Controlling the electronic coupling between electron donor and acceptor subunits in a dyad is pivotal for the development of novel organic materials, for instance, thermally activated delayed ...fluorescence (TADF) materials and triplet photosensitizers. Herein, we prepared two compact electron donor/acceptor dyads based on phenothiazine (PTZ) and naphthalimide (NI) with different conformation restrictions induced by the C–N (NI-N-PTZ) or C–C (NI-C-PTZ) linkers. The effect of electronic coupling (matrix elements, V DA) on the photophysical properties, especially the intersystem crossing (ISC) and the TADF, were investigated. NI-C-PTZ shows stronger ground-state electronic coupling (V DA = 2548 cm–1) compared to NI-N-PTZ (V DA = 870 cm–1). TADF was observed only for NI-N-PTZ due to its smaller electronic coupling. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy indicated the presence two triplet excited states and three ISC mechanisms in NI-N-PTZ with different electron spin polarizations (ESP): radical pair ISC (RP-ISC) and spin–orbital charge transfer ISC (SOCT-ISC) for one triplet state, and spin–orbital coupling ISC (SO-ISC) for another. Moreover, for the second one, an inversion of the electron spin polarization (ESP) was observed at 0.5–1.1 μs delay time. NI-N-PTZ represents a rare example for compact electron donor/acceptor dyad showing TADF emission in the red spectral region.
The topology of electronic and phonon band structures of graphene is well studied and known to exhibit a Dirac cone at the K point of the Brillouin zone. Here, we applied inelastic x-ray scattering ...(IXS) along with ab initio calculations to investigate phonon topology in graphite, the 3D analog of graphene. We identified a pair of modes that form a very weakly gapped linear anticrossing at the K point that can be essentially viewed as a Dirac cone approximant. The IXS intensity in the vicinity of the quasi-Dirac point reveals a harmonic modulation of the phonon spectral weight above and below the Dirac energy, which was previously proposed as an experimental fingerprint of the nontrivial topology. We illustrate how the topological winding of IXS intensity can be understood in terms of atomic displacements and highlight that the intensity winding is not in fact sensitive in telling quasi- and true Dirac points apart.
The main structural differences between the 18
th
century Persian Bulat knife and the modern knife made of Kh (ShKh15) type bearing steel are considered. Qualitative and quantitative characteristics ...of carbide banding of high-carbon steels manufactured using Damascus steel technologies have been established. The nature of carbide banding is described in detail based on the shape of the planar projection lines of Bulat (Damascus) structure. Studies have confirmed that the matrix structure of tempered martensite and lamellar troostite is isometric and is not oriented in the direction of deformation. The carbide structure of steels manufactured using Damascus steel technologies is strongly oriented in the direction of deformation. It is calculated that the degree of orientation of linear elements in the two-dimensional metallographic specimen plane approaches unity. The regular layered banding (score 4) patterns are consistent with the structure of the Eastern genuine Damascus steels having an increased content of phosphorus admixture (0.1–0.3% P). The Damascus structure of alloyed tool steels is characterized by uneven reticular-fibrous carbide banding patterns (score 5).
The main distinguishing features of the Eastern Damascus sabers are considered according to the topological affiliation of the blade, which include the classical forms of Persian Shamshir, Indian ...Talwar, and Turkish Kilij. The need to consider the regional factor was indicated, which describes the affiliation of the Damascus saber with a particular region, where it was mounted with a national hilt, such as an Afghan Pulwar or Arab Saif. The structure of the edge section of the 18th century Damascus saber blade (Pulwar) has a characteristic carbide banding. In the process of topographic studies of the patterned surface, it was revealed that the shape of the main components of the Damascus pattern consists of individual characteristic elements, such as knottiness, wave crests (dunes), and hollows. Based on the nature of the pattern, the saber blade belongs to the Khorasan variety. It has been established that genuine Damascus steel is an inhomogeneous high carbon steel with an increased phosphorus content. The stoichiometric composition of the excess cementite and abnormally coarse eutectic carbides has been studied. It was found that the Eastern saber blade with a Damascus pattern is a unique technological solution of its time. The Damascus blade does not only consist of one wootz-ingot. Typically, separate sections of each one third of the blade are made of a combination of non-uniform crucible steels with different chemistry and structure. Each third of the blade is responsible for its own unique properties and performance characteristics. A full cycle of reconstruction of the genuine Damascus structure was carried out under the laboratory conditions using experimental alloys No. 1 and No. 2. It was shown that carbide inhomogeneity patterns and stoichiometric composition of the excess carbide phases in alloys No. 1 and No. 2 are completely identical to the patterns of the genuine Damascus structure of the 18th century Pulwar saber blade.
Heusler compounds having D2d crystal symmetry gained much attention recently due to the stabilization of a vortexlike spin texture called antiskyrmions in thin lamellae of Mn1.4Pt0.9Pd0.1Sn as ...reported in the work of Nayak et al. Nature (London) 548, 561 (2017). Here we show that bulk Mn1.4Pt0.9Pd0.1Sn undergoes a spin-reorientation transition from a collinear ferromagnetic to a noncollinear configuration of Mn moments below 135 K, which is accompanied by the emergence of a topological Hall effect. We tune the topological Hall effect in Pd and Rh substituted Mn1.4PtSn Heusler compounds by changing the intrinsic magnetic properties and spin textures. A unique feature of the present system is the observation of a zero-field topological Hall resistivity with a sign change which indicates the robust formation of antiskyrmions.
Spiro rhodamine (Rho)-perylene (Pery) electron donor–acceptor dyads were prepared to study the spin–orbit charge transfer intersystem crossing (SOCT-ISC) in these rigid and sterically congested ...molecular systems. The electron-donor Rho (lactam form) moiety is attached via the N–C bond to the electron acceptor at either 1- or 3-position of the Pery moiety (Rho-Pery-1 and Rho-Pery-3). Severe torsion of the Pery moiety in Rho-Pery-1 was observed. The fluorescence of the two dyads is significantly quenched in polar solvents, and the singlet oxygen quantum yields (ΦΔ) are strongly dependent on solvent polarity (4–36%). Femtosecond transient absorption spectra demonstrate that charge separation (CS) takes 0.51 ps in Rho-Pery-1 and 5.75 ps in Rho-Pery-3, and the charge recombination (CR)-induced ISC is slow (>3 ns). Nanosecond transient absorption spectra indicate that the formation of triplet states via SOCT-ISC takes 24–75 ns for Rho-Pery-1 and 6–15 ns for Rho-Pery-3, and the distorted π-framework of the Pery moiety results in a shorter triplet lifetime of 19.9 vs 291 μs for the planar analogue. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism.
The phase of the primary electron spin echo signal was analyzed on model radical pairs (RPs) in a singlet spin state. An interpretation is given of the fact known in the literature that the echo ...signal of such pairs may not have a component with a “normal” phase, which is expected for the echo signal of non-interacting unpaired electrons in the RP. For the first time, it has been theoretically shown that forbidden EPR transitions can make a significant contribution to the phase shift of the magnetization of RP spins, caused by the dipole–dipole hyperfine interaction of unpaired RP electrons with magnetic nuclei. The “anomalous” phase of the spin echo signal of interacting spins in radical pairs is a demonstration that spin dynamics is accompanied by reversible transitions of dipole polarization (spin magnetization) and quadrupole spin polarization.
The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electron ...spin polarization (ESP) of the triplet state of the dimers, accessed with spin–orbit charge-transfer ISC, is different from that of the monomer (spin–orbit coupling-induced ISC). The TREPR spectra show that the triplet state initially formed by charge recombination is localized on either of two subunits, with different preference and ESP patterns. On the basis of the relative orientation of the respective zero field splitting principal axes, the T x state on one subunit and the T z state on another subunit in the dimer are overpopulated. The balance between the two triplet states is confirmed by the temperature dependency of the population ratio. No quintet state was detected with TREPR down to 20 K; thus, the recently proposed singlet fission ISC mechanism is excluded.
It is shown that the ancient Damascus structure of a Persian blade differs in composition and nature of the separation of layers from the multilayer structure of contemporary welding Damascus steel. ...Structural analysis of the step-by-step transformation of the cellular distribution of excess carbides within separate layered conglomerates, represented by a genuine Damascus troostite-carbide structure (65/35), has been performed on Damascus steel. In specimens of contemporary welding Damascus steel a stepwise process has been performed for transformation of ferrite-pearlite (40/60) structures into ferrite-martensite (30/70) structures. Continuous ferritic layers are transformed into accumulations of isolated recrystallized ferritic grains. Values of threshold stress intensity coefficient (∆
K
th
) are determined graphically, with which a fatigue crack does not propagate over the extent of a prescribed number of cycles. A critical value is recorded for stress intensity coefficient (∆
K
fc
) with which knife blades are broken. Behavior of a kinetic diagram linear section of the kinetic fatigue failure diagram (KFFD) is evaluated. In welding Damascus steel over a linear section areas of delayed fatigue crack growth are observed due to local delamination ahead of its front.
The cubic chiral helimagnets with the P213 space group represent a group of compounds in which the stable skyrmion-lattice state is experimentally observed. The key parameter that controls the energy ...landscape of such systems and determines the emergence of a topologically nontrivial magnetic structure is the Dzyaloshinskii-Moriya interaction (DMI). Chemical substitution is recognized as a convenient instrument to tune the DMI in real materials and has been successfully utilized in studies of a number of chiral magnets, such as MnSi, FeGe, MnGe, and others. In our study, we applied small-angle neutron scattering to investigate how chemical substitution influences the skyrmionic properties of an insulating helimagnet Cu2OSeO3 when Cu ions are replaced by either Zn or Ni. Our results demonstrate that the DMI is enhanced in the Ni-substituted compounds (Cu,Ni)2OSeO3, but weakened in (Cu,Zn)2OSeO3. The observed changes in the DMI strength are reflected in the magnitude of the spin-spiral propagation vector and the temperature stability of the skyrmion phase.
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