Oligothiophene- and Oligopyrrole-Mediated Aggregation of Gold

Apr 4, 2007 - Gold nanocrystals in a toluene solution were aggregated by several thiophene and pyrrole oligomers as shown by surface plasmon (SP) ...
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J. Phys. Chem. C 2007, 111, 5886-5892

Oligothiophene- and Oligopyrrole-Mediated Aggregation of Gold Nanoparticles G. Zotti,* B. Vercelli, and M. Battagliarin Istituto CNR per l’Energetica e le Interfasi, c.o. Stati Uniti 4, 35127 PadoVa, Italy

A. Berlin Istituto CNR di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy

V. Herna´ ndez and J. T. Lo´ pez Navarrete Departamento de Quı´mica Fı´sica, Facultad de Ciencias, UniVersidad de Ma´ laga, 29071-Ma´ laga, Spain ReceiVed: January 12, 2007; In Final Form: February 23, 2007

Gold nanocrystals in a toluene solution were aggregated by several thiophene and pyrrole oligomers as shown by surface plasmon (SP) spectroscopy and TEM analysis. The rate of aggregation can be controlled by varying the oligomer type and concentration. Two distinct regimes of aggregation kinetics are observed: (1) at low oligomer concentrations, aggregation is slow and dependent on the gold nanocrystal concentration with production of a SP band at ca. 700 nm and (2) at high oligomer concentrations, aggregation is very fast and produces a SP band at ca. 570 nm. The results suggest that, in general, at high oligomer concentrations, the linker molecules form more dense layers via perpendicular orientation toward the metal plane, whereas a parallel orientation is obtained at low concentrations. TEM analysis indicates in the high concentration regime that there is formation of spherical or dendritic superclusters depending on the aggregating oligomer. FTIR analysis of the terthiophene and dipyrrole gold aggregates has shown the presence of adsorbed ligand molecules with surface enhancements of some bands, giving features particular to parallel and perpendicular coordination, respectively.

Introduction

CHART 1

Among nanosized particles, gold nanoparticles (AuNPs) are particularly investigated due to their potential applications in sensors, nanoelectronic devices, biochemical tagging reagents, and catalysts.1 We are interested in their connection through conjugated pathways such as those provided by oligothiophenes or oligopyrroles and in generally conjugated polymers (CPs). The possible use of such structures in optoelectronic devices (such as solar cells, light emitting diodes, electronic memories, etc.) is being explored intensively.2 Moreover, the synthesis and processing of nanoparticles consisting of metallic nanocrystal cores and organic monolayer shells promise interesting technological applications in catalysis, drug delivery, microelectronics, medical diagnostics, and particularly in sensors.3 Here, we give a brief survey of the main literature results obtained from AuNPs with CPs. 3-Octylthiophene-2-thiol has successfully capped AuNPs, and these were copolymerized with 3-octylthiophene.4 Recently,5 AuNPs were bridged with some phosphine-terminated oligothiophene linkers. Electrochemical oxidation of these particles results in the deposition of thin films consisting of nanoparticles linked by oligothiophene moieties. The more recent literature reports the production of polythiophene-capped AuNPs from soluble poly(3-hexyl-thiophene)6 and soluble polyaniline as a reagent and host for metal nanoparticles, gold included.7 In this paper, we report the interaction of several oligothiophenes and oligopyrroles (see Chart 1) with tetraoctylammo* Corresponding author. Tel.: (+39)49-8295868; fax: (+39)49-8295853; e-mail: [email protected].

nium-bromide-stabilized AuNPs and the arrangement of the latter in aggregated structures. The investigation has been

10.1021/jp070263g CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007

Oligothiophene/Oligopyrrole-Mediated Aggregation performed with the use of surface plasmon (SP) spectroscopy, FTIR, and TEM analysis. Experimental Procedures Chemicals and Reagents. Toluene solutions of AuNPs (10-2 M gold concentration, λmax ) 525 nm), stabilized by tetraoctylammonium-bromide (TOABr, 50 mM), were prepared according to Schiffrin et al.8 From TEM analysis, the average particle size of the Au clusters is 5 ( 1 nm. AuNPs were routinely used after dilution to 10-3 M gold concentration. Transparent gold-coated supports were prepared from float glass slides by treatment with 3-mercaptopropyl-trimethoxysilane (MTS),9 which provides a surface bearing free thiol groups, and subsequently with a 10-3 M toluene solution of AuNPs for 18 h, which deposited a monolayer of them on the thiol surface. The oligomers 2,2′:5′,2′′-terthiophene-5-thiol (T3SH),10 6-(2,2′: 5′,2′′-terthiophen-5-yl)hexane-1-thiol (T3C6SH),11 diphenyl(2,2′:5′,2′′-terthiophen-5-yl)phosphane (T3PPh2),5 3,3′′-dihexyl-2,2′:5′,2′′-terthiophene (H2T3),12 3,4-dimethoxythiophene (DMT),13 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT),14 4,4′-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT(C6)2),15 3,4:3′,4′-bis(ethylenedioxy)-2,2′-bithiophene (EDT2),16 2-{2-[2-(2-methoxy-ethoxy)-ethoxy]-ethoxymethyl}-2,3-dihydro-thieno-[3,4-b][1,4]dioxine (EDT-EO),17 2-dodecyl-2,3-dihydro-thieno-[3,4-b][1,4]dioxine (EDT-C12H25),18 4H-dithieno[3,2-b:2,3′-d]pyrrole (DTP),19 6-(4H-dithieno[3,2-b:2,3′-d]pyrrol-4-yl)hexanoic acid (DTPC6CA),20 1H,1′H-[2,2′]bipyrrole (DP),21 and 1,1′-dihexyl-1H,1′H-[2,2′]bipyrrole (DHDP)22 were prepared as described in the literature. Regioregular head-totail coupled poly(3-octylthiophene) P3OT was produced chemically according to McCullough et al.23 Hexadecanethiol, pyrrole (P), 2,2′-bithiophene (T2), 2,2′:5′,2′′-terthiophene (T3), 3,4ethylenedioxythiophene (EDT), and all other chemicals were reagent grade and used as received. Apparatus and Procedure. Electronic spectra were obtained from a PerkinElmer Lambda 15 spectrometer. Solution spectra of aggregating species were run after fast mixing (ca. 30 s) of AuNPs and ligand solutions. Spectra were run in less than 30 s. FTIR spectra were taken with a PerkinElmer 2000 FTIR spectrometer. Gold aggregate samples for FTIR analysis were prepared as follows. Addition of 2 mg of T3 or DP to 5 mL of a 10-2 M AuNPs solution in gold produced the aggregate, which was separated by centrifugation, washed three times with 10 mL of toluene to remove excess ligand and TOABr, and dried. Then the black precipitate was finely ground with KBr (200 mg) and the FTIR spectrum of the obtained pellet run. TEM micrographs were taken with a JEM3010 (JEOL Ltd, J) high-resolution transmission electron microscope, operated at 300 kV. Specimens were prepared by placing a drop of the colloidal gold solution onto 200 mesh copper grids, covered with an amorphous carbon film, and allowing the solvent to evaporate. The mean diameters and size distributions of AuNPs were calculated on the basis of the measurement of at least 200 particles. Calculations. Density functional theory (DFT) calculations were carried out using the Gaussian 03 program24 running on a SGI Origin supercomputer. Becke’s three-parameter exchange functional combined with the LYP correlation functional (B3LYP)25 was employed because it has been shown that the B3LYP functional yields similar geometries for medium-sized molecules as those obtained from the MP2 calculations with the same basis sets.26 Moreover, the DFT force fields calculated using the B3LYP functional yield infrared spectra in very good

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5887 agreement with experiments.27 The standard 6-31G** basis set was used to obtain optimized geometries on isolated entities.28 For the resulting ground-state optimized geometries, harmonic vibrational frequencies and infrared intensities were calculated with the B3LYP functional. Calculated frequencies were uniformly scaled down by a factor of 0.96 for the 6-31G** calculations, as recommended by Scott and Radom.27 This scaling procedure typically has an adequate accuracy for a reliable assignment of the experimental data. Thus, all vibrational frequencies reported in this work are scaled values. The theoretical spectra were obtaining by convoluting the scaled frequencies with Gaussian functions (10 cm-1 width at the halfheight). The relative heights of the Gaussians were determined from the theoretical infrared intensities. Results and Discussion Aggregation of AuNPs by Terthiophene-Thiols and -Phosphines. In an attempt to produce a stable capping of AuNPs with oligothiophene thiols via the place exchange route,1 we added T3SH to a toluene solution of AuNPs. The ruby red color turned immediately blue with the subsequent formation of a dark gold precipitate in a clear yellowish solution. The result indicates that gold clusters are aggregated (instead of being capped) by the thiol. A similar behavior has been shown by T3C6SH, in which the thiol head and the T3 tail are spaced by a long alkyl chain. Moreover, the direct (ab initio) synthesis from tetrachloroauric acid and the previously mentioned thiols with sodium borohydride8 have given the same results. It thus appeared that the thiol moiety was not reactive enough in terthiophenes to cap the gold surface. Different from the thiols, the direct formation of phosphinegold nanoclusters (T3PPh2-Au) has been reported with the particle size resulting in being lower than 2 nm.5 We have checked whether this reaction occurs via place exchange also with the 5 nm nanoparticles used in this investigation. Yet, the test was negative in the sense that aggregation is obtained instead, much the same as for terthiophene thiols. Therefore, even phosphine adsorption does not preclude gold aggregation. Since we have confirmed that the direct synthesis of T3PPh2-Au nanoclusters from gold tetrachloroaurate occurs successfully as reported,5 it appears that the formation of phosphine complexes of low valent Au(I)29,30 is essential for the production of the clusters. In this respect, it must be remembered that oligophenylenevinylene-capped 1.5 and 4 nm gold clusters, obtained via the direct and place exchange routes, respectively, behave differently since in butanol, the former are stable where the latter aggregate.31 These results have suggested that the destabilizing agent of AuNPs is the terthiophene moiety itself. To confirm this hypothesis, we have checked the behavior of the non-substituted terthiophene molecule T3 toward the AuNPs as reported in the next section. Aggregation of AuNPs by Terthiophene. Upon addition of T3 to the toluene solution of AuNPs, the red color turns immediately blue, then with time, the solution changes to a black-gray suspension from which gold aggregates precipitate. In a more detailed form, AuNPs in a toluene solution (10-3 M), upon addition of T3 (10-2 M), show an evolution of the SP resonance band. As shown in Figure 1, the loss of the SP band at 525 nm and the appearance of a new band at ca. 570 nm are evident. The spectral evolution is accompanied by a color change from red to blue. This colorimetric feature is due to nanoparticle aggregation in the solution since aggregation causes the SP bands to red-shift from the visible to the near-

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Zotti et al. TABLE 1: Critical Concentration C0 for AuNPs Aggregating Ligands

Figure 1. SP spectra of AuNPs 10-3 M in toluene (-) before and (- - -) after addition of 10-2 M T3 and (‚‚‚‚) after subsequent addition of 10-2 M hexadecanethiol.

infrared region. In fact, the oscillating dipoles of neighboring particles influence the frequency of a central particle,30 producing a red-shift of the SP band. The solution is stable for some minutes before precipitation of a black powder is observed, indicating that the nanoparticle aggregates eventually become too large to remain in solution. Subsequent addition of a gold-capping thiol such as hexadecanethiol (either before or after gold precipitation) causes the immediate restoration of the color and spectral characteristics of the isolated AuNPs (Figure 1), which indicates that aggregation may be reversed. It appears that the shield of tetraoctylammonium bromide on the surface of AuNPs, which prevents particle aggregation, is substituted by monolayer patches of neutral T3 that link the particles together like double-sided tape. It is in fact reported that thiophene and terthiophene32,33 are adsorbed on gold surfaces in ethanol solution. In particular, terthiophene forms a monolayer with the terthiophene plane parallel to the gold surface.32 We have confirmed this result spectrophotometrically finding on gold-coated glass the formation of a T3 monolayer with a maximum at 365 nm and with an intensity of 6 × 10-3 au. From the extinction coefficient of T3 (2.5 × 104 M-1 cm-1 34) and considering that both sides of the glass slide are functionalized, the degree of coverage is ca. 1 × 10-10 mol cm-2. Since a dense ferrocene monolayer is formed by 4.5 × 10-10 mol cm-2 35 and the T3 molecule lies flat over the surface, the coverage may be considered complete. Aggregation of AuNPs by Other Ligands. Using the same procedure adopted for T3, AuNPs in a toluene solution have been added with different thiophene and pyrrole oligomers resulting in different evolutions of the SP resonance band. The results in terms of critical concentration (see definition in next section) are summarized in Table 1. Thiophenes. In a way similar to that of T3 but at higher concentrations, bithiophene T2 causes aggregation of AuNPs, whereas thiophene T is not effective. It is thus clear that the role of the oligomer length is crucial since the linker ability increases with its degree of oligomerization. The literature reports that EDT also forms a strongly chemisorbed monolayer by interaction of the π-system with gold.36 Accordingly, we have found that the SP spectrum broadens in 30-60 min upon addition of 10-2 M EDT2. In the case of monomeric EDT, a 10-2 M concentration is ineffective, but in a 1 M concentration or higher, it produces the blue color (band at 560 nm). The effect of the oligomer length is once more evident.

ligand

C0 (M)

P DP T T2 T3 EDT EDT2 EDT-EO 12-crown-4 EDT-C12H25 DMT CPDT DTP

5 × 10-2 10-4 >10-2 10-3 1.0 10-3 5 × 10-2 10-2 10-1 10-2 10-4

Addition of a short ethylenoxy chain to EDT, such as in EDT-EO, makes aggregation begin at a much lower concentration, which suggests that the ethylenedioxy part of EDT helps coordination. In fact, 12-crown-4 is effective in aggregating AuNPs and at an even lower concentration (10-2 M). On the contrary, the presence of a similarly long alkyl chain in EDT-C12H25 does not allow aggregation even at a 0.1 M concentration, as expected from efficient steric hindrance by the alkyl chain. Also, the presence of alkoxy substituents in the 3 and 4 positions of thiophene, such as in DMT, does not inhibit aggregation, which on the contrary appears to be even stronger than with EDT. Polythiophene monolayers have been reported to be produced by adsorption of soluble poly(3-octylthiophene) P3OT on gold.37 We have in fact found that the AuNPs in toluene do not aggregate upon addition of P3OT. This suggests that the alkyl side groups prevent self-interaction among gold particles. A test with the dihexyl-substituted terthiophene H2T3 has shown no spectral change after hours, thus confirming that the steric action of alkyl substituents is the factor that prevents aggregation. In a fashion similar to T3, CPDT and DTP (and its N-substituted homologues such as, e.g., DTPC6CA) cause aggregation. At difference 4,4′-dihexyl-substituted cyclopentadithiophene CPDT(C6)2 does not cause any change in the SP spectrum, giving further support to the suggested idea of steric stabilization. Pyrroles. Pyrroles also aggregate AuNPs. We found that pyrrole itself up to 10-2 M shifts the SP peak to 570 nm. At lower concentrations (typically 10-3 M), pyrrole produces a slow aggregation of free AuNPs to clusters at ca. 700 nm. Precipitation is observed only after several hours. Dipyrrole DP up to 10-4 M immediately causes a similar aggregation (red-shift to 580 nm) with follow-up precipitation of the aggregates, whereas the N-hexyl-substituted homologue NRDP does not cause any aggregation. It is thus shown that in the π-conjugated backbone, the sulfur atom is not specifically required to aggregate, whereas once more, alkyl substitution introduces steric restraints to this process. Phenylenes. The observed stability of AuNPs in toluene is a first indication that the phenyl moieties do not favor aggregation. This has been confirmed with R,R′-diphenyl, which does not change the SP band even in concentrations as high as 10-1 M. This fact suggests that a low aromaticity (which is broken in the surface adduct formation) or a minimum dipole are required for the linker molecule to promote the formation of aggregates. Finally, we must stress that in no case does ligand oxidation, which could make results uncertain, take place. The most easily oxidized ligand dipyrrole displays an oxidation potential (ca.

Oligothiophene/Oligopyrrole-Mediated Aggregation

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5889

Figure 2. Time evolution of SP spectra of AuNPs 2 × 10-3 M in toluene upon addition of 1.4 × 10-3 M pyrrole. Spectra recorded after (-) 0, (- - -) 15, and (‚‚‚‚) 30 min. Inset: time decay of SP band at 525 nm with curve fitting eq 1. (χ2 ) 1.2435 × 10-6 and R2 ) 0.99881).

Figure 3. SP spectra of AuNPs 10-3 M in toluene (-) before and (- - -) after 30 min addition of 0.1 M EDT.

0.2 V vs Ag/Ag+21) sufficiently positive that no oxidation is produced, given that the gold solution, coming from reductive procedures, shows a potential around -0.2 V. Moreover, in all cases where complete flocculation occurred, the clear solution did not show optical features attributable to an oxidized ligand. Aggregation Kinetics of AuNPs. Addition of pyrrole (10-3 to 10-2 M) to AuNPs in a toluene solution (10-3 M) causes the progressive decrease of the free-cluster SP resonance band at 525 nm and an increase of a broad band around 700 nm due to the SP of aggregated clusters (Figure 2). A clear isosbestic point at 600 nm marks the passage from free to aggregated AuNPs. We have followed the time evolution of the bands before precipitation is observed, as recently reported for the chain formation of gold nanorods.38 The decay of the free-cluster SP band absorbance A (Figure 2) follows a power-law dependence on time of the type

A ) A0 - ktb

(1)

with b ) 0.35 ( 0.05. This result by itself does not allow discrimination between chemically controlled and diffusioncontrolled kinetics. Previous studies on the aggregation of gold colloids39 have in fact revealed changes between reaction-limited and diffusion-limited aggregation with analogous power-law dependences on time. The reaction of pyrrole with AuNPs proceeds in a progressive manner (no steps), and an excellent fit is obtained when eq 1 is applied to the data. The rate constant k for a gold concentration of 10-3 M is 0.024 min-b (0.003 s-b) independent from the TOABr concentration in the range of (5-50) × 10-3 M and from the pyrrole concentration in the range of (1-20) × 10-3 M (lower concentrations of pyrrole do not produce aggregation). Increasing the gold concentration from 1 × 10-3 to 4 × 10-3 M approximately doubles the aggregation rate. For pyrrole concentrations g10-1 M, an immediate change to blue (due to a band around 570 nm) and a steady value of absorbance indicates a new type of faster reaction, and a concentration of 5 × 10-2 M (critical concentration C0) marks the passage from one regime to the other. A similarly slow kinetics is followed by, for example, EDT (see spectra in Figure 3) in the range of 0.1-0.5 M (lower concentrations do not produce aggregation). The rate constant k (for a gold concentration of 10-3 M) is 0.02 min-b as for pyrrole, which suggests that this slow process, independent from the nature of the incoming linker, either is diffusion controlled

Figure 4. TEM micrographs of 5 nm AuNPs (a and b) before and (c and d) after T3 addition. The latter show parallel alignment of adjacent particles.

or involves the displacement of the AuNP-stabilizing TOABr shell as the rate-determining step of the aggregation. Also in this case, high ligand concentrations (g1 M) cause an immediate change to blue (570 nm) as sign of a faster aggregation. TEM Analysis of Aggregated AuNPs. To explore the dependence of gold nanocrystal arrangement on the aggregant, solutions of AuNPs free or capped with T3 or DP were deposited on a flat carbon-coated copper grid, and TEM images of the resultant nanocrystals were collected after the solvent evaporated (see Figures 4 and 5). The images (Figure 4) reveal that the colloids capped with T3 (Figure 4c,d) form densely packed 3-D structures with a spherical outline, some 2-D layers being apparent. In contrast, in the case of bare AuNPs (Figure 4a,b), disordered 3-D structures are present. The measured diameter of the T3 aggregated spheres is in the range of 100-300 nm. The overall behavior is similar to that observed for thioethers,40 and a similar spherical aggregation has been recently observed also with 1,9nonanedithiol.41 Since the orientation of the T3 chains is well-defined, there is a tendency to form highly ordered close-packed layers. From the ordered domains of nanoparticles, the interparticle edge-toedge distance was evaluated as 1.3 ( 0.4 nm. This distance reflects the presence of T3 molecules between the nanoparticles. That the nanoparticles in the aggregate are individually isolated

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Zotti et al.

Figure 7. FTIR spectra of (a) DP-Au aggregate and (b) DP. (c) Simulated spectrum of DP.

Figure 5. TEM micrograph of 5 nm AuNPs after DP addition.

Figure 6. FTIR spectra of (a) T3-Au aggregate and (b) T3. (c) Simulated spectrum of T3.

is consistent with the picture of molecular bridging between the nanoparticles revealed by the adsorption data. In contrast, for simple salt-induced aggregation of nanoparticles, these are usually fused together because of the collapse of electrical double layers. Differently from T3, DP forms irregular dendritic superclusters (Figure 5) constituted by spherically shaped aggregates with diameters in the range of 20-50 nm. The formation of dendrites may be due to the faster aggregation and/or the stronger linkage given by the DP molecules. FTIR Analysis of Aggregated AuNPs. The aggregation of AuNPs by the conjugated ligands may involve the ligand as a simple promoter of gold aggregation or as an active component of the aggregate. FTIR analysis of AuNPs after aggregation promoted by T3 or DP ligands has allowed us to confirm the presence of the ligand in the aggregate and to determine some of its characteristics. Samples were prepared as reported in the Experimental Procedures. The FTIR spectra of T3 and DP are displayed in Figures 6 and 7, respectively, in AuNPs (a), as free ligands (b), and as the theoretical B3LYP/6-31G** spectrum (c). In Figures S1 and S2 of the Supporting Information are sketched the eigenvectors associated with the most important bands. In general, the theoretical spectra reproduce the main tendencies of the experimental features even when the absorption spectra are

recorded in the solid state. In the calculated infrared spectrum of T3, the region below 900 cm-1 is dominated by four normal modes at 814, 798, 772, and 669 cm-1. Thus, in view of the calculated eigenvectors, the multiplets of bands at 814 and 798 cm-1 must be assigned to the in-plane symmetric and antisymmetric aromatic C-S stretching modes, respectively. On the other hand, the C-H out-of plane bending vibrations whose wavenumbers are characteristic of the substitution positions appear in the 800-600 cm-1 region. We can easily identify the feature near 770 cm-1 as the Cβ-H out-of-plane bending vibration, whereas the doublet of bands centered at 670 cm-1 is due to the CR-H out-of-plane mode. In the calculated infrared spectrum of DP, the sets of mediumweak bands between that at 1224 and that at 873 cm-1 can be described as different in-plane C-H deformations with large contributions of the in-plane bending vibrations of the N-H bonds, especially in the modes calculated at 1224, 1085, and 873 cm-1. Finally, the strong modes calculated at 740 and 674 cm-1 arise again from the Cβ-H and CR-H out-of-plane C-H bending vibrations, respectively. Optical properties of molecules are altered dramatically when they are adsorbed on or near some rough metal surfaces. Since the discovery, the surface-enhanced infrared absorption SEIRA has been thoroughly investigated.42-44 It has been proposed that the enhancement is due to a strong electromagnetic field amplified through the excitation of collective electron resonance (localized plasma oscillation) of the small metal island.45 The infrared surface selection rule for molecules at and near a smooth surface of an extended metal is based on the high electronic screening of all metals in the infrared spectral region. Because of the screening currents parallel to the surface within the metal, the electric vector parallel to the surface is very small as compared to the perpendicular component. Therefore, the incident radiation will essentially interact with the molecular vibrations that have dipole changes oriented perpendicularly to the surface. In SEIRA, molecular vibrations having transition dipole components perpendicular to the surface are selectively observed.46 The infrared spectrum of the T3-Au aggregate is compared with the pristine T3 spectrum in Figure 6. If the average molecular orientation had the molecular plane parallel to the surface of the metal islands, then most of its in-plane vibrations would be perpendicular to the relevant electric vector of light and will exhibit low absorptions. In contrast, out-of-plane vibrational dipoles would be parallel with the field, and the corresponding modes should be enhanced in the spectrum.

Oligothiophene/Oligopyrrole-Mediated Aggregation SCHEME 1

Taking the 814 and 772 cm-1 vibrations as typical markers for the in-plane and out-of-plane modes, it can be seen in the figure that the 814/772 relative intensity is largely decreased in the SEIRA spectrum. The results indicate the average orientation of the thiophene planes of T3 with respect to the Au surface near 0°. Obviously, the weak in-plane absorptions cannot originate from species adsorbed at flat islands but have to be assigned to species located at surface defects or to species in the second or multilayers with the molecular plane no more than parallel to the metal surface. Opposite of T3, in DP, the very strong bands at 740 and 674 cm-1 assigned to the out-ofplane C-H bending modes are practically missing in the SEIRA spectrum, indicating that DP is adsorbed vertically with the C2 axis parallel (for the anti conformation) to the Au surface. Formation and Structure of Oligoheterocycle-Assembled AuNPs. According to the previous32 and present results, terthiophene links to gold with the terthiophene plane parallel to the gold surface using its three sulfur atoms and the π-conjugated carbons in line.32 Such moieties may link two adjacent gold surfaces (i.e., a single terthiophene may act as a adhesive sheet between them). For this reason, cyclopentadithiophene and N-carboxyhexyl-dithienopyrrole cause aggregation, whereas the 4,4′-dihexyl-substituted cyclopentadithiophene and dihexylterthiophene, which bear bulky substituents, do not. Since pyrroles cause aggregation as well, it is clear that the sulfur atom is not specifically required to aggregate. As in the case of thiophene, in pyrroles alkyl substitution introduces steric restraints even to inhibit the process completely. Scheme 1 illustrates the results by showing the assembling processes in an idealized fashion using pyrrole as an example. The kinetic investigation has suggested that the mechanism of aggregate formation involves in fact two steps: (1) a slow step at low ligand concentrations and (2) a fast step at higher concentrations. Below a critical concentration C0, gold nanospheres possessing the anchoring adsorbate interlock slowly to yield the aggregates. The formation of the strongly red-shifted absorption band at ca. 700 nm is attributed to the selective coupling of the plasmon oscillations in tightly interlinked nanospheres. Above the critical concentration C0, the aggregation is very fast and leads to aggregates characterized by a lower redshift (band at ca. 570 nm). From this result, it may be concluded that in the superstructures formed under these conditions, AuNPs are less closely spaced. It is possible that at high oligomer concentrations, the linker molecules form more dense layers, possibly via perpendicular (rather than parallel) orientation toward the metal plane. In fact, the orientation of thiophene adsorbed on gold has been suggested as either parallel (1) or perpendicular (2) depending on the conditions.32 Moreover, the cofacial interaction between thiophene molecules, expected from a more compact assembly (type 2), has been evidenced by STM observations on 111 gold faces.33 The progression from a parallel to an upright position has also been found from FTIR analysis.47 Finally, the SEIRA spectra we recorded for the stable forms of T3- and DP-Au aggregates have evidenced clearly the formation of layers of flat and perpendicular adsorbed molecules.

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5891 From the C0 values, listed in Table 1, we can draw a series of conclusions: (1) from the pyrrole, thiophene, and 3,4ethylenedioxythiophene series, it results that C0 decreases as the oligomer length is increased, which points to a higher adsorption constant for the longer oligomer (ca. 1000-fold increase from monomer to dimer). This result is in fact reasonable since longer oligomers provide multiple coordination sites. (2) C0 increases in the series pyrrole < 3,4-ethylenedioxythiophene < thiophene, which suggests that a higher polarity of the heteroaromatic molecule favors adsorption to gold, as confirmed by the absence of adsorption by phenylenes. 3,4Ethylenedioxythiophene presents oxygen atoms available for further coordination. It appears that coordination of the heteroatoms to gold is the key factor for adsorption. Considering that the previous sequence follows that of increasing aromaticity, also the facile loss of aromaticity accompanying the heteroatom coordination to gold may favor pyrrole over, for example, thiophene. (3) C0 is 100 times lower for DTP than for CPDT, which indicates that the bithiophene-bridging nitrogen atom favors adsorption to gold. Also, in this case, the presence of additional coordinating atoms increases adsorption and favors aggregation. Conclusion AuNPs capped with a tetraoctylammonium bromide monolayer and dissolved in toluene solution, upon addition of sterically free oligothiophenes or oligopyrroles, form aggregated superstructures. The interactions between AuNPs and oligomers were investigated using SP absorption spectroscopy, FTIR, and TEM. On the basis of these studies, it was concluded that the aggregate formation proceeded in two ways, characterized by a different spacing among the aggregated clusters, depending on the linker type and concentration. The main outcome of this investigation is that polyconjugated heterocyclic oligomers have been found to aggregate AuNPs, producing assemblies that can be readily disassembled. Therefore, self-assembly of AuNPs on different nonsterically crowded polyconjugated polymers appears to be feasible. Moreover, the discovery that hindrance to aggregation is provided by alkyl substitution opens up the way to direct adsorption of thiol-functionalized oligothiophenes and oligopyrroles on preformed AuNPs of definite size, thus allowing the facile obtainment of new gold-polymer superstructures. These topics are the object of our ongoing research, and the results will be presented in forthcoming papers. Acknowledgment. The authors thank Dr. G. Schiavon of the CNR for helpful discussions and S. Sitran of the CNR for his technical assistance. Research at the University of Ma´laga was supported by MEC of Spain through Research Projects BQU2003-03194 and CTQ2006-14987-C02-01 and by the Junta de Andalucı´a for our FQM-159 scientific group. Supporting Information Available: Figures S1 and S2 of B3LYP/6-31G** eigenvectors calculated for some of the vibrational normal modes of T3 and DP, respectively. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) See, for example (a) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (b) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka,

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