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1. Hybrid conjugated polymers with alternating dithienosilole or dithienogermole and tricoordinate boron units

   https://doi.org/10.1039/c7py01790a  

   Adachi, Yohei ; Ooyama, Yousuke ; Ren, Yi ; Yin, Xiaodong ; Jäkle, Frieder ; Ohshita, Joji ( January 2018 , Polymer Chemistry)

   Conjugated polymers composed of tricoordinate boron and π-conjugated units possess extended conjugation with relatively low-lying LUMOs arising from p B –π interactions. However, donor–acceptor (D–A) polymers that feature triorganoboranes alternating with highly electron-rich donors remain scarce. We present here a new class of hybrid D–A polymers that combine electron-rich dithienosiloles or dithienogermoles with highly robust tricoordinate borane acceptors. Polymers of modest to high molecular weight are readily prepared by Pd-catalyzed Stille coupling reaction of bis(halothienyl)boranes and distannyldithienosiloles or -germoles. The polymers are obtained as dark red solids that are stable in air and soluble in common organic solvents. Long wavelength UV-vis absorptions at ca. 500–550 nm indicate effective π-conjugation and pronounced D–A interactions along the backbone. The emission maxima occur at wavelengths longer than 600 nm in solution and experience further shifts to lower energy with increasing solvent polarity, indicative of strong intramolecular charge transfer (ICT) character of the excited state. The powerful acceptor character of the borane comonomer units in the polymer structures is also evident from cyclic voltammetry (CV) analyses that reveal relatively low-lying LUMO levels of the polymers, enhancing the D–A interaction. Density functional theory (DFT) calculations on model oligomers further support these experimental observations.
2. Enhancing the performance of a fused-ring electron acceptor via extending benzene to naphthalene

   https://doi.org/10.1039/c7tc04520d  

   Zhu, Jingshuai ; Wu, Yang ; Rech, Jeromy ; Wang, Jiayu ; Liu, Kuan ; Li, Tengfei ; Lin, Yuze ; Ma, Wei ; You, Wei ; Zhan, Xiaowei ( January 2018 , Journal of Materials Chemistry C)

   We compared an indacenodithiophene(IDT)-based fused-ring electron acceptor IDIC1 with its counterpart IHIC1 in which the central benzene unit is replaced by a naphthalene unit, and investigated the effects of the benzene/naphthalene core on the optical and electronic properties as well as on the performance of organic solar cells (OSCs). Compared with benzene-cored IDIC1, naphthalene-cored IHIC1 shows a larger π-conjugation with stronger intermolecular π–π stacking. Relative to benzene-cored IDIC1, naphthalene-cored IHIC1 shows a higher lowest unoccupied molecular orbital energy level (IHIC1: −3.75 eV, IDIC1: −3.81 eV) and a higher electron mobility (IHIC1: 3.0 × 10 −4 cm 2 V −1 s −1 , IDIC1: 1.5 × 10 −4 cm 2 V −1 s −1 ). When paired with the polymer donor FTAZ that has matched energy levels and a complementary absorption spectrum, IHIC1-based OSCs show higher values of open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency relative to those of the IDIC1-based control devices. These results demonstrate that extending benzene in IDT to naphthalene is a promising approach to upshift energy levels, enhance electron mobility, and finally achieve higher efficiency in nonfullerene acceptor-based OSCs.
3. Benzobisthiadiazole-based high-spin donor–acceptor conjugated polymers with localized spin distribution

   https://doi.org/10.1039/D1MA00144B  

   Sabuj, Md Abdus ; Huda, Md Masrul ; Sarap, Chandra Shekar ; Rai, Neeraj ( May 2021 , Materials Advances)

   Stable organic semiconductors (OSCs) with a high-spin ground-state can profoundly impact emerging technologies such as organic magnetism, spintronics, and medical imaging. Over the last decade, there has been a significant effort to design π-conjugated materials with unpaired spin centers. Here, we report new donor–acceptor (D–A) conjugated polymers comprising cyclopentadithiophene and cyclopentadiselenophene donors with benzobisthiadiazole (BBT) and iso-BBT acceptors. Density functional theory calculations show that the BBT-based polymers display a decreasing singlet–triplet energy gap with increasing oligomer chain length, with degenerate singlet and triplet states for a N = 8 repeat unit. Furthermore, a considerable distance between the unpaired electrons with a pure diradical character disrupts the π-bond covalency and localizes the unpaired spins at the polymer ends. However, replacing the BBT acceptor with iso-BBT leads to a closed-shell configuration with a low-spin ground-state and a localized spin density on the polymer cores. This study shows the significance of the judicious choice of π-conjugated scaffolds in generating low- ( S = 0) and high-spin ( S = 1) ground-states in the neutral form, by modulation of spin topology in extended π-conjugated D–A polymers for emergent optoelectronic applications.
4. The crucial role of end group planarity for fused-ring electron acceptors in organic solar cells

   https://doi.org/10.1039/C9QM00314B  

   Rech, Jeromy J. ; Bauer, Nicole ; Dirkes, David ; Kaplan, Joseph ; Peng, Zhengxing ; Zhang, Huotian ; Ye, Long ; Liu, Shubin ; Gao, Feng ; Ade, Harald ; et al ( July 2019 , Materials Chemistry Frontiers)

   Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π–π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π–π stacking distance by ∼1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π–π stacking between end groups)more »play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.« less
5. Exploration and development of gold- and silver-catalyzed cross dehydrogenative coupling toward donor–acceptor π-conjugated polymer synthesis

   https://doi.org/10.1039/C8PY01162A  

   Kang, Lauren J. ; Xing, Liwen ; Luscombe, Christine K. ( January 2019 , Polymer Chemistry)

   π-Conjugated polymers are materials of interest for use in organic electronics. Within these polymers, donor–acceptor polymers are favorable for solar cell applications due to improved charge mobility, better absorption in the low energy region of the solar spectrum, and tunable band gaps. One of the barriers to commercializing these donor–acceptor materials is that their synthetic pathways are complex because of the alternating repeat units in the polymer. To address this, the application of cross dehydrogenative coupling (also called oxidative CH/CH cross-coupling) toward the synthesis of donor–acceptor polymers was explored. In this work, the roles of specific reagents in a one-pot gold- and silver-catalyzed cross dehydrogenative coupling and the factors that contribute to selectivity for cross-coupling rather than homo-coupling are analyzed. Based on our results, we postulate that the percentage of alternating repeat units in the final polymer is affected by the increased reactivity of the dimer that forms in the initial stages of the polymerization compared to the monomer, which ultimately may be exploited to control the ratio of electron-rich to electron-poor monomers.