketopyrrolopyrrole − 5,15-diacetylenyl-10,20-diaryl-porphyrinato zinc oligomers nPyDPP-PAr [n = 1–3, Ar = methyl-benzothiophene (BT) or methyl thiophene (T) or methylbenzene (B)] were designed. Density functional theory calculations were performed to theoretically study the molecular geometry, electronic structure, and charge transport properties of the seven donor-acceptor compounds. The effects of oligomer length and different Ar groups on the crystal packing mode and charge transport properties were systematically studied. Thiophen substitution reduces the electron injection barrier, diminishes the reorganization energy, enlarges the charge transfer integral, and therefore shows three-to-one role for ambipolar semiconductor with large charge transfer mobility. Extending the π-conjugated backbone not only improves the semiconductor performance, but also makes the ambipolar performance more balanced. 3PyDPP-PT and 3PyDPP-PBT show high hole/electron transfer mobility as high as 8.28/7.90 cm2 V−1 s−1 and 9.00/9.79 cm2 V−1 s−1, respectively.

Conjugated Polymers for Gene Delivery and Photothermal Gene Expression.

Gene therapy has attracted great attention due to its applications in disease treatment. The key point for gene therapy is how to improve the efficiency of gene delivery and precisely control gene expression. Conjugated polymers (CPs) have hydrophobic π-conjugated backbones and modifiable side chains, which provide desired photophysical properties and have been applied in the field of bio-sensing, biomedical and gene-based therapies. Herein, we summarize the strategies of DNA delivery and small interfering RNA (siRNA) delivery based on cationic conjugated polymers, in addition, the strategies for precisely control the expression of target gene by photothermal-responsive conjugated polymers for cancer therapy and gene editing are described. Finally, future challenges of efficient and safe gene delivery strategies are presented, and perspectives in the precise control of gene expression are also discussed.

Facile Preparation of a Thienoisoindigo-Based Nanoscale Covalent Organic Framework with Robust Photothermal Activity for Cancer Therapy.

The covalent organic frameworks (COFs) so far are usually built with small aromatic subunits, which makes their absorption spectra mainly located in the high-energy part of the visible region. In this work, we have developed a COF with a low band gap by integrating electron-deficient thienoisoindigo and electron-rich triphenylamine. The intramolecular charge-transfer effect combining the extended length of the π-conjugated backbone of COF endow it with broad absorption even to the second near-infrared region. After optimizing the solvent, a uniform size and colloidal stable COF is obtained. Benefiting from the coplanar structure of the monomer, this COF achieves a considerable photothermal conversion efficiency (PCE) of 48.2%. With these advantages, it displays convincing cancer cell killing effect upon laser irradiation in vitro or in vivo. This work provides a simple and practical method to acquire promising a COF-based phototherapy reagent that is applied in biomedicine field.

Structure-Transport Properties Governing the Interplay in Humidity-Dependent Mixed Ionic and Electronic Conduction of Conjugated Polyelectrolytes.

Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest in the field of energy storage and conversion, optoelectronics, and bioelectronics. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a π-conjugated backbone imparting electronic transport characteristics along with side chains composed of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure-transport properties of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4-7. UV-vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveals that the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σi) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σe) follows a non-monotonic profile. The values of σe continue to rise with increasing RH reaching a local maximum after which σe begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering a decrease in σe. This drop in σe is attributed to two factors. First, disruption of the locally ordered π-stacked domains observed through in situ humidity-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σe. A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σi (1.0 × 10-1 S/cm at 95% RH) while not substantially disrupting σe (1.7 × 10-2 S/cm at 85% RH and 8.0 × 10-3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.

Molecular Engineering of CoII Porphyrins with Asymmetric Architecture for Improved Electrochemical CO2 Reduction.

The electrochemical reduction of carbon dioxide (CO2 ) based on molecular catalysts has attracted more attention, owing to their well-defined active sites and rational structural design. Metal porphyrins (PorMs) have the extended π-conjugated backbone with different transition metals, endowing them with unique CO2 reduction properties. However, few works focus on the investigation of symmetric architecture of PorMs as well as their aggregation behavior to CO2 reduction. In this work, a series of CoII porphyrins (PorCos) with symmetric and asymmetric substituents were used as model of molecular catalysts for CO2 reduction. Owing to the electron donating effect of 2,6-dimethylbenzene (DMB), bandgaps of the complexes became narrower with the increasing number of DMB. As electrocatalysts, all PorCos exhibited promising electrocatalytic CO2 reduction performance. Among the three molecules, asymmetric CoII porphyrin (as-PorCo) showed the lowest onset potential of -288 mV and faradaic efficiencies exceeding 93 % at -0.6 V vs. reversible hydrogen electrode, which is highly competitive among the reported state-of-art porphyrin-based electrocatalysts. The CO2 reduction performance depended on π-π stacking between PorCo with carbon nanotubes (CNTs) and adjacent PorCos, which could be readily controlled by atomically positioned DMB in PorCo. Density functional theory calculations also suggested that the charge density between PorCo and CNT was highest due to the weak steric hindrance in as-PorCo, providing the new insight into molecular design of catalysts for efficient electrochemical CO2 reduction.

Side-Chain-Tuned Molecular Packing Allows Concurrently Boosted Photoacoustic Imaging and NIR-II Fluorescence.

It is generally considered that photoacoustic imaging (PAI) and fluorescence imaging (FLI) cannot be enhanced concurrently, as they are dependent on competitive photophysical processes at the single-molecule level. Herein, we reveal that BDTR9-OC8 and BDTR9-C8, which have identical π-conjugated backbones but are substituted by side chains of different rigidity, show distinct phototheranostic properties in the aggregated state. The NIR-II FLI and PAI brightness of BDTR9-C8 nanoparticles are enhanced by 4.6 and 1.4 times compared with BDTR9-OC8 nanoparticles. Theoretical calculations and GIWAXS analysis revealed that BDTR9-C8 with rigid side chains shows a relative amorphous condensed state, which will benefit the efficient transportation of photo-generated excitons and phonons, subsequently enhancing the FLI and PAI signals. Besides, both nanoparticles exhibit excellent photothermal conversion efficiency due to their strong light-harvesting capability and are considered effective photothermal therapy materials. This work provides an illuminating strategy for material design in the future.

Chain Stiffness of Donor–Acceptor Conjugated Polymers in Solution

The optoelectronic properties of conjugated polymers are dictated by their chain conformations, which depend on the interplay of delocalization of electrons along the π-conjugated backbone and the intrachain interactions of pendant side chains. Here, we leverage small-angle neutron scattering to measure the chain shapes of several classes of commonly used, high mobility donor–acceptor conjugated polymers in dilute solution. We find that these model conjugated polymers are semiflexible with persistence lengths ranging from several to hundreds of nanometers, dependent on the molecular structure of the polymer, indicating the importance of repeat unit geometry, particularly side-chain size and branching, on the overall chain conformations. The measured persistence lengths show good agreement with those calculated according to dihedral distributions predicted from density functional theory. Larger persistence lengths are shown to correlate with increased charge-carrier mobility, signifying the importance of rational molecular design to obtain high persistence length organic semiconductors and thus advantageous optoelectronic properties.

Enhanced interfacial electron transfer kinetics between Co2+/3+ complexes and organic dyes with free space near their backbone.

Fast electron transfer (ET) between surface-bound dye molecules and electron donor molecules dissolved in electrolytes with simultaneous reduction in recombination rates are crucial to improve the photon-to-electron conversion efficiency of photo-electrochemical technologies. Here, the electron transfer characteristics of a new dye molecule PX47 with only two alkyl chains placed in the anti configuration of the π-conjugated quarterthiophene backbone is studied. It is anticipated that the appropriate free space between the alkyl chains allowed the approach of the Co(c1-bpy)3 redox mediator to near the backbone of the dye anchored to a TiO2 electrode even at complete coverage of the TiO2 surface, thereby enhancing electronic coupling. The ET kinetics measured by transient absorption spectroscopy were enhanced by a factor of six between PX47 and Co(c1-bpy)3 as compared to the structurally similar MK2 dye with four alkyl chains. The ET rates between PX47 and the larger nonyl-substituted (Co(c9-bpy)3 or tert-butyl substituted Co bipyridine (Co(dtb-bpy)3) were reduced by half and one third as compared to Co(c1-bpy)3, respectively, indicating a blocking effect of longer or bulky substituents on the redox mediators. For MK2 with four alkyl chains near the backbone, the ET rate was very similar between Co(c1-bpy)3 and Co(dtb-bpy)3 and was enhanced for (Co(c9-bpy)3, the latter explained by trapping the mediator inside the dye layer due to alkyl-alkyl interactions. Unexpectedly, two distinctly different recombination rates were measured between the oxidized Co(c1-bpy)3 mediators and TiO2 electrons in the PX47-TiO2 samples, which is explained by two possible arrangements of the PX47 on the TiO2 surface. The dominant arrangement allowed the adsorption of Co(c1-bpy)3 on the TiO2 surface enhancing recombination. The findings suggest that by strategically placing alkyl chains around the electronic units of dye molecules, the redox mediator can be intercalated into the dye layer increasing proximity and better electronic coupling. Although this work presents an effective strategy to enhance ET rates by designing structurally complementary electron donor-acceptor pairs, additional design strategies to further reduce recombination in such open dye structures are needed.

Conjugated polymers for biomedical applications.

Conjugated polymers (CPs) are a series of organic semiconductor materials with large π-conjugated backbones and delocalized electronic structures. Due to their specific photophysical properties and photoelectric effects, plenty of CPs with varied chemical structures and functions are quickly evolving in the diverse biomedical field, such as fluorescence imaging, photodynamic therapy, photothermal therapy, etc. In addition, the functionalized side chains of CPs could contribute to the expected water-solubility, biocompatibility, biological response, etc. CPs can also be prepared into nanoparticles for acquiring controllable particle size and dispersion through the common synthesis procedure. In this review, we focus on the latest developments of CPs in biotherapy, biological regulation, biological response and bioprinting applications. The major challenges and outlooks of CPs for applications in the biomedical field are also discussed.

Reusable Polyacrylonitrile‐Sulfur Extractor of Heavy Metal Ions from Wastewater (Adv. Funct. Mater. 51/2021)

Heavy Metal Ion Recovery In article number 2105845, Guiyin Xu, Ju Li, and co-workers prepare a highly porous and functionalized filter with a cyclic π-conjugated backbone and electronic conductivity PAN-S grafted on an industrially produced melamine foam. Selective affinity of Hg2+, Pb2+, Cu2+, and Cd2+ is achieved, and the chemically adsorbed metal ions can be electrochemically desorbed and reduced on counter-electrode, achieving the highest possible waste concentration. This paradigm offers a sustainable and affordable approach for heavy meal ion removal and recovery from wastewater.

Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal-Organic Framework Linker.

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10-11 to 2.48 ns at 2.0 × 10-3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

Improving the thermoelectric performances of polymer via synchronously realizing of chemical doping and side-chain cleavage

Side-chain cleavage is an effective strategy to reconstruct the arrangement of polymer main chains, thus improving their electrical properties and corrosion stability. However, simultaneously realizing side-chain cleavage and chemical doping of polymer semiconductors have not been reported. Herein, a series of p-type polymer semiconductors with the incorporation of ester-cleavage groups (PBDTTT-TETx, X is the molar ratio of the cleavage ester groups and nondegradable alkyl groups) were designed and synthesized. The introduced functional ester side-chains on PBDTTT-TET0.5 can be degraded by a commonly used p-type dopant FeCl3 readily, resulting in the maximum 120-fold enhancement in electrical conductivity and 15-fold increase in power factor at the FeCl3 concentration of 30 mM, compared to the reference one (PBDTTT-PET0.5 with the nondegradable ester side-chains). Scanning electron microscopy, atomic force microscopy measurements and X-ray diffraction reveal that the increase of crystalline domains along with the side-chain cleavage contributes to the formation of (bi)polaron network. The improved doping efficiency can be reflected by their ultraviolet photoelectron spectroscopy and UV–vis-NIR spectroscopy results. Moreover, a further optimized power factor can be achieved for PBDTTT-TET0.05, which is 5.8 times higher than that of PBDTTT-TET. Therefore, molecule design strategy by incorporating FeCl3-cleavage ester groups into the π-conjugated backbones provided an effective approach to establish advanced TE materials.

Photobleaching Kinetics of MEH-PPV in Solution: The Role of Conformational Disorder.

Semiconductor polymers are the foundation of organic electronics due to their remarkable optical features, ability to form a thin film, and low cost compared to silicon. However, some of them have intense photobleaching under UV-blue radiation, compromising several applications. In this context, we have investigated the conformational disorder effect on the real-time photobleaching kinetics of a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/chloroform solution under deep-blue radiation. Our results pointed out that a 405 nm diode laser initially causes a significant conformational disorder in the π-conjugated backbone of MEH-PPV as revealed by the Huang-Rhys factor. As a result, a new vibrational mode arises with an energy separation of 230 meV, indicating the substitution of the vinyl (C═C) by carbonyl (C═O) bonds. Then, the conformational disorder reaches a maximum value at some tens of minutes, which is inversely proportional to the polymer concentration, and after that, a random chain scission occurs. Consequently, the effective conjugation length of MEH-PPV in chloroform decreases from nine to three coplanar repetitive units after 1 h of excitation, producing a drastic drop in photoluminescence. Finally, we show that the photobleaching steps are mapped through the conformational disorder.