Anisotropic spin transport and photoresponse characteristics detected by tip movement in magnetic single-molecule junction

Orientation-dependent transport properties induced by anisotropic molecules are enticing in single-molecule junctions.Here,using the first-principles method,we theoretically investigate spin transport properties and photoresponse characteristics in trimesic acid magnetic single-molecule junctions with different molecular adsorption orientations and electrode contact sites.The transport calculations indicate that a single-molecule switch and a significant enhancement of spin transport and photoresponse can be achieved when the molecular adsorption orientation changes from planar geometry to upright geometry.The maximum spin polarization of current and photocurrent in upright molecular junctions exceeds 90%.Moreover,as the Ni tip electrode moves,the tunneling magnetoresistance of upright molecular junctions can be increased to 70%.The analysis of the spin-dependent PDOS elucidates that the spinterfaces between organic molecule and ferromagnetic electrodes are modulated by molecular adsorption orientation,where the molecule in upright molecular junctions yields higher spin polarization.Our theoretical work paves the way for designing spintronic devices and optoelectronic devices with anisotropic functionality base on anisotropic molecules.

利用高空间分辨针尖增强拉曼散射识别氮掺杂富勒烯催化的氧还原反应中间体

氮掺杂富勒烯(C_(59)N)催化剂在氢燃料电池的氧还原反应中表现出良好的活性.然而,C_(59)N上发生的氧还原反应路径的中间体和催化活性位点尚未被直接表征,阻碍了对C_(59)N催化剂在氧还原反应中活性增强机制的理解.本文通过在模拟计算中考虑空间限制等离子体的不均匀分布,从理论上提出高空间分辨针尖增强拉曼散射可以有效地识别C_(59)N上氧还原反应的不同中间体构型.通过调整聚焦的空间限制等离子体位置,氧还原反应中与O_(2)-C_(59)N相互作用有关联的振动模式可以被高空间分辨针尖增强拉曼散射光谱直接选择出来,并且得到增强.此外,选择出来的振动模式对应的高空间分辨针尖增强拉曼散射图像在吸附位点周围给出了拉曼热点,提供了氧还原反应在C_(59)N上催化活性位点的原位观测细节。这些发现为今后通过高分辨率高空间分辨针尖增强拉曼散射技术在分子尺度上探索催化系统提供了良好的参考.

Effect of aggregation on thermally activated delayed fluorescence and ultralong organic phosphorescence:QM/MM study

Aggregation-induced thermally activated delayed fluorescence(TADF)phenomena have attracted extensive attention recently.In this paper,several theoretical models including monomer,dimer,and complex are used for the explanation of the luminescent properties of(R)-5-(9H-carbazol-9-yl)-2-(1,2,3,4-tetrahydronaphthalen-1-yl)isoindoline-1,3-dione((R)-ImNCz),which was recently reported[Chemical Engineering Journal 418129167(2021)].The polarizable continuum model(PCM)and the combined quantum mechanics and molecular mechanics(QM/MM)method are adopted in simulation of the property of the molecule in the gas phase,solvated in acetonitrile and in aggregation states.It is found that large spin–orbit coupling(SOC)constants and a smaller energy gap between the first singlet excited state and the first triplet excited state(△E_(st))in prism-like single crystals(SC_(p)-form)are responsible for the TADF of(R)-lmNCz,while no TADF is found in block-like single crystals(SC_(b)-form)with a larger △E_(st).The multiple ultralong phosphorescence(UOP)peaks in the spectrum are of complex origins,and they are related not only to ImNCz but also to a minor amount of impurities(ImNBd)in the crystal prepared in the laboratory.The dimer has similar phosphorescence emission wavelengths to the(R)-lmNCz-SC_(p) monomers.The complex composed of(R)-lmNCz and(R)-lmNBd contributes to the phosphorescent emission peak at about 600 nm,and the phosphorescent emission peak at about 650 nm is generated by(R)-lmNBd.This indicates that the impurity could also contribute to emission in molecular crystals.The present calculations clarify the relationship between the molecular aggregation and the light-emitting properties of the TADF emitters and will therefore be helpful for the design of potentially more useful TADF emitters.

Density Functional Theory Study on Raman Spectra of Rhodamine Molecules in Different Forms

玫瑰精分子是为与拉曼光谱学有关的应用的最使用的染料之一。我们系统地学习了玫瑰精 6G 的拉曼系列，玫瑰精 123，并且玫瑰精 B (RhB ) 分子使用密度功能的理论。它与 BP86 被发现那功能 cationic 玫瑰精分子的计算拉曼系列在对在水的答案的相应试验性的系列的好同意。相反的离子，氯，和特定的氢契约的参与在能部分在答案并且在金表面上解释 RhB 的拉曼系列之间的观察差别的 RhB 的拉曼系列上有显著效果，这被显示出。它也显示对表面的精确描述提高了仍然在金属表面上为玫瑰精分子散布的拉曼要求考虑界面的相互作用导致的变化。

Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions

Spin-dependent transport in ferromagnet/organic-ferromagnet/metal junctions is investigated theoretically.The results reveal a large tunneling magnetoresistance up to 3230%by controlling the relative magnetization orientation between the ferromagnet and the central organic ferromagnet.The mechanism is explained by distinct efficient spin-resolved tunneling states in the ferromagnet between the parallel and antiparallel spin configurations.The key role of the organic ferromagnet in generating the large magnetoresistance is explored,where the spin selection effect is found to enlarge the difference of the tunneling states between the parallel and antiparallel configurations by comparing with the conventional organic spin valves.The effects of intrinsic interactions in the organic ferromagnet including electron–lattice interaction and spin coupling with radicals on the magnetoresistance are discussed.This work demonstrates a promising potential of organic ferromagnets in the design of high-performance organic spin valves.

Theoretical verification of intermolecular hydrogen bond induced thermally activated delayed fluorescence in SOBF-OMe

Thermally activated delayed fluorescence(TADF)molecules have attracted great attention as high efficient luminescent materials.Most of TADF molecules possess small energy gap between the first singlet excited state(S_(1))and the first triplet excited state(T_(1))to favor the up-conversion from T_(1)to S_(1).In this paper,a new TADF generation mechanism is revealed based on theoretical simulation.By systematic study of the light-emitting properties of SOBF-OMe in both toluene and in aggregation state,we find that the single SOBF-OMe could not realize TADF emission due to large energy gap as well as small up-conversion rates between S_(1)and T_(1).Through analysis of dimers,we find that dimers with intermolecular hydrogen bond(H-bond)are responsible for the generation of TADF,since smaller energy gap between S_(1)and T_(1)is found and the emission wavelength is in good agreement with experimental counterpart.The emission properties of SOBF-H are also studied for comparison,which reflect the important role of H-bond.Our theoretical results agree ith experimental results well and confirm the mechanism of H-bond induced TADF.

Current spin polarization of a platform molecule with compression effect

Using the first-principles method,the spin-dependent transport properties of a novel platform molecule containing a freestanding molecular wire is investigated by simulating the spin-polarized scanning tunneling microscope experiment with Ni tip and Au substrate electrodes.Transport calculations show that the total current increases as the tip gradually approaches to the substrate,which is consistent with the conductance obtained from previous experiment.More interestingly,the spin polarization(SP)of current modulated by compression effect has the completely opposite trend to the total current.Transmission analyses reveal that the reduction of SP of current with compression process originates from the promotion of spin-down electron channel,which is controlled by deforming the molecule wire.In addition,the density of states shows that the SP of current is directly affected by the organic–ferromagnetic spinterface.The weak orbital hybridization between the Ni tip and propynyl of molecule results in high interfacial SP,whereas the breaking of the C≡C triple of propynyl in favor of the Ni–C–C bond induces the strong orbital hybridization and restrains the interfacial SP.This work proposes a new way to control and design the SP of current through organic–ferromagnetic spinterface using functional molecular platform.

Theoretical design of single-molecule NOR and XNOR logic gates by using transition metal dibenzotetraaza[14]annulenes

The idea of replacing traditional silicon-based electronic components with the ones assembled by organic molecules to further scale down the electric circuits has been attracting extensive research focuses.Among the molecularly assembled components,the design of molecular logic gates with simple structure and high Boolean computing speed remains a great challenge.Here,by using the state-of-the-art nonequilibrium Green’s function theory in conjugation with first-principles method,the spin transport properties of single-molecule junctions comprised of two serially connected transition metal dibenzotetraaza[14]annulenes(TM(DBTAA),TM=Fe,Co)sandwiched between two single-walled carbon nanotube electrodes are theoretically investigated.The numerical results show a close dependence of the spin-resolved current-voltage characteristics on spin configurations between the left and right molecular kernels and the kind of TM atom in TM(DBTAA)molecule.By taking advantage of spin degree of freedom of electrons,NOR or XNOR Boolean logic gates can be realized in Fe(DBTAA)and Co(DBTAA)junctions depending on the definitions of input and output signals.This work proposes a new kind of molecular logic gates and hence is helpful for further miniaturization of the electric circuits.

Bias-induced reconstruction of hybrid interface states in magnetic molecular junctions

Based on first-principles calculations,the bias-induced evolutions of hybrid interface states inπ-conjugated tricene and in insulating octane magnetic molecular junctions are investigated.Obvious bias-induced splitting and energy shift of the spin-resolved hybrid interface states are observed in the two junctions.The recombination of the shifted hybrid interface states from different interfaces makes the spin polarization around the Fermi energy strongly bias-dependent.The transport calculations demonstrate that in theπ-conjugated tricene junction,the bias-dependent hybrid interface states work efficiently for large current,current spin polarization,and distinct tunneling magnetoresistance.But in the insulating octane junction,the spin-dependent transport via the hybrid interface states is inhibited,which is only slightly disturbed by the bias.This work reveals the phenomenon of bias-induced reconstruction of hybrid interface states in molecular spinterface devices,and the underlying role of conjugated molecular orbitals in the transport ability of hybrid interface states.

Perspective for aggregation-induced delayed fluorescence mechanism:A QM/MM study

To enhance the potential application of thermally activated delayed fluorescence(TADF)molecular materials,new functions are gradually cooperated to the TADF molecules.Aggregation induced emission can effectively solve the fluorescence quenching problem for TADF molecules in solid phase,thus aggregation-induced delayed fluorescence(AIDF)molecules were recently focused.Nevertheless,their luminescent mechanisms are not clear enough.In this work,excited state properties of an AIDF molecule DMF-BP-DMAC[reported in Chemistry-An Asian Journal 14828(2019)]are theoretically studied in tetrahydrofuran(THF)and solid phase.For consideration of surrounding environment,the polarizable continuum method(PCM)and the combined quantum mechanics and molecular mechanics(QM/MM)method were applied for solvent and solid phase,respectively.Due to the increase of the transition dipole moment and decrease of the energy difference between the first single excited state(S1)and the ground state(S0),the radiative rate is increased by about 2 orders of magnitude in solid phase.The energy dissipation of the non-radiative process from S1 to S0 is mainly contributed by low-frequency vibrational modes in solvent,and they can be effectively suppressed in aggregation,which may lead to a slow non-radiation process in solid phase.Both factors would induce enhanced luminescence efficiency of DMF-BP-DMAC in solid phase.Meanwhile,the small energy gap between S1 and triplet excited states results in high reverse intersystem crossing(RISC)rates in both solvent and solid phase.Therefore,TADF is confirmed in both phases.Aggregation significantly influences both the ISC and RISC processes and more RISC channels are involved in solid state.The enhanced delayed fluorescence should be induced by both the enhanced fluorescent efficiency and ISC efficiency.Our calculation provides a reasonable explanation for experimental measurements and helps one to better understand the luminescence mechanism of AIDF molecules.