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.
IntroductionSupercapacitors are electrochemical energy storage devices that store charge through fast, reversible redox reactions, enable load-leveling, regenerative energy harvesting, and high power applications. [1][2][3][4][5][6] The energy density of a supercapacitor (E = CV 2 /2) is linearly dependent on the specific capacitance C and proportional to the square of the operational voltage V. The main strategies to increase energy depend upon the mechanism of charge storage, whether capacitive or Faradaic. Capacitive electrodes store charge on the surface; hence, research is focused on strategies to increase the surface area, typically based on high surface area carbons. [7][8][9] Faradaic materials undergo redox reactions and offer the ability to tune the operating voltage. There has been considerable success in tuning the cathode voltage and delivering high capacitance systems operating at the upper limit of prototypical nonaqueous electrolytes. [1,7] However, the device voltage and energy remain limited by a lack of complementary high power electrodes for the anode. [10,11] In the absence of new high voltage electrolytes, there is no room to further extend the cathode potential, because over-potential may lead to hazardous runaway reactions between the highly oxidized cathodes and flammable nonaqueous electrolytes. In contrast, it is generally safe for nonaqueous electrolytes to operate down to −2 V relative to the Ag/AgCl electrode, and the development of anode materials is critical to increase the energy densities of supercapacitors.Consequently, there is an urgent need for strategies aimed at extending the operational voltage toward negative potentials to improve the energy density and cell voltage of supercapacitors. As an alternative to metal oxides, redox-active macromolecules offer mechanical flexibility, low-cost, and scalability relevant for small and large scale applications. [12][13][14][15][16] In radical polymers, [17][18][19][20] the redox sites are at pendant radical groups distributed along a polymer backbone. Due to the insulating nature of the backbone and low conductivity, devices based on these materials require blending with additional conductive materials for electron transport, ultimately lowering the electrode energy Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n-type redox-active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open-shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge-discharge cycles, exceeding other n-dopable organic materials. Redox cycling proce...
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...
Metal-free bis-cyclopentadithiophene-based molecular sensitizers with varying halogens (Cl, Br, and I) installed at a terminal position in conjugation with the dye frontier molecular orbital π-system are studied in combination with two cobalt redox shuttles (RSs): with and without a nonmetal-coordinated nitrogen atom (pyrazine or pyridine) accessible to the halide-decorated dyes. This systematic study employs UV−vis absorption, cyclic voltammetry, density functional theory calculations, and nanosecond transient absorption spectroscopy to probe the influence of possible halogen-bonding between the dyes and the pyrazine-based RS on electron-transfer reactions and effects on dye-sensitized solar cell performances. The results of this study imply a possible halogen-bonding event occurring between the halogenated dyes and the halogen binding RS with substantial effects on electrontransfer reaction rates.
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