Enhanced Third-Order Nonlinear Optical Properties in Dendrimer

Third-order NLO effects were investigated by degenerate four-wave mixing ... The results show an enhancement of the third-order nonlinear susceptibili...
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NANO LETTERS

Enhanced Third-Order Nonlinear Optical Properties in Dendrimer−Metal Nanocomposites

2005 Vol. 5, No. 12 2379-2384

Ying Wang, Xinbing Xie, and Theodore Goodson, III* Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109 Received July 20, 2005; Revised Manuscript Received October 4, 2005

ABSTRACT Nonlinear optical (NLO) and time-resolved fluorescence spectroscopic measurements of dendrimer−metal nanocomposites (DNCs) are reported. Third-order NLO effects were investigated by degenerate four-wave mixing (DFWM) measurements for DNCs and DNCs incorporated into thin polymeric films. The results show an enhancement of the third-order nonlinear susceptibility for the chromophore-functionalized dendrimer− metal nanocomposites. Investigations of the mechanism of the enhanced NLO effect suggested a strong contribution because of the metal’s local field. These results show the potential usefulness of dendrimer−metal nanocomposites for nonlinear optical effects and biophotonic applications.

Investigations of materials with large and fast third-order nonlinear optical (NLO) responses are of current interest because of the possibility of their use in technological applications such as all-optical switching, signal processing, and fast optical communications.1 Organic π-conjugated polymers and oligomers,2,3 organometallic complexes,4-6 and organometallic dendrimers7 have shown promise in this regard. These materials may provide a viable alternative to traditional inorganic crystalline NLO materials (such as lithium niobate (LiNbO3)) because of their ease in chemical alteration and reduction in cost. Organometallic systems are particularly attractive for third-order NLO effects because they offer lowlying transitions, large molecular hyperpolarizability, and fast switchable NLO properties.8 Recently, new enthusiasm for the use of organic-inorganic hybrid materials has emerged with the motivation of future NLO applications in biotechnonlogy,9,10 in photovoltaic cells,11 and in nanophotonic materials.12 This has led to the characterization of the NLO properties in various organic-inorganic hybrids.13-18 It has been demonstrated already that one may engineer hybrid materials for enhanced linear and NLO effects.16,18 For example, the third-order NLO response of a polymer was found to be enhanced by two-orders of magnitude by introducing a metal nanoparticle assembly.19 The enhanced second harmonic generation (SHG) properties have also been investigated as a function of metal nanoparticle topology.20 Local field effects have been suggested (both experimentally and theoretically) to play an important role in the enhancement process in both real and imaginary parts of the thirdorder nonlinear susceptibility.21,22 The basis of the local field enhancement effects results from the various changes in the * Corresponding author. E-mail: [email protected]. 10.1021/nl051402d CCC: $30.25 Published on Web 10/26/2005

© 2005 American Chemical Society

transition moments and collective excitations in different metal nanoparticle topologies. The details of this process have been investigated by time-resolved spectroscopy.23 An increase (or decrease) of fluorescence quantum yields and lifetimes of dye molecules in the proximity of nanoparticles have been reported.24,25 Although it may be clear that the changes in the metal topology and interactions between the organic π-electrons with the metal’s surface plasmon resonance (SPR) do affect the linear and second-order nonlinear properties in the system,16 the mechanism for an enhanced third-order NLO effect in organic-inorganic hybrid materials is not as certain. In the case of third-order NLO effects where the absorptive (imaginary) part of the nonlinear susceptibility is dominant, it is reasonable to suggest that the SPR and intermolecular interactions would play a major role in determining the magnitude of the overall NLO response. The dispersion of the real and imaginary parts of the third-order nonlinear susceptibility (χ3) and the strength of the local field factors should be affected. Therefore, it is important to probe the different parameters that may affect the nonlinear enhancement process in a systematic fashion both synthetically and with nonlinear optical and time-resolved spectroscopic measurements. Such investigations would further suggest the utilization of third-order NLO effects for biophotonics. Indeed, the χ3 in the condensed phase may be affected by internal dynamical processes of the molecule-particle framework. Thus, χ3 effects may be used for molecular sensing,26 peptide and protein low-frequency mode investigations,27 and high contrast agents in imaging of biological samples.28 It is crucial to have synthetic (chemical) control of the metal nanoparticle topologies and organic systems of interest in order to probe the important parameters that contribute

Scheme 1.

Synthesis of Model Compound LRh-AEA and the Chromophore Functionalized PAMAM Dendrimer (G2-LRh)

to the nonlinear enhancement process systematically. Although there are many choices for the two components, gold nanoparticle (because of the possibility of utilizing the multiple-binding sites of the dendrimer) and poly(amidoamine) (PAMAM) dendrimer (because of the possibility for using the structural branches to control, grow, and functionalize the metal nanoparticle) are good choices for a systematic study. When compared to the doping of metal nanoparticles into polymers or glass,29 the NLO materials fabricated by dendrimer-metal nanocomposites can better control the size of metal nanoparticles because of the unique properties of the dendrimer. The peripheral functionalization of dendrimers using luminescent chromophores was generally performed with various hydrophobic moieties such as phenyl, naphthyl, pyrenyl, and dansyl chromophores.30 However, for drug delivery and biological studies it is essential to have a bifunctional dendrimer, which can be used as drug delivery carrier and sensor or imaging probe simultaneously. Therefore, the prerequisite is to functionalize the water-soluble dendrimer with a more hydrophilic chromophore. The challenge for such a functionalized dendrimer is purification, particularly for the chromophore that possesses aggregation and adsorptive properties such as rhodamine B. Shown in Scheme 1 are the synthetic routes for both the model compound (LRh-AEA) and a lissamine rhodamine B functionalized PAMAM dendrimer (G2-LRh). The exhaustive dialysis and gel permeation filtration were the most effective ways of G2-LRh purification. Specifically, exhaustive dialysis was used initially to remove excess reactants and then gel permeation filtration or precipitation of G2-LRh from 30% CH3Cl/CH3OH with ether was applied for the removal of partly dye functionalized dendrimers. Characterization of G2-LRh was made by 1H NMR and MALDI-TOF mass spectral analysis. The NMR spectrum shows a high degree of surface functionalization according to the number of protons. The MALDI-TOF gives an average molecular weight of 11 910 (M + 5H+). 2380

The DNCs were prepared by mixing solutions containing the appropriate dendrimer and a gold salt. The gold ions formed complexes with the interior and/or exterior functional groups of the dendrimer. Subsequent reduction of the metal ions with NaBH4 leads to the formation of gold nanoparticles that may be encapsulated within the interior of dendrimer and/or stabilized by multiple dendrimers.31 The formation of gold nanoparticles was illustrated by a surface plasmon band centered at approximately 520 nm (see Figure 1a). Shown in Scheme 2 is the procedure used for the DNC’s thin film preparation. The polymer matrices (PMMA and PVB, see the Supporting Information) were used for doping DNCs. To form homogeneous and transparent films, the gold salt was first extracted into the organic phase using tetraoctylammonium bromide. The appropriate orange/red organic solution was mixed with the PAMAM dendrimer in methanol. Subsequent reduction of the metal ions with borane dimethylamine in pyridine leads to a precipitate. The precipitate was washed with dichloromethane, dissolved into methanol, and then centrifuged to remove large aggregates of gold nanoparticles. The supernatant was mixed with a polymer solution along with a small portion of pyridine. The addition of pyridine into the film solution may facilitate the dispersion of dendrimer-gold nanocomposites into the polymer matrix. To probe the interactions between the peripheral chromophores of the dendritic architecture and the gold nanoparticles, attention must be given first to the linear optical properties of the dendrimer-metal nanocomposites. As in our previous reports,14 absorption spectra have been utilized to characterize the metal nanoparticle topologies. Shown in Figure 1a is the absorption spectrum of G2-LRh, G2-Au, and G2-LRh-Au in a methanol/water (1:9) solvent. The G2-LRh-Au exhibits a broader spectrum and larger absorption compared to the corresponding G2-LRh, which indicates the formation of gold nanoparticles. In the visible region, one of the absorption peaks of G2-LRh is found at Nano Lett., Vol. 5, No. 12, 2005

Figure 1. (a) Absorption spectrum of G2-LRh-Au, G2-LRh, and G2-Au. (b) Fluorescence spectra of G2, G2-LRh dendrimers, and dendrimer nanocomposites G2-LRh-Au and G2-Au. Scheme 2. Synthetic Procedure for DNCs Incorporated into Thin Film

approximately 525 nm and this is assigned to the π f π* transition of the rhodamine chromophore. The SPR maximum of the gold nanoparticle is located near 520 nm as well (Figure 1a). Thus, the chemical mechanism may operate in DNCS by mixing of molecular and metal nanoparticle states to produce new states. It is possible for these new states to make a contribution to the enhanced optical nonlinearity. Noted that the ratio of [Au]/per chromophore in G2-LRhAu hybrid system is too low (∼2.5) to induce any aggregates of LRh-G2 on gold nanoparticles; that is, no blue or redshifted spectrum was observed. Because the model compound is unable to adsorb on gold nanoparticles, it is likely that the gold nanoparticles in the DNCs were encapsulated by the branches of dendrimers. It is very possible that the gold atoms on the surface of the nanoparticles possess unoccupied orbitals for nucleophiles to donate electrons. Strong electron donors such as the PAMAM dendrimer contain the amine functionality in the branch center and donate electrons to the vacant orbitals on the gold surface. The distances between the three-ring aromatic system (lissamine rhodamine B) and the metal surface can be estimated to be less than ∼1.0 nm. At such a short distance, various deactivation pathways of the surface-bound LRh-G2 will take place. As shown in Figure 1b, there is substantial fluorescence quenching (90%) of LRh-G2, and energy and/or electron transfers may be the main reason for the quenching. It should be noted that we did not find any green and/or red emission from the G2 dendrimer or the G2-Au nanocomposites. However, we do Nano Lett., Vol. 5, No. 12, 2005

observe the NLO properties of dendrimer-gold nanocomposites13,14,17 and the enhancement of third-order NLO effects in dye-functionalized dendrimer nanocomposites (see below). The measurements of third-order nonlinear susceptibility (χ3) of materials of interest were carried out by a degenerate four-wave mixing (DFWM) method. The DFWM signal is a function of input laser intensity, that is, Is ) qI3laser, where q is proportional to the square of path length L, the modulus of the third-order nonlinear susceptibility, |χ3|, and is inversely proportional to the fourth power of the linear refractive index, n.32 As shown in Figure 2, the log-log plot of DFWM signal versus the input intensity gave a slope of 3.0 ( 0.05 over the range of incident laser powers used. This suggests that all materials possess third-order nonlinear optical properties except for the dendrimer G2, whose DFWM signal is too weak (smaller than CS2) to be measured accurately. The third-order nonlinear susceptibility (χ3) of the DNCs is obtained from the DFWM data using the expression33-34 3 χ3s ) χCS × 2

( ) ( )( ) ns 2 qs × nCS2 qCS2

1/2

lCS2 ls

(1)

where n is linear refractive index and l is the path length. The χ3 of CS2 is 2.73 × 10-12 esu.32 Table 1 summarizes the χ3 values of the G2-LRh and various dendrimer-metal nanocomposites. In solutions, the χ3 values of G2-Au, G2-LRh, and G2-LRh-Au were 2381

Figure 2. Dependence of DFWM signal (Is) upon laser intensity (IL).

found to be 4.5 × 10-12, 5.6 × 10-12, and 14 × 10-12 esu, respectively. Obviously, the χ3 value of G2-LRh-Au is not a simple additive effect of the G2-Au and G2-LRh moieties. The normalized χ3 values of G2-LRh-Au are approximately one order of magnitude larger than that of G2-Au and G2-LRh. To explain the cooperative NLO effects, one must take into account both the local field effects and excited-state interactions. Let us first consider the local field enhancement mechanism. At the SPR excitation (532 nm), the gold nanoparticles create an enhanced electric field. Subsequently, the chromophore will sense the higher electric field. Thus, the fluorescence intensity of the chromophore may be enhanced35 and this situation is similar to the local field enhancement processes observed for organic molecules in surface-enhanced raman scattering (SERS).36 Theoretical estimates of the local field enhancement factors near 550 nm give a result close to ∼100.37 For our particular metal particles in the dendrimer nanocomposites, the particles are nearly spherical and are in the size range of ∼5 nm.31 This would lead to an enhancement factor that is in the same order of magnitude as our experimental result of about a factor of 10.38,39 However, for the case of excited-state interactions, gold nanoparticles will modify the partitioning of the excitation energy into various radiative and nonradiative decay channels.35,40-43 From experiments and theoretical calculations,41-43 the fluorescence quenching of lissamine dye molecules in the vicinity of gold nanoparticle is caused by the increased nonradiative rate Rnonrad because of energy

transfer and the decreased radiative rate Rrad, which may happen if the molecular dipole and the dipole induced by the gold nanoparticle radiate out of phase. Indeed, our nanocomposites (G2-LRh-Au) are similar to the system of the lissamine rhodamine B-Au hybrid;42 the fluorescence quenching observed from time-resolved measurements (see below) illustrated that the excitation energy has been transferred from G2-LRh to gold nanoparticles. The Fo¨rster resonance energy transfer (nonradiative dipole-dipole coupling mechanism) mode may not be appropriate for calculating the distance between dye and nanoparticles because it does not consider the local field effects. The excitation energy transfer efficiency may be small because of the strong overlap between the emission spectrum of the gold nanoparticles44 (the absorption coefficient: σabs ) 1.52 × 107 M-1 cm-1 for 5 nm) and absorption spectrum of G2-LRh. The nonlinearity produced by the G2-LRh-Au hybrid can be enhanced as the polarization (related to dielectric constant) of the dye increased. Table 1 also displays the χ3 values of the DNCs incorporated into thin films. As shown in eq 2, the χ3 of the metal nanoparticles depends on volume fraction, p, the nonlinear susceptibility term of the metal nanoparticle itself (χ3m) and the local field factor ( f(ω)) relevant to the dielectric constant. χ3 ) pf (ω)2 | f (ω)|2χ3m

(2)

The larger volume fraction of the metal nanoparticle in G2-Au-PVB film (4.85%) gives the bigger χ3 value (177 × 10-12 esu) when compared to the G2-Au-PMMA film (volume fraction: 3.47%, χ3 ) 116 × 10-12 esu). The thirdorder NLO responses of the nanocomposites in thin films are much larger than those in solutions. For example, the χ3 value of the G2-Au-PMMA film (116 × 10-12 esu) is approximately 26 times larger than the corresponding G2Au in solution (4.5 × 10-12 esu). The large volume fraction ( p) of the metal nanoparticle in thin film and the different dielectric constant may contribute to the larger χ3 of DNCs incorporated into thin films. It should be noted that the χ3 of DNCs films fabricated by such a simple method are comparable to the similar-sized gold nanocluster composites that were prepared by the complicated ion implantation technique.45 In general, there are three major mechanisms for the χ3 effects of gold nanoparticles.46 The first is through an interband electric-dipole transition (∼10-13s) between the states of d valence bands and the states of the s-p conduction band. The second is a hot electron excitation near SPR with

Table 1. Third-Order Nonlinear Susceptibility (χ3) of Dendrimer-Gold Nanocomposites χ3

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materials

(×10-12 esu)

G2-Au G2-LRh-Au G2-LRh G2-Au-PMMA film G2-Au-PVB film

4.5 ( 0.90 14 ( 2.1 5.6 ( 1.12 116 ( 18.5 177 ( 21.4

χ3 (N, the number density of the molecule) N (esu molecule-1) 1.5 × 10-30 (per Au) 1.16 × 10-25 (per G2-LRh), 2.33 × 10-29 (per Au) 4.65 × 10-26

Nano Lett., Vol. 5, No. 12, 2005

Figure 3. (a) Time-resolved fluorescence dynamics of LRh-AEA and G2-LRh in a decay time period of 400 ps. (b) Time-resolved fluorescence dynamics of LRh-AEA, G2-LRh, and G2-LRh-Au in a decay time period of 2.5 ps.

a response time of several picoseconds. The third is thermal contribution, which has a very slow response time. It is likely that the third (thermal) mechanism will have a contribution here. To further provide information regarding the effect of intermolecular interactions (G2-LRh and gold nanoparticles) on the enhanced NLO properties, we carried out ultrafast time-resolved fluorescence upconversion measurements. The fluorescence and fluorescence anisotropy decays have been utilized to characterize the excited-state dynamics of the materials.47 Figure 3 shows the normalized fluorescence dynamics of LRh-AEA, G2-LRh, and G2-LRh-Au at various decay time periods (λex ) 400 nm, λem ) 585 nm). The fast decay component observed in G2-LRh (Figure 3a) is associated with the energy transfer from the monomeric chromophore to nonradiative traps created by intrachromophoric interactions.48 In the short decay time period (Figure 3b), the decay curve of G2-LRh was fitted by a biexponential equation with a rise time of ∼100 fs and a long decay time of ∼1.3 ps. The fact that the rise time feature was observed only in the G2-LRh system and not in the model compound (LRh-AEA) further indicates the strong intramolecular interactions. However, the fluorescence decay of G2-LRh-Au is significantly different from what was observed for G2-LRh. The decay curve of G2-LRh-Au can be fitted by a two-exponential function with decay time constants of ∼234 fs and 1.9 ns. The short- and long-living components42 can be assigned to nanoparticle surface-bound G2-LRh and free G2-LRh. The nonradiative decay process, that is, the fast decay observed in the hybrid system, is a consequence of energy and/or electron transfers from G2LRh to metal nanoparticles. In the first case, the gold nanoparticles accept the transferred energy so that oscillator strength can be increased to produce additional electronic field for G2-LRh. As a result, the nonlinearity of the DNCs can be enhanced. For the case of electron transfer, electrons are transferred from dye to gold nanoparticles. This process may lead to strong charge delocalization between G2-LRh and gold nanoparticles or form a stable charge-separation state. The dipole moment (polarization or dielectric constant) of the entire system would become larger so that the χ3 of Nano Lett., Vol. 5, No. 12, 2005

DNCs can be enhanced as well. As discussed above, the third (thermal) mechanism will have a contribution here but may not be the dominant effect. The excited-state lifetimes of lissamine-Au are in the picosecond regime,42 suggesting that the excitation near the SPR is a factor that strongly affects the polarization of the hybrid. Indeed, at such close distances with strong intermolecular interactions, the changes in excited-state dynamics of the chromophore-metal particle assembly do correlate with an enhancement of third-order optical nonlinearity (χ3) observed in DNCs. In summary, we have synthesized a chromophore-functionalized dendrimer, dendrimer-metal nanocomposites, and dendrimer-metal nanocomposites incorporated into thin films. The linear and nonlinear optical properties were investigated by steady-state and time-resolved fluorescence spectroscopy as well as DFWM measurements. The mechanism for the enhanced optical nonlinearity is suggestively related to local field enhancement. The understanding of the mechanism is very important in the particular design (control of the distance and the dipole orientation between dye and metal) and applications of DNCs in photonics and biophotonics. Acknowledgment. T.G.III acknowledges the Army Research Office and the National Science Foundation for support. Supporting Information Available: Experimental procedures for the synthesis and characterization of model compound LRh-AEA, lissamine rhodamine B functionalized dendrimer, dendrimer-gold nanocomposites, and the nanocomposites incorporated into thin films, and steady-state, time-resolved fluorescence spectroscopy and DFWM measurements This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Handbook of Optics IV, Fiber Optics & Nonlinear Optics, 2nd ed.; Bass, M., Enoch, J. M., Stryland, E. W. V., Wolfe, W. L., Eds.; McGraw-Hill: New York, 2001. (2) Schulz, M.; Tretiak, S.; Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 2000, 122, 452. 2383

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NL051402D

Nano Lett., Vol. 5, No. 12, 2005