Benzotrithiophene-Based Donor–Acceptor Copolymers with Distinct Supramolecular Organizations

Naphthodithiophene-Fused Porphyrins: Synthesis, Characterization, and Impact of Extended Conjugation on Aromaticity

The fusion of tetrapyrroles with aromatic heterocycles constitutes a useful tool for manipulating their opto-electronic properties. In this work, the synthesis of naphthodithiophene-fused porphyrins was achieved through a Heck reaction-based cascade of steps followed by the Scholl reaction. The naphthodithiophene-fused porphyrins display a unique set of optical and electronic properties. Fusion of the naphtho[2,1-b:3,4-b']dithiophene to porphyrin (F2VTP) leads to a ~20% increase in the fluorescence lifetime, which is accompanied, unexpectedly, by a more than two-fold drop in the emission quantum yield (ϕ=0.018). In contrast, fusion of the isomeric naphtho[1,2-b:4,3-b']dithiophene to porphyrin (F3VPT) results in a ~1.5-fold increase in the fluorescence quantum yield (ϕ=0.13) with a concomitant ~30 % increase in the fluorescence lifetime. This behavior suggests that fusion of the porphyrin with the naphthodithiopheno-system mainly affects the radiative rate constant in the Q-state deactivation pathway, where the effects of the isomeric naphtho[2,1-b:3,4-b']dithiophene- versus naphtho[1,2-b:4,3-b']dithiophene-fusion are essentially the opposite. Interestingly, nucleus-independent chemical shifts analysis revealed a considerable difference between the aromaticities of these two isomeric systems. Our results demonstrate that subtle structural differences in the fused components of the porphyrin can be reflected in rather significant differences between the photophysical properties of the resulting systems.

Benzotrithiophene-based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization

Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.

AIEgens/Mitochondria Nanohybrids as Bioactive Microwave Sensitizers for Non-Thermal Microwave Cancer Therapy

Aggregation-induced emission luminogens (AIEgens) are widely used as photosensitizers for image-guided photodynamic therapy (PDT). Due to the limited penetration depth of light in biological tissues, the treatments of deep-seated tumors by visible-light-sensitized aggregation-induced emission (AIE) photosensitizers are severely hampered. Microwave dynamic therapy attracts much attention because microwave irradiation can penetrate very deep tissues and sensitize the photosensitizers to generate reactive oxygen species (ROS). In this work, a mitochondrial-targeting AIEgen (DCPy) is integrated with living mitochondria to form a bioactive AIE nanohybrid. This nanohybrid can not only generate ROS under microwave irradiation to induce apoptosis of deep-seated cancer cells but also reprogram the metabolism pathway of cancer cells through retrieving oxidative phosphorylation (OXPHOS) instead of glycolysis to enhance the efficiency of microwave dynamic therapy. This work demonstrates an effective strategy to integrate synthetic AIEgens and natural living organelles, which would inspire more researchers to develop advanced bioactive nanohybrids for cancer synergistic therapy.

Selenophene-containing heterotriacenes by a C-Se coupling/cyclization reaction

A new novel family of tricyclic sulfur and/or selenium-containing heterotriacenes 2-4 with an increasing number of selenium (Se) atoms is presented. The heterotriacene derivatives were synthesized in multistep synthetic routes and the crucial cyclization steps to the stable and soluble fused systems were achieved by copper-catalyzed C-S and C-Se coupling/cyclization reactions. Structures and packing motifs in the solid state were elucidated by single crystal X-ray analysis and XRD powder measurements. Comparison of the optoelectronic properties provides interesting structure-property relationships and gives valuable insights into the role of heteroatoms within the series of the heterotriacenes. Electrooxidative polymerization led to the corresponding poly(heterotriacene)s P2-P4.

Hole transporting materials for perovskite solar cells: a chemical approach

Photovoltaic solar cells based on perovskites have come to the forefront in science by achieving exceptional power conversion efficiencies (PCEs) in less than a decade of research. This "still young" generation of solar cells is currently rivalling, in PCEs, well-established technologies, such as cadmium telluride (CdTe) and silicon. Further improvements in device stability by means of innovative materials are yet to come, with technology becoming closer to meeting the market requirements. Emerging from this groundbreaking discovery, a great number of charge transporting materials have flourished, which is particularly true for hole transporting materials (HTMs). The huge number of molecules prepared stem from design and engineering of a wide variety of new and also chemically modified old molecules where organic synthesis has played a fundamental role. In this review, the contribution of chemistry through those synthetic protocols used for producing new and innovative HTMs from relatively simple organic molecules is presented in a rational and systematic manner. The variety and impact of synthetic strategies followed, the structure-property relationship and stability, conductivity and device performance are highlighted from a chemical viewpoint.

Self-Organization and Charge Transport Properties of Selenium and Tellurium Analogues of Polythiophene

A series of conjugated polymers comprising polythiophene, polyselenophene, and polytellurophene with branched 3,7-dimethyloctyl side chains, well-matched molecular weight, dispersity, and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV, with polytellurophene having the lowest potential. Field-effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude, with polytellurophene having the highest mobility (2.5 × 10^-2 cm² V^-1 s^-1 ). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure, their solid-state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical, polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline, even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group-16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material, and that heavy group-16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.

New D-A1-D-A2-Type Regular Terpolymers Containing Benzothiadiazole and Benzotrithiophene Acceptor Units for Photovoltaic Application

Two novel regular terpolymers that are of D-A1-D-A2 type and contain benzothiadiazole and 2,5-dibromo-8-dodecanoylbenzo[1,2-b:3,4-b':5,6-d″]trithiophene (P1) or 2,8-dibromo-5-dodecanoylbenzene[1,2-b:3,4-b':5,6-d″]trithiophene (P2) acceptor units with the same thiophene donor were synthesized through Stille coupling, and their optical and electrochemical properties were investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels of these terpolymers indicate that there is sufficient LUMO offset with PCBM for efficient exciton dissociation, and their deeper HOMO levels ensure the high open-circuit voltage for the resultant bulk heterojunction solar cells. Measurements on the solar cell devices also confirm that compared to those based on P2 the devices based on P1 possess a higher short-circuit photocurrent (J_sc) as well as a higher fill factor (FF), which is attributed to the lower bandgap and higher hole mobility for P1, whereas the V_oc is higher for the devices that are based on P2, which may be a result of P2 having a lower HOMO energy level than P1. The optimized polymer solar cells fabricated using P1:PC₇₁BM (DIO/CF) and P2:PC₇₁BM (CF/DIO) for the active layers showed a PCE of 7.19% and 6.34%, respectively. Atomic force microscopy (AFM) images of P1:PC₇₁BM blend films show that they exhibit more suitable morphology with favorable interpenetrating networks, which favors high J_sc and FF. Moreover, P1 exhibits a more crystalline nature than P2 that also favors the charge transport. This may be a result of better molecular packing, more distinct phase separation of the blended films, as well as a reduction of charge recombination.

Structures and photoelectric properties of five benzotrithiophene isomers-based donor-acceptor copolymers

In this paper, we have investigated the structures, electronic and optical properties of five conjugated copolymers (BTT1-BTz, BTT2-BTz, BTT3-BTz, BTT4-BTz and BTT5-BTz) featuring benzotrithiophene (BTT) isomers as donor units and benzothiadiazole (BTz) as acceptor units, linked through thiophene spacers, employing many-body perturbation theory (MBPT). We have explored the isomer effects by configuration of the sulfur atoms in BTT units, aimed to get insight into how the structural modifications to the conjugated backbone can influence the molecular structures and electronic properties of conjugated polymers. Using the trimer as the computational model, the calculated low and high energy absorption bands (660 and 413 nm) for BTT1-BTz agree well with the experimental ones (645 and 430 nm) with a small offset of ~15 nm. On the basis of our calculations, it is found that the backbones of these polymers display different coplanarities, with the dihedral angles between the two neighboring rings varying from 12.3° to 79.0°. Importantly, both BTT1-BTz and BTT2-BTz exhibit intense adsorption around 660 and 623 nm, indicating their promising application in solar cells, whereas BTT3-BTz and BTT4-BTz display the intense adsorption at 569 and 551 nm, which are also usable in the tandem solar cells. BTT5-BTz has narrow and weak adsorption in the visible and infrared region, implying it is not conducive to the sunlight absorption. The blue shift of about 150 nm from BTT1-BTz to BTT5-BTz is suggested to be originated from the shorter effective conjugation lengths.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

HOMO stabilisation in π-extended dibenzotetrathiafulvalene derivatives for their application in organic field-effect transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB-TTF) central core and a 2,1,3-chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB-TTF, have been synthesised as active materials for organic field-effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron-withdrawing 2,1,3-chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution-processed single-crystal transistors exhibit high performance with a hole mobility up to 0.04 cm(2)  V(-1)  s(-1) as well as good ambient stability.

Highly coplanar very long oligo(alkylfuran)s: a conjugated system with specific head-to-head defect

Well-defined monodisperse conjugated oligomers, which have planar backbones and are free from the disturbance of substituents, attract broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer 16F-6C6 together with the related nF-2C6 (n = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time, we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause considerable twists in its oligo- or polythiophene analogues. We present that the properties of these rigid oligo(alkylfuran)s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, 16F-6C6 was observed to be stable and highly fluorescent. Experimental and computational studies of the redox states of these oligo(alkylfuran)s reveal that 16F-6C6 has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet (⟨S2⟩ = 0.989) is 3.8 kcal/mol more stable than the closed-shell dication.

Synthesis and photovoltaic properties of new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b′′] trithiophene donor and different acceptor units (P1 and P2)

A power conversion efficiency of about 5.26% has been achieved for the device based onP1:PC71BM.

Rational design of benzotrithiophene-diketopyrrolopyrrole-containing donor-acceptor polymers for improved charge carrier transport

Two new donor-acceptor polymers containing benzo[2,1-b:3,4-b':5,6-c'']trithiophene (BTT) as donor and diketopyrrolopyrrole (DPP) as acceptor are synthesized and applied in OFETs. By tuning the alkyl substituents of the polymers, a striking difference in packing order, thin-film arrangement, and charge carrier transport is observed. The polymer without substituents at the BTT exhibits a hole mobility two orders of magnitude higher than that with alkyl chains therein.