Search

A combination of UV-vis-NIR spectroscopy, femtosecond Z-scan measurements, and time-dependent density functional theory (TD-DFT) calculations have been used to comprehensively investigate the linear optical and nonlinear optical (NLO) properties of pi-delocalizable metal-functionalized oligo(phenyleneethynylene)s. A range of unsymmetrically or symmetrically end-functionalized mono-, di-, tri-, penta-, hepta-, and nona(phenyleneethynylene)s were synthesized, with larger examples bearing varying numbers of 2,5-di(hexyloxy)phenyl groups to ensure sufficient solubility of the metal complex derivatives. The effect of incorporating varying numbers of solubilizing substituents in the OPE bridge, peripheral group modification, OPE lengthening, coligand variation, and metal location in the OPE on the linear optical properties has been established, with the first three molecular modifications resulting in significant changes in the optical absorption maxima. TD-DFT calculations reveal that the most intense transition in the linear optical spectra is localized on the OPE bridge and involves excitation from acetylenic to cumulenic molecular orbitals that are not greatly spatially separated from one another. The nonlinear optical properties are dominated by two-photon absorption, which for all but 1,4-{trans-[RuCl(dppm)(2)]C[triple bond]C}(2)C(6)H(4) appears as a band around 11,400 cm(-1) and a sharp increase of nonlinear absorption at frequencies >17,000 cm(-1). Surprisingly, there is relatively little influence of the length of the OPE bridge on the magnitude of the two-photon absorption cross sections, which are in the range 300-1000 GM.

Lewis addition between the metalloligand [RuCp(C[triple bond]Cpy-4)(dppf)] (1) (dppf = (C(5)H(4)PPh(2))(2)Fe) and MCl(2)(CH(3)CN)(2) gives [RuCp(C[triple bond]Cpy-4)(dppf)](2)[MCl(2)] (M = Pd (2) and Pt (3)), all of which have been spectroscopically and crystallographically characterized. The mixed-metal adducts show the d(6)-d(8)-d(6) metal alignment maintained by the trans disposition of the directionally rigid 4-ethynylpyridine at the centrosymmetric square-planar metal center. Electrochemical studies of these and the related known complexes trans-[RuCl(C[triple bond]Cpy-4)(dppm)(2)] (4) and [trans-RuCl(C[triple bond]Cpy-4)(dppm)(2)](2)[MCl(2)] (dppm = Ph(2)PCH(2)PPh(2)) (M = Pd (5) and Pt (6)) suggest that the spacer-linked heterotrinuclear network is able to stabilize the oxidized Ru(III) better than the metalloligand precursor. Cubic nonlinear optical responses are also generally higher for the heterometallic complexes. The gamma(real) value of the PdRu(2) dppm complex 5 is among the largest for linear organometallic complexes.

A methodology for achieving versatile and facile ligand exchange by post-fabrication chemical treatment in PbS nanocrystal:poly(3-hexylthiophene) (P3HT) hybrid composite photovoltaic devices is demonstrated. We report a considerable improvement of the photovoltaic performance after post-fabrication chemical treatment using acetic acid to produce PbS nanocrystals surrounded by short-length ligands. Annealing induced morphological and photovoltaic performance changes in the resulting composite devices were investigated as a function of the annealing time.

Triphenylamine derivatives bridged by methylene units afford a near planar molecular platform in a series of one dimer and two oligomers exhibiting increased structural rigidity compared to that of their parent triphenylamines. This series of dendrimers show significantly enhanced two-photon absorptions that are up to 3-fold that of triphenylamines with similar molecular size and structure.

The wavelength dependence of the cubic nonlinearity of ligated platinum-terminated polyynes trans, trans-{(p-MeC6H4)3P}2(p-MeC6H4)Pt(C[triple bond]C)n Pt(p-C6H4Me){P(p-C6H4Me)3}2 (n = 3-6, 8, 10, 12) has been examined by femtosecond Z-scan studies in the wavelength range 520-1500 nm and the results rationalized by density functional theory calculations on the model complexes trans, trans-(H3P)2(C6H5)Pt(C[triple bond]C)n Pt(C6H5)(PH3)2 (n = 2-8, 10, 12). Although the final states for one- and two-photon transitions are not the same in these centrosymmetric molecules, the Z-scan studies reveal coincidences in one-photon absorption with features in the frequency dependencies of both real and imaginary parts of the cubic hyperpolarizability, as well as inflections in the frequency dependencies of the real part of gamma that correspond to resonances in the imaginary part of gamma. The theoretical studies suggest that the linear absorption spectra are dominated by X(1)A g --> n(1)B(3u) transitions, with the first state of B(3u) symmetry playing a steadily diminishing role upon oligoyne chain lengthening. The theoretical studies also predict a red-shift of two-photon absorption (TPA) profile with increasing conjugation length, and a significant enhancement on proceeding from the shortest to the longest chromophore, trends that are observed experimentally. The experimental low-energy TPA maxima for these complexes can be approximated by a simple Gaussian profile. The sp-carbon chain-length dependence of linear and nonlinear absorption maxima enable an estimate (neglecting saturation) of 660 and 1000 nm for the infinite carbon chain, carbyne.

We report an approach to produce predefined patterns of quantum dots and multipod nanocrystals using optical lithography for direct writing of films for optoelectronic and electronic devices. To obtain photopatternability, the nanostructures (for example, CdSe, CdTe, and PbSe nanocrystals) were functionalized by incorporation of the functional ligand t-butoxycarbonyl (t-BOC) which has an acid-labile moiety. This change in the surface chemistry results in the ability to photopattern the semiconductor nanocrystals where desired for a number of optoelectronic device geometries. We demonstrate that the ultimate resolution (line width and spacing) of this technique is below 5 microm (the limit of our optical apparatus used for writing).

We study experimentally the nonlinear dynamics of two-color optical vortex beams in the presence of second-harmonic generation combined with the effects of photo- and thermal refraction, as well as self- and induced-phase modulation. We use an iron-doped lithium niobate crystal as a nonlinear medium for the vortex propagation and observe experimentally, depending on the laser wavelength, a decay of a double-charge vortex, splitting and reshaping of background beam, pattern formation, and controllable nonlinear rotation of a vortex pair.

We report pH sensing for biological applications based on surface enhanced Raman scattering (SERS) from silver nanoparticles functionalized with 2-aminothiophenol (2-aminobenzenethiol, 2-ABT). pH-dependent SERS spectra of the attached 2-ABT molecules enable one to sense the pH over the range of 3.0-8.0. We have performed the first demonstration of SERS detection in living cells kept in different pH buffer solutions and show that the pH sensitivity is retained in that case. Thus, the nanoparticles can be used as probes delivering spatially localized chemical information from biological environments and provide a new way to monitor chemical changes at cellular level.

The linear (absorption and emission) and nonlinear optical (NLO) properties of a series of D(3) [(Fe(II), Ru(II), Ni(II), Cu(II), Zn(II)] octupolar metal complexes featuring the 4,4'-bis[(dibutylamino)styryl]-2,2'-bipyridine ligand are reported. Zinc(II), nickel(II), and copper(II) complexes exhibit similar absorption spectra in the visible region (lambda(ILCT) = 474-476 nm) which are assigned to intraligand charge-transfer (ILCT) bands. The quadratic and cubic NLO properties are strongly influenced by the nature of the metallic center. Harmonic light scattering studies at lambda = 1.91 microm reveal that these chromophores display large first hyperpolarizabilities beta(1.91) in the range of (211-340) x 10(-30) esu; replacing the Zn(II) metal ion by Ni(II) or Cu(II) results in a decrease of the static beta(0) coefficient by a factor of 1.5-1.6. Z-scan measurements at 765 and 965 nm reveal relatively large two-photon absorption cross-sections [650 < sigma(2) < 2200 GM], showing that both beta and sigma(2) values can be tuned by simple modification of the metal ion.

This paper presents methodology developed for the computational modeling and design of negative refractive index materials (NIMs) based on molecular chirality. An application of the methodology is illustrated by ab initio computations on two organometallic molecules which constitute the monomer units of a chiral polymer. Comparisons with experimental data for the polymer are made. Even though the resulting chirality parameter for the pristine material is small, it is shown that negative index can be achieved by introducing sharp plasmonic resonances with metal nanoparticle inclusions.

A series of RuII or FeII trischelate complex salts containing N-methyl/aryl-2,2':4,4' ':4',4' ''-quaterpyridinium ligands that has previously been subjected to quadratic nonlinear optical studies (Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 13399) has now been investigated for two-photon absorbing behavior. Z-scan measurements using a 750 nm laser afford reasonably large two-photon absorption (2PA) cross-sections sigma2 of ca. 62-180 GM for the RuII complexes, but only very weak 2PA is observed for the FeII compounds. The excited-state and 2PA properties of the representative chromophore [RuII(Me2Qpy2+)3]8+ (Me2Qpy2+=N' ',N' ''-dimethyl-2,2':4,4' ':4',4' ''-quaterpyridinium) have also been investigated by using semiempirical intermediate neglect of differential overlap/multireference-determinant single and double configuration interaction computations with the optimized geometry obtained via density functional theory. The calculated sigma2 value of ca. 624 GM at 1.70 eV for this metal-to-ligand charge-transfer chromophore is about 10 times larger than that obtained from the Z-scan studies.

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.

The nonlinear absorption coefficient of As(2)S(3) glass has been measured to be 2.0 cm/GW for femtosecond pulses at 800 nm. Femtosecond laser structuring via two photon absorption in bulk As(2)S(3) glass by erasable and permanent photo-darkening is demonstrated using both holographic and direct multi-beam laser writing.

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations.

Compounds of the form Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)2(X2bipy = 4,4'-X(2)-2,2'-bipyridine, X = Me 3a, Br 3b, I 3c) have been synthesised from the mono-alkynyl precursors Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)Cl (X = Me 2a, Br 2b, I 2c); the former are the first ruthenium bis-alkynyl compounds that also contain a bipyridyl ligand. Spectroelectrochemical investigation of 3a shows that the metal is readily oxidised to form the ruthenium(III) compound 3a+, and will also undergo a single-electron reduction at each nitro group to form 3a2-. ESR and UV/visible spectra of these redox congeners are presented. We also report the synthesis of [Ru(Me2bipy)(PPh3)2(-C triple bond CBut)(N triple bond N)][PF6] during the attempted synthesis of Ru(Me2bipy)(PPh3)2(-C triple bond CBut)2, and report its X-ray crystal structure and IR spectrum. X-Ray crystal structures of 3b and 3c(as two different solvates) are presented, and the nature of the intermolecular interactions seen therein is discussed. Z-Scan measurements on Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, But, Ph, C6H4Me) are also reported, and show that Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, Ph) exhibit moderate third-order non-linearities.

We report the first Bragg gratings fabricated by Focused Ion Beam milling in optical waveguides. We observe striking features in the optical transmission spectra of surface relief gratings in silicon-on-insulator waveguides and achieve good agreement with theoretical results obtained using a novel adaptation of the beam propagation method and coupled mode theory. We demonstrate that leaky Higher Order Modes (HOM), often present in large numbers (although normally not observed) even in nominally single mode rib waveguides, can dramatically affect the Bragg grating optical transmission spectra. We investigate the dependence of the grating spectrum on grating dimensions and etch depth, and show that our results have significant implications for designing narrow spectral width gratings in high index waveguides, either for minimizing HOM effects for conventional WDM filters, or potentially for designing devices to capitalize on very efficient HOM conversion.

We present simple modifications of the classic Z-scan technique for the investigations of nonlinear chiroptical effects, i.e. nonlinear circular birefringence and two-photon circular dichroism. Two methods for studying these effects: a "polarimetric Z-scan" and a "polarization modulated Zscan" are described in detail. These techniques were applied to estimate the order of magnitude of the effects for several different materials.

The dispersion of cubic nonlinearity in the organometallic dendrimer 1,3,5-(3,5-{trans-[(dppe)2(4-O2NC6H4CC)RuCC]}2C6H3CCC6H4-4-CC)3C6H3 can be understood in terms of an interplay of two-photon absorption and absorption saturation. Simple dispersion relations reproduce the behavior of both the real and imaginary components of the hyperpolarizability.

We show here that a thin film of a nonlinear conjugated polymer: poly(p-phenylenevinylene) (PPV) is capable of efficiently generating the third harmonic of femtosecond pulses of light at the near infrared (including telecommunication bands) wavelengths, giving coherent, low divergence beams in the visible range. By using more than one fundamental beam overlapping in space and in time on the PPV film, autocorrelation signals are observed as well as higher order mixing signals due to X(5) and higherorder nonlinear susceptibilities. These signals can be used for applications such as short laser pulse diagnostics and photonic signal processing and they provide information about the nonlinear properties of the material itself.

A combination of cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, time-dependent density functional theory (TD-DFT), and Z-scan measurements employing a modified optically transparent thin-layer electrochemical (OTTLE) cell has been used to identify and assign intense transitions of metal alkynyl complexes at technologically important wavelengths in the oxidized state and to utilize these transitions to demonstrate a facile electrochromic switching of optical nonlinearity. Cyclic voltammetric data for the ruthenium(II) complexes trans-[RuXY(dppe)(2)] [dppe = 1,2-bis(diphenylphosphino)ethane, X = Cl, Y = Cl (1), Ctbd1;CPh (2), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (3); X = Ctbd1;CPh, Y = Ctbd1;CPh (4), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (5)] show a quasi-reversible oxidation at 0.50-0.60 V (with respect to ferrocene/ferrocenium 0.56 V), which is assigned to the Ru(II/III) couple. The ruthenium(III) complex cations trans-[RuXY(dppe)(2)](+) were obtained by the in situ oxidation of complexes 1-5 using an OTTLE cell. The UV-vis-NIR optical spectra of 1(+)-5(+) contain a low-energy band in the near-IR region ( approximately 8000-16 000 cm(-)(1)), in contrast to 1-5, which are optically transparent at wavelengths < 22 000 cm(-)(1). TD-DFT calculations have been applied to model systems trans-[RuXY(PH(3))(4)] [X = Cl, Y = Cl, Ctbd1;CPh, or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh; X = Ctbd1;CPh, Y = Ctbd1;CPh or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh] to rationalize the optical spectra of 1-5 and 1(+)-5(+). The important low-energy bands in the electronic spectra of 1(+)-5(+) are assigned to the promotion of an electron from either a chloride p orbital or an ethynyl p orbital to the partially occupied HOMO. These absorption bands have been utilized to demonstrate a facile switching of cubic nonlinear optical (NLO) properties at 12 500 cm(-)(1) (corresponding to the wavelength of maximum transmission in biological materials such as tissue) using the OTTLE cell, the first electrochromic switching of molecular nonlinear refraction and absorption, and the first switching of optical nonlinearity using an electrochemical cell.

Cubane-type clusters of formula [Mo(3)CuS(4)Cl(4)(dmpe)(3)](PF(6)) (4), [Mo(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (5), and [W(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (6) have been prepared by reacting the incomplete cuboidal trimers [Mo(3)S(4)Cl(3)(dmpe)(3)](PF(6)) (1), [Mo(3)S(4)Br(3)(dmpe)(3)](PF(6)) (2), and [W(3)S(4)Br(3)(dmpe)(3)](PF(6)) (3), respectively, with CuX (X = Cl or Br) or the mononuclear copper complex [Cu(CH(3)CN)(4)](+) in THF. The reaction takes place without global changes in the metal oxidation states, and compounds 4-6 with a [M(3)CuS(4)](5+) core possess 16 e(-) for metal-metal bonding. X-ray structural analysis of 4-6 revealed an effective C(3v) symmetry for the M(3)Cu unit with the M-M distances being statistically the same for M = Mo or W. However, the M-Cu distance is 0.04 and 0.1 A longer than the M-M bond length for Mo and W, respectively. There is no significant structural rearrangement of the ligand-metal bonding in proceeding from [M(3)S(4)X(3)(dmpe)(3)](+) to [M(3)CuS(4)X(4)(dmpe)(3)](+). The cyclic voltammograms of the [Mo(3)CuS(4)] cubane clusters show one quasi-reversible reduction process at E(1/2) = -0.31 V for 4 and at E(1/2) = -0.23 V for 5 and one irreversible reduction at -0.69 and -0.58 V for 4 and 5, respectively. The tungsten cluster 6 shows a unique quasi-reversible reduction wave at E(1/2) = -0.71 V. The incorporation of copper into the incomplete [M(3)S(4)] cuboidal complexes produces a decrease of the reduction potential for both molybdenum and tungsten. Absorption spectra of 1-6 are broadly similar; replacing Mo by W in proceeding from 2 to 3 or from 5 to 6 and replacing Br by Cl in proceeding from 2 to 1 or from 5 to 4 results in a blue shift of the three UV-visible absorption bands. All six clusters exhibit optical limiting, as measured by the Z-scan technique at 523 nm using 40 ns pulses. The power-limiting mechanism remains obscure, but under the conditions employed, threshold-limiting fluence decreases on replacing W by Mo on proceeding from 3 to 2 or 6 to 5 and on proceeding from tetranuclear cluster (4-6) to trinuclear precursor (1-3, respectively). For all six clusters, values of the excited-state cross section sigma(eff) are larger than those of the corresponding ground-state cross section sigma(0); i.e., all clusters are efficient optical limiters.

beta-Diketonato complexes [Tl{CH{C(O)R}(2)}] or PPN[Au{CH{C(O)R}(2)}(2)] [where PPN = (Ph(3)P)(2)N] react with an excess of CS(2) to give, respectively, [Tl(2){S(2)C=C{C(O)R}(2)}](n)() [R = Me (1) or Ph (2)] or (PPN)(2)[Au(2){&mgr;-kappa(2)-S(2)C=C{C(O)R}(2)}(2)] [R = Me (3.PPN) or Ph (4.PPN)]. The gold complexes 3.PPN and 4.PPN can also be obtained from PPN[AuCl(2)] and the corresponding thallium 1,1-ethylenedithiolate. PPN[AuCl(4)] reacts with 1 to give PPN[Au{kappa(2)-S(2)C=C{C(O)Me}(2)}(2)] (5) which, in turn, reacts with PhICl(2), Br(2) or I(2) (1:1) to give PPN[AuX(2){kappa(2)-S(2)C=C{C(O)Me}(2)}] [X = Cl (6), Br (7), or I (8)]. By reacting 6 with TlCF(3)SO(3) and 1,10-phenanthroline (phen) in 1:2:1 molar ratio, the cationic complex [Au{kappa(2)-S(2)C=C{C(O)Me}(2)}(phen)]CF(3)SO(3) (9) can be obtained. The crystal structures of 5 and 6.0.5Me(2)CO have been determined. Third-order nonlinearities of 3.PPN, 3.Pr(4)N, 5, and 6 have been evaluated by the Z-scan technique; an increase in nonlinearity upon extending the pi-system is observed, with the anionic dimetallacycle containing two gold and four sulfur atoms in 3 responsible for the largest nonlinearities (approximately 100 x 10(-36) esu).

Poly(indenofluorene) shows a strong degenerate four-wave mixing (DFWM) response when it is excited with 100-fs pulses at 800 nm. The DFWM signal scales with the 1.5 power of the input intensity, which we interpret as being due to absorption saturation phenomena. The saturation was studied by open-aperture Z scan in dilute solutions of poly(indenofluorene) in chloroform. The changes in the absorption coefficient alpha are described by the formula alpha = alpha(0)/[1 + (I/I(sat))(1/2)], where I(sat) is the saturation intensity, which is found to be of the order of 100>MW/cm(2).