hyper-Rayleigh Scattering Research Articles

Electrochemical, Spectroelectrochemical, and Molecular Quadratic and Cubic Nonlinear Optical Properties of Alkynylruthenium Dendrimers1

A combination of cyclic voltammetry (CV), UV-vis-NIR spectroscopy and spectroelectrochemistry, hyper-Rayleigh scattering (HRS) [including depolarization studies], Z-scan and degenerate four-wave mixing (DFWM) [including studies employing an optically transparent thin-layer electrochemical (OTTLE) cell to effect electrochemical switching of nonlinearity], pump-probe, and electroabsorption (EA) measurements have been used to comprehensively investigate the electronic, linear optical, and nonlinear optical (NLO) properties of nanoscopic pi-delocalizable electron-rich alkynylruthenium dendrimers, their precursor dendrons, and their linear analogues. CV, UV-vis-NIR spectroscopy, and UV-vis-NIR spectroelectrochemistry reveal that the reversible metal-centered oxidation processes in these complexes are accompanied by strong linear optical changes, "switching on" low-energy absorption bands, the frequency of which is tunable by ligand replacement. HRS studies at 1064 nm employing nanosecond pulses reveal large nonlinearities for these formally octupolar dendrimers; depolarization measurements are consistent with lack of coplanarity upon pi-framework extension through the metal. EA studies at 350-800 nm in a poly(methyl methacrylate) matrix are consistent with the important transitions having a charge-transfer exciton character that increases markedly on introduction of peripheral polarizing substituent. Time-resolved pump-probe studies employing 55 ps, 527 nm pulses reveal absorption saturation, the longest excited-state lifetime being observed for the dendrimer. Z-scan studies at 800 nm employing femtosecond pulses reveal strong two-photon absorption that increases significantly on progression from linear complex to zero- and then first-generation dendrimer with no loss of optical transparency. Both refractive and absorptive nonlinearity for selected alkynylruthenium dendrimers have been reversibly "switched" by employing the Z-scan technique at 800 and 1180 nm and 100-150 fs pulses, together with a specially modified OTTLE cell, complementary femtosecond time-resolved DFWM and transient absorption studies at 800 nm suggesting that the NLO effects originate in picosecond time scale processes.

Hyper-Rayleigh scattering and continuum generation of salt induced aggregates of Silver nanoparticles: The effect of cation size (Li +, Na + and K +)

The effect of cation size (Li +, Na + and K +, counterion Cl −) on the hyper-Rayleigh scattering (HRS) and continuum generation capability of salt induced aggregates of silver nanoparticles (AgNPs) was studied using femtosecond laser pulses. It was observed that while on addition of NaCl and KCl, HRS intensities of AgNPs were greatly enhanced (∼50×), upon addition of LiCl this enhancement was rather modest (∼4×). Continuum generation was also observed only from NaCl and KCl induced aggregates. These observations suggest that smaller size and hence higher ionic charge density of the Li + ion is not conducive to the formation of aggregates.

Hyper-Rayleigh scattering in electrochemically synthesized Ag–Au coupled clusters

Electrochemically synthesized bimetallic (Ag–Au) nanoclusters (coupled) of average sizes of6 and 12 nm were studied for coherent second harmonic generation (SHG) in differentorganic media. Significant enhancement in SHG intensity of these nanoclusters in toluene isobserved due to its lower dielectric constant. By hyper-Rayleigh scattering (HRS) theβ value isfound to be 1.62 × 10−20 esu for 6 nm size coupled clusters, which is four to five orders of magnitude higher than thatreported for gold particles.

Synthesis, Characterisation and Molecular Hyperpolarisabilities of Pseudo‐Octahedral Hydrido(nitrile)iron(II) Complexes for Nonlinear Optics: X‐ray Structure of [Fe(H)(dppe)2(4‐NCC6H4NO2)][PF6]·CH2Cl2

AbstractA series of ionic pseudo‐octahedral trans‐hydrido(nitrile)iron(II) complexes with the general formula [Fe(H)(dppe)2(4‐NCR)][PF6] [dppe = 1,2‐bis(diphenylphosphanyl)ethane; R = acceptor‐substituted conjugated ligand] have been synthesised by chloride abstraction from the starting compound trans‐[FeHCl(dppe)2] and fully characterised. First hyperpolarisabilities (β) have been determined by hyper‐Rayleigh scattering (HRS) at the fundamental wavelength of 1072 nm and the high near‐resonant values obtained (up to 1130 × 10–30 esu) are interpreted in terms of the two‐level model (TLM) and are correlated with IR and NMR spectroscopic data. Wavelength‐dependent HRS has been performed in the 1072–1580 nm range for two of the compounds, namely [Fe(H)(dppe)2{4‐NC(CH)(CH)C6H4NO2}][PF6] and [Fe(H)(dppe)2{4‐NCC6H4(CH)(CH)C6H4NO2}][PF6]. These results clearly show the two‐photon resonance but also the shortcomings of the TLM when it comes to deriving reliable static β values. A structural study of the compound [Fe(H)(dppe)2(4‐NCC6H4NO2)][PF6] by X‐ray diffraction shows that it crystallises in the centrosymmetric monoclinic space groupP21/n, with four molecules in the unit cell and a pairwise antiparallel alignment of the dipoles. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Antiparallel-Aligned Neutral-Ground-State and Zwitterionic Chromophores as a Nonlinear Optical Material

Efficient noncentrosymmetric arrangement of nonlinear optical (NLO) chromophores with high first-order hyperpolarizability (beta) for increased electro-optical (EO) efficiency has proven challenging as strong dipolar interactions between the chromophores encourage antiparallel alignment, attenuating the macroscopic EO effect. This work explores a novel approach to simultaneously achieve large beta values while providing an adjustable dipole moment by linking a strong neutral-ground-state (NGS) NLO chromophore with positive beta to a zwitterionic (ZWI) chromophore with negative beta in an antiparallel fashion. It is proposed that the overall beta of such a structure will be the sum of the absolute values of the two types of chromophores while the dipole moment will be the difference. Molecules 1-3 were synthesized to test the feasibility of this approach. Molecular dynamics calculations and NMR data supported that the NGS chromophore component and the ZWI chromophore component self-assemble to an antiparallel conformation in chloroform. Calculations showed that the dipole moment of 1 is close to the difference of the two component chromophores. Hyper-Rayleigh scattering (HRS) studies confirmed that the first hyperpolarizability of 1 is close to the sum of the two component chromophores. These results support the idea that an antiparallel-aligned neutral-ground-state chromophore and a zwitterionic chromophore can simultaneously achieve an increase in beta and a decrease of the dipole moment.

X-Shaped Electro-optic Chromophore with Remarkably Blue-Shifted Optical Absorption. Synthesis, Characterization, Linear/Nonlinear Optical Properties, Self-Assembly, and Thin Film Microstructural Characteristics

A novel type of "X-shaped" two-dimensional electro-optic (EO) chromophore with extended conjugation has been synthesized and characterized. This chromophore is found to exhibit a remarkably blue-shifted optical maximum (357 nm in CH(2)Cl(2)) while maintaining a very large first hyperpolarizability (beta). Hyper-Rayleigh Scattering (HRS) measurements at 800 nm provide a beta(zzz) value of 1840 x 10(-30) esu. Self-assembled thin films of this chromophore were fabricated via a layer-by-layer chemisorptive siloxane-based approach. The chromophoric multilayers have been characterized by transmission optical spectroscopy, advancing contact angle measurements, synchrotron X-ray reflectivity, atomic force microscopy, and angle-dependent polarized second harmonic generation spectroscopy. The self-assembled chromophoric films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO response (chi(2)(333) approximately 232 pm/V at 1064 nm; r(33) approximately 45 pm/V at 1310 nm). This work demonstrates an attractive approach to developing EO materials offering improved nonlinearity-transparency trade-offs.

Ferroelectric domains in nitrobenzene-nitromethane solutions measured by hyper-Rayleigh scattering

Hyper-Rayleigh scattering (HRS) spectra were measured for liquid solutions of C6H5NO2 and CH3NO2 at T=300 K. The depolarized HRS spectra at small frequency shift are dominated by two components due to reorientation of the nitrobenzene molecules. One is a Lorentzian with spectral width nu1=0.16-0.45 cm(-1) and corresponding orientation relaxation time tau=33-12 ps. The second component is a narrow spike with spectral width <2 MHz and corresponding relaxation time tau>80 ns, attributed to HRS from slowly relaxing ferroelectric domains. The dipole order parameter g0=0.053+/-0.005, saturation parameter p=0.9+/-0.1, and volume V=20+/-6 nm3 for these domains in nitromethane were determined from measurements of the nitrobenzene-concentration dependence of the intensity ratio for these two spectral components. Orientation of the 230 nitromethane molecules within each domain is inhomogenous but highly ordered.

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.

First hyperpolarizabilities of unprotected and polymer protected copper nanoparticles prepared by laser ablation

In this Letter, we report the first hyperpolarizability of Cu nanoparticles in organic media with and without any stabilizer. Copper nanoparticles of ∼9 nm size were prepared by laser ablation of a Cu-metal target immersed in isopropanol both in the presence and absence of polyvinyl pyrrolidone (PVP, a stabilizer and a surface modification agent). The first hyperpolarizability of the nanoparticles was measured by the hyper-Rayleigh scattering (HRS) technique. Both types of nanoparticles show large second harmonic response and the β values are (414 ± 19) × 10 −30 esu/atom 1/2 for the unprotected and (808 ± 12) × 10 −30 esu/atom 1/2 for the PVP protected Cu-nanoparticles.

Chemical Applications of Hyper-Rayleigh Scattering in Solution

Hyper-Rayleigh or second harmonic Rayleigh scattering (HRS or SHLS) has been employed to measure equilibrium physical properties of molecules in solution. It has been utilized in our laboratory to obtain the dissociation constants of weak organic acids such as substituted benzoic and cinnamic acids in solution. Partition coefficient, supramolecular structure formation and stoichiometry, critical micelle concentration, and binding or association constant of a molecule with another molecule have all been measured by the HRS technique by us. We find that both the symmetry of the molecule and its concentration in the medium play an important role in finding new chemical applications of the technique.

Polar Cofacially Fixed Sandwich Complexes: Do They Demonstrate Second Harmonic Generation (SHG)?

AbstractFor the purpose of possible second harmonic generation (SHG) a cationic and a neutral sandwich unit were cofacially arranged in a three‐step synthesis starting from 1,8‐diiodonaphthalene. First, 1‐cyclopentadienyl‐8‐iodonaphthalene (2) was formed, then the neutral ferrocenyl substituent was fixed in the 8‐position by a Negishi cross‐coupling reaction. The deprotonation of the cyclopentadienyl substituent, and the subsequent coordination of the half‐sandwich fragments ML = [Fe(η5‐C5Me5)]+, [Rh(η5‐C5Me5)]2+, [Ir(η5‐C5Me5)]2+, [Ru(η6‐C6H6)]2+ to the cyclopentadienyl anion revealed the desired dinuclear complexes 1‐[(η5‐cyclopentadienediyl)(η5‐pentamethylcyclopentadienyl)iron(II)]‐8‐ferrocenylnaphthalene (5), 1‐[(η5‐cyclopentadienediyl)(η5‐pentamethylcyclopentadienyl)rhodium(III)]‐8‐ferrocenylnaphthalene hexafluorophosphate (6PF6), 1‐[(η5‐cyclopentadienediyl)(η5‐pentamethylcyclopentadienyl)iridium(III)]‐8‐ferrocenylnaphthalene hexafluorophosphate (7PF6), and 1‐[(η6‐benzene)(η5‐cyclopentadienediyl)ruthenium(II)]‐8‐ferrocenylnaphthalene hexafluorophosphate (8PF6). The neutral complex 5 was oxidized to the paramagnetic cation 1‐[(η5‐cyclopentadienediyl)(η5‐pentamethylcyclopentadienyl)iron(III)]‐8‐ferrocenylnaphthalene hexafluorophosphate (5PF6). Compounds 3, 5PF6, 6PF6, and 7PF6 were characterized by X‐ray structure determination; the neutral compound 3 crystallizes in the space group P21/c, whereas all of the cationic dinuclear complexes crystallize in the chiral space group C2221. A cyclic voltammetry study points to a predominant “through‐space” interaction between the cationic sandwich unit and the neutral ferrocene substituent. The compounds 5PF6, 6PF6, 7PF6, and 8PF6 were subjected to hyper‐Rayleigh scattering (HRS) and Kurtz‐powder measurements. In both studies no SHG intensity could be observed. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Syntheses and Quadratic Optical Nonlinearities of Ruthenium(II) Complexes with Ethynyl-ConnectedN-Methylpyridinium Electron Acceptors

We have prepared a number of new dipolar complexes containing ethynyl or buta-1,3-diynyl units linking electron-rich {Ru(II)(NH3)5}2+, trans-{Ru(II)(NH3)4L}+ (L = pyridine or N-methylimidazole), or trans-{Ru(II)Cl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers to pyridinium electron acceptors. In acetonitrile solutions at 295 K, the new complexes display unusual blue-shifting of their metal-to-ligand charge-transfer (MLCT) bands as the conjugation is extended, in a fashion similar to that of the corresponding ethenyl systems. Hyper-Rayleigh scattering (HRS) and Stark spectroscopic measurements provide direct and indirect estimates of static first hyperpolarizabilities beta0, and both the linear and nonlinear optical (NLO) properties are temperature- and medium-dependent. Thus, at 77 K in butyronitrile glasses, the MLCT bands display more normal red shifts upon conjugation extension. While the Stark-derived beta0 values generally increase as n (the number of ethynyl units) increases from 0 to 2, the HRS data show maximization at n = 1 for two of the ammine series but an increase upon moving from n = 1 to 2 for the pdma complexes. Comparisons with the analogous ethenyl chromophores show that the latter generally display larger beta0 values, whether determined via HRS or Stark data, and the inferiority of the ethynyl systems in terms of NLO response is more pronounced when n = 2. This differing behavior is attributable primarily to larger increases in the transition dipole moment mu12 (and, hence, donor-acceptor pi-electronic coupling) on elongation in the ethenyl chromophores.

First hyperpolarizabilities of vinylogue organometallic sesquifulvalene chromophores: A DFT study

AbstractStatic first hyperpolarizabilities of vinylogue mono‐ and bimetallic sesquifulvalene chromophores, [(η5‐C5H5)M{μ−(η5‐C5H4)CHCH(η6‐C7H7)}]+ (M = Fe, Ru, and Os) and [(η5−C5H5)M{μ−(η5‐C5H4)CHCH(η6‐C7H7)}Cr(CO)3]+ (M = Fe, Ru, and Os) have been calculated within DFT theoretical approach. The results agree well with the recent Hyper‐Rayleigh scattering (HRS) measurements. The coordinated transition metals reduced the magnitude of dipole moment and first hyperpolarizability. The metal‐to‐ligand charge‐transfer contributes to the first hyperpolarizability and, as a result, the rational substitution of coordinated metal could modulate the first hyperpolarizability of the organometallic chromophores. The study would be helpful to understand optical nonlinearities in organometallic complexes. © 2006 Wlley Periodicals, Inc. Int J Quantum Chem, 2006

Organometallic complexes for nonlinear optics. Part 36. Quadratic and cubic optical nonlinearities of 4-fluorophenylethynyl- and 4-nitro-( E)-stilbenylethynylruthenium complexes

The complexes trans-[Ru(C C-4-C 6H 4F)X(dppe) 2] [X = Cl ( 1), C CPh ( 2), C C-4-C 6H 4NO 2 ( 3)], trans-[Ru{C C-4-C 6H 4-( E)-CH CH-4-C 6H 4NO 2}X(dppe) 2] [X = C CPh ( 4), C C-4-C 6H 4C CPh ( 5)], and [C 6H 3-1,3-{C C- trans-[RuCl(dppe) 2]} 2-5-(C C-4-C 6H 4F)] ( 6) have been synthesized and the identity of 1 confirmed by a single-crystal X-ray diffraction study. Cyclic voltammetry reveals a metal-centered oxidation, the potential of which is largely invariant on alkynyl ligand replacement across the series 1– 5; the diruthenium complex 6 shows two oxidation processes, consistent with weakly interacting metal centers. Hyper-Rayleigh scattering (HRS) studies at 1064 nm using ns pulses suggest quadratic nonlinearities for 3– 5 that are amongst the largest thus far for organometallic complexes, a trend maintained with the two-level-corrected data. HRS studies at 800 nm using fs pulses and amplitude modulation to remove multi-photon fluorescence contributions reveal significant fluorescence-free nonlinearities for 3– 5; the frequency-independent nonlinearities calculated from the 800 nm results are suggestive of fluorescence contributions to the 1064 nm data. Z-scan studies at 820 nm reveal cubic nonlinearities that increase with the size of the π-system, although error margins are significant.

Hyper-Rayleigh scattering spectrum of liquid nitromethane

The vertical vertical (VV), horizontal vertical (HV), and vertical horizontal (VH) hyper-Rayleigh scattering (HRS) spectra were measured for liquid CH3NO2 at T= 300 K. The main HRS spectral component has a width upsilon1=1.28 +/- 0.04 cm(-1), which gives an orientation relaxation time pi=4.1 +/- 0.1 ps in good agreement with other experiments. However, the VH spectrum also contains a previously unobserved strong narrow peak at zero-frequency shift, absent from the VV and HV spectra, which is due to a slowly relaxing longitudinal orientation mode. The upper bound on the width of this peak is 5 MHz, which corresponds to a relaxation time pi > 30 ns.

Synthesis, characterization, and hyperpolarizability measurements of main‐chain azobenzene molecules

AbstractA series of new AB type azobenzene monomers based on various substituted phenols and higher order fused/extended aromatic rings were synthesized and their hyperpolarizability tensor β determined by hyper‐Rayleigh scattering (HRS) measurement in methanol. The electron donor (OH) and acceptor units (COOH) were kept constant in the series, but the effective conjugation length was varied by varying the number and position of substituents as well as the number of aromatic rings. The effect of substitution of the phenolic ring on the β value was investigated and it was found to range from 15 × 10−30 to 42 × 10−30 esu. The effect of intramolecular hydrogen bonding on the nonlinear optical (NLO) property was also examined. The nonlinearity was in the following order of phenol derivative: α‐naphthol &gt; phenyl phenol &gt; 2,6‐dimethyl phenol &gt; o‐cresol &gt; cardanol &gt; phenol &gt; β‐naphthol. The unusually low values for the β‐naphthol‐based chromophore compared with its isomer (α‐naphthol) could be rationalized based on hydrogen bonding of the o‐hydroxyl group with the β nitrogen of the azo bridge. These azobenzene NLO chromophoric monomers were polymerized to form main‐chain polymers with a head to tail structure. The polymers had high thermal stability and rather low solubility in common organic solvents. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4455–4468, 2005

Hyper-Rayleigh scattering of neutral and charged helicenes

Oxidation effects on the hyper-Rayleigh scattering (HRS) second-order nonlinear optical responses of homo- and hetero-helicenes have been investigated using the time-dependent Hartree–Fock approach and the Austin Model 1 semi-empirical Hamiltonian. Upon oxidation, this approach predicts a substantial increase of the HRS response, which, with the exception of tetrathia-[7]-helicene and pentathia-[9]-helicene, is accompanied by a decrease of the depolarization ratio. In the case of helicenes bearing thiophene units, the HRS increase is associated with the presence of a substantial axial contribution to the first hyperpolarizability, whereas the radial component is otherwise dominant.

Two-photon excited fluorescence depolarisation experiments: II. The proper response function for analysing TCSPC data

The instrumental response function (IRF) needed for the analyses of fluorescence time-correlated single-photon counting (TCSPC) experiments is usually determined by measuring the Rayleigh scattering (RS) of colloidal particles. Since RS is a one-photon process the response function obtained is referred to as a one-photon IRF (OP-IRF). Hence, in a two-photon excited (TPE) fluorescence experiment the hyper-Rayleigh scattering (HRS) might suit the determination of the two-photon IRF better [A. Habenicht, J. Hjelm, E. Mukhtar, F. Bergstrom, L.B.-Å. Johansson, Chem. Phys. Lett. 354 (2002) 367]. In the present study, we explore the importance of OP- and TP-IRF in fluorescence depolarisation studies. POPOP in ethanol and n-octanol is analysed when using OP- and TP-IRF. It is demonstrated that the TP-IRF yields significantly better statistics.

Acido- and Phototriggered NLO Properties Enhancement

In this paper, acido- and phototriggered enhancement of the nonlinear optical (NLO) properties are achieved by combining the 10-(2-arylethenyl)indolino[2,1-b]oxazolidine unit with various styrylic residues. The synthetic pathway allows the preparation of phenyl, naphthyl, anthryl, and phenanthryl derivatives, which have been shown to display photochromic and acidochromic behavior at ambient temperature. The multiaddressable molecular switches are characterized by a large contrast of the NLO response along the reversible transformations. Hyper-Rayleigh scattering (HRS) experiments clearly indicate the strong influence of the electrodonating substituent borne by the aromatic system. Theoretical calculations have been performed for representative molecules showing a good agreement with the experimental hyperpolarizabilities. This enables a better understanding of the NLO responses in terms of structural and electronic parameters for both closed and protonated open forms.

Study of intermolecular interactions in liquid nitrobenzene by depolarized hyper-Rayleigh scattering

We employed depolarized hyper-Rayleigh scattering (HRS) to investigate the intermolecular interactions in liquid nitrobenzene. By comparing the depolarization ratios of the second-harmonic scattered light from neat nitrobenzene and mixtures of nitrobenzene and methanol of varying mixing ratios, we demonstrated the existence of a coherent component of HRS in liquid nitrobenzene. The coherent component was found to essentially disappear at a sufficiently high dilution of the nitrobenzene liquid. We also observed that both localized orientational correlation and delocalized libron excitation contribute to coherent HRS in liquid nitrobenzene. The delocalized contribution to coherent HRS was found to diminish much more readily with the introduction of interstitial foreign molecules than the localized contribution.