Dual modulation of a molecular switch with exceptional chiroptical properties

The ability to modulate the chiroptical properties of optically active molecules induced by external stimuli such as light, heat, and electrical fields allows for the design and development of molecular switches, memory devices, sensors, and photonic devices. A helical o-terphenyl compound functionalized with photoresponsive azobenzene and electroactive imide groups is designed as a dual-mode chiroptical molecular switch. Its exceptional optical activity (e.g., [alpha]436 = -9500) can be changed and modulated through photoisomerization of the azobenzene moiety using UV and visible light. Reversible modulation by electrochemical means was also achieved through the redox reaction occurring at the imide group. Large chiroptical read-out signals were observed during the redox cycles as indicated by the molar ellipticity values as high as 285,000 deg.cm2.dmol-1. Exceptionally high optical activity and large responses to both light and electrical bias make this chiral molecule suitable for the development of new molecular switches, sensors, and other optical devices.     

Gold nanoparticle networks with photoresponsive interparticle spacings

Photoresponsive gold nanoparticle networks were prepared by functionalizing them with azobenzene derivatives. A network can be formed when a linker molecule constituting the azobenzene moiety suitably derivatized on either side with gold surface sensitive groups such as thiols and amines is added to the nanoparticle solution. It is shown that the interparticle spacing in the networks could be controlled by the reversible trans-cis isomerization of the azobenzene moiety induced by UV and visible light, respectively. The photoinduced variation in the interparticle spacings is inferred by the changes in the optical spectra of the gold nanoparticles which display a red or blue shift in the surface plasmon resonance peak depending on a decrease or increase in the interparticle spacing, respectively. Transmission electron microscopy images are in consonance with the evidence from the optical spectra.     

How can azobenzene block copolymer vesicles be dissociated and reformed by light?

The underlying mechanism of UV light-induced dissociation and visible light-induced reformation of vesicles formed by an azobenzene diblock copolymer was investigated. These processes were studied in situ by monitoring changes in optical transmittance of the vesicular solution while being exposed to UV or visible light irradiation. The results indicate that the UV-induced dissociation of the vesicles results from their thermodynamic instability due to a shift of the hydrophilic/hydrophobic balance arising from the trans-cis isomerization, while their reaggregation takes place upon visible light irradiation that shifts the hydrophilic/hydrophobic balance in the opposite direction after the reverse cis-trans isomerization. The study suggests a specific design principle for obtaining UV light-dissociable and visible light-recoverable vesicles based on azobenzene block copolymers. On one hand, the structure of azobenzene moiety used in the hydrophobic block should have a small (near zero) dipole moment in the trans form and a significantly higher dipole moment in the cis form, which ensures a significant increase in polarity of the hydrophobic block under UV light irradiation. On the other hand, the hydrophilic block should be weakly hydrophilic. The conjunction of the two conditions can make the light-induced shift of the hydrophilic/hydrophobic balance important enough to lead to the reversible change in vesicular aggregation.     

甲氧基苯基偶氮衍生物电子结构与二阶NLO性质的HF/FF方法研究

采用量子化学HF方法在6-31G水平上优化6个甲氧基苯基偶氮衍生物分子的几何构型，利用HF／6-31G。方法计算它们的偶极矩、电荷分布、前线分子轨道能级并结合有限场(FF)方法计算二阶非线性光学系数．结果表明，偶氮苯衍生物分子具有很好的共轭性，在给吸电子基团作用下，电荷转移明显，展现示出较强的极性．偶氮苯衍生物分子与苯乙烯、Schiff碱类衍生物相似，也具有很好二阶非线性光学活性，同时六元杂环取代的偶氮苯衍生物分子二阶非线性光学系数比未取代的大，五元杂环取代结果相反．     

Giant Hysteretic Single‐Molecule Electric Polarisation Switching above Room Temperature

Continual progress has been achieved in information technology through unrelenting miniaturisation of the single memory bit in integrated ferromagnetic, ferroelectric, optical, and related circuits. However, as miniaturisation approaches its theoretical limit, new memory materials are being sought. Herein, we report a unique material exhibiting single‐molecule electric polarisation switching that can operate above room temperature. The phenomenon occurs in a Preyssler‐type polyoxometalate (POM) cluster we call a single‐molecule electret (SME). It exhibits all the characteristics of ferroelectricity but without long‐range dipole ordering. The SME affords bi‐stability as a result of the two potential positions of localisation of a Tb³⁺ ion trapped in the POM, resulting in extremely slow relaxation of the polarisation and electric hysteresis with high spontaneous polarisation and coercive electric fields. Our findings suggest that SMEs can potentially be applied to ultrahigh‐density memory 1 and other molecular‐level electronic devices operating above room temperature. 2     

Optical switching of coupled plasmons of Ag-nanoparticles by photoisomerisation of an azobenzene ligand

The optical switching of coupled plasmons of silver nanoparticles derivatised with a photoisomerisable azobenzene ligand is presented. It is shown that nanoparticle clusters, linked with an azobenzene dithiol molecule, display switchable optical properties. The photoisomerisation of the linker molecule was used to vary the separation between nanoparticles, which was monitored by changes in the UV-Vis-spectra of the plasmon band of adjacent nanoparticles. A red-shift due to the appearance of a coupled longitudinal plasmon band was observed resulting from the formation of nanoparticle clusters. The maximum absorbance wavelength of this secondary plasmon band was altered by isomerisation of the linker and the spectral changes observed were in good agreement with theory and earlier measurements for gold. Evidence of energy transfer between a nanoparticle and an azobenzene terminated monothiol attached to it was also observed in the UV-Vis spectra.     

Photoswitching behavior of a novel single molecular tip for noncontact atomic force microscopy designed for chemical identification

A tripod molecule with an azobenzene arm was designed as a single molecular tip for noncontact atomic force microscopy (NC-AFM). The azobenzene moiety showed photoisomerization that enabled measurements of the same position of the sample by different tip apexes with different interactions. Photoswitching behavior of the molecule synthesized and adsorbed on Au surfaces was examined and reversible switching between the trans- and cis forms was successfully confirmed by NC-AFM measurements.     

Optically Triggered Control of the Charge Carrier Density in Chemically Functionalized Graphene Field Effect Transistors

Field effect transistors (FETs) based on 2D materials are of great interest for applications in ultrathin electronic and sensing devices. Here we demonstrate the possibility to add optical switchability to graphene FETs (GFET) by functionalizing the graphene channel with optically switchable azobenzene molecules. The azobenzene molecules were incorporated to the GFET channel by building a van der Waals heterostructure with a carbon nanomembrane (CNM), which is used as a molecular interposer to attach the azobenzene molecules. Under exposure with 365 nm and 455 nm light, azobenzene molecules transition between cis and trans molecular conformations, respectively, resulting in a switching of the molecular dipole moment. Thus, the effective electric field acting on the GFET channel is tuned by optical stimulation and the carrier density is modulated.     

Comparative studies of the trans-cis photoisomerizations of azobenzene and a bridged azobenzene

Using density-functional-based molecular dynamics simulations, we have performed comparative studies of the trans-cis isomerizations of azobenzene and bridged azobenzene (B-Ab) 5,6-dihydrodibenzo[c,g][1,2]diazocine induced by nπ* electronic excitation. The quantum yields found in our calculations, 45% for the bridged azobenzene versus 25% for azobenzene, are consistent with the experiment. Both isomerization processes involve two steps: (1) Starting from the trans structure, each molecule moves on its S(1) excited-state potential energy surface, via rotation around the NN bond, to an avoided crossing near the S(1)/S(0) conical intersection, where de-excitation occurs. (2) Subsequently, in the electronic ground state, there is further rotation around the NN bond, accompanied by twisting of the phenyl rings around their CN bonds, until the cis geometry is achieved. Because of its lower symmetry and smaller initial CNNC dihedral angle, the bridged azobenzene has a much shorter lifetime for the S(1) excited state, about 30 fs, as compared to about 400 fs for azobenzene. However, we find that the complete isomerizations have approximately the same time scales. Although the bridging feature in trans-B-Ab does not hinder rotation around the NN bond in step 1, it makes twisting of the two phenyl rings around the CN bonds much slower in step 2.     

Can octupolar molecules be poled by an external electric field?

Octupolar molecules are generally believed to be of potential use in developing nonlinear optical materials owing to the fact that they do not easily form molecular aggregates. This is often put against the conjectured drawback that electric fields have no poling, or ordering, effect for this class of molecules because of the lack of a permanent ground state dipole moment. In this paper, we analyze this notion in some detail and present results from molecular dynamics computer simulations of an ensemble of a prototypical octupolar molecule, the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) molecule, dissolved in chloroform. It is found that TATB molecules indeed show rather significant dipole moments in solutions because of the dual action of the thermal motions of the atoms and the strong intermolecular interactions. Applied electric fields accordingly show significant effects on the orientations of the molecular dipole moments. We also find that TATB molecules can aggregate because of the strong hydrogen-bonding interactions between the molecules, though they lack a static permanent dipole moment. Thus, the simulation results for TATB molecules in solution present us with a totally different notion about the collective properties of octupolar molecules. Taking account of quantum chemistry results, we found that the collective molecular nonlinear optical (NLO) properties are enhanced after the onset of the electric field, showing significant anisotropic characteristics.     

On the dipole moments and first-order hyperpolarizability of N,N-bis(4-bromobutyl)-4-nitrobenzenamine

The nonlinear optical molecule N,N-bis(4-bromobutyl)-4-nitrobenzenamine was synthesized. The ground state dipole moment was determined by the Debye-Guggenheim method. A solvent mixture of acetonitrile and toluene was used for the solvatochromic determination of the excited state dipole moment. Excited state has a high value for the dipole moment which indicated a higher degree of charge transfer from the donor to the acceptor moiety on excitation by light. The first hyperpolarizability (beta(ijk)) of the molecule was evaluated assuming the two level model of the first hyperpolarizability.     

Experimental measurement of the induced dipole moment of an isolated molecule in its ground and electronically excited states: indole and indole-H2O

Reported here are measurements of the magnitude and orientation of the induced dipole moment that is produced when an indole molecule in its ground S(0) and electronically excited S(1) states is polarized by the attachment of a hydrogen bonded water molecule in the gas phase complex indole-H(2)O. For the complex, we find the permanent dipole moment values mu(IW)(S(0)) = 4.4 D and mu(IW)(S(1)) = 4.0 D, values that are substantially different from calculated values based on vector sums of the dipole moments of the component parts. From this result, we derive the induced dipole moment values mu(I) (*)(S(0)) = 0.7 D and mu(I) (*)(S(1)) = 0.5 D. The orientation of the induced moment also is significantly different in the two electronic states. These results are quantitatively reproduced by a purely electrostatic calculation based on ab initio values of multipole moments.     

Electromagnetic fields around silver nanoparticles and dimers

We use the discrete dipole approximation to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local electric field (E-field) enhancement near the particle surfaces. The results are used to determine what conditions are most favorable for producing enhancements large enough to observe single molecule surface enhanced Raman spectroscopy. Most of the calculations refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addition, for the dimer calculations we study the influence of dimer separation and orientation, especially for dimers that are separated by a few nanometers. We find that the largest /E/2 values for dimers are about a factor of 10 larger than those for all the monomers examined. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given /E/2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest /E/2 values we have calculated for spacings of 2 nm or more is approximately 10(5).     

Confined plasmons in nanofabricated single silver particle pairs: experimental observations of strong interparticle interactions

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-size effects. The discrete dipole approximation (DDA), which is a finite-element method, describes the experimental results well even at d < 20 nm, including particles that have metallic bridges.     

Molecule‐specific determination of atomic polarizabilities with the polarizable atomic multipole model

Recently, many polarizable force fields have been devised to describe induction effects between molecules. In popular polarizable models based on induced dipole moments, atomic polarizabilities are the essential parameters and should be derived carefully. Here, we present a parameterization scheme for atomic polarizabilities using a minimization target function containing both molecular and atomic information. The main idea is to adopt reference data only from quantum chemical calculations, to perform atomic polarizability parameterizations even when relevant experimental data are scarce as in the case of electronically excited molecules. Specifically, our scheme assigns the atomic polarizabilities of any given molecule in such a way that its molecular polarizability tensor is well reproduced. We show that our scheme successfully works for various molecules in mimicking dipole responses not only in ground states but also in valence excited states. The electrostatic potential around a molecule with an externally perturbing nearby charge also exhibits a near‐quantitative agreement with the reference data from quantum chemical calculations. The limitation of the model with isotropic atoms is also discussed to examine the scope of its applicability. © 2012 Wiley Periodicals, Inc.     

Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy

We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV--visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.     

Environment-dependent ultrafast photoisomerization dynamics in azo dye

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.     

Photoswitchable Adsorption in Metal–Organic Frameworks Based on Polar Guest–Host Interactions

Reversible remote‐controlled switching of the properties of nanoporous metal–organic frameworks (MOFs) is enabled by incorporating photoswitchable azobenzene. The interaction of the host material with different guest molecules, which is crucial for all applications, is precisely studied using thin MOF films of the type Cu₂(BDC)₂(AzoBipyB). A molecule‐specific effect of the photoswitching, based on dipole–dipole interactions, is found.     

Optical properties of current carrying molecular wires

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the nonequilibrium Green's function method we derive expressions for the absorption line shape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge-transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction.