Presence Of Heavy Atoms Research Articles

Electromagnetically tunable spin-valley-polarized current via anomalous Nernst effect in monolayer of jacutingaite.

Monolayer jacutingaite (Pt2HgSe3) exhibits remarkable properties, including significant spin-orbit coupling and a tunable band gap, attributed to its buckled honeycomb geometry and the presence of heavy atoms. In this study, we explore the spin- and valley-dependent anomalous Nernst effect (ANE) in jacutingaite under the influence of a vertical electric field, off-resonance circularly polarized light (OCPL), and an antiferromagnetic exchange field. Our findings, within the low-energy approximation, reveal the emergence of a perfectly spin-polarized ANE with the application of appropriate OCPL and a perfectly valley-polarized ANE under an antiferromagnetic exchange field. Leveraging the robust spin-orbit coupling inherent in monolayer jacutingaite, our study highlights the potential to attain perfectly spin-valley-polarized Nernst currents across a wide range of Fermi energy levels by combining these fields in pairs with a suitable strength. The findings can be used for the development of spin-valley-based optoelectronic devices.&#xD.

Catalyzing Singlet Fission by Transition Metals: Second versus Third Row Effects.

The synthesis and characterization of platinum(II) and palladium(II) complexes bearing two (dimers Pt(Lpc)2Cl2 and Pd(Lpc)2Cl2), one (monomers Pt(Lpc)(Lref)Cl2 and Pd(Lpc)(Lref)Cl2), or no (reference compounds Pt(Lref)2Cl2 and Pd(Lref)2Cl2) pentacene-based pyridyl ligands are presented. Photophysical properties of the dimers are probed by means of steady-state and time-resolved transient absorption measurements in comparison to the monomer and model compounds. Our results document that despite enhanced spin-orbit coupling from the presence of heavy atoms, intramolecular singlet fission (iSF) is not challenged by intersystem crossing. iSF thus yields correlated triplet pairs and even uncorrelated triplet excited states upon decoherence. Importantly, significant separation of the two pentacenyl groups facilitates decoupling of the two chromophores. Furthermore, the mechanism of iSF is altered depending on the respective metal center, that is, Pt(II) versus Pd(II). The dimer based on Pt(II), Pt(Lpc)2Cl2, exhibits a direct pathway for the iSF and forms a correlated triplet pair with singlet-quintet spin-mixing within 10 ns in variable solvents. On the other hand, the dimer based on Pd(II), Pd(Lpc)2Cl2, leads to charge transfer mixing during the population of the correlated triplet pair that is dependent on solvent polarity. Moreover, Pd(Lpc)2Cl2 gives rise to a stable equilibrium between singlet and quintet correlated triplet pairs with lifetimes of up to 170 ns. Inherent differences in the size and polarizability, when contrasting platinum(II) with palladium(II), are the most likely rationale for the underlying trends.

Mechanism of Enhanced Triplet-Triplet Upconversion in Organic Molecules.

We use time-dependent density functional theory (TDDFT) to investigate the mechanism of efficient triplet-triplet upconversion (TTU) in certain organic materials. In particular, we focus on materials where some singlets are generated in a two-step spin-nonconserving process (T1 + T1 → T2 → S1). For this mechanism to contribute significantly, the intersystem crossing (ISC) from the high-lying triplet to the singlet (T2 → S1) must outcompete the internal conversion (IC) to the low-lying triplet (T2 → T1). By considering multiple families of materials, we show that the T2 → S1 ISC can be enhanced in a number of ways: the substitution of electron-donating (ED) and electron-withdrawing (EW) groups at appropriate positions; the substitution of bulky groups that distort the molecular geometry; and the substitution of heavy atoms that enhance the spin-orbit coupling (SOC). In the first two cases, the enhancements are consistent with El-Sayed's rule in that rapid T2 → S1 ISC requires significant differences in the characters of the S1 and the T2 wavefunctions. Together, these effects enable a wide tunability of T2 → S1 ISC rates over at least 5 orders of magnitude. Meanwhile, the T2 → T1 IC is inhibited in these systems due to the large T2 - T1 energy gap >0.5 eV, which entails a high energy barrier to the T2 → T1 IC and the prediction of a slow rate regardless of the substituents or the presence of heavy atoms. In this way, tuning the T2 → S1 ISC appears to provide an effective strategy to achieve systematic improvement of TTU materials.

Spin–orbit effects on the electronic and optical properties of lead iodide

Lead iodide (PbI2) has gained much interest due to its direct electronic gap in the visible range and layered crystal structure. It has thereby been considered as a promising material for applications in atomically thin optoelectronic devices. In this work, we present a detailed investigation of the effect of spin–orbit coupling (SOC) that arises from the presence of heavy atoms on the electronic and optical properties of PbI2 using first-principles calculations based on density-functional theory and many-body perturbation theory. We find that SOC not only alters the bandgap but also induces the mixing of orbital characters, resulting in a significant change in the overall band structure and charge carrier effective masses. Moreover, the band orbital mixing caused by SOC results in the dramatic change in optical transition matrix elements and, correspondingly, the absorption spectrum. Our experimentally measured absorption spectra validate the calculation results and demonstrate the importance of SOC in the optical processes of PbI2. Our findings provide insights that are important for the potential use of PbI2 as a material platform for visible optoelectronic devices.

New Insights into the Exploitation of BODIPY Derivatives as Organic Photocatalysts

AbstractBoron‐dipyrromethene complexes (BODIPYs) are attracting a growing interest for their notable photophysical properties. Specifically, their application in organic photocatalysis is receiving particular attention thanks to their ability to undergo both electron and energy transfer processes, thus acting as promising photoredox catalysts and photosensitizers. Although the number of examples employing these organic dyes is constantly increasing, there are not so many studies that correlate their chemical composition to their photocatalytic activity. In this work, a rationally designed structure‐activity relationship study was performed by selecting a synthetically relevant atom transfer radical addition (ATRA) reaction as a benchmark. We demonstrated how the presence of heavy atoms in the chromophore's core turned out to be essential for achieving high reactivity levels. On the contrary, electron‐withdrawing groups in position 8 (meso) undermined the catalytic performances, in agreement with the proposed reaction mechanism.

Clustering and halogen effects enabled red/near-infrared room temperature phosphorescence from aliphatic cyclic imides

Pure organic room temperature phosphorescence (RTP) materials become increasingly important in advanced optoelectronic and bioelectronic applications. Current phosphors based on small aromatic molecules show emission characteristics generally limited to short wavelengths. It remains an enormous challenge to achieve red and near-infrared (NIR) RTP, particularly for those from nonaromatics. Here we demonstrate that succinimide derived cyclic imides can emit RTP in the red (665, 690 nm) and NIR (745 nm) spectral range with high efficiencies of up to 9.2%. Despite their rather limited molecular conjugations, their unique emission stems from the presence of the imide unit and heavy atoms, effective molecular clustering, and the electron delocalization of halogens. We further demonstrate that the presence of heavy atoms like halogen or chalcogen atoms in these systems is important to facilitate intersystem crossing as well as to extend through-space conjugation and to enable rigidified conformations. This universal strategy paves the way to the design of nonconventional luminophores with long wavelength emission and for emerging applications.

BODIPY-Based Polymers of Intrinsic Microporosity for the Photocatalytic Detoxification of a Chemical Threat.

Effective heterogeneous photocatalysts capable of detoxifying chemical threats in practical settings must exhibit outstanding device integrity. We report a copolymerization that yields robust, porous, processible, chromophoric BODIPY (BDP; boron-dipyrromethene)-containing polymers of intrinsic microporosity (BDP-PIMs). Installation of a pentafluorophenyl at the meso position of a BDP produced reactive monomer that when combined with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephthalonitrile (TFTPN) yields PIM-1. Postsynthetic modification of these polymers yields Br-BDP-PIM-1a and -1b─polymers containing bromine at the 2,6-positions. Remarkably, the brominated polymers display porosity and processability features similar to those of H-BDP-PIMs. Gas adsorption reveals molecular-scale porosity and Brunette-Emmet-Teller surface areas as high as 680 m2 g-1. Electronic absorption spectra reveal charge-transfer (CT) bands centered at 660 nm, while bands arising from local excitations, LE, of BDP and TFTPN units are at 530 and 430 nm, respectively. Fluorescence spectra of the polymers reveal a Förster resonance energy-transfer (FRET) pathway to BDP units when TFTPN units are excited at 430 nm; weak phosphorescence at room temperature indicates a singlet-to-triplet intersystem crossing. The low-lying triplet state is well positioned energetically to sensitize the conversion of ground-state (triplet) molecular oxygen to electronically excited singlet oxygen. Photosensitization capabilities of these polymers toward singlet-oxygen-driven detoxification of a sulfur-mustard simulant 2-chloroethyl ethyl sulfide (CEES) have been examined. While excitation of CT and LEBDP bands yields weak catalytic activity (t1/2 > 15 min), excitation to higher energy states of TFTPN induces significant increases in photoactivity (t1/2 ≅ 5 min). The increase is attributable to (i) enhanced light collection, (ii) FRET between TFTPN and BDP, (iii) the presence of heavy atoms (bromine) having large spin-orbit coupling energies that can facilitate intersystem crossing from donor-acceptor CT-, FRET-, or LE-generated BDP singlet states to BDP-related triplet states, and (iv) polymer triplet excited-state sensitization of the formation of CEES-reactive, singlet oxygen.

Decreasing Structural Dimensionality of Double Perovskites for Phase Stabilization toward Efficient X-ray Detection.

Halide double perovskites have attracted substantial attention for optoelectronic applications owing to their low toxicity and high stability. However, double perovskites have strict requirements in terms of the halide type, thus rendering many of their properties unchangeable, including the band gap, atomic number, and carrier transport. By introducing long-chain organic amines, the chloride site of double perovskites can be completely replaced by bromide. Using this strategy, two dimensions silver-indium-bromide double perovskites (PEA)4AgInBr8 and (i-BA)4AgInBr8 were successfully synthesized [(PEA)+ = C6H5(CH2)2NH3+, (i-BA)+ = CH(CH3)2CH2NH3+]. Density functional theory calculations and spectroscopy characterizations were performed to unveil the semiconducting behaviors and photoluminescence properties of the title compounds. Electrical characterization confirms their good carrier-transport property (μτ product: 2.0 × 10-3 cm2 V-1) and low dark current. Moreover, the presence of heavy atoms, together with the ultrastable baseline contributes to a high X-ray detection sensitivity (185 μC Gyair-1 cm-2), greater than that of most previous double-perovskite detectors. Our work lays the foundation for broadening the family of potential double perovskites, creating a new path for the realization of long-sought perovskites with low toxicity and high stability that retain good optoelectronic performance.

Ultrasensitive 3D Aerosol-Jet-Printed Perovskite X-ray Photodetector.

X-ray photon detection is important for a wide range of applications. The highest demand, however, comes from medical imaging, which requires cost-effective, high-resolution detectors operating at low-photon flux, therefore stimulating the search for novel materials and new approaches. Recently, hybrid halide perovskite CH3NH3PbI3 (MAPbI3) has attracted considerable attention due to its advantageous optoelectronic properties and low fabrication costs. The presence of heavy atoms, providing a high scattering cross-section for photons, makes this material a perfect candidate for X-ray detection. Despite the already-successful demonstrations of efficiency in detection, its integration into standard microelectronics fabrication processes is still pending. Here, we demonstrate a promising method for building X-ray detector units by 3D aerosol jet printing with a record sensitivity of 2.2 × 108 μC Gyair-1 cm-2 when detecting 8 keV photons at dose rates below 1 μGy/s (detection limit 0.12 μGy/s), a 4-fold improvement on the best-in-class devices. An introduction of MAPbI3-based detection into medical imaging would significantly reduce health hazards related to the strongly ionizing X-rays' photons.

Кинетика затухания фосфоресценции при наличии статистического распределения молекул по скоростям излучательной и безызлучательной дезактивации триплетных возбуждений

Approximate formulas of phosphorescence decay kinetics under the conditions of the statistical scatter of molecules over rate constants of radiative or non-radiative deactivation of triplet excitation have been obtained. It is shown that the phosphorescence decay function has different form for these two cases. The obtained difference in kinetics was used to establish the reason for the deviation from the exponential law and faster decay of the sensitized phosphorescence of naphthalene and diphenyl in n-paraffin matrices ( T = 77 K) in the presence of heavy atoms in the donor.

Quantifying Triplet State Formation in Zinc Dipyrrin Complexes.

Photocatalysis is a promising method to harness solar energy and use it to form fuels and other high-value chemicals, but most sensitizers used in photocatalytic reactions are complexes of rare and expensive metals such as ruthenium and iridium. Zinc dipyrromethene complexes have potential to be a more earth-abundant alternative, but their photophysical properties are largely unexplored. In this study, triplet state formation was quantified in two zinc dipyrromethene complexes, with and without heavy atoms, by transient absorption spectroscopy. Without heavy atoms, the triplet quantum yield was 16% in toluene and 27% in THF. With the addition of heavy I atoms, the triplet quantum yield increased to 62-63% and was insensitive to solvent polarity. The fact that in the absence of heavy atoms the triplet yield is affected by solvent polarity and in the presence of heavy atoms it is not suggests that triplet formation occurs through different pathways in the two complexes. These triplet yields meet or exceed those of successful organic photosensitizers, illustrating the potential for zinc dipyrromethene complexes as photosensitizers.

Size, structure, and luminescence of Nd2Zr2O7 nanoparticles by molten salt synthesis

Pyrochlore materials with novel properties are in demand with multifunctional applications such as optoelectronics, scintillator materials, and theranostics. Many reports have already indicated the importance of the synthesis technique for Nd2Zr2O7 (NZO) nanoparticles (NPs); however, no explanation has been provided for the reason behind the nature of its phase selectivity. Here, we have explored the structural and optical properties of the NZO NPs synthesized by a molten salt synthesis method. We have synthesized size-tunable NZO NPs and correlated the particle size with their structural behavior and optical performance. All NZO NPs are stabilized in defect fluorite phase. Neutron diffraction provided insight on the behavior of oxygen in the presence of heavy atoms. We have collected bright amalgam of blue and green emission on UV irradiation due to the presence of oxygen vacancies from these NPs. We have carried out in situ XRD and Raman investigations to observe the temperature-induced phase transformation in a controlled argon environment. Interestingly, we have not observed phase change for the molten salt synthesized fluorite NZO NPs; however, we observed phase transformation from a precursor stage to pyrochlore phase by in situ XRD directly. These observations provide a new strategy to synthesize nanomaterials phase-selectively for a variety of applications in materials science.

Features of deactivation of excited states of cationic polymethine dye in the matrices of halogen-containing derivatives of poly-N-epoxypropyl carbazole

Singlet and triplet states of the cationic polymethine dye in polymers that are derivatives of poly-N-epoxypropyl carbazole with heavy atoms of different chemical nature were studied. It was shown that the presence of heavy atoms in the structure of the polymer leads to an appearance and rise of the dye phosphorescence due to the growth of the spin-orbit interaction in the dye molecule and increasing in the rate of singlet-triplet transitions. Also, the appearance of recombination luminescence indicated the formation and recombination of electron-hole pairs, which is an additional channel of relaxation of excited states of the dye in hole conducting polymer films. It was shown that external magnetic field leads to decreasing in the intensity of recombination luminescence. The time-dependent behavior of the magnetic effect is the result of competition between the singlet and triplet channels of the formation of the electron-hole pairs.

Revisiting Stabilities of Cubic Zincblende IV‐IV Materials From Density Functional Theory

The electronic structure of cubic zincblende (ZB) IV‐IV compounds are treated traditionally through first‐principles models neglecting relativistic effects, regardless the presence of heavier atoms like Ge, Sn, or Pb. Applying relativistic first‐principles plane wave pseudopotential methods, we revisit here the thermodynamic and dynamical stability of ZB‐IV‐IV materials: SiC, GeC, SnC, PbC, SiGe, SiSn, SiPb, GeSn, PbGe, and PbSn. Our results evince that except 3C‐SiC, all other IV‐IV compounds in the ZB phase exhibit positive formation enthalpy, thus manifesting thermodynamic instability. PbC, and SnC divulge huge thermodynamic instability, due to the high value of their elastic strain formation energy; whereas SiGe, GeSn, and PbSn show weak thermodynamic instability due to their insufficient chemical formation energy. Furthermore, except 3C‐SiC, we found that all other ZB‐IV‐IV compounds are dynamically unstable.

Modeling and Prediction of Drug Dispersability in Polyvinylpyrrolidone-Vinyl Acetate Copolymer Using a Molecular Descriptor

The expansion of a novel in silico model for the prediction of the dispersability of 18 model compounds with polyvinylpyrrolidone-vinyl acetate copolymer is described. The molecular descriptor R3m (atomic mass weighted 3rd-order autocorrelation index) is shown to be predictive of the formation of amorphous solid dispersions at 2 drug loadings (15% and 75% w/w) using 2 preparation methods (melt quenching and solvent evaporation using a rotary evaporator). Cosolidified samples were characterized using a suite of analytical techniques, which included differential scanning calorimetry, powder X-ray diffraction, pair distribution function analysis, polarized light microscopy, and hot stage microscopy. Logistic regression was applied, where appropriate, to model the success and failure of compound dispersability in polyvinylpyrrolidone-vinyl acetate copolymer. R3m had combined prediction accuracy greater than 90% for tested samples. The usefulness of this descriptor appears to be associated with the presence of heavy atoms in the molecular structure of the active pharmaceutical ingredient, and their location with respect to the geometric center of the molecule. Given the higher electronegativity and atomic volume of these types of atoms, it is hypothesized that they may impact the molecular mobility of the active pharmaceutical ingredient, or increase the likelihood of forming nonbonding interactions with the carrier polymer.

Investigation of organic magnetoresistance dependence on spin-orbit coupling using 8-hydroxyquinolinate rare-earth based complexes

The recently discovered organic magnetoresistance effect (OMAR) reveals the spin-dependent behavior of the charge transport in organic semiconductors. So far, it is known that hyperfine interactions play an important role in this phenomenon and also that spin-orbit coupling is negligible for light-atom based compounds. However, in the presence of heavy atoms, spin-orbit interactions should play an important role in OMAR. It is known that these interactions are responsible for singlet and triplet states mixing via intersystem crossing and the change of spin-charge relaxation time in the charge mobility process. In this work, we report a dramatic change in the OMAR effect caused by the presence of strong intramolecular spin-orbit coupling in a series of rare-earth quinolate organic complex-based devices. Our data show a different OMAR lineshape compared with the OMAR lineshape of tris(8-hydroxyquinolinate) aluminum-based devices, which are well described in the literature. In addition, electronic structure calculations based on density functional theory help to establish the connection between this results and the presence of heavy central ions in the different complexes.

Coherent Chromatic Effect in the Transmission Electron Microscope.

Under the assumption of local atomic scattering, elastic electron scattering at finite scattering angles implies a small but finite energy loss. This energy loss, which under conventional imaging conditions in high-resolution transmission electron microscopy is of the order of 0.1meV and thus negligible, increases by more than 2 orders of magnitude if light elements are investigated at sub-Ångström resolution. For a microscope of finite chromatic aberration, the energy loss leads to an element-specific chromatic effect which increases with the instrument resolution and with decreasing mass of the scattering atom. Despite that this effect is small, it can degrade the achievable image contrast. However, the effect can be considered in the optimization of the phase-contrast imaging conditions and even be beneficial to enhance the relative image contrast of light atoms in the presence of heavy atoms.

Study of intersystem crossing mechanism in organic materials

AbstractIntersystem crossing rate from singlet excited state to triplet excited state of an organic molecule has been derived using exciton‐spin‐orbit‐molecular vibration interaction as a perturbation operator. Incorporation of heavy metal atom enhances the spin‐orbit interaction and hence the intersystem crossing rate because it depends on the square of the heaviest atomic number. We found that in the presence of heavy atom the singlet‐triplet energy difference still plays an influential role in the intersystem crossing process. The derived exciton‐spin‐orbit‐molecular vibration interaction operator flips the spin of the singlet exciton to triplet exciton after photoexcitaion from the singlet ground state with the assistance of molecular vibrational energy. From this operator an expression for the intersystem crossing rate is derived and calculated in some organic solids. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fluorescence in rhoda- and iridacyclopentadienes neglecting the spin-orbit coupling of the heavy atom: the ligand dominates.

We present a detailed photophysical study and theoretical analysis of 2,5-bis(arylethynyl)rhodacyclopenta-2,4-dienes (1a–c and 2a–c) and a 2,5-bis(arylethynyl)iridacyclopenta-2,4-diene (3). Despite the presence of heavy atoms, these systems display unusually intense fluorescence from the S1 excited state and no phosphorescence from T1. The S1 → T1 intersystem crossing (ISC) is remarkably slow with a rate constant of 108 s–1 (i.e., on the nanosecond time scale). Traditionally, for organometallic systems bearing 4d or 5d metals, ISC is 2–3 orders of magnitude faster. Emission lifetime measurements suggest that the title compounds undergo S1 → T1 interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be ΔHISC = 28 kJ mol–1 (2340 cm–1) for 1a, which is supported by density functional theory (DFT) and time-dependent DFT calculations [ΔHISC(calc.) = 11 kJ mol–1 (920 cm–1) for 1a-H]. However, below 233 K a second, temperature-independent ISC process via spin–orbit coupling occurs. The calculated lifetime for this S1 → T1 ISC process is 1.1 s, indicating that although this is the main path for triplet state formation upon photoexcitation in common organometallic luminophores, it plays a minor role in our Rh compounds. Thus, the organic π-chromophore ligand seems to neglect the presence of the heavy rhodium or iridium atom, winning control over the excited-state photophysical behavior. This is attributed to a large energy separation of the ligand-centered highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) from the metal-centered orbitals. The lowest excited states S1 and T1 arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T1 state associated with a large S1–T1 energy separation favor an “organic-like” photophysical behavior.

Toward Spin–Orbit Coupled Diabatic Potential Energy Surfaces for Methyl Iodide Using Effective Relativistic Coupling by Asymptotic Representation

The theoretical treatment of state-state interactions and the development of coupled multidimensional potential energy surfaces (PESs) is of fundamental importance for the theoretical investigation of nonadiabatic processes. Usually, only derivative or vibronic coupling is considered, but the presence of heavy atoms in a system can render spin-orbit (SO) coupling important as well. In the present study, we apply a new method recently developed by us (J. Chem. Phys. 2012, 136, 034103, and J. Chem. Phys. 2012, 137, 064101) to generate SO coupled diabatic PESs along the C-I dissociation coordinate for methyl iodide (CH3I). This is the first and mandatory step toward the development of fully coupled full-dimensional PESs to describe the multistate photodynamics of this benchmark system. The method we use here is based on the diabatic asymptotic representation of the molecular fine structure states and an effective relativistic coupling operator. It therefore is called effective relativistic coupling by asymptotic representation (ERCAR). This approach allows the efficient and accurate generation of fully coupled PESs including derivative and SO coupling based on high-level ab initio calculations. In this study we develop a specific ERCAR model for CH3I that so far accounts only for the C-I bond cleavage. Details of the diabatization and the accuracy of the results are investigated in comparison to reference ab initio calculations and experiments. The energies of the adiabatic fine structure states are reproduced in excellent agreement with ab initio SO-CI data. The model is also compared to available literature data, and its performance is evaluated critically. This shows that the new method is very promising for the construction of fully coupled full-dimensional PESs for CH3I to be used in future quantum dynamics studies.