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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. 

We present a comparative study of the tribological properties of Pd-, Pt-, and Zr-based bulk metallic glasses (BMG-Pd, BMG-Pt, and BMG-Zr, respectively) under unlubricated conditions. In particular, micro-tribometry is utilized with a 52,100 steel ball, showing that BMG-Pt exhibits a significantly higher coefficient of friction (COF) (0.58 ± 0.08) when compared with BMG-Pd (0.30 ± 0.02) and BMG-Zr (0.20 ± 0.03). Topographical roughness on and off wear scars is characterized via atomic force microscopy (AFM), with results that do not correlate with the observed frictional behavior. On the other hand, scanning electron microscopy (SEM) is utilized to reveal contrasting wear mechanisms for the three samples: while BMG-Pd and BMG-Zr exhibit predominantly abrasive wear, there is evidence of adhesive wear on BMG-Pt. Consequently, the occurrence of adhesive wear emerges as a potential mechanism behind the observation of relatively high coefficients of friction on BMG-Pt, suggesting stronger interactions with steel when compared with the other BMG samples. 

Abstract We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.°2 away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on 2021 December 8. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV γ -ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the γ -ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and γ -ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed γ -ray spectral cutoff in both the leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.