Phenylazoimidazole as a possible optical molecular switch: An ab initio study

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the phenylazoimidazole optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the current through the trans form is significantly larger than through the cis form, which suggests this system has attractive potential application in future molecular switch technology.     

Coherent electron transport through an azobenzene molecule: a light-driven molecular switch

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.     

Hard Sphere Diffusion Behaviour of Polymer Translocating through Interacting Pores

The translocation of polymer chain through a small pore from a high concentration side (cis side) to a low concentration side (trans side) is simulated by using Monte Carlo technique. The effect of the polymer--pore interaction on the translocation is studied. We find a special interaction at which the decay of the number of polymer chain, N, at the cis side obeys Fick's law, i.e. N decreases exponentially with time. The behaviour is analogous to the diffusion of hard sphere.     

CNDO calculations of some thioamides—II

CNDO calculation is made for thioformamide, N-methylthioformamide (both in cis and trans forms) and N, N-dimethyl thioformamide. The charges, bond orders, dipolemoments and ionization potentials of molecules are discussed and comparison made with corresponding amides. The barrier to internal rotation about C-N bond for thioformamide and N-chloroacetamide is reported along with changes in the charges on atoms and dipolemoment of molecules with rotation. The trans form of N-methyl thioformamide is found to be more stable than cis isomer by 4.9 Kcal/mole.     

Theoretical investigation on surface‐enhanced Raman evidence for conformation transition of dimercaptoazobenzene adsorbed on gold nanoclusters

Probing and controlling the configurations and mechanical motions of the azobenzenes adsorbed on the metal substrates are preliminary for their applicability in various functional devices. In this work, we presented a detailed investigation of Raman properties of the dimercaptoazobenzene (DMAB) bound to gold nanoclusters using density functional calculations. It is demonstrated that the spectral features of the trans conformation of DMAB are quite different from the cis conformation, and the Raman intensities of the trans‐DMAB are much larger. Magnitude of chemical enhancement for the adsorbed trans‐DMAB is found to be close to or less than that for the adsorbed cis‐DMAB for the molecule–cluster complexes. This change trend can be, to a large extent, governed by the energy difference between the highest occupied energy level of the molecule and the lowest unoccupied energy level of the gold. Moreover, it is further demonstrated that differences in Raman intensities of the two conformations can be amplified for the cluster–molecular–cluster junctions, and thus chemical enhancement is much larger for the trans conformation than the cis conformation, possibly facilitating the experimental identification of the trans/cis DMAB. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic studies on laser-induced gratings in azobenzene-doped polymer film

Studies on the dynamic behaviours of laser-induced gratings through two-photon absorption and single-photon absorption were carried out in an azobenzene-doped PMMA film. The experimental results exposed the structures of energy levels and the relaxation between them, in which different lifetimes of the triplet states of trans and cis isomers of azobenzene molecule were recognized. The temporal characteristics of the diffracted signal from the induced-gratings showed also the dynamic processes of the photoisomerization reaction between two isomers.     

First-principles study of the electronic transport properties of the anthraquinone-based molecular switch

By applying non-equilibrium Green’s function (NEGF) formalism combined with first-principles density functional theory (DFT), we have investigated the electronic transport properties of the anthraquinone-based molecular switch. The molecule that comprises the switch can be converted between the hydroquinone (HQ) and anthraquinone (AQ) forms via redox reactions. The transmission spectra of these two forms are remarkably distinctive. Our results show that the current through the HQ form is significantly larger than that through the AQ form, which suggests that this system has attractive potential application in future molecular switch technology.     

First-principles study on the transport properties of phenyl dithiol oligomers

We present first-principles studies on the transport properties of oligomers, polyphenyl dithiol, PP(n)DTs, sandwiched between two Al(1 0 0) electrodes. The variation of the current-voltage curve for PP(2)DT in different tilt angles is investigated systematically. The results indicate that PP(2)DT can be functioned as a molecular switch controlled by molecular conformation. We also study the variation of the equilibrium conductance and current-voltage properties of PP(n)DT as a function of n.     

Far-off resonance conditional phase-shifter using the ac Stark shift

We suggest a conditional phase-shifter that achieves a phase shift of π radians between two weak laser beams with a total energy density on the level of 1000 photons per atomic cross-section. The two laser beams interact through the simple nonlinear technique of ac Stark shifting the common ground state of a V-type system. We find that this switch can operate in the far-off resonance regime, with low absorption and high phase accumulation. Additionally, the bandwidth of this switch can be increased independently of the energy requirement.     

用1H-NMR-Eu (fod)3位移谱研究3-(2,2,2-三氯乙烯基)-2,2-二甲基环丙烷羧酸甲酯的立体构型

本文用¹H-NMR-Eu （fod）₃位移谱鉴定了(1RS)-3-(2,2,2-三氯乙烷基)-2,2-二甲基环丙烷羧酸甲酯的顺、反异构体,测出其三元碳环质子的耦合常数³J=5.5Hz (顺式)、4Hz (反式),并以此确定构型。根据质子极限位移的测定值与假接触位移公式,求出两种配位物合理的配位参数,并由此计算出其顺、反异构体的羰基与Eu（fod）₃配位能力的比值。     

Isomerization mechanisms from cis to trans form in polyacetylene

Mechanisms of cis-trans irreversible isomerizations occurring in both thermal treatment and doping of polyacetylene are presented and examined by means of MINDO/3 molecular orbital calculations on some finite polyenes. It is predicted that the thermal isomerization proceeds along the internal rotation of (-CH=CH-) unit around two carbon-carbon double bonds via the transition state having a biradical nature. Meanwhile, the isomerization by doping is rationalized by the mechanism that the cis segment between two doping sites is converted into the trans-cisoid form which rotates easily around two single bonds to yield some trans forms in (CH)_x chain.     

Rotational barrier and thermodynamical parameters of furfural,thiofurfural, and selenofurfural in the gas and solution phases: theoretical study based on density functional theory method

This work aims to study the trans and cis conformers of furfural, thiofurfural and selenofurfural in the gas and solution phases. Assuming that there is equilibrium between these conformers, the transition state has also been investigated. All computations have been done using density functional theory method with B3LYP as the functional and 6-311++G(d,p) as the basis sets. The optimized molecular structures and related parameters of these conformers are reported. The infrared wavenumbers and Raman activities of these conformers are also reported with appropriate assignments. The energy differences between the trans and cis conformers, associated barriers and thermodynamical parameters have been derived from the computations. It is found that the structural parameters are not much different in the gas and solution phases. However, in the gas phase, the trans conformer is always more stable, but increasing the polarity of the solvent leads to the cis conformer becoming more stable. The rotational barrier is always larger than the energy difference and both of them increase when the solvent becomes more polar. Some of the results for furfural compare satisfactorily with literature and therefore the data from this work will be useful for thiofurfuraldehyde and selenofurfuraldehyde, as their literature is limited.     

A possible salicylideneanilines-based optical molecular switch induced by a reversible hydrogen transfer: an ab initio study

The electronic transport properties of the salicylideneanilines-based molecular optical switch are investigated using a nonequilibrium Green's function formalism combined with first-principles density functional theory. The molecule that comprises the switch can convert between the enol and keto tautomeric forms upon photoinduced excited state hydrogen transfer in the molecular bridge. Theoretical results show that the current through the enol form is significantly larger than that through the keto form, which realize the on and off states of the molecular switch. The physical origin of the switching behaviour is interpreted based on the spatial distributions of molecular orbitals and the HOMO-LUMO gap. Furthermore the effect of the donor/acceptor substituent on the electronic transport through the molecular device is also discussed in detail. The switching performance can be improved to some extent through the acceptor substituent.     

Reversible electron-induced cis-trans isomerization mediated by intermolecular interactions

Reversible isomerization processes are rarely found when organic molecular switches are adsorbed on metal surfaces. One obstacle is the large energy difference of the isomeric forms, since usually the most planar conformer has the largest adsorption energy. In the example of an imine derivative, we show a strategy for also stabilizing the non-planar isomer by intermolecular bonding to its neighbors. Tunneling electrons from the tip of a scanning tunneling microscope can then be used to induce reversible switching between the trans?and cis-like state. Supported by model force-field calculations, we illustrate that the most probable cause of the enhanced stability of the three-dimensional cis?state at specific adsorption sites is the electrostatic interaction with N sites of the neighboring molecule.     

The Studies of 13C NMR Chemical Shifts and Induced Lanthanide Shift Reagent Cis-trans Isomerization of N-Cinamylidene-hexyl,t-butyl and Substituted Aryl-Amines

The Carbon-13 chemical shifts of N-Cinamylidenehexyl, t-butyl and sixteen substituted aryl amines were analysed. The lanthanide induced carbon-13 chemical shifts of the cis and trans of four N-cinamylidene substituted anilines were analysed. The spectra N-cinamylidene hexylamine showed only the trans form. The cis/trans ratio of N-cinamylidene aryl amines increased as the LSR concentration increased.     

Steady-state fluorescence and photo-isomerization study of 1-methyl-4′-(p-N,N dimethyl-amino styryl) pyridinium iodide

Photochemical and thermal trans/cis isomerization reactions are reported for 1-methyl-4′-(p-N,N dimethyl-amino styryl) pyridinium iodide, Cy, which is synthesized in the trans configuration. In a basic solution the trans form, Cy_tr, cannot isomerise directly to the cis form. Its protonated form, Cy_trH⁺, is active and reacts photochemically from trans to cis isomer, Cy_cH⁺. The quantum yields Φ_tc and Φ_ct are determined in water. Deprotonation process of Cy_cH⁺ yields the cis isomer, Cy_c, which can thermally revert to the stable trans form. The rate constant and the activated parameters of the thermal reaction are also determined. Due to irreversibility of the thermal reaction , a complete molecular reaction cycle is performed in one direction. To get more information on the spectral properties of protonated form, its absorption and fluorescence spectra were investigated in sixteen neat polar protic and aprotic solvents. Absorption energy correlates linearly with hydrogen bond acceptor ability of the solvent. Another linear correlation was found between fluorescence energy of CyH⁺ and free energy for transferring the proton to the surrounding solvent, ΔG_t^o.     

Design and simulation of a novel 32 × 32 photonic bandgap power switch based on SOI waveguide

In this paper, we propose a novel 32 × 32 photonic bandgap (PBG) power switch based on silicon-on-insulator (SOI) substrate. The 32 × 32 PBG power switch merges the design concept of PBG structure and multimode interference (MMI) structure. With controlling the voltages of 256 blocks for changing the refractive index, we can demonstrate that the light propagation direction can be switched to the assigned output port as a 32 × 32 power switch. According to the simulations, the power switching control tables are searched and selected for successful switching. The size of our designed 32 × 32 PBG switch can be reduced to 252 × 6040 μm which is much smaller than the conventional opto-electronic switches.     

Theoretical analysis of the tautomer composition of cytosine isolated in an Ar matrix

The IR spectra of cytosine and its five tautomers are calculated and analyzed in the B3LYP/6-31G(d) approximation. Comparison of calculated and experimental spectra showed that, in the isolated state, there occur four tautomeric forms, cis, trans, trans-amino-hydroxy, and cis-imino-oxo, as well as one canonical form of cytosine.     

CNDO calculations of some thioamides—III

CNDO calculations are made for thioacetamide, N-methyl-thioacetamide (both in cis and trans forms) and N, N dimethylthioacetamide. The charges, bond orders and dipole moments of the molecules are discussed. The contributions of changes in pi-energy and monoatomic energy to the barier to internal rotation about C-N bond in thioacetamide are calculated. The assignments of first and second ionization potentials and photoelectron spectra of thioamides are discussed.     

Spectroscopic investigations of a novel push-pull azo compound embedded in rigid polymer

A new heterocyclic push-pull azo compound-in-poly(methymethacrylate) (PMMA) film has been made by means of the spin-coating method. The spectroscopic properties of the films have been investigated with the steady-state absorption spectra, and steady-state fluorescence and femtosecond time-resolved fluorescence spectra in the first time, which is an important characteristic for the application of the film. The excited singlet (S₁) state lifetimes for trans and cis isomers of the film at room temperature have been measured. The excited triplet (T₁) state lifetime of cis isomer of the film has been obtained. The electronic structure of the film has been explained. The results show that the aggregate state of the azo molecules greatly influences its absorption spectra.