Theoretical investigation of the second-order nonlinear optical properties of helical pyridine-pyrimidine oligomers

The structure and second-order nonlinear optical properties of a series of helical pyridine-pyrimidine oligomers recently synthesized by Barboiu and Lehn have been investigated theoretically by combining molecular mechanics and quantum chemistry approaches. In the absence of substituents, the hyper-Rayleigh scattering response (betaHRS) and the projection of the first hyperpolarizability on the dipole moment (beta||) exhibit periodic variations with chain length, but these nonlinear optical responses remain small. The first hyperpolarizabilities can, however, be enhanced by adding substituents. The greatest enhancement is obtained by substituting the pyrimidine groups by donor groups. Moreover, regular distributions of the donor groups around the helices enable the design of supramolecular structures exhibiting dipolar, octupolar or Lambda-shaped nonlinear optical characters, evident from the values of the depolarization ratios in hyper-Rayleigh scattering. Therefore this theoretical investigation demonstrates the potential of helical structures for the organization of chromophores in such a way that they exhibit large and specific second-order nonlinear optical responses.     

Second-order Nonlinearities of CdS Nanoparticles Studied by Hyper-Rayleigh Scattering Technique

Optical nonlinearities of semiconductor nanoparticles are of great interest recently. So fartheir third-order nonlinear optical (NLO) properties have been widely studied. However,there are only few studies on second-order NLO properties, because it is believed that thecentrosymmetry or near-centrosymmetry of spherical nanoparticles eliminate their firstorder hyperpolarizability 6 values to'zero or near zero. And for a long time it remains aproblem to directly study the second-order=NLO pro…     

Alkynyl expanded donor-acceptor calixarenes: geometry and second-order nonlinear optical properties

A number of wide- and narrow-rimmed functionalized alkynylcalix[4]arenes have been synthesized by Sonogashira coupling. With respect to their optical properties, these donor-acceptor systems are treated as ensembles of covalently linked, electronically independent tolane subchromophores. Linear UV/visible and fluorescence spectroscopic investigations revealed that the charge-transfer character of the electronic transitions in calixarenes, and also the second-order nonlinear optical (NLO) properties depend on the electron-withdrawing nature of the terminal ethynylphenyl substituent (NO(2), CF(3), H). The nitro derivatives display high values of the quadratic hyperpolarizability beta. Not only do the (nonlinear) optical properties of the target compounds depend on the number and relative disposition of the subchromophores, but also on the geometry of the calixarenes. In particular, the opening angle of the calixarene cavity can be determined by the substitution pattern of the calixarene scaffold (wide- versus narrow-rim substitution) and the number of the acetylene functions introduced. Both the NLO properties and the conformational issues are conveniently assessed by using hyper-Rayleigh scattering (HRS) in solution, and supported by X-ray crystallography in the solid state.     

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.     

Ab initio investigation on the second-order nonlinear optical responses in keto-enol equilibria of salicylideneanilines

The keto-enol (K-E) tautomerization equilibrium, more precisely, the keto-amine/enol-imine equilibrium, has been investigated for a series of substituted salicylideneanilines in view of designing compounds with a contrast of second-order nonlinear optical properties. Substituting the salicylidene ring by an acceptor group or the other ring by a donor prevents the K form from being stable, whereas in the other cases, the K form can easily be converted to the E form due to the small activation barrier, figuring out in most cases that the K form is metastable. For a representative set of donor/acceptor substituents, the E and K forms present a sufficiently large contrast of beta to allow its detection by using electric-field-induced second harmonic generation or hyper-Rayleigh scattering. The largest beta values are mainly associated with species bearing a donor in the para position of the salicylidene ring and an acceptor on the other ring whereas the largest beta values are generally found for the E form.     

Linear and nonlinear optical properties of a macrocyclic trichromophore bundle with parallel-aligned dipole moments

A macrocyclic trichromophore bundle 1 with parallel-aligned dipole moments has been synthesized to study the influence of aggregation and orientation of a nonlinear optical (NLO) chromophore on its optical properties. The linear and nonlinear optical properties of 1 and a single chromophore standard 2 have been studied by UV-vis absorption, fluorescence, solvatochromic spectrometry, and hyper-Rayleigh scattering (HRS). Reduced first-order hyperpolarizability beta, hypsochromic shift, enhanced solvatochromic shifts, and fluorescence quenching for individual chromophores were observed when 1 was compared with 2. Analysis of the data showed that the transition dipole moment changes only slightly when the chromophores are parallel aligned in the bundle architecture. However, the apparent hyperpolarizability of the individual chromophores decreased significantly by about 20%. The reduction in beta for the individual chromophores in 1 is largely due to the hypsochromic shift, i.e., excitation energy increase of the interband (charge-transfer) energy gap and the reduced difference between the ground-state and excited-state dipole moments. The hypsochromic shift and fluorescence quenching are consistent with exciton theory. Possible reasons for the enhanced solvatochromic shift are discussed.     

Synthesis and nonlinear optical properties of linear and Λ-shaped pyranone-based chromophores

New, pyranone-based chromophores were synthesized and their (nonlinear) optical properties were measured. The chromophores were prepared by first condensing an electron withdrawing group with pyranone, followed by reaction with an aldehyde-functionalized π-conjugated bridged donor molecule. This approach enables one to easily incorporate the pyranone moiety and to prepare both linear and Λ-shaped chromophores. The (nonlinear) optical properties were measured using femtosecond hyper-Rayleigh scattering. These measurements demonstrated the advantages of this approach.     

Synthesis of 2-aryl-5-styrylphospholes: promising candidates for the phosphole-based NLO chromophores

2-Aryl-1-phenyl-5-styrylphospholes were prepared in good yields in a one-pot procedure from the corresponding 1,9-diarylnona-8-ene-1,6-diynes and dichloro(phenyl)phosphine via intermediary titanacyclopentadienes. According to the UV-vis absorption and fluorescence spectra of this class of compounds, the optical properties of the phosphole-vinylene-bridged pi-conjugated system have been revealed to depend strongly on the electronic character of the terminal functionalities. In particular, the polarizability at the excited-state has been found to be considerably greater than that in the ground state. High molecular hyperpolarizabilities obtained for the push-pull type of 2-aryl-5-styrylphospholes in the hyper-Rayleigh scattering measurements demonstrate the potential utility of the stilbene-type phosphole derivatives as a new class of second-order NLO chromophores.     

Gold-nanorod-based sensing of sequence specific HIV-1 virus DNA by using hyper-Rayleigh scattering spectroscopy

Infectious diseases caused by the human immunodeficiency virus (HIV) remain the leading killers of human beings worldwide, and function to destabilize societies in Africa, Asia, and the Middle East. Driven by the need to detect the presence of HIV viral sequence, here we demonstrate that the second-order nonlinear optical (NLO) properties of gold nanorods can be used for screening HIV-1 viral DNA sequence without any modification, with good sensitivity (100 pico-molar) and selectivity (single base-pair mismatch). The hyper-Rayleigh scattering (HRS) intensity increases 45 times when a label-free 145-mer, ss-gag gene DNA, was hybridized with 100 pM target DNA. The mechanism of HRS intensity change has been discussed with experimental evidence for higher multipolar contribution to the NLO response of gold nanorods.     

Hyper-Rayleigh scattering measurements of nonlinear optical chromophores at 1907 nm

Hyper-Rayleigh scattering measurements are performed on nonlinear optical chromophores using an excitation wavelength of 1907 nm. This wavelength is the longest that has yet been reported for the hyper-Rayleigh scattering experiment and allows measurements of the first hyperpolarizability of chromophores that are free of contributions from two-photon absorption induced fluorescence and reduces one and two photon resonance enhancement to a minimum. Using the 1907 nm setup we demonstrate good agreement between our results and those obtained with the electric field induced second harmonic generation (EFISHG) technique previously performed at this wavelength.     

Theoretical understanding of the increment of beta upon protonation of pyridine peripheral octupolar molecules: toward nonlinear optical sensors

In this article, we present a computational study of the nonlinear optical properties of pyridine-based octupolar molecules in their neutral and fully triprotonated states. The effect of the core substitution and the degree of conjugation with the periphery has been also established and confirms the possibility of fine-tuning the nonlinear optical response. Computations involving the time-dependent density-functional theory approach serve to further explore the existence of excited states with nonzero dipole moment. From these results, the origin of the high second-order nonlinear optical activity upon protonation is addressed.     

Surface forces and drag coefficients of microspheres near a plane surface measured with optical tweezers

Optical tweezers are widely used to measure molecular forces in biology. Such measurements are often influenced by a nearby surface that can perturb both the calibration of the tweezers as well as the hydrodynamic forces acting on microspheres to which the biomolecules are attached. In this study, we have used a very stable optical tweezers setup employing a recently developed calibration method (Toli?-N?rrelykke, S. F.; Sch?ffer, E.; Howard, J.; Pavone, F. S.; Jülicher, F.; Flyvbjerg, H. Rev. Sci. Instrum. 2006, 77 (10), 103101) to determine how the calibration of the tweezers and the forces on the microspheres depend on the height above the surface. We show that the displacement sensitivity of the tweezers is modulated by a standing light wave between the microsphere and the surface. We measured the dependence of the drag coefficient on height and compared it to exact and closed-form solutions to the Navier-Stokes equations. Also, we measured the surface force gradients in different salt solutions and for different surface blocking methods. For a given blocking method, our data suggest that microspheres can experience attractive and/or repulsive forces close to surfaces. For example, a Teflon layer reduces attractive interactions, and the presence of casein can lead to long-range repulsive interactions. These measurements are a prerequisite for the accurate measurement of normal forces with respect to an interface that occur in biological molecules held between surfaces.     

SCC-DFTB: what is the proper degree of self-consistency?

The approximate SCC-DFTB method (Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, Th.; Suhai, S.; Seifert, G. Phys. Rev. B 1998, 58, 7260) is derived from DFT by a second-order expansion of the total energy expression. In this article, basic approximations and assumptions underlying the DFTB method are discussed in detail, and further extensions to include third-order terms are proposed. Further, the SCC-DFTB and semiempirical NDDO formalisms are compared to elucidate similarities and differences.     

Theoretical and experimental study of optical second harmonic generation in new chiral thiolates salts

The second harmonic generation ability of new chiral thiolate salts is investigated. Aromatic thiolate anions ArS⁻ are expected from semi-empirical calculations with PM3 parameterization of the MNDO Hamiltonian to possess higher intrinsic polarizabilities than the parent neutral thiols ArSH.

Salts associating (hyper)polarizable aromatic thiolate anions with chiral cations, which ensure the noncentrosymmetry necessary to have second-order nonlinear optical effects have been synthesized. The first hyperpolarizabilities, β, of the salts are measured using hyper-Rayleigh scattering technique. Second harmonic generation powder tests (Kurtz and Perry method) carried out on various salts studied, were positive indicating that these materials crystallize in noncentrosymmetric space groups.     

Hyper-Rayleigh scattering studies of first-order hyperpolarizability of squarylium dye in solution

We have synthesized a squarylium dye based on 2,6-di-tert-butylpyrylium (full name given in the text). Its nonlinear optical properties was detected by the hyper-Rayleigh light scattering technique in a chloroform solution, although this type of chromophore has a symmetrical structure and a negligible permanent dipole moment. The internal reference method was employed to determine the first hyperpolarizability β value of the chromophore and further discussion is provided.     

Enhancement of Nonlinear Optical Scattering by Gold Nanoparticles through Aggregation-Induced Plasmon Coupling in the Near-Infrared

Gold nanoparticles (AuNPs) are regarded as promising building blocks in functional nanomaterials for sensing, drug delivery and catalysis. One remarkable property of these particles is the localized surface plasmon resonance (LSPR), which gives rise to augmented optical properties through local field enhancement. LSPR also influences the nonlinear optical properties of metal NPs (MNPs) making them potentially interesting candidates for fast, high resolution nonlinear optical imaging. In this work we characterize and discuss the wavelength dependence of the hyper-Rayleigh scattering (HRS) behavior of spherical gold nanoparticles (GNP) and gold nanorods (GNR) in solution, from 850 nm up to 1300 nm, covering the near-infrared (NIR) window relevant for deep tissue imaging. The high-resolution spectral data allows discriminating between HRS and two photon photoluminescence contributions. Upon particle aggregation, we measured very large enhancements (ca. 10⁴) of the HRS intensity in the NIR, which is explained by considering aggregation-induced plasmon coupling effects and local field enhancement. These results indicate that purposely designed coupled nanostructures could prove advantageous for nonlinear optical imaging and biosensing applications.     

Dipolar donor-acceptor-substituted schiff base complexes with large off-diagonal second-order nonlinear optical tensor components

The synthesis, characterization, and two-dimensional second-order nonlinear optical (NLO) response of a dipolar NiII donor- acceptor Schiff base complex and the related ligand are reported. Electric-field-induced second-harmonic generation and harmonic light (hyper-Rayleigh) scattering techniques, in combination with INDO/SCI-SOS theoretical calculations, were used to investigate the vector part of the hyperpolarizability tensor and the two-dimensional character of the molecular nonlinearity, respectively. Off-diagonal hyperpolarizability tensors can be related to charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis, while parallel transitions account for the diagonal hyperpolarizability tensor. The role of the metal center in enhancing the two-dimensional NLO response of such molecules is twofold since it acts both as the donor and the bridging moiety of the planar donor-(pi-conjugate-bridge)-acceptor system. These dipolar two-dimensional molecules are interesting candidates from the perspective of polarization-independent NLO materials.     

Investigating the interaction of crystal violet probe molecules on sodium dodecyl sulfate micelles with hyper-Rayleigh scattering

We report the use of the nonlinear optical technique of hyper-Rayleigh scattering to investigate the interaction of the cationic probe molecule crystal violet with micelles of sodium dodecyl sulfate. An absolute value of (847 +/- 80) x 10(-30) esu is measured at the fundamental wavelength of 870 nm for the molecular hyperpolarizability of crystal violet free in pure aqueous solutions. In aqueous solutions of sodium dodecyl sulfate, above and below the critical micelle concentration, the measured hyperpolarizability of crystal violet is weaker than in the solution free of sodium dodecyl sulfate. From the comparison with linear optical photoabsorption spectroscopy data, this difference is attributed to electrostatic interactions between the cationic crystal violet molecules and the negatively charged sodium dodecyl sulfate surfactant molecules present in excess. Polarization resolved hyper-Rayleigh scattering measurements are then performed to show that, below and above the critical micelle concentration, crystal violet molecules also undergo symmetry changes upon interaction with sodium dodecyl sulfate. Above the critical micelle concentration, the minimum fraction of micelles interacting with at least one CV molecule is estimated. For instance, for a crystal violet aqueous concentration of 150 microM, this fraction is larger than 7%.     

Highly unusual effects of pi-conjugation extension on the molecular linear and quadratic nonlinear optical properties of ruthenium(II) ammine complexes

We have used several techniques, including hyper-Rayleigh scattering and Stark spectroscopy, to investigate the effects of polyene chain length on the optical properties of complexes containing ruthenium(II) electron donor groups and pyridinium electron acceptors. In marked contrast with all other known donor-acceptor polyenes, conjugation extension beyond a single double bond in the dipolar complexes studied leads to blue-shifting of the intramolecular charge-transfer absorptions. Furthermore, the static first hyperpolarizabilities beta0 become maximized with trans-1,3-butadienyl linkages and then decrease in complexes with three CH=CH bonds. Our results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.     

Syntheses and spectroscopic and quadratic nonlinear optical properties of extended dipolar complexes with ruthenium(II) ammine electron donor and N-methylpyridinium acceptor groups

In this paper, we describe the extremely unusual optical properties of Ru(II)-based electron donor-acceptor (D-A) polyene and some closely related chromophores. For three different polyene series, the intense, visible d-->pi* metal-to-ligand charge-transfer bands unexpectedly blue-shift as the number of E-ethylene units (n) increases from 1 to 3, and the static first hyperpolarizabilities beta(0) determined via hyper-Rayleigh scattering and Stark spectroscopy maximize at n = 2, in marked contrast to other known D-A polyenes in which beta(0) increases steadily with n. Time-dependent density-functional theory and finite field calculations verify these empirical trends, which arise from the orbital structures of the complexes. This study illustrates that transition metal-based nonlinear optical chromophores can show very different behavior when compared with their more thoroughly studied purely organic counterparts.