Low-lying Lowest Unoccupied Molecular Orbital Research Articles

NIR‐II Absorption/Fluorescence of D–A π‐Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron‐Fused Azobenzene Complexes

Luminescence in the second near‐infrared (NIR‐II, 1,000–1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron‐acceptors with low‐lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor–acceptor (D–A) π‐conjugated polymers (CPs) with the NIR‐II emission property, however, limited kinds of molecular skeletons are still available. Herein, D–A CPs involving fluorinated boron‐fused azobenzene complexes (BAz) with enhanced electron‐accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to −4.42 eV, which is much lower than those of conventional strong electron‐acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR‐II. Furthermore, owing to the inherent solid‐state emissive property of the BAz skeleton, obvious NIR‐II fluorescence from the film (up to λFL = 1213 nm) and the nanoparticle in water (λFL = 1036 nm, brightness = up to 29 cm–1 M–1) were observed, proposing that our materials are applicable for developing next‐generation of NIR‐II luminescent materials.

NIR-II Absorption/Fluorescence of D-A π-Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron-Fused Azobenzene Complexes.

Luminescence in the second near-infrared (NIR-II, 1,000-1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron-acceptors with low-lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor-acceptor (D-A) π-conjugated polymers (CPs) with the NIR-II emission property, however, limited kinds of molecular skeletons are still available. Herein, D-A CPs involving fluorinated boron-fused azobenzene complexes (BAz) with enhanced electron-accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to -4.42 eV, which is much lower than those of conventional strong electron-acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR-II. Furthermore, owing to the inherent solid-state emissive property of the BAz skeleton, obvious NIR-II fluorescence from the film (up to λFL = 1213 nm) and the nanoparticle in water (λFL = 1036 nm, brightness = up to 29 cm-1 M-1) were observed, proposing that our materials are applicable for developing next-generation of NIR-II luminescent materials.

Metal Effect on Cationic [Cp2MH]+ (M = Group 4 and 5)-Mediated CO2 Hydrogenation in the Gas Phase.

Effective CO2 hydrogenation has recently attracted quite some attention for producing more valuable chemical oxygenates (such as methanol, formate) in mild conditions. However, the influence of the metal center on the CO2 activation remains unclear. First, electrospray ionization mass spectrometry (ESI-MS) was employed to explore the direct CO2 hydrogenation to formic acid mediated by [Cp2MH]+ (M = Zr, Hf) in the gas phase at room temperature. The key formate intermediate [Cp2M(O2CH)]+ (M = Zr, Hf) was confirmed by traveling wave ion mobility spectrometry (TWIMS). Second, to gain insights into the metal effect, the CO2 hydrogenation process involving Group 4 (i.e., Ti, Zr, Hf) transition metals was calculated along with Group 5 (i.e., V, Nb, Ta) by density functional theory (DFT) methods. The CO2 insertion process was found to be the rate-limiting step. For [Cp2TiH]+, [Cp2ZrH]+, [Cp2HfH]+, [Cp2VH]+, [Cp2NbH]+, and [Cp2TaH]+, the barriers are +7.7, +6.5, +5.9, +9.2, +8.0, and +6.3 kcal/mol, respectively. [Cp2HfH]+-mediated CO2 hydrogenation occurs the most rapidly, as revealed by MS. According to the orbital analysis on the CO2 insertion transition state, the electron-deficient metal center resulting in a low-lying lowest unoccupied molecular orbital (LUMO) could interact more favorably with the π bond of deformed CO2, which was also consistent with the natural bond orbital (NBO) results. Last but not the least, NBO charges on the metal centers were found to correlate linearly well with the CO2 insertion barriers rather than hydride affinity. Thus, the reactivity of different metal hydride complexes with CO2 to produce a formate could be estimated by the NBO charge on metals. Our findings might provide a series of candidates for the catalyst as well as guidance for catalyst design in mild CO2 hydrogenation.

Synergistic Use of All-Acceptor Strategies for the Preparation of an Organic Semiconductor and the Realization of High Electron Transport Properties in Organic Field-Effect Transistors.

The development of n-type organic semiconductor materials for transporting electrons as part of logic circuits is equally important to the development of p-type materials for transporting holes. Currently, progress in research on n-type materials is relatively backward, and the number of polymers with high electron mobility is limited. As the core component of the organic field-effect transistor (OFET), the rational design and judicious selection of the structure of organic semiconductor materials are crucial to enhance the performance of devices. A novel conjugated copolymer with an all-acceptor structure was synthesized based on an effective chemical structure modification and design strategy. PDPPTT-2Tz was obtained by the Stille coupling of the DPPTT monomer with 2Tz-SnMe3, which features high molecular weight and thermal stability. The low-lying lowest unoccupied molecular orbital (LUMO) energy level of the copolymer was attributed to the introduction of electron-deficient bithiazole. DFT calculations revealed that this material is highly planar. The effect of modulation from a donor-acceptor to acceptor-acceptor structure on the improvement of electron mobility was significant, which showed a maximum value of 1.29 cm2 V-1 s-1 and an average value of 0.81 cm2 V-1 s-1 for electron mobility in BGBC-based OFET devices. Our results demonstrate that DPP-based polymers can be used not only as excellent p-type materials but also as promising n-type materials.

A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer

Open AccessCCS ChemistryCOMMUNICATIONS6 Dec 2022A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer Chengcheng Dong, Juan Chu, Linzhu Cao, Fengchao Cui, Shuang Liang, Xintong Zhang, Xin Tao, Heng-guo Wang and Guangshan Zhu Chengcheng Dong Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Juan Chu Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Linzhu Cao Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Fengchao Cui Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Shuang Liang Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, School of Physics, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Xintong Zhang Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, School of Physics, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Xin Tao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Heng-guo Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author and Guangshan Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202351 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Radical-containing porous organic polymers (POPs) have drawn great interest in various applications. However, the synthesis of radical POPs remains challenging due to the unstable nature of organic radicals. Here, a persistent and stable three-dimensional silicon-diacetylene porous organic radical polymer was synthesized via a classic Eglinton homocoupling reaction of tetraethynylsilane. The presence of carbon radicals in this material was confirmed by electron paramagnetic resonance, and its paramagnetic behavior was analyzed by a superconducting quantum interference device. This unique material has a low-lying lowest unoccupied molecular orbital (LUMO) energy level (−5.47 eV) and a small energy gap (ca. 1.46 eV), which shows long-term cycling stability and excellent rate capability as an anode material for lithium-ion batteries, demonstrating potential application in energy fields. Download figure Download PowerPoint Introduction Porous organic radical polymers (PORPs) are a new class of porous materials, which introduce open-shell organic radical species into porous organic network structures. PORPs are of great interest in synthesis and various practical applications due to their unique properties in magnetic, electronic, optoelectronic, and biologic fields.1–8 However, targeted synthesis of persistent and stable PORPs with high surface areas, good thermal and chemical stability, and tunable functionalization is particularly challenging. A variety of protocols have been employed to achieve this goal, including the control of steric,9–11 electronic,12 and aggregated states13 to avoid covalent bonding of the organic radicals. So far, very few PORPs have been successfully synthesized.14–22 In 2015, Jiang’s group14 immobilized nitroxide radicals on the pore walls of a conventional imine-linked covalent organic framework (COF), which gave rise to radical-containing COFs for outstanding energy storage. Then, they found that oxidation of the pyrene unit of an sp2 carbon-conjugated COF, by iodine vapor, resulted in a pyrene radical cation-containing COF.15 Fang’s group16 has decorated 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical on the channel walls of three-dimensional (3D) COFs via a bottom-up approach, which resulted in the formation of 3D radical COFs. Another desirable strategy is using persistent and stable organic radicals as building blocks to construct PORPs, such as polychlorotriphenylmethyl-covalent organic radical framework (PTM-CORF),17 polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF),18 and phenanthroline-based cationic radical porous hybrid polymer (Phenc·+-PHP-2Br).19 The unique π-conjugated PORPs,20–22 were synthesized through a strategy to trap the spins in the organic network structure, which avoided the delocalizing and recombining processes. So far, the majority of PORPs reported are two-dimensional layered materials. We endeavor to design 3D PORPs because of their rigid framework, high surface area, and good physical and chemical stability. The above properties could potentially favor the stabilization of organic radical species in these materials, leading to the development of new PORPs with unique functionalities.23–26 From a structural point of view, sp3 hybridized C, Si, and Ge atoms, along with conjugated diacetylene derivatives, have been selected to serve as nodes and linear linkers, respectively, to construct new 3D porous organic polymers (POPs) by covalent bonds.27–29 The resulting acetylene-rich porous materials serve as promising electrode materials in lithium-ion batteries (LIBs).30–35 However, to the best of our knowledge, the radical characteristics of these materials have not yet been uncovered. In contrast, conjugated diacetylene-derived small-molecule compounds were reported to exhibit diradical resonance structure in the solid state.36,37 Based on these reports, we predict that a new 3D PORP could be constructed if the conjugated diacetylene units were introduced into 3D organic network structures. This turned out to be the case. Herein, we report the facile synthesis of a 3D Si-diacetylene PORP in the powder state via Eglinton38,39 homocoupling reaction of tetraethynylsilane. This unique material has a low-energy lowest unoccupied molecular orbital (LUMO), a strong extended absorption band up to 2000 nm, a small energy gap, and ferromagnetism at low temperature. The obtained 3D PORP exhibits excellent rate capability and long cycling stability as an anode material in LIBs. Results and Discussion We have initially designed three sp3 hybridized silicon-based POPs with similar structures, in which diacetylene ( DA), biphenyl ( BP), and phenylene diacetylene ( PDA) serve as linear linkers (Scheme 1). The 3D silicon-diacetylene POP Si-DA is synthesized via catalytic Eglinton homocoupling reaction of tetraethynylsilane. The analogous polymer Si-PDA is synthesized via Sonogashira cross-coupling reaction40 of tetraethynylsilane and 1,4-diidobenzene (Scheme 1). Polymer Si-BP (reported as porous aromatic framework [ PAF-3]25 or porous polymer network [ PPN-4]26) is prepared from a Yamamoto-type Ullman homocoupling reaction of tetrakis(bromophenyl)silane according to the procedures described in the literature. The full synthesis and characterization details for Si-DA, Si-PDA, and Si-BP are depicted in Supporting Information Schemes S1–S3 and Figures S1–S20. Scheme 1 | Synthesis of porous organic polymers Si-DA, Si-PDA, and Si-BP. Download figure Download PowerPoint The powder X-ray diffraction analysis showed that Si-DA is amorphous in nature (see Supporting Information Figure S5). According to Fourier transform infrared (FT-IR) analysis of Si-DA (Figure 1a), the stretching vibration of terminal C(alkyne)–H bond of tetraethynylsilane at 3284 cm−1 disappeared, indicating the complete coupling reaction of terminal alkyne carbons. The weak and broad stretching vibration peak of the C≡C bond was located around 2150 cm−1. The X-ray photoelectron spectroscopy (XPS) analysis of Si-DA (Figure 1b) shows a C 1s peak at 284.82 eV with a shoulder peak towards higher energies, which was deconvoluted into three peaks at 284.66, 285.39, and 288.19 eV. The peaks observed at 284.66 and 285.39 eV were assigned to C (sp2) and C (sp), respectively. The peak at 288.19 eV could be ascribed to C–O and C=O probably because the surface of the samples absorbed oxygen, which would subsequently react with exposed, uncoupled terminal acetylenic bonds.27–29,41,42 In addition, the signal of copper catalyst was not observed in the XPS analysis (see Supporting Information Figure S9). In the solid-state 13C NMR spectrum of Si-DA, a broad peak with low intensity was observed at around 126 ppm, which we assigned to C (sp2) due to the paramagnetic effect of the radical material and the low symmetry of the amorphous framework (see Supporting Information Figure S10). In the solid-state 29Si NMR spectra of Si-DA, a peak at −111 ppm was observed, which is similar with that of the monomer tetraethynylsilane (see Supporting Information Figure S11). This indicates that the chemical environment surrounding the silicon atoms was retained after the C–C coupling reaction. Thermogravimetric analysis plots revealed that Si-DA maintained its structural integrity under air atmosphere until 180 °C (see Supporting Information Figure S12). To investigate the pore-size distribution in Si-DA, N2-sorption isotherm measurements were performed at 77 K (Figure 1c). The isotherms are typical type II curves with a Brunauer–Emmett–Teller surface area of 779 m2 g−1, showing a microporous nature. The pore sizes calculated from the corresponding N2 isotherms using density functional theory (DFT) were mainly 1.8 and 4.2 nm. The morphology of Si-DA obtained from scanning electron microscopy images are irregular spherical particles (see Supporting Information Figure S16). The element distribution of carbon and silicon in Si-DA was clearly seen through transmission electron microscopy elemental mapping images (see Supporting Information Figure S17). The ultraviolet–visible-diffuse reflectance (UV–vis-DR) spectrum of Si-DA exhibits a stronger and more extended absorption ranging from 200 to 2000 nm (Figure 1d). The optical band gap of Si-DA was evaluated to be 1.30 eV as can be seen in Figure 1d.43 Figure 1 | (a) FT-IR spectra of tetraethynylsilane and Si-DA. (b) C 1s XPS spectra of Si-DA. (c) N2-adsorption isotherms and the pore-size distribution of Si-DA. (d) UV–vis-DR absorption spectrum of Si-DA. Download figure Download PowerPoint Si-DA was also characterized by electron paramagnetic resonance (EPR) in sealed quartz tubes under ambient conditions. The EPR spectrum of Si-DA powder shows a strong signal at g-factor ≈ 2.003,12,13,22,44 indicating the existence of carbon-centered radical species (Figure 2a). In contrast, analogous POP Si-PDA shows a very weak EPR signal, while Si-BP is EPR silent under similar measurement conditions. To further confirm the presence of the unpaired electrons in Si-DA, magnetization measurements were performed using superconducting quantum interference device (SQUID) magnetometry. The temperature-dependent magnetization is shown in Figure 2b. The magnetic susceptibility (χm) is significantly enhanced with decreasing temperature, which demonstrates a Curie-like paramagnetic character where the spin orientations become ordered at low temperature.8,20 From 300 to 50 K, the spins are paramagnetic and randomly oriented in Si-DA. This is a typical characteristic of paramagnetic materials. The measured magnetization (M) versus applied field (H) curves for Si-DA powder sample at 2 K illustrate the nonlinear M–H plots (Figure 2c), which became linear at 300 K (see Supporting Information Figure S20). These results reveal an enhanced magnetism at low temperature, which further confirms the radical feature of Si-DA. Furthermore, the magnetization at 2 K showed a substantial increase and saturation behavior at high magnetic fields, which also denotes a ferromagnetic phase transition at very low temperature.20,21,45,46 Figure 2 | (a) Solid-state EPR spectra of Si-DA, Si-PDA, and Si-BP at room temperature. (b) Temperature-dependent spin susceptibility (χm) of Si-DA determined by the SQUID. (c) M–H profiles of Si-DA at 2 K. (All magnetic data are processed by background correction. M, magnetization; H, applied field.) (d) Calculated spin density of the representative fragment of Si-DA. (e) The optimized geometries of nonradical (left, singlet) and diradical (right, triplet) structures with the corresponding bond length (Å). The isosurface of the spin density was drawn with an isovalue of 0.01 electrons/Å3. The α-spin and β-spin electrons are shown with the green and blue isosurfaces, respectively. Carbon, silicon, and hydrogen atoms are light gray, cyan, and white, respectively. Download figure Download PowerPoint To further understand the radical nature of Si-DA, DFT was used to calculate spin density and electronic structure. As reported by Ogawa et al.,36,37 the conjugated diacetylene units could exhibit open-shell diradical resonance structures (the triplet state) in the solid state. Here, we investigated the spin density distribution and geometry of the open-shell resonance structure with DFT calculations. We found for the open-shell resonance structure that the spin density is mainly distributed on C atoms neighboring Si atoms on the polymer skeleton (Figure 2d). In the optimized open-shell resonance structure, the original C≡C bond length changed from 1.216 to 1.287 Å, while the original C–C bond length is shortened from 1.358 to 1.286 Å, both indicating double bond characteristics (Figure 2e). The calculated energy levels of the singly occupied molecular orbital (−6.93 eV) and the LUMO (−5.47 eV) deliver a low energy gap (Egap) of 1.46 eV (see Supporting Information Figure S21). The unique features of the as-obtained 3D PORP Si-DA, including persistent radicals, permanent porosity, robust framework, and low energy gap, makes it a potential candidate for electrochemical energy storage.47,48 Consequently, we explored the potential application of Si-DA as the anode material for LIBs and demonstrated the important role of persistent radicals for Si-DA in comparison with nonradical POPs Si-BP and Si-PDA (see Supporting Information Figures S21–S26). The electrochemical property of Si-DA as the anode material for LIBs was first studied using cyclic voltammetry (CV) at a scan rate of 0.1 mV s−1 (Figure 3a). In the first cathodic sweep, peaks at 0.7, 0.9, and near 0 V, indicated a sequential lithiation process.49,50 The well-defined peak at 0.7 V is associated with the decomposition of the electrolyte and the formation of a solid electrolyte interface (SEI) layer.51–53 In the first anodic sweep, two dominant peaks at 0.15 and 0.98 V can correspond to the delithiation process.54 In the subsequent sweeps, as the formation of the SEI layer diminished, the two anodic peaks shifted to 0.13 and 0.93 V, respectively, and the CV curves overlapped, indicating the highly reversible redox reactions of Si-DA.55 The galvanostatic discharge/charge curves of Si-DA were further recorded at 50 mA g−1 (Figure 3b), which were in line with the CV curves. High initial discharge/charge capacities of 1308 and 503 mA h g−1 were obtained, corresponding to 38.5% of the initial Coulombic efficiency resulting from the formation of SEI on the surface of the electrode.56–59 Interestingly, significantly improved capacity (946 mA h g−1) was achieved with increased Coulombic efficiency (99.2%) up to 60 cycles after the activation process.60–62 Moreover, when using a higher activation rate of 200 mA g−1 (Figure 3c), Si-DA still showed the increasing capacity for the initial 80 cycles and a saturated capacity of 1161 mA h g−1 for an additional 220 cycles, which are higher than those of Si-PDA (493 mA h g−1) and Si-BP (490 mA h g−1). Even at an extremely higher current density of 20 A g−1, the capacity of Si-DA can still reach 200 mA h g−1 with excellent cycling stability for at least 10,000 cycles with Coulombic efficiency of >99.6% (Figure 3e). The possible activation process can be determined by the electrochemical impedance spectroscopy results of Si-DA anodes, in which the charge-transfer resistance (253 Ω for the first cycle) decreased to 45 Ω after 250 cycles (see Supporting Information Figure S23). Furthermore, Si-DA also possesses superior reversible capacities of 709, 649, 604, 520, 447, 382, 281, 212,and 159 mA h g−1 at current densities from 50 mA g−1 to 20 A g−1, which are significantly higher than the two counterparts (Figure 3d). The binding energy between the diradical structure and two Li ions is −6.13 eV, which is lower than that between the nonradical structure and two Li ions (−2.97 eV) (see Supporting Information Figure S27). These results imply that the presence of carbon centered radicals in Si-DA could enhance electrical conductivity and electrochemical properties, providing binding sites for lithium ions more rapidly, resulting in better LIB performances. Figure 3 | Electrochemical performance of Si-DA, Si-PDA, and Si-BP as anode materials in LIBs. (a) CV curves of Si-DA for the initial three cycles at 0.1 mV s−1. (b) The typical galvanostatic charge/discharge profiles between 3 and 0.01 V at 50 mA g−1. (c) Cycle performance of Si-DA, Si-PDA, and Si-BP at 200 mA g−1. (d) The rate performance of Si-DA, Si-PDA, and Si-BP at different current densities. (e) Cycling performance of Si-DA at 20 A g−1. Download figure Download PowerPoint Conclusion A novel 3D silicon-diacetylene PORP Si-DA has been synthesized via a classic Eglinton homocoupling reaction of tetraethynylsilane. This method provides a facile synthesis of amorphous diacetylene-linked silicon POPs with unique radical features such as high stability, low-energy LUMO, strong extended absorption up to 2000 nm, narrow band gap, and ferromagnetism at low temperature. Benefiting from the unique radical features, 3D PORP Si-DA exhibits high capacity, promising rate capability and is a potentially stable anode material for LIBs. Overall, our research could broaden the horizons for future 3D PORPs designs, which additionally helps the understanding of structure-property relationship for energy storage applications. Supporting Information Supporting Information is available and includes additional experimental details of POPs, various characterization experiments and results, computational experiments and results, and battery performance measurements and results. Conflict of Interest The authors declare no conflict of interest Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 22131004, U21A20330, and 52173195), the “111” project (grant no. B18012), Jilin Provincial Department of Science and Technology (grant no. 20210508048RQ) and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.

Green Synthesis of Lactone‐Based Conjugated Polymers for n‐Type Organic Electrochemical Transistors

AbstractAs new and better materials are implemented for organic electrochemical transistors (OECTs), it becomes increasingly important to adopt more economic and environmentally friendly synthesis pathways with respect to conventional transition‐metal‐catalyzed polymerizations. Herein, a series of novel n‐type donor–acceptor‐conjugated polymers based on glycolated lactone and bis‐isatin units are reported. All the polymers are synthesized via green and metal‐free aldol polymerization. The strong electron‐deficient lactone‐building blocks provide low‐lying lowest unoccupied molecular orbital (LUMO) and the rigid backbone needed for efficient electron mobility up to 0.07 cm2 V−1 s−1. Instead, polar atoms in the backbone and ethylene glycol side chains contribute to the ionic conductivity. The resulting OECTs exhibit a normalized maximum transconductance gm,norm of 0.8 S cm−1 and a μC* of 6.7 F cm−1 V−1 s−1. Data on the microstructure show that such device performance originates from a unique porous morphology together with a highly disordered amorphous microstructure, leading to efficient ion‐to‐electron coupling. Overall, the design strategy provides an inexpensive and metal‐free polymerization route for high‐performing n‐type OECTs.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Open AccessCCS ChemistryMINI REVIEW1 Oct 2021Recent Advances in Molecular Design of Organic Thermoelectric Materials Dongyang Wang, Liyao Liu, Xike Gao, Chong-an Di and Daoben Zhu Dongyang Wang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Liyao Liu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Xike Gao Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032 Google Scholar More articles by this author , Chong-an Di *Corresponding author: E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Daoben Zhu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101076 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic thermoelectric (OTE) materials have gained widespread attention because of their potential for wearable power generators and solid cooling elements. Nevertheless, the development of state-of-the-art OTE materials still suffers from limited molecular categories because of the rarity of molecular design strategies, which limits further development of this emerging field. Recently, many efforts have been devoted to developing molecular design concepts for high performance OTE materials. In this mini review, we present the fundamentals of the thermoelectric (TE) effect and the basic guidelines for designing OTE materials based on doped semiconductors. Subsequently, we provide an overview of the molecular design of conjugated backbones and side chains for enabling efficient TE conversion, including the effects of the molecular framework, heteroatomic substitution, and side chain length and polarity. Finally, we emphasize the developing tendency of molecular engineering towards high-performance and multifunctional OTE materials. Download figure Download PowerPoint Introduction Ever-growing wasted heat causes severe environmental issues and a global energy dilemma. Exploring eco-friendly energy technologies with sustainable electricity is of vital significance in alleviating the energy crisis. Thermoelectric (TE) materials can directly convert renewable heat into electricity via the Seebeck effect and have been considered as promising systems to meet this challenge.1–7 Additionally, TE materials can convert electricity to thermal energy, which could be also of considerable interest for spot-cooling devices via the Peltier effect. In the past 200 years, tremendous efforts have been made to improve the TE performance. Recently, TE materials have become increasingly important in satisfying the rising requirements for distributed energy used in emerging fields, such as health monitoring sensors and the Internet of Things (IoT). In the past decade, organic thermoelectric (OTE) materials have received increasing attention, not only because of their low-cost, mechanical process flexibility, molecular diversity, large-area processing methods, but also their intrinsically low thermal conductivity and excellent performance at room temperature.6,8–13 The TE performance is generally evaluated by the dimensionless figure of merit, ZT = S2σT/κ, which is composed of the Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and absolute temperature (T).14 Therefore, a promising OTE candidate should possess high σ, high S, and low κ, and thus be a good electrical conductor but a poor thermal conductor. These benchmarks imply that OTE materials can benefit from the molecular design of organic semiconductors (OSCs) such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and organic thin-film transistors (OTFTs).15–28 Despite early investigations of their thermopower properties before 1990, OTE materials experienced a slow development until 2010.29,30 Significant development in OTE materials has resulted from investigation of two conducting polymers, poly(3,4-ethlenedioxythiophene) (PEDOT) and poly(nickel 1,1,2,2-ethenetetrathiolate) [poly (Ni-ett)], which exhibited maximum ZT values greater than 0.1.31–34 In the past five years, increasing attention has also been paid to doped OSCs with high mobility. For instance, two diketopyrrolopyrrole (DPP) derivatives, namely selenium-substituted diketopyrrolopyrrole (PDPPSe-12) and aromatic-dicyanovinyl-dipyrrolo[3,4-c]pyrrole-1,4-diylidene)bis(thieno[3,2-b]thiophene (A-DCV-DPPTT) [DPPTT = dipyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2-b]thiophene)], have been reported to possess mobilities over 0.5 cm2 V−1 s−1 and exhibit a maximum ZT greater than 0.2.35,36 Regarding the structure–property relationship, an ideal OTE material should have tunable charge carrier concentration (n) and high charge carrier mobility (μ), which are highly relevant to the interaction between host molecules and dopants, position of the energy levels, and intermolecular packing order. The energy levels are directly bound to the molecular backbone structure, whereas the miscibility and self-assembly order are associated with side chain properties. Therefore, the molecular design of novel backbones and side chains is indispensable for fine-tuning the TE performance. Benefiting from increasing investigation of molecular design and chemical doping, tremendous progress has been made in this area. In fact, several insightful reviews of OTE materials have been published regarding material categories, processing techniques, and chemical doping.37–40 In this mini review, we summarize the recent molecular design strategies of small molecular and polymeric semiconductors for OTE applications by focusing on molecular backbone and side chain engineering. In the first section, we identify the critical role of molecular design in determining TE performance and highlight the principal requirements of the conjugated backbone and side-chain. In the second section, we focus on the conjugated backbone modulation for improving the TE performance. In the following section, we indicate the influence of side chains on the TE performance of OSCs. Finally, we highlight the current challenges and outlook for the future developments of OTE materials. Critical Role of Molecular Design in Determining TE Performance Fundamentals for efficient TE conversion The power generation of TE materials is realized by utilizing the Seebeck effect, which is induced by a balance between the diffusion of chemical potential and electrostatic repulsion arising from the buildup of charge. The resulting performance is typically evaluated by the figure of merit ZT and power factor PF as follows: ZT = S 2 σ κ T (1) PF = S 2 σ (2)in which σ, κ, and S can be described as: σ = n e μ (3) κ = κ C + κ L = L σ T + 1 3 C V ν l (4) S = k B e ∫ E − E F k B T σ ( E ) σ d E (5)For metals or degenerate semiconductors with parabolic band, S can be written as a function of n and effective mass (m*) in a form appropriate for an energy-independent scattering approximation. S = 8 π 2 k B 2 3 e h 2 m * T ( π 3 n ) 2 / 3 (6)where kB is the Boltzmann constant, EF is the Fermi level, h is the Planck constant, L is the Lorentz factor, Cv is the constant-volume specific heat, l is the scattering phonon mean free path, v is the phonon speed, and κ is composed of charge carriers transport heat (κC), and lattice phonons transport heat (κL).41 To maximize the ZT value of an OTE material, high S, high σ, and low κ are required. A high n can contribute to an increase in σ and κ. However, as shown in eqs 2 and 3, the increase in n usually results in an inevitably low S. The decoupling of these interrelated parameters is relatively nontrivial. The σ of OSCs is commonly governed by both n and their self-assembly structural order from the molecular orientation to the molecular alignment. Because charge transport can be strongly anisotropic due to the electrical coupling differences among the molecules, the π-electron coupling of closer intermolecular contacts for efficient charge transfer is required. Moreover, conjugated units with planar and rigid structures that stack with each other for efficient interchain electronic coupling can enhance charge transport. In contrast, S reflects the average entropy per charge carrier and directly relates to the density of states (DOS). Therefore, the S can be manipulated by tuning the shape of the DOS. For instance, narrowing the DOS is favorable to raising the asymmetry of the Fermi energy and enhancing S. Although the κ of OTE materials is relatively low, the mechanism of how κ changes with the diverse doping concentration and different morphology is still unclear. Therefore, it remains vital to acquire the limits of κ and its dependence on molecular properties. Basic rules for designing OTE materials Based on the aforementioned discussion, it is essential to manipulate interrelated properties at the molecular level (Figure 1). The molecular design of a conjugated backbone is powerful for tuning the energy level by incorporating diverse aromatic cores. Of particular note, the molecular structure is also crucial in affecting transition from single molecule to microscale and macroscale thin films. Consequently, ordered self-assembly has been widely employed for achieving efficient charge transport and high TE performance. Additionally, it is highly desirable to introduce novel variations to enable multifunctional OTE applications. In this context, we summarize three design rules for developing OTE materials. Figure 1 | Illustration of backbone design in OSCs toward TE applications. Download figure Download PowerPoint The frontier molecular orbitals (FMOs) in the conjugated molecules play vital roles in affecting intramolecular and intermolecular charge transport, doping efficiency, and stability. For OSCs, these are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In principle, the energy of the HOMO directly depends on the electron density and delocalization of the π electrons throughout a conjugated backbone.15 The spatial distribution of molecular orbital relies on efficient π-orbital overlap between two or more conjugated units. Moreover, the charge transfers mostly take place by removing electrons from donor molecules to the LUMO of n-type materials or accepting electrons from the HOMO of p-type materials to acceptor molecules. Therefore, modulation of FMOs is considered the most efficient strategy to achieve a high doping efficiency between host and dopant molecules. Looking beyond TE performance, ambient stability is also a basic requirement for doped OTE materials, which is mainly determined by the electron affinity (EA) values divided by ionization energy (IA).37 For example, in n-type materials, molecules with low EA values usually suffer from chemical instability due to the influence of water, oxygen, and other trap states; therefore, lowering the LUMO level is a general strategy to enhance their air stability. These restrictive relationships demonstrate that appropriate FMOs are crucial for OSCs to ensure efficient TE conversion. Previous studies performed on high mobility OSCs reveal that high crystallinity endows materials with efficient charge transport ability, which offers a basic guideline for designing OTE materials.42 In most cases, planar conjugated molecules contribute to a high degree of intrachain order and crystalline microstructures with intimate intermolecular packing modes.23,43 In principle, intimate intermolecular contacts can satisfy the need of coupled π-electronic levels and thus lead to efficient intermolecular charge transfer. Concerning chemical doping, the incorporation of specific side chains can improve miscibility between dopant and host molecules to enhance the doping efficiency. However, the additional side chains are generally highly insulating and prevent electronic communication between molecules. Consequently, a remaining issue for molecular design relies on the balance between the high-degree oriented backbone and side chains that are likely to prevent electronic contact. To develop multifunctional OTE materials, side chains with specific groups such as tetraethylene glycol and ether chains in the conjugated backbone are required to endow the devices with sensing, photosensitive, and stretchable behaviors.25,44–46 These materials can hardly be realized by using conventional molecular design strategy. In stretchable materials for example, the typical design principles, such as adding flexible conjugation breakers and incorporating longer and softer side chains, can lead to decreased crystallinity of films. Thus, it remains challenging to maintain good charge transport properties upon reduced interchain transport efficiency. In other words, the balance between functionality and OTE performance should be evaluated when taking advantage of novel side chains. Molecular Design of Conjugated Backbone The energy level determines the charge transport because the carriers with energy close to the chemical potential can participate in transport. Generally, the band gap of OSCs ranges from 1 to 3 eV, and the introduction of carriers can also affect the HOMO and LUMO. Therefore, the energy level and doping efficiency are critical in determining σ. Moreover, σ is affected by the π-electronic coupling level, which requires close intermolecular contacts to facilitate efficient charge transfer. Therefore, the conjugated framework with planar structures allows more efficient electronic coupling. In principle, the conjugated backbone strongly affects the electronic structure, energy level offset, and planarity of conjugated molecules (Figure 2), which is crucial for enhancing the n, intermolecular interactions, and charge transfer between the host materials and dopants. Evidently, the rational design of conjugated backbones is essential for exploring high-performance OTE materials. Figure 2 | Key factors for designing conjugated backbones for OTE materials: (1) electronic structure, (2) energy level offset, (3) planarity. Download figure Download PowerPoint Framework effect Conjugated frameworks are decorated with acceptors and donors according to the electron-donating or -withdrawing role. Various types of molecules, such as DPPs, imides, and amides, have acted as acceptors. Moreover, the conjugated framework is a key factor in determining the energy level, doping efficiency with dopants, and planarity of conjugated molecules. For instance, the specific molecular modulation from incorporating electron-rich or -deficient units can achieve the targeted n- and p-type DPPs-based OTE materials (Figures 3a and 3b). As a result, an in-depth understanding of the conjugated framework effects is considerably important.47–51 Figure 3 | The molecular design based on DPP units for (a) n-type and (b) p-type OTE materials. (c) Charge-transfer mechanism and (d) PF at diverse dopant concentrations of N-DMBI doped A-DCV-DPPTT and Q-DCM-DPPTT. Reprinted with permission from ref 36. Copyright 2017 American Chemical Society. Download figure Download PowerPoint The conjugated units are linked by two types of structures: one is the aromatic mode form with a single bond between the monomers, and the other is the quinoid form with a double bond connection. Despite the similar molecular geometry, the two diverse connection modes can lead to significant differences in electronic distribution along the framework, resulting in differences in energy levels and doping efficiency. For instance, Huang et al.36 reported two solution-processable DPPTT derivatives, A-DCV-DPPTT and quinoid-dicyanomethylene-dipyrrolo[3,4-c]pyrrole-1,4-diylidene)bis(thieno[3,2-b]thiophene (Q-DCV-DPPTT), for studying conjugated framework-dependent TE properties. The A-DCV-DPPTT and Q-DCV-DPPTT had diverse DPPTT units and different HOMO and LUMO values, in which A-DCV-DPPTT had a higher LUMO value of −3.9 eV than that of Q-DCV-DPPTT (−4.5 eV). In the film state, molecular interactions of A-DCV-DPPTT and Q-DCV-DPPTT with the dopant are different, and the singly occupied molecular orbital (SOMO) of the 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI)·was −2.4 eV, enabling efficient electron transfer between N-DMBI and A-DCV-DPPTT (Figure 3c). In addition, the electron transfer from N-DMBI• SOMO to the LUMO level of Q-DCV-DPPTT was difficult because the charge transfer occurred in the solution state before spin-coating of the film, resulting in lower doping efficiency. Accordingly, the high electron mobility and appropriate energy levels of the A-DCV-DPPTT incorporated aromatic structures for doped films had an σ of 5.3 S cm−1 and a high ZT value of up to 0.26. Notably, the performance was more than one order of magnitude higher than those of Q-DCV-DPPTT with a quinone structure (Figure 3d). Theoretical studies have shown that polarons are predominantly localized on the polymer chains. Therefore, the polaron delocalization length is important in macroscopic conductivity.52,53 Generally, ladder-type polymers typically exhibit two-dimensional geometry and thus have limited conformational freedom. Benefiting from these planar building blocks, the electron delocalization length in conjugated polymers can be improved significantly. Wang et al.54 studied polybenzimidazobenzophenanthroline (BBL) and poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] (Figure 4a); the former had a linear-“torsion-free”-ladder-type framework and the latter a distorted donor–acceptor structure. In comparison with P(NDI2OD-Tz), BBL presented a larger calculated polaron and spin delocalization lengths, leading to an easier intramolecular transfer and higher mobility. Consequently, BBL doped with tetrakis(dimethylamino)ethylene (TDAE) achieved n-type conductivity of up to 2.4 S cm−1, which was three orders of magnitude higher than that of P(NDI2OD-Tz) (Figure 4b). Sometimes, conjugated polymers with a rigid planar backbone instead of a disordered crystalline structure can also exhibit unexpected structural tolerance and good miscibility with dopants, resulting in both high n and μ.51 Figure 4 | The backbone effect for high-performance OTE materials. (a) Chemical structures of BBL and P(NDI2OD-T2) and (b) σ and S versus TDAE doping time for BBL and P(NDI2OD-T2). Reprinted with permission from ref 54. Copyright 2016 Wiley-VCH. (c) Chemical structures of BDPPV derivatives and (d) PF at different doping concentrations of doped BDPPV derivatives. Reprinted with permission from ref 56. Copyright 2015 American Chemical Society. (e) Chemical structures of PDPH and PDPF and (f) PF at different doping concentrations of doped PDPH and PDPF. Reprinted with permission from ref 57. Copyright 2018 Wiley-VCH. Download figure Download PowerPoint Substituent effect The electronic structure can be fine-tuned by introducing electron-withdrawing elements, such as halogen atoms on electron-rich moieties, leading to a deeper LUMO level and higher n-type doping efficiency.55 For instance, fluorine atoms can enhance the electron withdrawing ability via electrostatic and sigma inductive effects. Furthermore, the incorporation of fluorine atoms not only endows stability for doped materials, but also imparts high rigidity, extremely low surface energy, and various self-assembly properties in the solid state. For these reasons, the fluorine substitution effect has been considerably studied. Shi et al.56 reported three benzodifurandione-based poly(p-phenylene vinylene) (BDPPV) derivatives, BDPPV, ClBDPPV, and FBDPPV, which had varied LUMO levels and electron mobilities from the diverse halogen atoms (Figure 4c). BDPPV derivatives have low-lying LUMO levels, which makes them the most electron-deficient conjugated polymers. As the feasibility of chemical doping is inevitably accompanied by hydride or H atom transfer, the introduction of halogen atoms (Cl and F) efficiently lowered the LUMO levels of molecules and resulted in stronger C–H bond acceptors in ClBDPPV and FBDPPV. Therefore, the doping levels of the three derivatives could be tuned according to the different reactivities of the dopant molecules and the polymers substituted with halogen atoms. In addition, the doping efficiencies of ClBDPPV and FBDPPV were almost identical to those of BDPPV. Clear shifts of the Fermi level toward higher binding energies were observed, resulting in a dramatic enhancement of σ, of which the highest conductivity of FBDPPV reached 14 S cm−1. The enhancement of σ was mainly caused by the increase in doping levels, whereas ordering of polymers was not likely to be disturbed by the dopant. Consequently, FBDPPV exhibit a high PF of up to 28 μW m−1 K−2 (Figure 4d). Other studies have demonstrated that electron-withdrawing modification of the donor units can increase the EA of molecules and affect TE performance. Pei et al.57 reported two DPP-based donor–acceptor (D–A) copolymers, PDPH and PDPF, in which the former did not have electron-withdrawing modification of the donor moiety and the latter had halogen atoms as the electron-withdrawing group (Figure 4e). DPP is a widely investigated building block in OTE materials. After introducing the fluorine atoms on the donor moiety, PDPF presented not only lower HOMO and LUMO levels, but also multiple-polymer packing orientations. It thus can introduce diverse transition regions to accommodate more dopants and dopant cations, leading to enhanced doping efficiency and avoiding phase separation. Moreover, the fluorine substitution contributes to the formation of a hydrogen bonding network between units, resulting in a coplanar conformation with negligible dihedral angles. Benefiting from these features, the magnitude of both σ and PF of PDPF was enhanced by three orders of magnitude in comparison to PDPH. In addition, the design of cyano-functionalized bithiophene imide (BTI) building blocks can synergistically combine the benefits of both imide and cyano functionalities to afford such acceptors good solubility, excellent planar backbone, and high electron-deficiency.55 Heteroatomic substitution Heteroatomic substitution may induce efficient intermolecular orbital overlaps and increase the bandwidth, thus affecting the intermolecular interactions and energy levels. Although selenium and sulfur are in the same group, selenium has more electrons and is larger than sulfur. The corresponding HOMO levels are higher than those of the sulfur ones. Therefore, selenium substitution is a key strategy for fine-tuning the conjugated backbone. A clear example of the selenium substitution effect on TE performance was reported by Ding et al.,35 based on a DPP derivative named PDPPSe-12. In comparison with PDPPS-12, PDPPSe-12 exhibited a slightly higher HOMO to facilitate p-type doping. Although the introduction of a selenium atom contributes to an increased π–π stacking distance, a stronger intermolecular interaction in PDPPSe-12 was achieved because of the larger radius of the selenium atom than that of sulfur. Therefore, the more favorable intermolecular interaction and relatively large π–π stacking distance enabled the dopant to intercalate the domains without destroying the packing order. Consequently, FeCl3 doped PDPPSe-12 exhibited an excellent Hall mobility reaching 2.3 cm2 V−1 s−1 and maximum PF of 364 μW m−1 K−2, leading to a prominent ZT of 0.25. In addition, selenium substitution can also contribute to a stronger diradicaloid character and self-doping level.58 In comparison with the diradicaloid quinoidal quaterthiophene derivative 2DQQT-S, 2DQQT-Se exhibited a considerably higher degree of spin delocalization that was attributed to the lower aromaticity of the peripheral fused selenophene. As a result, the signal in the EPR spectra of 2DQQT-Se was broadened, leading to a significant enhancement in σ with a value of 0.29 S cm−1. These initial studies demonstrate the significance of heteroatomic substitution in enabling rationally designed OTE materials. Side Chain Engineering of OTE Materials Conjugated molecules usually have side chains, which can enhance their solubility in organic solvents. However, the side chain engineering not only enhances solubility, but also affects the intermolecular packing and the optoelectronic properties. In fact, the side chain effects on the OSC properties have been widely investigated regarding side chain length, charge (ionic), polarity (oligo (ethylene glycol) chains), electronegativity (fluoroalkyl chains), and alkyl chain branching points.16,18,19,22,23 In this section, we summarize side chain engineering including the length and polarity of side chains and self-doping properties, which affects the miscibility, self-assembly packing order, and self-doping properties (Figure 5). Figure 5 | Key factors for side chain engineering of OTE materials: (1) miscibility between host OSC and dopant, (2) self-assembly properties, and (3) self-doping. Download figure Download PowerPoint Side chain length There are two types of alkyl chains (linear and branched) for OSCs. Statistically, the appropriate linear alkyl side chains can enhance interchain interdigitation.59,60 The branched side chains generally restrain interchain interdigitation due to their bulky conformation. Concurrently, branched alkyl chains can endow better solubility. In addition, the variation of the branching point of alkyl chains can result in dramatic differences in carrier mobilities with subtle changes in molecular packing. This strategy can modulate the intermolecular interactions.23,61 Thus, the length and conformation of side chains not only have a great influence on the solubility and miscibility, but also affect the packing order and charge transport properties of OSCs. To investigate the effect of branching position on OTE performance, Wang and Takimiya62 demonstrated three copolymers, named pNB, pNB-Tz, pNB-TzDP, composed of naphthodithiophene diimide (NDTI) and BTI units (Figure 6a). Among them, pNB-Tz and pNB-TzDP are distinguished by the different branching points of the side chains, in which the side chain is modified from 2-decyltetradecyl (DT) to 3-decylpentadecyl (DP). In comparison, pNB-TzDP exhibited dramatically improved packing order and resulted in a higher μ of 0.55 cm2 V−1 s−1. Furthermore, the two polymers exhibited different molecular orientations (Figure 6b). The charge carriers of bimodal orientation can move around the grain boundaries due to the presence of multiple parallel and perpendicular orientations of the π–π stacking planes, which are generally called “3D conduction channels”. In addition, the bimodal orientation endowed better accommodation of dopant molecules with almost unchanged packing order and π–π stacking distance. Therefore, the fine-tuned branching point endowed the pNB-TzDP with high σ of 11.6 S cm−1 and prominent PF of 53.4 μW m−1 K−2 (Figure 6c). In addition, polythieno[3,4-b]thiophene-Tosylate (PTbT-Tos) with different alkyl lengths presented tunable σ ranging from 0.0001 to 450 S cm−1 at room temperature.63 The electrical behavior of PTbT-Tos was different from that of PEDOT-Tos, in which the quasi-reversible phase transformation occurred fr

Electron Transport in Naphthalene Diimide Derivatives

Two naphthalene diimides derivatives containing two different (alkyl and alkoxyphenyl) N-substituents were studied, namely, N,N′-bis(sec-butyl)-1,4,5,8-naphthalenetetracarboxylic acid diimide (NDI-s-Bu) and N,N′-bis(4-n-hexyloxyphenyl)-1,4,5,8-naphthalenetetracarboxylic acid diimide (NDI-4-n-OHePh). These compounds are known to exhibit electron transport due to their electron-deficient character evidenced by high electron affinity (EA) values, determined by electrochemical methods and a low-lying lowest unoccupied molecular orbital (LUMO) level, predicted by density functional theory (DFT) calculations. These parameters make the studied organic semiconductors stable in operating conditions and resistant to electron trapping, facilitating, in this manner, electron transport in thin solid layers. Current–voltage characteristics, obtained for the manufactured electron-only devices operating in the low voltage range, yielded mobilities of 4.3 × 10−4 cm2V−1s−1 and 4.6 × 10−6 cm2V−1s−1 for (NDI-s-Bu) and (NDI-4-n-OHePh), respectively. Their electron transport characteristics were described using the drift–diffusion model. The studied organic semiconductors can be considered as excellent candidates for the electron transporting layers in organic photovoltaic cells and light-emitting diodes

B←N-Incorporated Dibenzo-azaacenes as n-Type Thermoelectric Materials.

Organic thermoelectric materials play a vital role in flexible power generating applications, such as wearable electronics and sensor networks. While there is a wealth of research on p-type organic thermoelectric materials, developments on n-type counterparts as complementary are comparatively limited. Herein, we report a new kind of n-type small-molecule thermoelectric materials based on B←N-incorporated dibenzo-azaacenes 1,2-DBNA-2 and 1,2-DBNA-5. Because of the low-lying lowest unoccupied molecular orbital (LUMO) energy levels, 1,2-DBNA-2 and 1,2-DBNA-5 could be efficiently n-doped, and the rigid and almost planar skeleton could ensure good carrier transfer. When doped with a typical n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), 1,2-DBNA-5 exhibits a moderate conductivity of 0.01 S cm-1 and a power factor of 0.06 μW m-1 K-2 with a Seebeck coefficient of -244.4 μV K-1 in thermoelectric devices. These results not only demonstrate that B←N-incorporated dibenzo-azaacenes are a novel class of n-type thermoelectric materials but also highlight a new strategy to develop n-type organic thermoelectric materials.

Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission.

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near-infrared (NIR) emission (λPL =758-847 nm) with high absolute photoluminescence quantum yield (ΦPL =7-23%) originating from low-lying LUMO energy levels of the BAz moieties (-3.94 to -4.25 eV). Owing to inherent solid-state emissive properties of the BAz units, deeper NIR emission (λPL =852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D-A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron-accepting unit through fine-tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.

N-Type host materials based on nitrile and triazine substituted tricyclic aromatic compounds for high-performance blue thermally activated delayed fluorescence devices

Novel n-type host materials based on tricyclic aromatic compounds, dibenzo[b,d]furan and dibenzo[b,d]thiophene (2Trz6CNDBF and 2Trz6CNDBT), have been successfully synthesized and characterized for high-performance blue thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). Dibenzo[b,d]furan and dibenzo[b,d]thiophene were utilized as central molecular building blocks to achieve excellent thermal stability and high triplet energy (ET). Nitrile and diphenyltriazine functional groups were introduced at the 6- and 2- positions of the central building blocks to achieve low-lying lowest unoccupied molecular orbital (LUMO) energy levels, high ET, and excellent electron transport properties. UV–Vis absorption, low-temperature photoluminescence, and ultraviolet photoelectron spectroscopy analysis showed that 2Trz6CNDBF and 2Trz6CNDBT possessed high ET (2.95 and 2.88 eV) and low-lying LUMO energy levels (−3.43 and −3.16 eV) that were well-matched with a blue TADF emitter, 5CzCN. Moreover, the electron-only device (EOD) result revealed that 2Trz6CNDBF and 2Trz6CNDBT had excellent electron transport properties. Blue TADF OLEDs fabricated with a p-type host material, mCBP, and n-type host, 2Trz6CNDBF or 2Trz6CNDBT, exhibited lower driving voltages (3.34 and 3.26 V, respectively) than a TADF OLED with only a p-type host material, mCBP (3.83 V). Blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT exhibited superior external quantum efficiency (ηext, 15.6 and 14.7%), current efficiency (ηce, 33.8 and 32.7 cd A−1), and power efficiency (ηpe, 25.6 and 25.7 lm W−1), respectively. The ηext and ηce of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 70% and ηpe by approximately 150% compared to those of the TADF OLED with a single p-type host, mCBP. In addition, the device lifetimes of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 1000%. The efficient electron injection by the low-lying LUMO energy level, effective exciton confinement by high ET, and high thermal stability of the film morphology provided the enhanced efficiency and lifetime for blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT.

Core-Twisted Tetrachloroperylenediimides: Low-Cost and Efficient Non-Fullerene Organic Electron-Transporting Materials for Inverted Planar Perovskite Solar Cells.

Herein, core-twisted tetrachloroperylenediimides (ClPDIs) were introduced as new efficient electron-transporting materials (ETMs) to replace the commonly used fullerene acceptor PC61 BM in inverted planar perovskite solar cells (PSCs). ClPDI showed a low-lying lowest unoccupied molecular orbital (LUMO) energy level of -3.95 eV, which was compatible with the conduction band of CH3 NH3 PbI3-x Clx (-3.90 eV). In addition, the role of the length of the alkyl side chain at the imide position of ClPDI in modulating the molecular solubility, aggregation capacity for charge-transport properties, surface hydrophobicity, and PSC performance was investigated. The device based on ClPDI-C4 (ClPDI with n-butyl side chains) as ETM achieved a maximum power conversion efficiency (PCE) of 17.3 % under standard AM 1.5G illumination, which iwas very competitive with that of the reference device employing PC61 BM/C60 (PCE=17.2 %) as ETM. Moreover, the devices with ClPDIs as ETMs exhibited better device stability than that with PC61 BM/C60 . This work highlights the great potential of ClPDI derivatives as low-cost (≈2.0 USD g-1 ) and effective ETMs to obtain efficient solution-processed inverted PSCs. This class of ClPDI derivatives is expected further promote the performance and stability of PSCs after extended investigation.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes.

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX—phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Rigid Coplanar Polymers for Stable n-Type Polymer Thermoelectrics.

Low n-doping efficiency and inferior stability restrict the thermoelectric performance of n-type conjugated polymers, making their performance lag far behind of their p-type counterparts. Reported here are two rigid coplanar poly(p-phenylene vinylene) (PPV) derivatives, LPPV-1 and LPPV-2, which show nearly torsion-free backbones. The fused electron-deficient rigid structures endow the derivatives with less conformational disorder and low-lying lowest unoccupied molecular orbital (LUMO) levels, down to -4.49 eV. After doping, two polymers exhibited high n-doping efficiency and significantly improved air stability. LPPV-1 exhibited a high conductivity of up to 1.1 S cm-1 and a power factor as high as 1.96 μW m-1  K-2 . Importantly, the power factor of the doped LPPV-1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high-performance and potentially air-stable n-type polymer thermoelectrics.

Rigid Coplanar Polymers for Stable n‐Type Polymer Thermoelectrics

AbstractLow n‐doping efficiency and inferior stability restrict the thermoelectric performance of n‐type conjugated polymers, making their performance lag far behind of their p‐type counterparts. Reported here are two rigid coplanar poly(p‐phenylene vinylene) (PPV) derivatives, LPPV‐1 and LPPV‐2, which show nearly torsion‐free backbones. The fused electron‐deficient rigid structures endow the derivatives with less conformational disorder and low‐lying lowest unoccupied molecular orbital (LUMO) levels, down to −4.49 eV. After doping, two polymers exhibited high n‐doping efficiency and significantly improved air stability. LPPV‐1 exhibited a high conductivity of up to 1.1 S cm−1 and a power factor as high as 1.96 μW m−1 K−2. Importantly, the power factor of the doped LPPV‐1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high‐performance and potentially air‐stable n‐type polymer thermoelectrics.

Stereodefined Access to Lactams via Olefin Difunctionalization: Iridium Nitrenoids as a Motif of LUMO-Controlled Dipoles.

Reported herein is a general platform of a stereodefined access to γ-lactams via Cp*Ir-catalyzed olefin difunctionalization, where in situ generated Ir-nitrenoid is utilized as a key motif of 1,3-dipoles to enable amido transfer in a syn-selective manner. Computational studies suggested that the stereodefined process can be attributed to the proposed working mode of concerted [3 + 2] cyclization. Frontier molecular orbital (FMO) analysis implied that a low-lying lowest unoccupied molecular orbital (LUMO) of the Ir-imido fragment engages in the olefin interaction. Mechanistic understanding on the nitrene transfer process led us to develop mild catalytic protocols of stereoselective difunctionalization of alkenyl dioxazolones to furnish α-(haloalkyl)- or (oxyalkyl)lactam products which are of high synthetic and medicinal utility. Product stereochemistry ( threo and erythro) was found to be designated by the olefin geometry ( E/ Z) of substrates.

Synthesis of Stannole-Containing π-Conjugated Polymers by Post-Element Transformation of Organotitanium Polymer.

The synthesis of stannole-2,5-diyl-containing π-conjugated polymers by the post-element transformation of a regioregular organotitanium polymer is described. For example, a 1,1-diphenylstannole-containing polymer is obtained in 83% yield by the reaction of a regioregular organotitanium polymer, which is prepared from 1,4-bis(2-ethylhexyloxy)-2,5-diethynylbenzene and a low-valent titanium complex with diphenyltin dichloride at -50 °C to ambient temperature. The number-average molecular weight and molecular weight distribution (Mn and Mw /Mn ) of the stannole-containing polymer are estimated as 4800 and 1.8, respectively. The obtained polymer is found to have the extended π-conjugated backbone and relatively low-lying lowest unoccupied molecular orbital (LUMO) energy level (-3.12 eV), which is supported by its UV-vis absorption spectrum and cyclic voltammetric (CV) analysis. In addition, the stannole-containing polymer is found to be applicable to a chemosensor for fluoride anion where the color and photoluminescence intensity of the polymer solution exhibits a distinct change in the presence of a fluoride anion.

Single-strand and ladder-type polymeric acceptors based on regioisomerically-pure perylene diimides towards all-polymer solar cells

Since dibromo perylene diimides (DBPDIs) contain two isomers that are difficult to separate, isomeric effect on properties and applications of PDI polymers has been rarely studied. Few ladder-type polymers have been used in organic solar cells. Herein, single-strand polymers (PrPDIT) were synthesized by the polymerization of regioisomerically-pure 1,7-dibromo-PDIs (1,7-DBPDIs) and 2,5-bis(trimethylstannyl)thiophene. Furthermore, ladder-type polymers (FPrPDIT) were obtained by the highly efficient photo-induced cyclization of polymers PrPDIT. The effects of isomers and molecular weight on the absorption, electrochemistry, thermal stability, and photovoltaic application of polymers were studied. Pure 1,7-DBPDIs lead to polymers with molecular weight distribution as low as 1.08. These polymers with strong absorption in 300–700 nm, low-lying lowest unoccupied molecular orbital (LUMO) energy levels around −4.0 eV and high thermal stability could be excellent electron acceptors. Regioisomerically-pure polymers demonstrate better photovoltaic performance than that of mixed ones. The highest power conversion efficiency (PCE) of all-polymer solar cells based on PrPDIT and FPrPDIT is up to 5.95% and 4.52% respectively.

Core-expanded naphthalenediimide derivatives as non-fullerene electron transport materials for inverted perovskite solar cells

Abstract Two core-expanded naphthalenediimide (NDI) derivatives NDI3HU-DTYM2 (1) and (DTYM-NDI-DTYA)2 (2) with coplanar and twisted molecular backbones, respectively, were used as low-temperature solution-processed electron transport materials (ETMs) for CH3NH3PbI3-based inverted planar perovskite solar cells (PSCs). The high electron mobility as well as low-lying lowest unoccupied molecular orbital (LUMO) energy levels and good film-formation ability on perovskite layers make 1 and 2 both promising ETMs for PSCs. The PSCs devices based on 1 show a power conversion efficiency (PCE) of 8.7%, while the PSCs devices based on 2 that has twisted molecular backbone exhibit a PCE value of up to 12.9%, a short-circuit current density (Jsc) value of 22.80 mA cm−2 with negligible hysteresis, which is comparable to that of PCBM-based PSCs devices. Our results demonstrate that core-expanded NDI-based n-type organic semiconductors, especially those with twisted molecular backbones, have great potential for high-performance, low-cost, solution-processed non-fullerene ETMs in PSCs.

Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics

Recent efforts in organic photovoltaics (OPVs) have been devoted to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent generation. However, the low-lying lowest unoccupied molecular orbital (LUMO) level in narrow bandgap NFAs typically results in a small energy difference (ΔEDA) between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, leading to low open-circuit voltage (VOC). The trade-off between ΔEDA and photocurrent generation significantly limits the simultaneous enhancement of both VOC and short-circuit current density (JSC). Here, we report a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing N-ethyl thiabarbituric acid terminal group on each end of the indacenodithienothiophene (IDTT) core. When paired with a benchmark low-bandgap PTB7-th polymer donor, simultaneous enhancement of both ΔEDA and absorption spectral coverage was realized. The OPV devices yield a VOC of 1.01 V, corresponding to a low energy loss of ...