Small Energy Gap Research Articles

Persistent radical anions in panchromatic D-A hybrid heterostructures induced by anion-π interactions

The combination of N, N′-dipyridyl-tetrachloroperylene diimide (4Cl-DPPDI) buliding blocks with lead iodides in the presence of hydrogen iodides led to a novel D-A hybrid heterostructure with segregated iodoplumbate anions and 2-D H-bonded 4Cl-DPPDI networks as electron donors and acceptors. Due to the compact anion-π interactions as well as small energy gaps between the donors and acceptors, persistent and stable radical anions are generated in such a unique complex triggered by natural-light irradiation, which thus exhibits a panchromatic absorption.

Optical and magnetic properties of HoFeO3 nanocrystals prepared by a simple co-precipitation method using ethanol

Abstract This paper reports the study of optical and magnetic properties of HoFeO3 nanocrystals prepared by a simple co-precipitation method using ethanol. Single-phase HoFeO3 was formed after calcination of the precipitate at 650, 750 and 850 °C during 1 h. The XRD and TEM results showed that the average crystallite sizes of all HoFeO3 samples were in the range of 25–30 nm. The UV-Vis spectra showed strong UV and Vis absorption with a small optical energy gap (Eg = 1.8019 eV). The soft ferromagnetic behavior of HoFeO3 nanoparticles with magnetization ∼0.7–0.8 emu/g, coercivity ∼8–23 Oe, remanent magnetization less than 0.005 emu/g.

Efficient deep-blue thermally activated delayed fluorescence emitters based on diphenylsulfone-derivative acceptor

Efficient deep-blue thermally activated delayed fluorescence (TADF) emitters are still very scarce. It is available to render the color bluer by reducing the intensity of intramolecular charge transfer (ICT) between acceptor and donor. Here, attaching the acceptor of six-membered heterocyclic diphenylsulfone-derivative (2PTO) with donors (DMAc and DPAc), two compounds, 2-(9,9-dimethyl-acridin-10(9H)-yl)-10-phenyl-10H-phenothiazine 5,5-dioxide (DMAC2PTO) and 2-(9,9-diphenylacridin-10(9H)-yl)-10-phenyl-10H-phenothiazine 5,5-dioxide (DPAC2PTO) were designed and synthesized. Due to the largely twisted configuration and weak ICT, DMAC2PTO with small singlet-triplet energy gap (ΔEST) displayed TADF feature and deep-blue emission. Moreover, DPAC2PTO exhibited much lower photoluminescence quantum yield (PLQY) than DMAC2PTO. The doped device of DMAC2PTO achieved a maximum external quantum efficiency (EQEmax) of 15.2% with a deep-blue color coordinate (CIEx,y) of (0.154, 0.108), and showed a narrow emission spectra with a 52 nm full width at half maximum (FWHM). In addition, the non-doped device of DMAC2PTO reached the EQEmax of 8.36% with CIEx,y of (0.158, 0.157).

A Stable All-Thiophene-Based Core-Modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States.

A soluble and stable core-modified [38]octaphyrin, MC-T8, containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X-ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38π conjugation pathway. The dication MC-T82+ is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC-T84+ becomes aromatic again, with a shallow-bowl-shaped geometry. Both the neutral compound and the dication demonstrated open-shell diradical character with a small singlet-triplet energy gap (-2.70 kcal mol-1 for MC-T8 and -3.78 kcal mol-1 for MC-T82+ ), and they are stable owing to effective spin delocalization.

Current-phase relation in a short clean josephson junction model: application to MgB2

Motivated by recent data on high-quality MgB2 thin films implying that the smaller energy gap has l = 6 (i-wave) symmetry, we consider a simple model for an all-MgB2 symmetric Josephson Junction (JJ). The model assumes an arbitrary-strength delta-function barrier and one-dimensional current conduction. It is shown that in this context a nodal energy gap with i-wave symmetry acts as an isotropic energy gap (s-wave) with an amplitude modified by the energy-gap misalignment-angle with respect to the crystal principal axes. The corresponding exact Green’s function in momentum space is derived employing a novel approach. The ensuing current-phase relations in the strong and weak barrier-strengths limits are calculated and found to confirm known results, e.g., the Ambegaokar-Baratoff current-phase relation. Inspired by an HTS experiment that established the d-wave energy-gap symmetry, we propose a JJ-related experiment with a MgB2 bicrystal to confirm our premise that the smaller energy has i-wave symmetry.

Computational analysis on D–π–A based perylene organic efficient sensitizer in dye-sensitized solar cells

We designed and screened a series of metal-free organic dyes with different tunable donors (D1–D3) based on literature dye (3a). The D1–D3 organic dyes have been applied in dye-sensitized solar cells (DSSCs). Triphenylamine, thiophene and 2-cyanoacrylic acid groups as an electron donor (D), spacer (π) and electron acceptor (A), respectively, to form a D–π–A system. Density functional theory (DFT) and time-dependent DFT was employed to study the electronic charge distribution, intramolecular charge transfer, absorption spectra and photovoltaic (PV) parameters of the dyes. PV presentation of the dye molecules depends on tunable donors (D1–D3) classification. Results reveal that the replacing group of D3 dye has been red-shift absorption spectra and improves PV cell performance. Particularly, D2 and D3 dyes have smaller energy gaps and higher absorption spectra covering entire visible region, compared to D1 dye. Therefore, these results display that molecular tailoring is a promising scheme to improve D–π–A design of sensitizer for highly efficient DSSCs application.

Cooperative effect of hetero-nuclear MnNi+ cation enhancing C–H bond activation of cyclohexane: a theoretical study

Hetero-nuclear MnNi+ cation exhibits high catalytic activity in the C–H bond activation of cyclohexane in comparison with mono-nuclear Mn+ cation. The dehydrogenation reaction mechanistic investigations of cyclohexane catalyzed by MnNi+ and Mn+ have been carried out with density functional theory calculations. The activations of the first, third and fifth C–H bond of cyclohexane occur mainly because of the orbital interaction between the C–H σ bond of cyclic hydrocarbon and the empty d orbital of Mn. The smaller energy gaps of interacting orbitals result in stronger electron transfer between the related orbitals, which can elucidate MnNi+ cation is more reactive than Mn+. The coordination bonds between MnNi+ and cycloolefin in the second and third dehydrogenation processes of cyclohexane are another important factor for accelerating the dehydrogenation, which can stabilize the transition states for the activation of C–H bonds. The lower the transition state energy, the easier the C–H bond activation.

Quantum chemical investigation of modified coumarin-based organic efficient sensitizers for optoelectronic applications

One of the most major features in the improvement of dye-sensitized solar cells (DSSCs) is the survey and plan of high-efficiency and low-cost dyes. In this paper, a new series of metal-free coumarin-based (NKX-2311) dye molecules, namely (D1–D4) are investigated for use in DSSC applications. The dye molecules contain three parts, electron-donor (D) connected by the π-conjugated linker as a spacer and electron-acceptor (A)/anchoring group. Molecular structure, electronic absorption spectra and photovoltaic (PV) parameters of the D1–D4 dyes have been performed by using density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches are employed. The effects of chemical modification on the spectra and PV properties of the coumarin-modified organic dyes have been revealed. The performance of three functionals (B3LYP, CAM-B3LYP and ωB97XD) has been evaluated to get an experimental absorption wavelength of NKX-2311. From the functionals, B3LYP method was better matched with the absorption wavelength of NKX-2311. Therefore, D1–D4 dye molecules excited state calculations of the absorption spectra have been investigated by using B3LYP functional with 6-31G(d) basis set. The results show that D1, D3 dyes are smaller energy gap and absorption spectra are systematically red-shifts. The theoretical results have a display that the lowest occupied molecular orbitals (LUMOs) and highest occupied molecular orbitals (HOMOs) of the D1–D4 dyes can be confirming positive response on the electron injection and dye regeneration process. The NLO property of the D1–D4 dyes can be derived as polarizability and first-order hyperpolarizability. The calculated values of D1 and D3 dyes are the best applicants for NLO performance. The conclusion of this study will deliver a useful reference to the future proposal and beneficial to get DSSCs with superior power conversion efficiency (PCE).

The effect of group-substitution on the sensitization properties of alkynylrhenium(I) tricarbonyl diimine complexes adsorbed to TiO2(101) film surface: a theoretical study.

A series of dyes are designed by adding the different electron-donating (-CH3, -NH2, -OH) and electron-withdrawing groups (-Br, -Cl, -NO2) to the different ancillary ligands in the alkynylrhenium(I) tricarbonyl diimine complexes [Re(CO)3(N^N){C≡C-C6H4-CH=C(CN)(COOH)}], where N^N = 1,10-phenanthroline (phen)(1) and then investigated the sensitization properties of dyes linked to the TiO2(101) surface. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to study the electronic structure, frontier molecular orbitals, and absorption spectral properties. The effect of group-substitution on sensitization properties is obvious. When the dye molecules are combined with TiO2(101) surface, not only the absorptions of some sensitizers containing -CH3 or -OH groups have red shift but also the electrons can be directly injected into the TiO2 conduction band from the dye molecules compared with the parent molecular 1. The results indicate that the designed dyes containing electron-donating groups have smaller energy gaps, better light-harvesting efficiency, sufficient driving force, and higher charge transfer efficiency as appropriate dye sensitizers. We hope it can provide valuable hints so that we can design more efficient dye sensitizers in DSSCs.

Hot Electron Cooling in Silicon Nanoclusters via Landau-Zener Nonadiabatic Molecular Dynamics: Size Dependence and Role of Surface Termination.

We develop a new express methodology for modeling excited-state dynamics occurring in dense manifolds of electronic states in atomistic systems. The approach leverages a modified Landau-Zener formula, the neglect of a back-reaction approximation, and the highly efficient density functional tight-binding method. We study the hot electron dynamics in a series of H- and F-terminated silicon nanocrystals (NCs) containing up to several hundred atoms. We explain the slower electron cooling dynamics in F-terminated NCs by the larger energy gaps between the adjacent electronic states in these systems as well as their slower fluctuations. We conclude that both the mass and chemical identity of the surface termination groups equally influence the electron dynamics, on average. However, the mass effect becomes dominant for higher-energy excitations. We find that the electron decay dynamics in F-terminated NCs has a greater sensitivity to the mass of the surface ligands than do the H-terminated NCs and explain this observation by the details of the electron-phonon coupling in the systems. We find that in the H-terminated NCs, electronic transitions in the cooling process occur predominantly between the surface states, whereas in F-terminated Si NCs, both surface and NC core states are coupled to the nuclear vibrations. We find that electron energy relaxation is accelerated in larger NCs and attribute this effect to the higher densities of states and smaller energy gaps in these systems.

Doping-induced topological phase transition in Bi: The role of quantum electronic stress

Charge doping is an essential means to tailor a materials' properties. However, besides moving the Fermi level, charge doping is generally not expected to induce topological phase transition (TPT). Here, using first-principles calculations, we demonstrate an electron doping-induced TPT in bulk Bi from a higher-order topological insulator (HOTI) to a TI. The underlying mechanism is revealed to be driven by an electron doping-induced quantum electronic stress (QES), which in turn induces a highly anisotropic lattice expansion to close/reopen the small energy gap in Bi band structure. Our finding significantly resolves an outstanding controversy concerning the topological characterization of bulk Bi among existing experiments and theories, and explains the physical origin of the topological order in Bi (111) thin films. It sheds new light on the fundamental understanding of topological properties of small band gap materials in relation to doping and QES.

A Theoretical Study of the Solid Solution Phases of Li<sub>x</sub>FePO<sub>4</sub>

For the next generation Li-battery anode materials LiFePO4, the forming of the solid solution phase LixFePO4 and the related charge/discharge mechanism are the high light topics recently. In the paper, ab-initio calculation was applied to study the formation and electronic structure of the solid solution of LixFePO4, and a Charge/Discharge model of LiFePO4 was set up based on the calculation results. Due to the high formation energy, LixFePO4 separates into FePO4 and LiFePO4 in bulk system under room temperature. The single solid solution phase LixFePO4 could exists in the nanoscale particle, and it is due to that the relative larger lattice mismatch energy. The nanoscale particle materials should have a good rating performance due to the forming of LixFePO4 in solid solution phase, of which the partially occupied state and the small energy gap between the VBM and the defect state could improve the intrinsic electronic conductivity. In bulk materials, the medium region, which is composed of LixFePO4, is very narrow between the two phases FePO4 and LiFePO4. There is a electron potential well in the region, of which the bottom is at the side of LixFePO4 (x<0.5). The number of electron in the well highly affects the lithium insertion and extraction. In order to efficiently transfer the electron between the potential well and the out circuit, an electron conductor network or layer should be coated on the LiFePO4 particle.

Understanding the nature of quinoidal and zwitterionic states in carbazole-based diradicals.

We report two carbazole-based diradicals, out of which the m-isomer shows a large diradical character y0 (0.89) and a small singlet-triplet energy gap ΔES-T (-0.98 kcal mol-1), whereas the p-isomer exhibits smaller y0 (0.79) but a much larger ΔES-T (-6.16 kcal mol-1). DFT calculations reveal that this tendency is also suitable for nitrogen and carbon-centered diradicals.

DFT study and NBO analysis of solvation/substituent effects of 3-phenylbenzo[d]thiazole-2(3H)-imine derivatives

In this work, to determine natural bond orbital (NBO) analysis, solvation and substituent effects for electron-releasing substituents (CH3, OH) and electron-withdrawing derivatives (Cl, NO2, CF3) in para positions on the molecular structure of the synthesized 3-phenylbenzo[d]thiazole-2(3H)-imine derivatives 1?6 (H (1), CH3 (2), Cl (3), OH (4), CF3 (5), NO2 (6)) in the selected solvents (acetone, toluene, and ethanol) and gas-phase employing polarizable continuum method (PCM) model were studied at the M06-2x/6- -311++G(d,p) level of theory. The relative stability of the studied compounds was affected by the possibility of intramolecular interactions between substituents and the electron donor/acceptor centers of the thiazole ring. Furthermore, atomic charges electron density, chemical thermodynamics, energetic properties, dipole moments, and the nucleus-independent chemical shifts (NICS) of the studied compounds and their relative stability are considered. The dipole moment values and the HOMO?LUMO energy gap reveal the different charge transfer possibilities within the considered molecules. Frontier molecular orbital (FMO) analysis revealed that compound 6 has very small HOMO-LUMO energy gaps in the considered phases, and thus is kinetically less stable. The obtained HOMO-LUMO energy gap corresponds to intramolecular hyperconjugative interactions ?? ?*. Finally, NBO analysis is carried out to demonstrate the charge transfer between localized bonds and lone pairs.

Spectral and Structural Perspectives of 1,3,3-Trimethyl-2,6-diphenylpiperidin-4-one

In this work, we report spectral, structural and computational studies on 1,3,3-trimethyl-2,6-diphenylpiperidin-4-one (TMP). The molecular structure of the title compound in the ground state has been investigated by DFT-B3LYP method with 6-311G (d,p) basis set. The optimized structural parameters of the title compound acquired from DFT method has been found in accord with the single crystal XRD values. Vibrational and Mulliken analysis, frontier molecular orbitals and molecular electrostatic potential of the title compound have also been calculated and discussed. The small energy gap (EHOMO-ELUMO) show that the charge transfer occurs within the molecule.

Quantum simulations of excited states with active-space downfolded Hamiltonians.

Many-body techniques based on the double unitary coupled cluster (DUCC) ansatz can be used to downfold electronic Hamiltonians into low-dimensional active spaces. It can be shown that the resulting dimensionality reduced Hamiltonians are amenable for quantum computing. Recent studies performed for several benchmark systems using phase estimation (PE) algorithms for quantum computers demonstrated that these formulations can recover a significant portion of ground-state dynamical correlation effects that stem from the electron excitations outside of the active space. These results have also been confirmed in studies of ground-state potential energy surfaces using quantum simulators. In this letter, we study the effectiveness of the DUCC formalism in describing excited states. We also emphasize the role of the PE formalism and its stochastic nature in discovering/identifying excited states or excited-state processes in situations when the knowledge about the true configurational structure of a sought after excited state is limited or postulated (due to the specific physics driving excited-state processes of interest). In this context, we can view PE algorithms as an engine for verifying various hypotheses for excited-state processes and providing statistically meaningful results that correspond to the electronic state(s) with the largest overlap with a postulated configurational structure. We illustrate these ideas on examples of strongly correlated molecular systems, characterized by small energy gaps and high density of quasidegenerate states around the Fermi level.

9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole—A new donor building-block in the design of sensitizers for dye-sensitized solar cells

The novel donor building-block - 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole was designed and employed in the synthesis of dye-sensitized solar cell (DSSCs). An effective, high-yielding synthesis of 4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole from 1,2,3,4,4a,9a-hexahydrocarbazole was realized. Three new metal-free organic sensitizers, containing the new donor building block were prepared by a stepwise approach from 4,7-dibromobenzo[c][1,2,5]chacogenadiazoles. A 2,1,3-Benzothiadiazole dye containing hexahydrocarbazole donor, thiophene as π-spacer and cyanoacrylate as anchoring electron acceptor showed photovoltaic properties higher than the well-known WS-2 sensitizer with PCE = 5.86 %. Although benzoxa- and –selenadiazole dyes have a bathochromic shift (24−30 nm) in the UV–vis spectra, and smaller energy gap Eg (about 0.1 eV), they have lower photovoltaic parameters, including PCE of 1.5–2.3 %. Introducing a new donor 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole into the construction of the DSSCs has broadened possibilities for the optimization of their photovoltaic properties.

Intra-layer atomic ordering and semi-conductivity in CsAMS2 (A = Li, Ag; M = Co, Fe)

Three layered compounds CsLiCoS2, CsAgCoS2 and CsLiFeS2 have been synthesized by conventional solid-state reaction method. All of three compounds exhibit semiconducting behavior in electrical transport with small thermal activation energy gap. The crystal structure of CsLiCoS2 is isostructural to ThCr2Si2-type CsCo2S2 with random mixture of Li+ and Co2+. The latter two compounds exhibit √2 ​× ​√2 ​× ​1 superstructure of ThCr2Si2-type because the monovalent Ag+/Li+ and divalent Co2+/Fe2+ independently occupy inequivalent sites in [AMS2]- layer. Through comprehensive investigation of the crystallographic lattice in CsAMS2, a phase boundary between disorder and ordered phases may be identified, which highlights the importance of the atomic size and lattice mismatch in determining the crystal structure.

Spin-Vibronic Model for Quantitative Prediction of Reverse Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Systems.

Computationally predicting reverse intersystem crossing (RISC) rates is important for designing new thermally activated delayed fluorescence (TADF) materials. We report a method that can quantitatively predict RISC rates by explicitly considering the spin-vibronic coupling mechanism. The coupling element of the spin-vibronic Hamiltonian is obtained by expanding the spin-orbit and the non-Born-Oppenheimer terms to second order and is then brought into the Golden Rule rate under the Condon approximation. The rate equation is solved directly in the time domain using a correlation function approach. The contributions of the first-order direct spin-orbit coupling and the second-order spin-vibronic coupling to an RISC rate can be quantitatively analyzed in a separate manner. We demonstrate the utility of the method by applying it to a representative TADF system, where we observe that the spin-vibronic portion is substantial but not dominant especially with a relatively small triplet-singlet energy gap. Likewise, our method may elucidate the physical background of efficient nonradiative transitions from the lowest triplet to a higher lying singlet in other purely organic TADF systems, and it will be of great utility toward designing new such molecules.

AIEgens with bright mechanoluminescence and thermally activated delayed fluorescence derived from (9H-carbazol-9-yl)(phenyl)methanone

Two compounds derived from (9H-carbazol-9-yl)(phenyl)methanone (TPhBCz and TPh2BCz) showing characteristics of AIE and TADF were synthesized and investigated. Small energy gap (ΔEST) between S1 and T1 may account for the TADF properties of the compounds. Moreover, TPhBCz showed strong mechanoluminescence (ML) without any pretreatment, while there was no ML for the TPh2BCz substituted with two bromine groups. The careful investigation on the data of single crystal structures and theoretical calculations demonstrated that the compact molecular packing as well as abundant and intense intermolecular interactions of the crystal lead to the prominent ML performance of TPhBCz. These results help to understand the unique ML process and provide a design and synthetic strategy for new materials exhibited AIE, TADF, and ML simultaneously.