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In this review, the molecular design and stimuli-responsive approach of activatable NIR-II organic fluorophores are introduced. Their applications in biomedical fields, ranging from biosensing to bioimaging and theranostics, are also highlighted.

Aggregation-induced emission (AIE) nanoparticles have been widely applied in photodynamic therapy (PDT) over the past few years. However, amorphous nanoaggregates usually occur in their preparation, resulting in loose packing with disordered molecular structures. This still allows free intramolecular motions, thus leading to limited brightness and PDT efficiency. Herein, we report deep-red AIE nanocrystals (NCs) of DTPA-BS-F by following the facile method of nanoprecipitation. It is observed that DTPA-BS-F NCs possess not only a high photoluminescence quantum yield value of 8% in the deep-red region (600-850 nm) but also an impressive reactive oxygen species (ROS) generation efficiency of up to 69%. Moreover, DTPA-BS-F NCs targeting dual-organelles of lysosomes and nucleus to generate ROS are also achieved, thus boosting the PDT effect in cancer therapy both

The emission efficiency of organic semiconductors (OSCs) often suffers from aggregation caused quenching (ACQ). An elegant solution is aggregation-induced emission (AIE), which constitutes the design of the OSC so that its morphology inhibits quenching π–π interactions and non-radiative motional deactivation. The light-emitting electrochemical cell (LEC) can be sustainably fabricated, but its function depends on motion of bulky ions in proximity of the OSC. It is therefore questionable whether the AIE morphology can be retained during LEC operation. Here, we synthesize two structurally similar OSCs, which are distinguished by that 1 features ACQ while 2 delivers AIE. Interestingly, we find that the AIE-LEC significantly outperforms the ACQ-LEC. We rationalize our finding by showing that the AIE morphology remains intact during LEC operation, and that it can feature appropriately sized free-volume voids for facile ion transport and suppressed non-radiative excitonic deactivation.

Light utilization is the vital first step of photosynthesis for photoautotrophic organisms. Boosting the growth and yield of photosynthetic organisms is critical in sustainable food and biofuel pro...

Stimulated emission depletion (STED) microscopy has become a useful tool for visualization and dynamic monitoring at an ultra-high resolution in biological research and material science. For STED technology, fluorescent probes are irreplaceable in the imaging process. Among these probes, organic fluorescent probes have superior photo-stability, high brightness, large Stokes’ shifts and excellent biocompatibility, thus are widely applied in STED microscopy. Based on this consideration, this review presents the recent advances on organic fluorescent probes for STED microscopy, including typical organic fluorescent probes, aggregation-induced emission luminogens (AIEgens), polymer dots, and other nanoparticles. The applications of organic fluorescent probes in biological imaging, such as in live-cell, live-tissue, and in vivo imaging, as well as in material monitoring at the nanoscale using STED microscopy, are also included. This review provides the guidelines for the design of new materials that can be used to enhance the imaging performance of STED microscopy, thus leading to real-world applications.

Aggregation-induced emission(AIE) luminogens(AIEgens) with high brightness in aggregates exhibit great potentials in biological imaging, but these AIEgens are seldom applied in super-resolution biological imaging, especially in the imaging by using the structural illumination microscope(SIM). Based on this consideration, we synthesized the donor-acceptor typed AIEgen of DTPA-BTN, which not only owns high brightness in the near-infrared(NIR) emission region from 600 nm to 1000 nm(photoluminescence quantum yield, PLQYs=11.35%), but also displays excellent photo-stability. In addition, AIE nanoparticles based on 4,7-ditriphenylamine-[1,2,5]-thiadiazolo[3,4-c]pyridine(DTPA-BTN) were also prepared with highly emissive features and excellent biocompatibility. Finally, the developed DTPA-BTN-based AIE nanoparticles were applied in the super-resolution cellular imaging via SIM, where much smaller full width at half-maximum values and high signal to noise ratios were obtained, indicating the superior imaging resolution. The results here imply that highly emissive AIEgens or AIE nanoparticles can be promising imaging agents for super-resolution imaging via SIM.

To obtain lipid droplets (LDs)-specific super-resolution cellular imaging and deep-penetrated tissue imaging, an imaging platform of DTPA-BT-M with aggregation-induced emission (AIE) characteristics for stimulated emission depletion (STED) nanoscopy and two-photon fluorescence (TPF) microscopy was developed here. Benefiting from its excellent AIE properties, superior fluorescence properties in a deep-red region are observed for DTPA-BT-M with a high photoluminescence quantum yield of up to 33.96% in the solid state. In addition, DTPA-BT-M also exhibits large Stokes’ shift, outstanding photostability, excellent biocompatibility and high LD specificity. Furthermore, a large two-photon absorption cross section of up to 1581 GM is also achieved in DTPA-BT-M due to its symmetrical D–A–D architecture. Therefore, the developed AIE luminogen is simultaneously applied in STED nanoscopy and TPF microscopy, leading to an ultra-high resolution (full width at half maximum value, FWHM = 95 nm) in DTPA-BT-M-stained LDs via STED nanoscopy and also a much deep penetration of ∼300 μm for lung tissue in BALB/C nude mice by TPF microscopy. The results here indicate that the easily synthesized DTPA-BT-M can be an impressive imaging platform for LDs-specific super-resolution cell imaging and two-photon tissue imaging, thus providing the possibility to further understand LDs' roles in the metabolic process in biological research.

Stimulated emission depletion (STED) nanoscopy is a promising fluorescence microscopy to detect unresolvable structures at the nanoscale level and then achieve a superior imaging resolution in materials science and biological research. However, in addition to the optimization of the microscope, luminescent materials in STED nanoscopy are also of great significance to obtain imaging, visualization and even long-term tracking at an ultra-high resolution (less than 100 nm), but this is seldom summarized. Based on this consideration, recent progress on STED fluorophores for super-resolution imaging is outlined here, including inorganic fluorophores, fluorescent proteins, organic luminescent materials, aggregation-induced emission (AIE) luminogens, and fluorescent nanoparticles. Characteristics of these aforementioned STED fluorophores are also included and compared to provide a deep understanding of the relationship between the properties in luminescent materials and their performance in STED imaging. According to the results on such luminescent materials, it is anticipated that guidelines to select proper probes and even develop new materials for super-resolution imaging via STED nanoscopy will be provided here, finally promoting the development of super-resolution imaging in both materials science and biological research.

It is noted that developing a single-molecule-triggered photo-theranostic platform for precise diagnosis and efficient cancer therapy is of great significance, but it still remains a difficult issue. In particular, it is challenging to balance the radiative decay (fluorescence) and non-radiative decay (photothermal effects) pathways for developed theranostic systems. Hence, the donor–acceptor–donor (D–A–D)-type aggregation-induced emission luminogen (AIEgen) of DTPA-BBTD is designed in this study. This molecule exhibits excellent absorption in the first near infrared (NIR-I) window as well as high brightness in the second near infrared (NIR-II) region, together with the favorable photoluminescence quantum yield (PLQY) of 1.51% compared with the commercial NIR-II fluorescent dye IR 26 used as a reference. In addition, owing to its efficient twisted intramolecular charge transfer effects and loose molecular packing when aggregated, DTPA-BBTD also exhibits an efficient photothermal conversion capability, thus providing the single-molecule-triggered photo-theranostic platform. It is observed that DTPA-BBTD-based AIE dots are successfully applied for NIR-II fluorescence imaging and NIR-I photoacoustic imaging to visualize the vasculature of mice and observe tumors in vivo, respectively. Moreover, photothermal therapy by using the developed AIE dots under irradiation is finally applied to well hinder tumor growth. This result demonstrates that manipulating the molecular backbone in AIEgens to balance the radiative decay and non-radiative decay can be an efficient strategy for constructing high-performance photo-theranostic platforms for precise cancer diagnosis and therapy.

Organelle-specific imaging and dynamic tracking in ultrahigh resolution is essential for understanding their functions in biological research, but this remains a challenge. Therefore, a facile strategy by utilizing anion-π

As an emerging antitumor strategy, photodynamic therapy (PDT) has attracted intensive attention for the treatment of various malignant tumors owing to its noninvasive nature and high spatial selectivity in recent years. However, the therapeutic effect is unsatisfactory on some occasions due to the presence of some unfavorable factors including nonspecific accumulation of PS towards malignant tissues, the lack of endogenous oxygen in tumors, as well as the limited light penetration depth, further hampering practical application. To circumvent these limitations and improve real utilization efficiency, various enhanced strategies have been developed and explored during the past years. In this review, we give an overview of the state-of-the-art advances progress on versatile nanoplatforms for enhanced PDT considering the enhancement from targeting or responsive, chemical and physical effect. Specifically, these effects mainly include organelle-targeting function, tumor microenvironment responsive release photosensitizers (PS), self-sufficient O2 (affinity oxygen and generating oxygen), photocatalytic water splitting, X-rays light stimulate, surface plasmon resonance enhancement, and the improvement by resonance energy transfer. When utilizing these strategies to improve the therapeutic effect, the advantages and limitations are addressed. Finally, the challenges and prospective will be discussed and demonstrated for the future development of advanced PDT with enhanced efficacy.

Nanozymes are a new generation of artificial enzymes that address the limitations of natural enzymes, with numerous reports on the development of high performance nanozymes for various applications. Herein, the phase regulation of network-like MoSe2 from the metal 1T phase to semiconductor 2H phase was achieved under different reaction conditions by a facile but efficient microwave-assisted solvothermal method. The coexisting 1T/2H-MoSe2 with edge enriched and defective nanostructures showed great enhancement of the peroxidase-like properties. This high enzymatic activity may arise from optimized active sites of 1T/2H-MoSe2, effectively leading to a rapid charge transfer within the substrate. In addition, chitosan functionalization not only greatly improves the dispersion stability of MoSe2, but also significantly increases its peroxidase-like activity, probably due to its enhanced affinity to hydrogen peroxide (H2O2). Based on these results we have established a highly sensitive colorimetric assay for the detection of H2O2 and glucose in human serum.

AIE nanocrystals with high brightness in the deep-red region were facilely prepared by kinetically controlled nanoprecipitation. These nanocrystals were then applied in super-resolution cellular imaging via STED nanoscopy.

Currently, bright aggregation-induced emission luminogens (AIEgens) with high photoluminescence quantum yields (PLQYs) in the NIR-II region are still limited, and thus an efficient strategy to enhance NIR-II fluorescence performance through tuning molecular aggregation is proposed here. The synthesized donor–acceptor tailored AIEgen (DTPA-TBZ) not only exhibits an excellent absorptivity in the NIR-I region, but also good fluorescence signals in the NIR-II region with an emission extending to 1200 nm. Benefiting from such improved intramolecular restriction and aggregation, a significant absolute PLQY value of 8.98% was obtained in solid DTPA-TBZ. Encouragingly, the resulting AIE dots also exhibit a high relative PLQY of up to 11.1% with IR 26 as the reference (PLQY = 0.5%). Finally, the AIE dots were applied in high performance NIR-II fluorescence imaging and NIR-I photoacoustic (PA) imaging: visualization of abdominal vessels, hind limb vasculature, and cerebral vessels with high signal to background ratios was performed via NIR-II imaging; Moreover, PA imaging has also been performed to clearly observe tumors in vivo. These results demonstrate that by finely tuning molecular aggregation in DTPA-TBZ, a good NIR-I absorptivity and a highly emissive fluorescence in the NIR-II region can be achieved simultaneously, finally resulting in a promising dual-modal imaging platform for real-world applications to achieve precise cancer diagnostics., A highly efficient dual-modal imaging platform by using bright AIE dots was constructed to achieve precise cancer diagnostics.

Dongfeng Dang, Yanfeng Zhang, Shujiang Ding and Zhicheng Zhang introduce the Materials Chemistry Frontiers themed collection on materials chemistry research at Xi’an Jiaotong University.

To investigate the relationships between molecular structures and their photovoltaic performance in solar cells, molecular engineering of push-pull small molecules from D-A to A-D-π-D-A architectures is proposed here using benzodithiophene, diketopyrrolopyrrole and fluorine-substituted phenyl groups as D, A and conjugated π units, respectively. In contrast to BDT-DPP, A-D-π-D-A structured DBDT-DPP-BF not only exhibits largely improved optical properties and matched energy levels, but also displays much ordered molecular packing and balanced hole/electron mobility in transistors. Therefore, compared with the inferior photovoltaic performance for BDT-DPP (PCE = 0.29%), the maximum PCE value of 5.27% with a significantly improved Jsc value of 12.01 mA/cm2 is finally achieved in DBDT-DPP-BF-based solar cells, indicating that manipulating molecular backbone from D-A to A-D-π-D-A architectures in the benzodithiophene and diketopyrrolopyrrole-contained push-pull small molecules here is an efficient method to enhance their photovoltaic performance in solar cells.

Graphene has attracted enormous attention due to its unique physical properties and attractive applications in many fields. However, it is an ongoing challenge to develop a facile and low-cost method for the large scale preparation of high-quality graphene (HQGr). In this work, we have developed an improved liquid-phase exfoliation method to mass produce HQGr. This method is quite simple but efficient by exfoliation of graphite in organic solvent with the co-assistance of sodium citrate and water. Remarkably, the concentration of as-exfoliated HQGr was as high as 0.71 mg/mL under optimal conditions, while the oxygen content in HQGr was only 2.39%. After annealing at 500 °C for 2 h in argon atmosphere, the mean conductivity of annealed HQGr was as high as 1.4 × 104 S m−1. Therefore, this facile method for liquid-phase exfoliation of graphite has excellent potential in the industrial-scale production of HQGr for numerous applications in energy storage, optical and electronic fields.

Non-conjugated fluorescent materials have attracted much research attention in recent years due to their great theoretical significance and practical value. But the working mechanism for this extraordinary photophysical phenomenon is still unclear and in debate. In this work, three non-conjugated polypeptides exhibited excitation independent clusteroluminescence (CL) with an emission peak at 440 nm. Experimental and theoretical analyses proved that the (n,π*) transition of the amide unit plays a crucial role in the CL. Notably, the processes of polymerization and clusterization are proposed as effective ways to activate the (n,π*) transition, which could increase the CL efficiency. This work not only gains further insights into the CL mechanism but also provides effective ways to design and manipulate non-conjugated fluorescent materials.

Human population is now faced with grand challenges such as global warming, food shortage and energy sustainability, which could be partially solved by massively increasing the growth and yield of photosynthetic organisms which capture the light energy to convert carbon dioxide and water into usable chemical energy. Cyanobacteria and eukaryotic microalgae are considered as attractive targets to be exploited by the algal factory because of their fast growth, low cost cultivation, less arable land and the diversity of high-value chemical substances produced. Many optical approaches have been introduced to increase the efficiency in artificial culturing systems, such as adding a luminescent layer that absorbs ultraviolet light and emits photosynthetic active radiation for cyanobacteria. In this work, we introduced luminogens with aggregation-induced emission characteristics (AIEgens) into the growth medium of a marine cyanobacteria. These hydrophobic AIEgens formed highly emissive luminogenic aggregates in the aqueous medium and dispersed around the cyanobacteria. Remarkedly, the number of cyanobacteria incubated in the medium with AIE aggregates was 5-fold more than the control group after 14-day culturing. The increased photosynthetic active radiation and the change of cyanobacteria protein expression in photosynthesis and metabolism might be the reason. Our study is the first using organic luminogenic aggregates as optical engineering inside the growth medium to dramatically increase the growth of cyanobacteria and demonstrated that AIEgens is promising technologies in the development of algal factories.

Combining doxorubicin with thermal therapy in the clinic has led to startling results in the treatment of problematic cancers. Here, we describe a multimo dal multi-walled carbon nanotube material that combines tumor targeting, doxorubicin delivery, and photothermal therapy for localized cancer treatment. The agent was constructed layer-by-layer from polypyrrole and gold nanoparticles on multi-walled carbon nanotubes. The gold surface was modified with tumor targeting folic acid terminated PEGylated chains, which also provide water-dispersibility, biocompatibility and should extend the half-life in blood. The material has a high loading/unloading capacity for the cytotoxic agent doxorubicin. Release of the doxorubicin, combined with the photothermal properties of the material that induces localized hyperthermia, leads to efficient cancer cell death.

Super-resolution imaging (SRI) is of great significance to break the Abbe's diffraction limit, enabling visualization and dynamic monitoring with ultra-high resolution in both material science and biology. Fluorophores play an important role in SRI. Among them, aggregation-induced emission (AIE) luminogens (AIEgens) are one of the most promising imaging agents for SRI. They are easy to be synthesized and possess high tailoring ability, high brightness in aggregated states, large Stokes' shift, good photo-stability, and also excellent biocompatibility. Based on this consideration, the recent progress on AIEgens for SRI is summarized here. The working principles of AIEgens and several SRI techniques are also introduced to illustrate the material requirements. The new achievements in AIEgens research for SRI, including their chemical structures, optical properties, and applications, are highlighted in detail. The goal is to provide the readers with an overview of the recent advances in AIEgens for SRI. It is also anticipated that a deep understanding of the relationship between the AIEgens' properties and their imaging performance can be obtained. This review also aims to provide a set of guidelines to develop new AIEgens for fluorescence imaging to achieve superior resolution, thus resulting a better understanding of their mechanism in both biology and material science.

The development of new luminescent materials has allowed us to gain new knowledge and opened a new opportunity for scientific achievement and social development. In this work, luminogens, namely, (...

Currently, the commonly developed organic luminescent materials (OLMs) usually exhibit poor luminescent performance in aggregated solid states compared with their well-dissolved solution states, making it a tough goal to achieve the highly emissive dual-state emission. To overcome this limitation, a "self-isolated enhanced emission" (SIEE) strategy through flexible alkyl chains to suppress the emission-quenched π-π stacking in solids is proposed here and, based on this guideline, remarkable emission efficiency with photoluminescence quantum yields up to 99.72 % in solution and 77.46 % in the solid state are achieved for the SIEE constructed DBBT-C8, which is then successfully used in solid-state displays and data encryption.

A novel tetra-armed D-A type small molecule 4BTF-DTPA-T was designed here by utilizing the thiophene connected ditriphenylamine (DTPA) core and fluorinated benzothiadiazole (BTF) arms. To make a better comparison, tri-armed (star-shaped) 3BTF-TPA was also synthesized. Although a similar absorption spectra covering from 300 to 600 nm was observed for 3BTF-TPA and 4BTF-DTPA-T, much higher extinction coefficient was achieved for tetra-armed 4BTF-DTPA-T in both solution and thin films. Bulk hetero-junction (BHJ) organic solar cells were also fabricated to investigate their photovoltaic performance and as anticipated, a moderate PCE value of 2.61% with a Voc of 1.00 eV was observed in 3BTF-TPA-based solar cells. However, when the tetra-armed 4BTF-DTPA-T was employed as the electron-donors, much enhanced Jsc and FF values up to 9.33 mA/cm2 and 44.80% were achieved for its homogeneous morphology in solar cells, finally leading to the maximum PCE value of 3.68%, indicating that the tetra-armed small molecules could be the promising electron-donors for solution-processed organic solar cells.

To achieve the highly emissive features and overcome the troublesome photobleaching for fluorescent organic molecules, a facile and versatile strategy named “self-isolation enhanced emission (SIEE)” was developed to prevent the π–π stacking of organic fluorophores by linking alkyl chains on their conjugated backbones. As a proof-of-concept, one or two octyl groups were grafted onto the backbone of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (termed as DTBT-0), resulting in two different molecules, termed as DTBT-1 and DTBT-2, respectively. Compared with DTBT-0, DTBT-1 and DTBT-2 exhibited remarkably enhanced fluorescent properties in both aggregated thin films and nanoparticles, demonstrating that the SIEE method could isolate the fluorophores effectively and then prevent their π–π stacking to achieve the impressive fluorescent properties. After proper surface modification, excellent water dispersibility, biocompatibility, and improved resistance to photobleaching were also achieved for highly emissiv...

To release the steric-strain in largely fused molecular backbone and achieve the much planar molecules for efficient organic solar cells, a facile method was proposed by dividing the rigidly fused dibenzosexithiophene (DBST) units in FT3 with large molecular skeletons to two smaller building blocks to construct the electron donor-acceptor type compound FT4. As observed, much improved molecular co-planarity was achieved for FT4 in contrast to that of FT3, then leading to the higher charge carrier mobilities up to 6.7 × 10 −4 cm 2 V −1 s −1 by SCLC method. Meanwhile, FT4-based BHJ solar cells also exhibited much enhanced photovoltaic performance with the maximum PCE value of 3.07% and higher J sc value of 8.23 mA/cm 2 , which is only 2.13% and 5.67 mA/cm 2 for FT3-based solar cells. The results here indicated that the strain-released method, especially in the highly fused molecules, is beneficial to further improve their co-planarity, then leading to the high charge carrier mobilities and much enhanced photovoltaic performance in solar cells.

Novel non-fullerene acceptors with an A 1 -A-A 1 framework were designed and synthesized, in which benzothiadiazole and diketopyrrolopyrrole building blocks were employed as the A unit and naphthalimide units were incorporated as the A 1 units. Both acceptor systems exhibited a broadened absorption spectra from 300 nm to 700 nm in contrast to that of fullerene derivatives. Meanwhile, high-lying LUMO energy levels were also observed for both developed acceptors. To investigate photovoltaic properties of the acceptors, bulk-heterojunction organic solar cells were fabricated using an established electron donating polymer as the donor material. As anticipated, efficient non-fullerene OSCs with high V oc values up to 1.01–1.12 V were achieved when the new molecules were employed as electron acceptors, leading to the PCE values of 1.23% and 1.64% in their solar cells. Our results here demonstrate that the A 1 -A-A 1 type small molecule containing naphthalimide building blocks in molecular backbone could be the promising electron acceptor in organic solar cells.

Organic fluorophores for stimulated emission depletion (STED) nanoscopy usually suffer from quenched emission in the aggregate state and inferior photostability, which largely limit their application in real-time, in situ, and long-term imaging at an ultrahigh resolution. Herein, an aggregation-induced emission (AIE) luminogen of DP-TBT with bright emission in solid state (photoluminescence quantum yields = 25%) and excellent photostability was designed to meet the requirements in STED nanoscopy. In addition to its excellent fluorescence properties, DP-TBT could also easily form self-assembling helixes and finally be well-visualized by super-resolution STED nanoscopy. The observations showed that helical fibers of DP-TBT as dashed lines had a much decreased fiber width with also a full width at half-maximum value of only 178 nm, which is ∼6 times higher than solid lines obtained by confocal microscopy (1154 nm). The STED nanoscopic data were also used to reconstruct 3D images of assembled helixes. Finally, by long-term tracking and dynamic monitoring, the formation and growth of helical fibers by DP-TBT in self-assembly processes were successfully obtained. These findings imply that highly emissive AIEgens with good photostability are highly suitable for real-time, in situ, and dynamic imaging at super-resolution using STED nanoscopy.

Bi-armed Cz(BT-T-3Cz) 2 , tri-armed TPA(BT-T-3Cz) 3 and tetra-armed SF(BT-T-3Cz) 4 with planar terminals were designed, in which carbazole, triphenylamine and spiro-fluorene units were employed as the electron-rich core, respectively, and benzothiadiazole units were introduced as the electron-deficient arms. As displayed, when the electron-deficient arms in molecules increase, improved thermal properties and increased extinction coefficient were obtained. Also as anticipated, the spatial configurations of SMs could affect their photovoltaic performance significantly. The linear Cz(BT-T-3Cz) 2 -based solar cells exhibited a PCE value of 3.0%, however, tri-armed TPA(BT-T-3Cz) 3 and tetra-armed SF(BT-T-3Cz) 4 displayed better photovoltaic properties with the PCE values up to 5.25% and 4.37% in their corresponding devices, respectively, indicating that altering the spatial configurations in molecules is an efficient method to enhance their photovoltaic performance in OSCs.

Spirobifluorene-linked three-dimensional small molecules of SF(DPP)4 and SF(TDPP)4 containing four diketopyrrolopyrrole arms were designed and synthesized. Both molecules displayed a broad and strong absorption in the region of 300–700 nm with suitable HOMO and LUMO energy levels as well as high charge carrier mobility. Meanwhile, solution-processed organic solar cells with a device structure of ITO/PEDOT:PSS/active layer/Ca/Al was also fabricated to investigate the photovoltaic properties of SF(DPP)4 and SF(TDPP)4. In contrast to that of star-shaped TPA(DPP)3 in solar cells, enhanced photovoltaic performance with an impressive open circuit voltage of 0.95 V and short-circuit current density of 9.12 mA/cm2 was achieved, leading to the maximum power conversion efficiency of 3.72 %. Our result here demonstrated that the three-dimensional-type small molecule in a D–A framework is a promising candidate as electron-donor materials for efficient organic solar cells.

Two novel D-A-A type regular terpolymers of PBDT-DTQ and PIDT-DTQ were designed and synthesized, in which benzodithiophene and indacenodithiophene building blocks were employed as the D unit, and quinoxaline building block was introduced as the A unit. Owing to the strong intra-molecular charge transfer effect in polymer backbone, much broader absorption spectra covering from 320 to 800 nm with the narrowed band-gap were obtained for the developed D-A-A type polymers in contrast to their corresponding D-A type polymers. On the other hand, compared with PBDT-DTQ, PIDT-DTQ exhibited a deeper HOMO energy level and also higher charge carrier mobility. To investigate the photovoltaic properties of PBDT-DTQ and PIDT-DTQ in detail, bulk hetero-junction polymer solar cells with a structure of ITO/PEDOT: PSS/Active Layer/Ca/Al were fabricated. PBDT-DTQ-based solar cells exhibited a moderate PCE value of 3.84%, however, an increased J sc of 11.42 mA/cm 2 , V oc of 0.86 V and FF of 65.77% was achieved for PIDT-DTQ-based device, leading to the maximum PCE up to 6.41%. Our results here demonstrated that using the regular terpolymers as electron donor materials could be an efficient way to broaden the absorption of polymers and improve the photovoltaic performance of PSCs.

Utilizing indacenodithiophene (D) and quinoxaline (A) building blocks, novel D–A–π–A type regular terpolymers of PIDT-DTQ-TT were designed with a conjugated π spacer of thieno[3,2-b]thiophene unit in the polymer backbone. In contrast to PIDT-DTQ with the thiophene unit as the spacer, PIDT-DTQ-TT displayed a much broader absorption with suitable HOMO energy levels and high charge carrier mobility. Also an enhanced Jsc of 12.43 mA cm−2 and a Voc of 0.83 V comparable to that of PIDT-DTQ-based devices were observed in PIDT-DTQ-TT-based solar cells, indicating that the balance between Jsc and Voc in devices can be achieved when the spacer in the polymer backbone was altered individually in D–A–π–A type regular terpolymers. Finally, a maximum power conversion efficiency value of 6.63% was achieved without any post-treatment in solar cells. Our results here demonstrate that tuning the conjugated spacers in D–A–π–A type regular terpolymers individually could be an efficient method to balance the trade-off between Jsc and Voc, leading to an enhanced photovoltaic performance in solar cells.

Isomeric TF1 and TF2 with highly fused thiophene cores were designed and synthesized here, in which a highly planar molecular structure was obtained for TF1 with the face-to-face sulfur atoms in the lateral region and a twisted molecular backbone was observed for TF2 with the back-to-back sulfur atoms. It is worth noting that different intermolecular interactions dominated in TF1 and TF2 caused by their isomeric thiophene cores, in which strong π–π stacking was achieved for TF1, whereas sulphur-involved nonbonding intermolecular interactions dominated in TF2, leading to the different fluorescence behaviors and also the altered liquid crystalline phases. Finally, typical P-type charge transport behaviors were achieved in both TF1- and TF2-based solution-processed OFETs. Also owing to the much ordered molecular packing in TF1, a higher charge carrier mobility of 3.7 × 10−3 cm2 V−1 s−1 was achieved for TF1-based OFETs compared to TF2-based OFETs.

A novel di(dodecylthiophene)- alt -2,3-di(3-octoxylphenyl)-5,8-dithieno[3,2- b ]thiophene-6,7-difluoroquinoxaline copolymer (PBT-TTQ-F) and its counterpart of PBT-TTQ-H without the substituted fluorine atom were designed and synthesized. Significant effect of the attaching fluorine atoms in the polymer backbone on photovoltaic properties of copolymers was observed. In the optimized bulk hetero-junction polymer solar cells with a structure of ITO/PEDOT: PSS/copolymers: PC 71 BM (1:2, w/w )/Ca/Al, PBT-TTQ-F exhibited a higher power conversion efficiency (PCE) of 3.05% with a short-circuit current ( J sc ) of 9.48 mA/cm 2 and a fill factor ( FF ) of 48.6% under an illumination of AM 1.5, 100 mW/cm 2 . The PCE, J sc and FF values are 1.2–1.7 times higher than those of the PBT-TTQ-H-based devices. Our results demonstrated that photovoltaic properties could be significantly enhanced by copolymeric fluorination in PCSc.

Two novel 5,8-disubstituted benzo[1,2- b :4,5- b ′]dithiophene- alt -2,3-di(4-methoxylphenyl)-6,7-dioctyloxy-5,8-dithienyl-quinoxaline copolymers of PBDTDT-(Qx-2)-O and PBDTDT(Qx-2)-T were designed and synthesized, in which the substituents are branch dioctyloxy and di(octylthienyl), respectively. Their optical, thermal, electro-chemical and photovoltaic properties were investigated. Compared to the reported analogues, both one- and two-dimensional (1D and 2D) copolymers exhibited outstandingly increasing open-circuit voltages ( V oc ) about 0.91–0.95 V in their bulk hetero-junction polymer solar cells (PSCs) with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) acceptor. The maximum power conversion efficiency (PCE) of 6.31% with V oc of 0.95 V, short-circuit current of 10.82 mA cm −2 and fill factor of 61.4% was obtained in the 2D-conjugated polymer of PBDTDT(Qx-2)-T/PC 71 BM-based device. The PCE value is 1.42 times higher than that of the 1D-conjugated polymer of PBDTD(Qx-2)-O/PC 71 BM-based device. Our work demonstrates that V oc values of the quinoxaline-type copolymers based PSCs can remarkably be increased by appending dioctyloxy chains at 6,7-positions of quinoxaline.

In order to investigate the effect of thiophene spacers in backbone between electron-rich thieno[3,2- b ]thiophene (TT) and electron-deficient benzothiadiazole (BT) units on the photovoltaic properties, novel two-dimensional (2-D) polymers with appending conjugated dioctylthiophene units in the 3,6-positions of TT were designed and synthesized, in which one or two thiophene spacers were embedded between TT and BT, respectively. The polymer with two thiophene spacers exhibited a broader absorption and higher power conversion efficiency in the bulk hetero-junction polymer solar cells. The best performance with efficiency up to 3.34% and short-circuit current values of 12.6 mA/cm 2 was obtained, which are 2.1 and 3.0 times higher than those corresponding values for the polymer with only one thiophene spacer. Our work indicates that increasing the numbers of thiophene spacers between TT and BT moieties in the backbone is an efficient method to enhance the photovoltaic performance of their 2-D polymers in polymer solar cells.

The novel D–A–π–A–D type molecules, FT1 and FT2, with varied molecular conformation were designed utilizing different fused hetero-aromatic units as the π-bridge building centers. FT1 with a dibenzo-quinquethiophene (DBQT) building center exhibited an eagle-shaped molecular conformation, whereas, FT2 with a dibenzo-sexithiophene (DBST) building center displayed a linear-shaped molecular conformation. In contrast to FT1, FT2 exhibited a much more planar molecular structure and more ordered inter-chain packing. An enhanced power conversion efficiency of 1.71% and short current density of 5.78 mA cm−2 were obtained in the FT2/PC71BM-based OSCs, which were 1.7 times and 1.5 times the corresponding values of the FT1/PC71BM-based cells. Our work demonstrated that the fused hetero-aromatic building centers can significantly affect the molecular conformation and that the photovoltaic performance of small molecules can also be improved by altering the molecular conformation in OSCs.

Two star-shaped molecules of TPA(BT–2Cz)3 and TPA(BT–3Cz)3 with a triphenyl-amine (TPA) core were designed and synthesized, in which 2- and 3-carbazolyl units (2Cz and 3Cz) are attached to the benzothiadiazole (BT) arms as planar terminal groups, respectively. Although both molecules show a similar broad UV-Vis absorption covering from 300 nm to 560 nm, different photovoltaic performances were observed owing to the varied linked positions of terminal carbazolyl units. In contrast to TPA(BT–2Cz)3-based solar cells, solution-processable organic solar cells (OSCs) based on TPA(BT–3Cz)3 exhibited better photovoltaic performance with the maximum power conversion efficiency up to 3.94% and an enhanced short-circuit current of 8.80 mA cm−2. This work further demonstrates that tuning the substituted position of the terminal carbazolyl group can significantly improve the photovoltaic performance of its resulting small molecules.

Isomeric star-shaped molecules of TPA(T–BT–3Cz)3 and TPA(BT–T–3Cz)3 with different moiety sequences in their arms based on the triphenylamine (TPA) core were designed and synthesized in order to study the influence of inserting thiophene (T) units in their arms and the effects of their positions on the photo-physical and photovoltaic properties of their molecules. For comparison, parent molecules of TPA(BT–3Cz)3 were also prepared with three arms containing 4,7-benzothiadiazole (BT) and 3-carbazolyl (3Cz) units. After the thiophene unit was inserted, the molecular planarity, absorption spectra and hole mobility improved significantly in the resulting TPA(BT–T–3Cz)3 and TPA(T–BT–T–3Cz)3. However, only TPA(BT–T–3Cz)3 exhibited improved photovoltaic properties in contrast to TPA(BT–3Cz)3 in the solution-processable solar cells with a device structure of ITO/PEDOT:PSS/SMs:PC71BM/Ca/Al. A maximum power conversion efficiency (PCE) of up to 5.07% with a short-circuit current density (Jsc) of 10.08 mA cm−2, an open circuit voltage (Voc) of 0.90 V and a fill factor (FF) of 54.05% was obtained in the TPA(BT–T–3Cz)3-based solar cells. To the best of our knowledge, this PCE is one of the highest PCE values for star shaped molecules with the TPA core in organic solar cells. Our work further indicates that the PCE and Jsc values of organic solar cells with TPA-based star-shaped molecules can be increased by inserting T units and tuning the moiety sequence of their arms.

Novel donor-acceptor-type molecules, endo-DBST(BT-Cz)(2) and exo-DBST(BT-Cz)(2), were designed with benzothiadiazole-carbazole (BT-Cz) based arms and isomeric cores of dibenzosexithiophene (DBST), in which sulfur atoms at two lateral thienos in DBST are face-to-face for the endo-derivative and back-to-back for exo-derivative. In contrast to the endo-type, the exo-form exhibited highly crystalline properties, leading to an increased hole mobility up to 1.1 x 10(-4) cmV(-1)s(-1). A significant effect of the isomeric DBST cores on the photovoltaic properties was also observed in the organic photovoltaic (OPV) cells. The maximum power conversion efficiency of 2.13% with a fill factor up to 61.41% was obtained for the exo-DBST(BT-Cz)(2)-based devices, which are 1.2 times higher than that in the endo-DBST(BTCz)(2)-based cells. Our results demonstrate that tuning the isomeric core would increase the carrier mobility significantly and consequently enhance the photovoltaic performance of the resulting molecules in OPV cells.

Novel wide band-gap polymer of PBTFT containing dibenzosexithiophene-alt-bithiophene backbone was designed and synthesized via the Stille cross-coupling reaction. This polymer exhibited good thermal stability, well coplanar backbone and a broad absorption band from 350 nm to 610 nm with a wide optical band-gap of 2.02 eV. The polymer solar cells (PSCs) based on the PBTFT:PC71BM active layer showed the power conversion efficiency of 3.0% with an open circuit voltage of 0.70 V, a short-circuit current of 7.94 mA cm(-2) and a fill factor of 53.98% under the illumination of AM.1.5, 100 mW cm(-2). Holes mobility up to 0.028 cm(2) V-1 s(-1) with an on-off ratio of 1.0 x 10(6) was obtained in the PBTFT-based organic field-effect transistors (OFETs). Our work indicates that the dibenzosexithiophene-alt-bithiophene based copolymer can be efficiently applied in PSCs and OFETs. (C) 2014 Elsevier Ltd. All rights reserved.

A class of low band-gap two-dimensional conjugated polymers of PBDTT-FQ PBDTT-TQ PBDTT-BTQ and PBDTT-TTQ was designed and synthesized, which contains the same di(alkylthiophene)-substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) and 6,7-difluoro-quinoxaline (Q) units, as well as various conjugated spacers of furan, thiophene, bithiophene and thieno[3,2-b]thiophene in the main chain. Significant effect of the varied spacers between the BDTT and Q units on the thermal, optical, electrochemical and photovoltaic properties was investigated and observed for these two-dimensional copolymers in the polymer solar cells. The maximum power conversion efficiency of 5.9% with a short circuit current of 13.7 mA/cm(2) and a fill factor of 0.56 was obtained for the PBDTT-TQ with thiophene spacer in the bulk hetero-junction PSCs using [6,6]-phenyl-C-71-butyric acid methyl ester as acceptor. (C) 2014 Elsevier B.V. All rights reserved.

Two-dimensional low band-gap copolymers of P1 and P2 with a framework of (2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene)- alt -(2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2 H ,5 H )-dione) (DTTT- alt -DTPP) were synthesized, in which two 2,3-dioctylthiophene-5-yl groups were pended into the 3-positions of two thiophenyl rings close to the thieno[3,2-b]thiophene (TT) unit for P1 and the 3, 6-positions of the TT unit for P2, respectively. The influence of pending positions on optophysical and photovoltaic properties were studied. Similar broad absorption spectra from 350 to 950 nm were observed with a low band-gap of 1.25 eV. However, significantly different photovoltaic properties were obtained in the bulk heterojunction polymer solar cells with a device structure of ITO/PEDOT:PSS/active layer/LiF/Al. The P1-based devices showed a power conversion efficiency (PCE) of 0.79%, but the P2-based devices displayed a notably increasing PCE up to 2.12% under an AM 1.5G irradiation with an intensity of 100 mW cm −2 . Our results indicate that altering pendent position of alkyl thiophene is effective way to improve the photovoltaic properties for the two-dimensional copolymers.

A novel tris(dibenzoylmethanato)[5-(2-(4- tert -butylbenzenyl)-5-benzenyl-1,3,4-oxadiazole-4′)-1,10-phenanthroline]europium(III) [Eu(DBM) 3 (BuOXD-Phen)] containing an electron-transporting oxadiazole-functionalized phenanthroline ligand was synthesized and characterized. Its UV–vis absorption and photoluminescence (PL), as well as the electroluminescence (EL) in polymer light-emitting devices (PLEDs) were investigated. The double-layer PLEDs with a configuration of ITO/PEDOT:PSS (50 nm)/PVK (40 nm)/PFO:PBD (30%):Eu(DBM) 3 (BuOXD-Phen) (1–8 wt %) (80 nm)/Ba (4 nm)/Al (150 nm) were fabricated. Saturated red Eu 3+ ion emission, based on the 5 D 0 → 7 F 2 transition, is centered at a wavelength of 614 nm with a full width at half maximum (FWHM) of 10 nm. The highest external quantum efficiency (QE ext ) of 1.26% at current density of 1.65 mA cm −2 , with a maximum brightness of 568 cd m −2 at 137.8 mA cm −2 was achieved from the device at 1 wt % dopant concentration.