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Organic semiconductors based on conjugated donor-acceptor (D–A) polymers are a unique platform for electronic, spintronic, and energy-harvesting devices. Understanding the electronic structure of D–A polymers with a small band gap is essential for developing next-generation technologies. Here, we investigate the electronic structure and optical spectra of cyclopentadithiophene-based closed/open-shell D–A polymers using density functional theory and the Bethe–Salpeter equation based on G$$_0$$${}_0$W$$_0$$${}_0$approximation. We explored the role of different acceptor units and chemical substitutions on the structural changes and, more importantly, electronic, optical, and dielectric behavior. We found that the computed first exciton peak of the polymers agreed well with the available experimentally measured optical gap. Furthermore, D–A polymers with open-shell character display higher dielectric constant than the closed-shell polymers. We show that the exceptional performance of polycyclopentadithiophene-thiophenylthiadiazoloquinoxaline (PCPDT-TTQ) as a scalablen-type material for Faradaic supercapacitors can be partly ascribed to its elevated dielectric constant. Consequently, these D–A polymers, characterized by their high dielectric constants, exhibit significant potential for various applications, including energy storage, organic electronics, and the production of dielectric films.

 

High-spin ground-state polyradicals are an important platform due to their potential applications in magnetic and spintronic devices. However, a low high-to-low spin energy gap limits the population of the high-spin state, precluding their application at room temperature. Also, design strategies delineating control of the ground electronic state from a closed-shell low-spin to open-shell polyradical character with a high-spin ground state are not well established. Here, we report indacenodinaphthothiophene isomers fused with a 6,6-dicyanofulvene group showing a high-spin quintet ground state. Density functional theory calculations indicate that the syn - and anti -configurations have a closed-shell low-spin singlet ground state. However, the linear -configuration displays a high-spin quintet ground state, with the energy difference between the high-spin quintet to the nearest low-spin excited states calculated to be as large as 0.24 eV (≈5.60 kcal mol −1 ), exhibiting an exclusive population of the high-spin quintet state at room temperature. These molecules are compelling synthetic targets for use in magnetic and spintronic applications. 

High-spin ground-state organic materials with unique spin topology can significantly impact molecular magnetism, spintronics, and quantum computing devices. However, strategies to control the spin topology and alignment of the unpaired spins in different molecular orbitals are not well understood. Here, we report modulating spin distribution along the molecular backbone in high-spin ground-state donor–acceptor (D–A) conjugated polymers. Density functional theory calculations indicate that substitution of different heteroatoms (such as C, Si, N, and Se) alters the aromatic character in the thiadiazole unit of the benzobisthiadiazole (BBT) acceptor and modulates the oligomer length to result in high-spin triplet ground-state, orbital and spin topology. The C, Si, and Se atom substituted polymers show a localized spin density at the two opposite ends of the polymers. However, a delocalized spin distribution is observed in the N substituted polymer. We find that the hybridization (sp 3 vs. sp 2 ) of the substituent atom plays an important role in controlling the electronic structure of these materials. This study shows that atomistic engineering is an efficient technique to tune the spin topologies and electronic configurations in the high-spin ground-state donor–acceptor conjugated polymers, compelling synthetic targets for room-temperature magnetic materials. 

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. 

Dye-sensitized solar cells (DSCs) have drawn a significant interest due to their low production cost, design flexibility, and the tunability of the sensitizer. However, the power conversion efficiency (PCE) of the metal-free organic dyes is limited due to the inability of the dye to absorb light in the near-infrared (NIR) region, leaving a large amount of energy unused. Herein, we have designed new DSC dyes with open-shell character, which significantly red-shifts the absorption spectra from their counterpart closed-shell structure. A small diradical character ( y < 0.10) is found to be beneficial in red-shifting the absorption maxima into the NIR region and broadening up to 2500 nm. Also, the open-shell dyes significantly reduce the singlet–triplet energy gaps (Δ E ST ), increase the total amount of charge-transfer to the semiconductor surface, reduce the exciton binding energy, and significantly increase the excited-state lifetimes compared to the closed-shell systems. However, the closed-shell dyes have higher injection efficiency with increased intramolecular charge transfer (ICT) character. Our study reveals the design rule for open-shell DSC dyes to be able to absorb photons in the NIR region, which can increase the efficiency of the solar cell device. 

Abstract Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.