Nonlinear optical properties of polydiacetylene with donor-acceptor substitution block

The elongation finite-field (elongation-FF) method is applied to donor/acceptor substituted polydiacetylenes (PDAs) for the estimation of substituent effects on nonlinear optical (NLO) properties. The first hyperpolarizability (beta) and the second hyperpolarizability (gamma) of PDA with separated donor and acceptor substitution blocks have much larger values than those of the other substituted PDAs. For the PDAs with donor and acceptor substitution blocks, the relationship between the NLO properties and the block period is examined. It is shown, from the local density of states, that gamma of a system with a quantum well structure has a maximum value at a certain block size. This indicates that by tuning the size of proper block it is possible to achieve the largest gamma value in block polymers. Furthermore, the through-space/bond interaction analysis is performed to examine the pi-conjugation effects on the NLO properties for particular substituted PDA. It is demonstrated by our quantitative analysis that beta is affected by electron transfers in the molecule, and the quantum well structure is critical for gamma improvement.     

Monitoring biological membrane-potential changes: a CI QM/MM study

In recent decades, new less-invasive, nonlinear optical methods have been proposed and optimized for monitoring fast physiological processes in biological cells. One of these methods allows the action potential (AP) in cardiomyocytes or neurons to be monitored by means of second-harmonic generation (SHG). We now present the first, to our knowledge, simulations of the dependency of the intensity of the second harmonic (I(SHG)) on variations of the transmembrane potential (TMP) in a cardiomyocyte during an action potential (AP). For this, an amphiphilic potential-sensitive styryl dye molecule with nonlinear optical properties was embedded in a dipalmitoylphosphatidylcholine (DPPC) bilayer, replacing one of the phospholipid molecules. External electrical fields with different strengths were applied across the membrane to simulate the AP of a heart-muscle cell. We used a combined classical/quantum mechanical approach to model the structure and the spectroscopic properties of the embedded chromophore. Two 10 ns molecular dynamics (MD) simulations provided input geometries for semiempirical molecular orbital (QM/MM) single-point configuration interaction (CI) calculations, which were used to calculate the wavelengths and oscillator strengths of electronic transitions in the di-8-ANEPPS dye molecule. The results were then used in a sum-over-states treatment to calculate the second-order hyperpolarizability. The square of the hyperpolarizability scales with the intensity of the second harmonic, which is used to monitor the action potentials of cardiomyocytes experimentally. Thus, we computed changes in the intensity of the second harmonic (DeltaI(SHG)) as function of TMP changes. Our results agree well with experimental measurements.     

Numerical coupled Liouville approach: Application to second hyperpolarizability of molecular aggregate

In our previous study [Int. J. Quant. Chem., to appear], we have developed a novel numerical calculation scheme for a dynamics of quantum network for linear molecular aggregates under intense time‐dependent electric fields. In this approach, each molecule is assumed to be an electric dipole arranged linearly with an angle from the longitudinal axis, and the molecular interactions are taken into account by adding the radiations from these dipoles to the external electric fields. The effects of the radiations from all the dipoles involve the intermolecular distance, the speed of light, retarded polarization, and its first‐ and second‐order time derivatives at the position of each dipole. The quantum dynamics is performed by solving coupled Liouville equations composed of the Liouville equation for each dipole. In the present study, we develop a calculation approach of nonperturbative second hyperpolarizability γ in our novel approach and examine the γ of dimer models composed of two‐state molecules under the one‐photon near resonant intense laser fields. Similar phase transition‐like behavior in the field‐intensity dependence of the γ is observed. We also investigate the second hyperpolarizability spectra in the three‐photon resonant region for dimers composed of three‐state molecules, which mimic the electronic states of allyl cation. Contrary to the one‐photon resonant case, phase transition‐like behavior is not observed in the intensity dependence of γ in the three‐photon resonant region. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 295–306, 1999     

The first hyperpolarizability of p-nitroaniline in 1,4-dioxane: a quantum mechanical/molecular mechanics study

In this work we have investigated the first hyperpolarizability of pNA in 1,4-dioxane solution using a quantum mechanics/molecular mechanics (QM/MM) model. The particular model adopted is the recently developed discrete solvent reaction field (DRF) model. The DRF model is a polarizable QM/MM model in which the QM part is treated using time-dependent density-functional theory and local-field effects are incorporated. This allows for direct computation of molecular effective properties which can be compared with experimental results. The solvation shift for the first hyperpolarizability is calculated to be 30% which is in good agreement with the experimental results. However, the calculated values, both in the gas phase and in solution, are by a factor of 2 larger than the experimental ones. This is in contrast to the calculation of the first hyperpolarizability for several small molecules in the gas phase where fair agreement is found with experimental. The inclusion of local-field effects in the calculations was found to be crucial and neglecting them led to results which are significantly larger. To test the DRF model the refractive index of liquid 1,4-dioxane was also calculated and found to be in good agreement with experiment.     

The polarizability and the second hyperpolarizability of tetrakis(phenylethynyl)ethene and several of its lithiated derivatives

The polarizability (α) and the second hyperpolarizability (γ) of tetrakis(phenylethynyl)ethene (TPEE) are compared and analyzed in connection with the properties (α, γ) of a series of selected/designed molecules having different conjugation patterns. Several lithiated derivatives of TPEE are designed and shown to have very enhanced second hyperpolarizabilities; for example, one of the lithiated TPEE has a 1.6×10³ times larger second hyperpolarizability than that of benzene. The potential of the proposed derivatives for applications in photonics is noted. The polarizabilities and the hyperpolarizabilities of the considered molecules have been computed employing the PM3 method which has been proven to be adequate for the present comparative study. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 177–187, 1999     

Structure of organized organic mono- and multi-layer films

A review is presented of some interesting structural properties of monomolecular layers at the air/water interface (Langmuir films) and of organized multilayer structures of organic molecules (Langmuir-Blodgett films). In particular multilayer films of bipolar lipids from archaebacteria and of proteins are considered, together with single-electron junction quantum effects in parallel, special emphasis is given to some recently developed experimental techniques based on X-ray scattering. supramolecular assemblies consisting of multilayers containing nanoparticles.     

Amphoteric doping of carbon nanotubes by encapsulation of organic molecules: electronic properties and quantum conductance

In order to investigate and optimize the electronic transport processes in carbon nanotubes doped with organic molecules, we have performed large-scale quantum electronic structure calculations coupled with a Green's function formulation for determining the quantum conductance. Our approach is based on an original scheme where quantum chemistry calculations on finite systems are recast to infinite, non-periodic (i.e., open) systems, therefore mimicking actual working devices. Results from these calculations clearly suggest that the electronic structure of a carbon nanotube can be easily manipulated by encapsulating appropriate organic molecules. Charge transfer processes induced by encapsulated organic molecules lead to efficient n- and p-type doping of the carbon nanotube. Even though a molecule can induce p and n doping, it is shown to have a minor effect on the transport properties of the nanotube as compared to a pristine tube. This type of doping therefore preserves the intrinsic properties of the pristine tube as a ballistic conductor. In addition, the efficient process of charge transfer between the organic molecules and the nanotube is shown to substantially reduce the susceptibility of the pi electrons of the nanotube to modification by oxygen while maintaining stable doping (i.e., no dedoping) at room temperature.     

The effects of geometry on the hyperpolarizability

Extensive studies in the past have focused on precise calculations of the nonlinear-optical susceptibility of thousands of molecules. In this work, we use the broader approach of considering how geometry and symmetry alone play a role, irrespective of molecular constraints. We investigate the nonlinear optical response of potential energy functions that are given by a superposition of force centers (representing the nuclear charges) that lie in various planar geometrical arrangements. We find that for certain specific geometries, such as an octupolarlike molecule with donors and acceptors of varying strengths at the branches, the hyperpolarizability is near the fundamental limit. In these cases, the molecule is observed to be well approximated by a three-level model, consistent with the three-level ansatz previously used to calculate the fundamental limits. However, when the hyperpolarizability is below the apparent limit (about a factor of 30 below the fundamental limit), the system is no longer representable by a three-level model, where both two-level and many-state models are found to be appropriate, depending on the symmetry.     

Three-dimensional visualization of the first hyperpolarizability tensor

With polarization dependent second harmonic generation (SHG) microscopy becoming a more popular method for investigating the structure of biological materials, there is a need to develop tools with which to understand and interpret the observed SHG properties. Quantum mechanical calculations of the hyperpolarizability tensor have become a popular method for understanding the SHG properties of biomolecules. Visualization of the full hyperpolarizability tensor, termed the unit sphere representation, has been developed to provide insight and intuition on the relationship between SHG properties and molecules. A single vector representation is also presented, which approximates the SHG properties of molecules for certain cases, where the anisotropy is negligible.     

Molecular dynamics simulations applied to electric field induced second harmonic generation in dipolar chromophore solutions

Electric field induced second harmonic generation (EFISH) is an important experimental technique in extracting the first hyperpolarizability of an organic chromophore molecule. Such experiments are carried out in solutions with chromophore molecules dissolved in some common solvents. A known fact is that the first hyperpolarizabilities extracted from EFISH experiments are subject to the use of local field factors. In this work, we apply simulations to study the EFISH properties of chromophore solutions. By combining quantum chemistry calculations with the results derived from molecular dynamics simulations, we show how macroscopic EFISH properties can be modeled, using 4-(dimethylamino)-4'-nitroazobenzene dissolved in chloroform as a demonstration case. The focus of the study is on deriving accurate local field factors. We find that the local field approach applies very well to dipolar solutions, such as the one studied here, but that the local field factors derived are much smaller than the commonly used Onsager or Lorentz local field factors. Our study indicates that many of the reported first hyperpolarizabilities for dipolar molecules from EFISH experiments are most probably underestimated because the Onsager/Lorentz approach, commonly used in extracting the molecular first hyperpolarizability, neglects the effects of the shapes of dipolar chromophore molecules on the local field factors.     

Nonlinear response properties of atomic hydrogen under quantum plasma environment: A time-dependent variation perturbation study on hyperpolarizability and two-photon excitations

Pilot calculations on the frequency-dependent nonlinear response property, viz. the electric dipole hyperpolarizability of atomic hydrogen under quantum plasma environment, have been performed using an external oscillatory electric field. Fourth-order perturbation theory within a variational scheme is adopted to obtain the hyperpolarizability within and beyond normal dispersion region. Two-photon absorption from the ground state is explicitly obtained from the pole positions of nonlinear response of the system and studied up to principal quantum number n = 4 . Ground and perturbed wave functions of appropriate symmetries are represented by linear combination of Slater-type orbitals. Exponential cosine-screened Coulomb potential is used to simulate the quantum plasma environment. With respect to plasma strength, the nonlinear response properties are considerably enhanced. Results are compared with those under classical plasma environment represented by screened Coulomb potential. Departure from Coulomb potential results in lifting of the accidental degeneracy in the respective two-photon excited states beyond n = 2 . For free hydrogen atom, the transition energies and the radial density profiles of the respective two-photon excited states match exactly with those obtained from analytical wave functions.     

The molecular orientation of para-sexiphenyl on Cu(110) and Cu(110) p(2x1)O.

Controlling the molecular growth of organic semiconductors is an important issue to optimize the performance of organic devices. Conjugated molecules, used as building blocks, have an anisotropic shape and also anisotropic physical properties like charge transport or luminescence. The main challenge is to grow highly crystalline layers with molecules of defined orientation. The higher the crystallinity, the closer these properties reach their full intrinsic potential, while the orientation determines the physical properties of the film. Herein we show that the molecular orientation and growth can be steered by the surface chemistry, which tunes the molecule-substrate interaction. In addition, the oxygen reconstruction of the surface, demonstrates the flexibility of the organic molecules to adopt a given surface corrugation and their unique possibility to release stress by tilting.     

Polarizability and first-order hyperpolarizability of cyclic imides

Dipole moment, polarizability, and first-order hyperpolarizability of cyclic imides (maleimide, succinimide, phthalimide and some of their derivatives) have been investigated using ab initio and density functional theory calculations. It is found that 4,5-dichloro-, and 3,4,5,6-tetrachlorophthalimide have highest mean polarizabilities and total hyperpolarizabilities among the studied molecules. Furthermore, polarized continuum model has been employed to investigate solvent effects on the nonlinear optical (NLO) properties of succinimide; results indicate that solvent polarity has considerable influence on the NLO response of the molecules.     

Effect of strong electron correlation on the efficiency of photosynthetic light harvesting

Research into the efficiency of photosynthetic light harvesting has focused on two factors: (1) entanglement of chromophores, and (2) environmental noise. While chromophores are conjugated π-bonding molecules with strongly correlated electrons, previous models have treated this correlation implicitly without a mathematical variable to gauge correlation-enhanced efficiency. Here we generalize the single-electron/exciton models to a multi-electron/exciton model that explicitly shows the effects of enhanced electron correlation within chromophores on the efficiency of energy transfer. The model provides more detailed insight into the interplay of electron correlation within chromophores and electron entanglement between chromophores. Exploiting this interplay is assisting in the design of new energy-efficient materials, which are just beginning to emerge.     

含1,3-二硫-2叶立德烯的系列有机发色团的二阶非线性光学性质的理论研究

设计了12个含有1,3-二硫-2叶立德烯的具有二阶非线性光学性质的有机发色团,对所研究分子用AM1方法进行几何构型优化,用INDO/CI方法获得基态到各激发态的垂直跃迁能和振子强度,即电子光谱;在此基础上用SOS公式计算系列分子的二阶非线性光学系数β_ijk,并从微观上对这一系列分子进行比较,为进一步探索综合性能较好的NLO有机物提供了理论指导.     

Acido-triggered nonlinear optical switches: benzazolo-oxazolidines

This work is the continuation of our previous experimental and theoretical studies aiming at designing efficient nonlinear optical (NLO) switches derived from the benzazolo-oxazolidine core. Here, we report the synthesis and the characterization of the linear and nonlinear optical properties of benzothiazolo[2,3-b]oxazolidine acidochromes by means of hyper-Rayleigh scattering as well as quantum chemical calculations. It is shown that these new derivatives incorporating a benzothiazole subunit exhibit very high static first hyperpolarizability values in their acido-generated form. On the basis of previously reported NLO responses of indolino- and benzimidazolo-oxazolidines, structure-properties relationships within the benzazolo-oxazolidine series are proposed.     

Rational enhancement of second-order nonlinearity: bis-(4-methoxyphenyl)hetero-aryl-amino donor-based chromophores: design, synthesis, and electrooptic activity

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborator's, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.     

Quantum tunneling of magnetization and related phenomena in molecular materials

Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.