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194.9, 157.1 (m), 153.5, 140.6, 136.2, 120.8 (q, J = 275 Hz), 105.9, 60.8, 56.2;anal. calcd. for C24H28F6NiO6S4: C 40.41, H 3.96; found: C 40.32, H 3.04.

Compound 7: FABMS: m/z 450 [M+1]+; 1H NMR (300 MHz, d6-DMSO):d = 5.47 (m, 2H), 3.47 (s, 4H), 1.74 (m, 8H), 1.57 (m, 8H), 1.48 (m, 8H),1.47±1.20 (m, 20H).

Compound 8: FABMS: m/z 372 [M+1]+; 1H NMR (300 MHz, d6-DMSO):d = 3.96 (d, J = 7.2 Hz, 4H), 3.65 (s, 2H), 1.94 (m, 2H), 1.35±1.18 (m, 16H),0.84 (t, J = 7.4 Hz, 12H).

Received: August 18, 1997Final version: November 6, 1997

±[1] U. T. Mueller-Westerhoff, B. Vance, D. I. Yoon, Tetrahedron 1991, 47,

909.[2] P. Cassoux, L. Valade, H. Kobayashi, A. Kobayashi, R. A. Clark, A. E.

Underhill, Coord. Chem. Rev. 1991, 110, 115.[3] J. S. Miller, A. J. Epstein, Angew. Chem. Int. Ed. Engl. 1994, 33, 385.[4] A.-M. Giroud-Godquin, P. M. Maitlis, Angew. Chem. Int. Ed. Engl.

1994, 30, 375.[5] C. S. Winter, S. N. Oliver, J. D. Rush, C. A. S. Hill, A. E. Underhill, J.

Appl. Phys. 1992, 71, 512.[6] C. S. Winter, S. N. Oliver, R. J. Manning, J. D. Rush, C. A. S. Hill,

A. E. Underhill, J. Mater. Chem. 1992, 2, 443.[7] C. A. S. Hill, A. Charlton, A. E. Underhill, S. N. Oliver, S. Kershaw,

R. J. Manning, B. J. Ainslie, J. Mater. Chem. 1994, 4, 1233.[8] S. D. Cummings, L.-T. Cheng, R. Eisenberg, Chem. Mater. 1997, 9, 195.[9] A. Vogler, H. Kunkely, Angew. Chem. Int. Ed. Engl. 1982, 21, 77.

[10] U. T. Mueller-Westerhoff, B. Vance, in Comprehensive CoordinationChemistry, Vol. 2 (Ed: G. Wilkinson), Pergamon, Oxford 1987, p. 595.

[11] T. R. Miller, I. G. Dance, J. Am. Chem. Soc. 1973, 95, 6970.[12] G. N. Schrauzer, V. P. Mayweg, J. Am. Chem. Soc. 1965, 87, 1483.[13] Crystal data for 4: C32H48NiS4, monoclinic, space group P21/n, a =

7.048(2), b = 27.927(10), c = 17.880(5) �, b = 93.09(3)�, V = 3514(2) �3,r = 1.278 g cm±3, Z = 2, c-scan absorption correction (0.89±1.00); of4603 unique reflections (2y < 50�) measured, 1703 with I > 2s(I) wereused in the refinement, R = 0.069, wR(F2) = 0.191 (370 variables).

[14] Crystal data for 6: C24H22F6NiO6S4, triclinic, space group P1, a =8.555(6), b = 11.834(5), c = 14.783(3) �, a = 99.48(2)�, b = 94.38(4)�, g =107.05(4)�, V = 1390(1) �3, r = 1.679 g cm±3, Z = 2, c-scan absorptioncorrection (0.82±1.00); of 4903 unique reflections (2y < 50�) measured,2850 with I > 2s(I) were used in the refinement, R = 0.041, wR(F2) =0.108 (370 variables).

[15] X-ray crystal data for 7: C28H50N2S2, monoclinic, space group C2/c, a= 25.124(4), b = 6.925(2), c = 16.424(4) �, b = 92.30(2)�, V =2855(1) �3, r = 1.109 g cm±3, Z = 8, c-scan absorption correction(0.85±1.00); of 1859 unique reflections (2y < 50�) measured, 1141 withI > 2s(I) were used in the refienement, R = 0.087, wR(F2) = 0.238 (146variables).

[16] R. D. Schmitt, R. M. Wing, A. H. Maki, J. Am. Chem. Soc. 1969, 91,4393.

[17] L.-T. Cheng, private communication.[18] G. H. Cross, D. Bloor, T. L. Axon, M. Farsari, D. Gray, D. Healy, M.

Swann, M. Szablewski, Proc. SPIEÐInt. Soc. Opt. Eng. 1994, 2285, 11.[19] S. R. Marder, L.-T. Cheng, B. G. Tiemann, A. C. Friedli, M. Blan-

chard-Desce, J. W. Perry, J. Skindhùj, Science 1994, 263, 511.[20] In dichloromethane, acetone, and dimethylsulfoxide, the absorption

lmax of 3 is 812, 763, and 751 nm, respectively; lmax of 5 is 830, 798,and 779 nm, respectively; and lmax of 6 is 845, 848, and 917 nm, re-spectively.

[21] S. D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 1996, 118, 1949.[22] E. G. McRea, J. Phys. Chem. 1957, 61, 562.[23] J. L. Oudar, D. S. Chemla, J. Chem. Phys. 1977, 66, 2664.[24] J. L. Oudar, J. Chem. Phys. 1977, 67, 446.[25] D. R. Kanis, M. A. Ratner, T. J. Marks, Chem. Rev. 1994, 94, 195.[26] T. Thami, P. Bassoul, M. A. Petit, J. Simon, A. Fort, M. Barzoukas, A.

Villaeys, J. Am. Chem. Soc. 1992, 114, 915.[27] K. L. Kott, C. M. Whitaker, R. J. McMahon, Chem. Mater. 1995, 7,

426.[28] M. S. Paley, J. M. Harris, H. Looser, J. C. Baumert, G. C Bjorklund, D.

Jundt, R. J. Tweig, J. Org. Chem. 1988, 54, 3774.[29] C. Bosshard, G. Knöpfle, P. Pr�tre, P. Günter, J. Appl. Phys. 1992, 71,

1594.[30] A. Davison, R. H. Holm, Inorg. Synth. 1967, 10, 8.[31] U. T. Mueller-Westerhoff, M. Zhou, J. Org. Chem. 1994, 59, 4988.

Nonlinear Optical Properties of PolyelectrolyteThin Films Containing Gold NanoparticlesInvestigated by Wavelength DispersiveFemtosecond Degenerate Four Wave Mixing(DFWM)**

By Wolfgang Schrof,* Stanislav Rozouvan,Edward Van Keuren, Dieter Horn, Johannes Schmitt, andGero Decher

Metallic nanoparticles have attracted a lot of interest be-cause of their fascinating optical and electronic properties,useful, for example, in colorants[1] or catalysts.[2] As theparticle size reaches the nanometer scale not only is thesurface-to-volume ratio drastically changed, but a transi-tion from metal to insulator also occurs, and effects result-ing from quantum confinement may be observed. The colorof metal nanoparticulate systems is dominated by surfaceplasmon optical resonance due to collective electronic exci-tation at the interface between metal nanoparticle and di-electric matrix.[3]

The surface plasmons are not only responsible for the lin-ear optical properties, but govern nonlinear optical (NLO)phenomena as well.[4,5] Richard et al.[6] were the first to ob-serve enhanced values of NLO properties resulting fromthe effect of surface plasmon enhancement. The opticallyinduced damping of surface plasmons in gold colloids wasinvestigated by pump-and-probe experiments in the femto-second time range.[7] In the present work we present femto-second wavelength dispersive degenerate four wave mixing(DFWM) experiments to elucidate the dispersion of thew(3) nonlinearity and its relaxation behavior around theplasmon band. These experimental findings will be dis-cussed in terms of a simple model for third-order nonli-nearity of nanoparticle systems.

Films containing metallic nanoparticles can be preparedin different ways, such as the sol-gel process,[8] the block co-polymer micelle route,[9] evaporation,[10] and ion implanta-tion.[11] A newly developed technique embeds the particles

±

[*] Dr. W. Schrof, Dr. S. Rozouvan, Dr. E. Van Keuren, Dr. D. HornBASF AG, Polymer LaboratoryD-67056 Ludwigshafen (Germany)

Dr. J. Schmitt, Prof. G. DecherUniversite Louis PasteurInsitut Charles SadronF-67083 Strasbourg (France)

Dr. J. SchmittMPI für Kolloid- und GrenzflächenforschungD-12489 Berlin (Germany)

[**] This work was performed within the Japanese ªNonlinear PhotonicsMaterialsº project under the management of the Japan High PolymerCenter as a part of the Industrial Science and Technology FrontierProgram supported by the New Energy and Industrial TechnologyDevelopment Organization. G.D. and J.S. acknowledge support fromthe UniversitØ Louis Pasteur, the Centre National de la RechercheScientifique, and the BMBF (Grant No. 10843).

into polyelectrolyte layers,[12] resulting in a very controlledbuild-up of the nanoparticle film well-suited to optical in-vestigations.

DFWM Experiments: In order to tune the femtosecondDFWM experiment across the surface plasmon resonance,a light source consisting of a femtosecond Ti/sapphire oscil-lator (Coherent Mira 900F), a regenerative Ti/sapphire am-plifier (Coherent RegA 9000), and an optical parametricamplifier (Coherent 9400) was used (see Fig. 1). This pro-duced 70 fs pulses tunable from 500 to 700 nm with a repe-tition rate of 300 kHz, allowing for effective lock-in detec-tion in order to improve the signal-to-noise ratio of theDFWM signal (see Fig. 1) without generating thermo-opti-cal effects.

In the ªFolded Box CARSº configuration[13] two of thethree forward beams are used for the generation of a re-fractive index grating in the sample, while the third beam isused for the detection of this grating, i.e., the light-inducedchange of the refractive index. In order to synchronize allpulses, two of the three beams passed through computer-controlled delay stages. The three collinear beams were fo-cused onto the same sample spot by a lens with a 100 mmfocal length, leading to a focal diameter of approximately20 mm.

The deflected beam, e.g., the DFWM signal, was de-tected by a silicon photodiode (Hamamatsu followed by aStanford Research preamplifier SR 620) after passing aspatial filter. Lock-in techniques (Stanford ResearchSR530) were used to improve the signal-to-noise ratio ofthe DFWM signal. Two of the incoming beams werechopped by a mechanical chopper at different frequencies(f1 and f2) and the signal was detected at the frequency sum(f1 + f2). This maximum amplitude of the DFWM signalwas compared with the maximum amplitude measured in a

reference sample under identical experimental conditions.CS2 in a quartz cuvette 1 mm thick was used as reference.

The w(3) value of the sample (s) was calculated by com-parison with the signal of the reference (ref) CS2 (w(3) = 6 ´10±13 esu) using Equation 1, where Leffs is the sample thick-ness, Leffref is the effective thickness of the reference, n isthe refractive index, a is the absorption coefficient of thesample, I4 is the DFWM intensity, and P is the average la-ser power.

w�3�s� w�3�

ref

n2s

n2ref

���������I4s

I4ref

s ���������P 3

refP 3

s

sae

aLeffs =2

�1ÿeÿaLeffs � Leffref (1)

Sample Preparation: Films incorporating gold nanoparti-cles were fabricated using the polyelectrolyte dipping tech-nique described in the Experimental section.[12] Scanningelectron microscopy of the Au particles adsorbed onto anamine functionalized Si wafer proved their spherical shape.Transmission electron microscopy of the Au particles ad-sorbed onto carbon-coated grids showed that the particleswere 15.3 ± 2.7 nm in diameter.

Au nanoparticles/polyelectrolyte (PE) multilayer filmswere prepared by depositing alternating layers of poly(styr-ene sulfonate sodium salt) (PSS, molecular weight 168 000)and poly(allylamine hydrochloride) (PAH, molecularweight 50 000±65 000) onto a poly(ethyleneimine)- (PEI)-modified glass substrate. These polyelectrolyte layers weredeposited from 3 ´ 10±3 M aqueous solutions containing1 M NaCl, using immersion times of 20 min, followed byrinsing with deionized water and drying after each secondlayer. The uppermost layer of PAH provided a positivelycharged surface for subsequent self-assembly of Au nano-particles using an immersion time of 5 h.

In this investigation a layered structure of glass/PEI/[(PSS/PAH)2Au/PAH]4 was used. From X-ray reflectivitymeasurements a layer thickness of 12.5 nm was found,which is smaller than the Au nanoparticle diameter, thusimplying a certain overlap between adjacent layers. Thesurface plasmon band characteristic of metallic nanoparti-cles was found around 540 nm.

The results of spectral DFWM measurements on ourgold nanoparticle sample are presented in Figures 2 and 3.Both w(3) and the figure of merit, w(3)/a, exhibit a sharp max-imum in a region of plasmon resonance. In contrast tomany organic materials, the nonlinearity drops at a muchfaster rate than the absorption coefficient as one movesaway from the resonance.

The NLO parameters of metallic nanoparticles em-bedded in an inert dielectric matrix system can be pre-dicted using the Maxwell±Garnett theory.[4,14] This theorypredicts strong enhancement of the NLO properties nearthe surface plasmon resonance. Considering the wave-length variation of the surface plasmon enhancement factoras the most important term in governing the optical proper-ties, a simple estimate gives a scaling of w(3) with the squareof the absorption coefficient a, which was observed in the

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Fig. 1. Optical layout of the frequency tunable DFWM experiment.

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DFWM measurements.[15,16] This model neglects wave-length variations in the values of w(3) and a for the metal it-self, i.e., it only considers the enhancement effects due tosurface plasmon resonance. Various models for the nonli-nearity of the metal itself give contributions from quantumconfinement of conduction band electrons (intraband con-tribution) (w(3) ~ 1/a3 where a is the particle radius), frominterband (d to sp) transitions, and from non-equilibriumdistribution of the hot electrons.[17] The latter two are pre-dicted to have negligible dependence on particle size.

Our measurements give a relation between the suscept-ibility w(3) and absorption coefficient governed by a fourthpower dependence: w(3) = 1.4 ´ 10±27´ a4 ± 0.15 (w(3) and a inesu units) with a maximum w(3) for the composite materialof approximately 6.3 ´ 10±10 esu at resonance. The relaxa-tion behavior of the DFWM signals at different wave-lengths around the plasmon peak is presented in Figure 4.The high-energy wing of the absorption is dominated by atwo-fold relaxation with short (100 fs) and long (1.8 ps) de-cay constants (see also Fig. 5), while the low-energy wingonly exhibits slow relaxation.

The different scaling between w(3) and a in our resultsand those in the literature may be due to the different pulsedurations used: 5 ps in the literature[15,16] and 100 fs in ourexperiments. In this case, for the decay times of these mate-rials, we are essentially measuring instantaneous nonlinea-rities, while the experiments in the literature would givethe integrated nonlinearity. The wavelength dependence ofthe decay times mentioned above would result in differentamounts of accumulated nonlinearity versus wavelengthmeasured in a long-pulse experiment. Our fourth-powerdependence means additional scaling between the internalvalues of a and w(3) for nonlinearities occurring on the time-scale of ~100 fs.

The two components (time dependencies) have beenmeasured in other nanoparticle systems and were attribut-

Fig. 2. Wavelength dispersion of w(3) value (squares) and absorption coeffi-cient a (solid line) for the gold nanoparticle film.

Fig. 3. Wavelength dispersion of w(3)/a value (triangles) and absorption coef-ficient a (solid line) for the gold nanoparticle film.

Fig. 4. DFWM decay curves of the gold nanoparticle polyelectrolyte film de-tected at different wavelengths. For better comparison all curves were nor-malized. The baseline for all curves is zero.

Fig. 5. DFWM decay curve (wavelength 525 nm) for a thin polyelectrolytefilm of gold nanoparticles. Relaxation constants fitted: t1 = 100 fs, t2 =1800 fs.

ed to rapid relaxation of the excitations in the conductionband with surplus energy by emission of LO phonons (fast-er component) and to the thermalization of the non-equi-librium distributions of hot electrons and/or interband re-combination (slower component).[4,7]

In summary, the wavelength tunability of the femtosec-ond DFWM experiment, extended to a range from 500 nmto 920 nm using an optical parametric amplifier pumped bya regenerative Ti/sapphire amplifier, allows us to investi-gate the NLO behavior of metal nanoparticles near the sur-face plasmon resonance. The very distinct spectral disper-sions observed were not apparent in earlier measurementswith longer pulse widths. We surmise that this is due to thedifferent time dependencies of the competing processes: in-traband, interband, and hot electron. However, it is alsopossible that the novel process used to fabricate these sam-ples has an additional influence on the NLO properties, forexample in interactions between the polyelectrolyte matrixand excitations in the metal. Future work will include in-vestigations in the near infrared (NIR) wavelength rangeas well as variations in the sample processing parameters.

Experimental

The Au nanoparticles were prepared by reducing chloroauric acid withsodium citrate in aqueous solution [18]. The stock solutions used contained1.0 g of HAuCl4 in 100 mL of water, and 1.0 g of trisodium citrate in100 mL of water. The Au solution was diluted by adding 1 mL to 100 mL ofwater and then heated to a gentle boil on a hotplate. To this solution 2.5 mLof the citrate solution was added causing a color change to red within 5 min.

Received: August 7, 1997Final version: October 20, 1997

±[1] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer,

Berlin 1995.[2] G. Schmid, Chem. Rev. 1992, 92, 1709.[3] J. C. Maxwell Garnett, Philos. Trans. R. Soc. London 1904, 203, 385;

1906, 205, 237.[4] K. Rustagi, D. Bhawalkar, Ferroelectrics 1990, 102, 367.[5] J. Haus, R. Inguva, Proc. SPIEÐInt. Soc. Opt. Eng. 1991, 1497, 350.[6] D. Richard, P. Roussignol, C. Flytzanis, Opt. Lett. 1985, 10, 511.[7] M. Perner, P. Bost, U. Lemmer, G. von Plessen, J. Feldmann, U. Beck-

er, M. Menning, M. Schmitt, H. Schmidt, Phys. Rev. Lett. 1997, 78,2192.

[8] M. Mennig, M. Schmitt, U. Becker, G. Jung, H. Schmidt, Proc. SPIEÐInt. Soc. Opt. Eng. 1994, 2288, 130.

[9] J. Spatz, A. Roescher, M Moeller, Adv. Mater. 1997, 8, 337.[10] M. Lee, P. Dobson, B. Cantor, Thin Solid Films 1992, 219, 199.[11] R. Magruder, L. Yang, R. Haglund, C. White, L. Yang, R. Dorsinville,

R. Alfano, Appl. Phys. Lett. 1993, 62, 1730.[12] J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E. Geer, R.

Shashidhar, J. M. Calvert, Adv. Mater. 1997, 9, 61.[13] P. N. Prasad, D. J. Williams, Introduction to NLO Effects in Molecules

and Polymers, Wiley, New York 1991.[14] J. Sipe, R. Boyd, Phys. Rev. A 1992, 46, 1614.[15] F. Hache, D. Richard, C. Flytzanis, J. Opt. Soc. Am. B 1986, 3, 1647.[16] K. Puech, W. Blau, A. Grund, C. Bubeck, G. Cardenas, Opt. Lett.

1995, 20, 1613.[17] F. Hache, D. Richard, C. Flytzanis, U. Kreibig, Appl. Phys. A 1988, 47,

347.[18] D. A. Handley, in Colloidal Gold: Principles, Methods and Applica-

tions (Ed: M. A. Hayat), Academic Press, San Diego, CA 1989, Ch. 2.

Opalescent Cholesteric Networks from ChiralPolyisocyanates in Polystyrene**

By Georg Maxein,* Harald Keller, Bruce M. Novak, andRudolf Zentel*

The cholesteric phase (chiral nematic phase) of liquidcrystals has some outstanding optical properties: It selec-tively reflects (Bragg reflection) visible light and exhibitsbrilliant colors if the pitch of the cholesteric helix coincideswith the wavelength of visible light (in the material) (l =np).[1] Due to the angular dependence of the reflection con-ditions (angle between incident light and the cholesterichelix), different colors are seen depending on the observa-tion angle. This creates shiny ªrainbow-likeº colors.Furthermore, the reflected light is right- or left-handed cir-cularly polarized, depending on the type of cholestericsuperstructure (helix) in the system. A right-handed chole-steric helix, for example, reflects only the right-handed cir-cularly polarized part of the incident light of wavelength l0

= np; the rest of the spectrum (l ¹ l0), together with theleft-handed circularly polarized light of wavelength l0, istransmitted.

The cholesteric phase's temperature sensitivity has beenutilized for various applications, such as colored thermal in-dicators.[2] To utilize the unique optical properties of chole-steric liquid crystals, it is not only desirable that the wave-length of the selective reflection is insensitive to tempera-ture but also that the sample itself is durable.

Recently, highly crosslinked cholesteric pigments havearoused a great deal of interest as dye pigments for cars oras ªcopy safeº colors for documents or money.[3] These cho-lesteric pigments have so far been prepared from cholester-ic monomers or oligomers by a photocrosslinking pro-cess.[4±6]

Helical biopolymers offer the same potential and a greatdeal is known about thermotropic and lyotropic phases ofpolypeptides[7] and cellulose derivatives[8] that exhibit se-lective reflection. With this polymeric approach, we re-

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±

[*] G. Maxein, Prof. R. Zentel[+]

Institut für Organische Chemie, Universität MainzJ.-J. Becher-Weg 18±20, D-55099 Mainz (Germany)

Prof. B. M. NovakPolymer Science & Engineering DepartmentUniversity of MassachusettsAmherst, MA 01003 (USA)

Dr. H. KellerBASF AG, ZKS/FD-67054 Ludwigshafen (Germany)

[+] Present address: BUGH Wuppertal, Chemie und Institut für Material-wissenschaften, Gauss Strasse 20, D-42097 Wuppertal, Germany.

[**] Georg Maxein thanks Bruce M. Novak for six ªcoolº and productivemonths in Amherst and Advanced Materials for the poster prize wonat BPS'97. Financial support from the BASF AG is gratefully acknowl-edged.