Electrodeposition of Oligothiophene-Linked Gold Nanoparticle Films

Jun 16, 2004 - Bryan C. Sih, Antje Teichert, and Michael O. Wolf*. Department of Chemistry, University of British Columbia,. Vancouver, British Columb...
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Chem. Mater. 2004, 16, 2712-2718

Electrodeposition of Oligothiophene-Linked Gold Nanoparticle Films Bryan C. Sih, Antje Teichert, and Michael O. Wolf* Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z1 Received November 28, 2003. Revised Manuscript Received May 4, 2004

The electrodeposition and characterization of conducting thin films of gold nanoparticles bridged with oligothiophene linkers are reported. Gold nanoparticles capped with the conjugated phosphines 5-diphenylphosphino-2,2′:5′,2′′-terthiophene (dcore ) 1.7 ( 0.3 nm) and 5-diphenylphosphino-3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (dcore ) 1.8 ( 0.4 nm) have been prepared. Electrochemical oxidation of these particles results in the deposition of thin films consisting of intact nanoparticles linked by oligothiophene moieties, in which the π-π* absorption of both the conjugated linker and the plasmon band are red-shifted relative to individual nanoparticles. Conductivities of both the unlinked and linked particles are measured and the latter are shown to have substantially higher conductivities than unlinked particles or gold nanoparticles linked by saturated alkyl chain linkers.

Introduction Nanoscale metal and semiconductor particles are of interest due to the unique catalytic, optical, and electronic properties arising from their size.1,2 These materials have been proposed for use in sensor, biomedical, and nanoelectronic devices, and many such applications require organization of these nanoparticles (NPs) into controlled and functional architectures. Considerable effort has been focused on creating two- and threedimensional networks of NPs using electrostatic interactions,3,4 hydrogen bonds,5 and saturated organic linkages.6-8 Where particles are linked by nonconjugated linkers, electron tunneling is the predominant mechanism for electrical conduction.9 The electrical conductivity (σ) depends primarily on the charge carrier population (n), the electronic coupling term (β, Å-1) and the activation energy barrier (EA, kJ mol-1) to electron transfer (eqs 1 and 2)

σ ) σ0 exp[-βδ] exp[-EA/RT]

(1)

σ0 ) neµ

(2)

where δ is the average interparticle distance (Å), R is the universal gas constant, T is the temperature (K), * Corresponding author. E-mail: [email protected]. (1) Schmid, G. Chem. Rev. 1992, 92, 1709. (2) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (3) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (4) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789. (5) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (6) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (7) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (8) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (9) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465.

e is the elementary charge on an electron, and µ is the mobility of the charge carriers. Thus, the electrical conductivities of these materials are expected to decrease as the length of the linker is increased due to higher activation energies, and this has been observed experimentally.10 It is therefore of interest to prepare systems with improved interparticle electrical conductivity with fixed particle-particle distances by varying β and EA. Previous studies have shown that conjugated linkers between an electrode and a molecular site provide improved electron coupling (β ) 0.4-0.6 Å-1)11-13 relative to purely saturated linkers (β ) 0.8-1.0 Å-1). Recently, Torma et al.14 used partially conjugated linkers to link Au NPs and showed that these materials have substantially lower EA values compared to Au NPs linked by nonconjugated linkers. The lower activation energies have been attributed to a different mechanism for interparticle electron transfer, which involves charge propagation through the organic bridging molecules. Bourgoin et al.15 obtained similar results by comparing the electrical conductivities of a monolayer of dodecanethiol-capped Au NPs with a monolayer of the same material where some of the ligands were exchanged with a terthiophene linker. The terthiophene-linked monolayer exhibited a conductivity several orders of (10) Simon, U.; Flesch, R.; Wiggers, H.; Schon, G.; Schmid, G. J. Mater. Chem. 1998, 8, 517. (11) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563. (12) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059. (13) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (14) Torma, V.; Vidoni, O.; Simon, U.; Schmid, G. Eur. J. Inorg. Chem. 2003, 1121. (15) Bourgoin, J.-P.; Kergueris, C.; Lefevre, E.; Palacin, S. Thin Solid Films 1998, 327-329, 515.

10.1021/cm035242n CCC: $27.50 © 2004 American Chemical Society Published on Web 06/16/2004

Electrodeposition of Au Nanoparticle Films

magnitude better than the unlinked Au monolayer. These studies suggest it would be of significant interest to link NPs using fully conjugated groups in three dimensions, thus allowing improved interparticle charge transfer and electronic communication. Datta et al.16 predicted that Au NPs linked together by “molecular wires” such as polythiophene (PT) or polyphenylenevinylene (PPV) would function as molecular ribbons useful in generating an interconnection network. To this end, NPs have been embedded in π-conjugated matrixes,17-20 and conductivity measurements on these hybrid materials show conductivities several orders of magnitude greater than the corresponding polymers without NPs present.18,20 These results demonstrate that gold NPs interact with the π-conjugated matrix; however, the electronic interactions and distribution of NPs in the matrix are illdefined. Recently, gold nanoparticles have also been capped with arenethiolates and the mixed saturated/ conjugated ligand substantially improved the interparticle conductivity.21 Linking Au NPs with π-conjugated groups also offers the possibility of tuning the conductivity of the material after assembly. Three-dimensional networks of NPs linked by saturated bridges have fixed conductivities dependent on interparticle distance. Once the network has been assembled, the linkers are fixed, and hence the conductivity cannot be varied. However, it is wellknown that the electrical conductivity of π-conjugated polymers such as polyacetylene and PT can be controlled via the degree of chemical or electrochemical doping of these materials.22,23 Therefore, in a network of Au NPs linked by π-conjugated bridges, in situ control of interparticle interactions and conductivity via doping of the polymer may be possible. We reasoned that a suitable approach to such hybrid materials was to prepare Au NPs with a conjugated unit attached to the surface via a suitable functional group, and linking these NPs by coupling of the conjugated units. NPs functionalized with electroactive pyrrolyl or ferrocenyl groups attached via an alkanethiolate group have been linked, however in these systems the linked NPs are separated by an insulating alkyl tether.24,25 Prior work in our group has shown that phosphines may be used to coordinate transition metals to conjugated oligothiophenes,26 and these complexes can be electropolymerized into π-conjugated polymers.27,28 Weare et al.29 developed a procedure for a single-step synthesis of (16) Datta, S.; Janes, D. B.; Andres, R. P.; Kubiak, C. P.; Reifenberger, R. G. Semicond. Sci. Technol. 1998, 13, 1347. (17) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Chem. Commun. 2001, 613. (18) Sarma, T. K.; Chowdhury, D.; Paul, A.; Chattopadhyay, A. Chem. Commun. 2002, 1048. (19) Peng, Z.; Wang, E.; Dong, S. Electrochem. Commun. 2002, 4, 210. (20) Breimer, M. A.; Yevgeny, G.; Sy, S.; Sadik, O. A. Nano Lett. 2001, 1, 305. (21) Wuelfing, W. P.; Murray, R. W. J. Phys. Chem. B 2002, 106, 3139. (22) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (23) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (24) Hata, K.; Fujihara, H. Chem. Commun. 2002, 2714. (25) Yamada, M.; Tadera, T.; Kubo, K.; Nishihara, H. J. Phys. Chem. B 2003, 107, 3703. (26) Clot, O.; Akahori, Y.; Moorlag, C.; Leznoff, D. B.; Wolf, M. O.; Batchelor, R. J.; Patrick, B. O.; Ishii, M. Inorg. Chem. 2003, 42, 2704. (27) Clot, O.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2000, 122, 10456.

Chem. Mater., Vol. 16, No. 14, 2004 2713 Scheme 1. Synthesis of 3 and 4

phosphine-stabilized gold NPs (dcore ≈ 1.5 nm), providing a suitable approach to incorporating conjugated capping groups via phosphine coordination. Here we report the preparation and characterization of oligothiophene-capped Au NPs and electrodeposited films of these hybrid materials. Experimental Section General. Chemicals were purchased from Aldrich except HAuCl4‚3H2O and 3,4-ethylenedioxythiophene which are from Strem Chemicals and Bayer, respectively. All reactions were performed using standard Schlenk techniques with dry solvents under a nitrogen atmosphere, and, unless otherwise noted, all reagents were used without further purification. Electrochemical measurements were conducted using a Pine AFCBP1 bipotentiostat. The working electrode was either a Pt disk, an indium tin oxide (ITO) thin film on glass, or Au (1000 Å) deposited on Si using a Cr (50 Å) adhesion layer. The counter electrode was a Pt coil wire and the reference electrode was a silver wire. An internal reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(n-Bu)4N]PF6 was used as a supporting electrolyte and was purified by triple crystallization from ethanol and dried at 90 °C under vacuum for 3 days. Methylene chloride used for cyclic voltammetry was purified by passing it through an activated alumina tower. Solution electronic absorption spectra were obtained on a Varian Cary 5000 UV-vis/NIR spectrometer in CH2Cl2, and solid-state absorption spectra were acquired on films deposited on ITO. Transmission infrared spectra were obtained on a Bomem MBseries spectrometer using KBr pellets. 1H and 31P NMR spectra were acquired on a Bruker AV-300 spectrometer, and spectra were referenced to residual solvent (1H ) or external 85% H3PO4 (31P). Energy-dispersive X-ray (EDX) analysis was performed on a Kevex Quantum light element X-ray detector equipped with a Quartz Xone X-ray analyzer. Elemental ratios for electrodeposited films on ITO/glass were compared to those of films of unlinked Au NPs cast from CH2Cl2 solution onto ITO/glass. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Leybold MAX200 equipped with an Al KR source with a pass energy of 192 eV, and the sampling area was 2 × 4 mm. Transmission electron microscopy (TEM) images were taken using a Hitachi H7600 electron microscope operating at 80-100 kV. NPs were dropcast from CH2Cl2 solutions onto Formvar-coated 300-mesh copper grids, whereas films of the linked NPs were electrodeposited directly onto 2000-mesh gold grids. The particle sizes were measured using the image processing program Quartz PCI 5 where a total of 150 particles were counted resulting in a mean core size. Scanning electron microscopy (SEM) images were taken using a Hitachi S4700 electron microscope operating at 5 kV. Films were imaged directly on ITO/glass electrodes used for electrodeposition. All interparticle distances used in this paper were either from literature or calculated using Hyperchem software via a semiempirical (AM1) geometrical optimization. Synthesis of NPs. The synthetic routes to the oligothiophene derivatives 3 and 4 are shown in Scheme 1. 5-diphen(28) Clot, O.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2001, 123, 9963. (29) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890.

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ylphosphino-2,2′:5′,2′′-terthiophene (3) was synthesized according to the literature procedure.18 5-diphenylphosphino3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (4) was synthesized in a similar fashion from 3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (2). Au NPs 5 and 6 capped with the oligothiophenes 3 and 4, respectively, were prepared using a literature procedure.29 5-Diphenylphosphino-3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (4). A solution of n-butyllithium in hexanes (12.8 mL, 1.6 M, 20.5 mmol) was added dropwise to a stirring solution of 3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene30 (2) (6.19 g, 18.8 mmol) in THF (100 mL) at -78 °C. The mixture was stirred for 1 h at -78 °C, and distilled PPh2Cl (5.1 mL, 28.1 mmol) was added dropwise. The reaction was then allowed to warm to room temperature and stirred for another 30 min, after which 1 M HCl was added to quench the reaction. The organic layer was separated, washed with water, and dried over MgSO4. The solvent was removed to yield the crude product, which was purified by chromatography on silica gel with hexanes/methylene chloride (7/3 v/v). Removal of the solvent gave 4 as a yellow powder. Yield: 5.1 g (52%). 1H NMR (300 MHz, CDCl3): δ 7.38 (m, 10H, Ph), 7.21 (s, 1H, Th), 7.20 (dd, 4H, Th), 7.01 (dd, 1H, Th), 4.33 (m, 4H, CH2). 31P{1H} NMR (121.5 MHz, CDCl3): δ -17.67 (s). Anal. C26H19O2PS3 requires C, 63.65; H, 3.90. Found: C, 63.76; H, 4.04%. Au146(3)56 (5). Phosphine 3 (1.5 g, 3.5 mmol) was added to a solution of HAuCl4‚3H2O (0.4 g, 1.0 mmol) and tetraoctylammonium bromide (0.63 g, 1.1 mmol) in a nitrogen-sparged distilled water/toluene (20:26 mL) mixture. The yellow solution was stirred vigorously for 20 min. NaBH4 (0.55 g, 14.6 mmol) was dissolved in 5 mL of distilled water and added dropwise to the yellow solution. The solution turned black rapidly and was stirred for an additional 3 h. The solvent was then removed in vacuo to yield crude 5, which was purified by reprecipitation (8×) from a solution of methylene chloride using hexanes. The pure product was obtained as a dark black powder. After purification no phase transfer catalyst or starting materials were observed by 1H NMR spectroscopy (Figure S1).31 Au201(4)86 (6). This material was prepared according to the same procedure used for 5 by using phosphine 4 instead of 3. Purification was by reprecipitation (6×) from a solution of methylene chloride using hexanes, followed by reprecipitation from methylene chloride with diethyl ether (5×). Pure 6 was obtained as a dark brown powder. After purification no phase transfer catalyst or starting materials were observed by 1H NMR spectroscopy (Figure S1).31 Electrodeposition. Electrochemical deposition was carried out in dry methylene chloride using a sealed glass threeelectrode electrochemical cell. A silver wire (reference electrode), platinum wire (counter electrode), and either a platinum disk, ITO/glass, or Au/Si (working electrode) were used. The Pt disk working electrode was polished prior to the experiment using a 0.05-µm alumina slurry, followed by sonication in water to remove traces of alumina from the Pt surface, washing with water, and drying. For ITO/glass or Au/ Si working electrodes, the surface was cleaned with acetone and dried in a vacuum before use. The deposition solution consisted of 15-20 mg of 5 or 6 dissolved in 6 mL of methylene chloride containing 0.1 M [(n-Bu)4N]PF6. Deposition was carried out by scanning from -0.5 to +1.8 V at 100 mV sec-1. Conductivity Measurements. Solid-state conductivity measurements for unlinked Au NPs 5 and 6 were carried out using interdigitated array electrodes (IDAs) kindly provided by Prof. Tim Swager (each array consists of 100 Au fingers with a 2-µm gap between fingers, 2005 µm finger length, and 3.5 µm finger height). For calculation of conductivities, the IDAs are treated as parallel plate electrodes, using the total area of all the electrode fingers facing each other across the 2-µm gap. The total area (ATOTAL) was calculated using eq 3 (30) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121. (31) See Supporting Information.

Sih et al. ATOTAL ) AFINGER(N - 1)

(3)

where N is the number of IDA fingers.32 Unlinked NPs 5 and 6 were dissolved in methylene chloride and dropcast onto the clean IDA electrodes from solution and then dried. The conductivities of the cast NPs were measured using linear potential sweeps, which produce linear current-potential (I-V) curves. The conductivity of 5 and 6 increased as more material was cast onto the IDAs and eventually leveled off; measurements were taken once conductivities no longer increased. Conductivities (σRT) were measured from the slope (∆I/∆E) of the I-V curve between ( 1000 mV and were calculated from eq 4

σRT )

d∆I ATOTAL∆E

(4)

where d is the IDA gap (2 µm). I-V scans were initiated at -1000 mV and scanned to 1000 mV at 100 mV sec-1; the I-V curves remained linear with no evidence of hysteresis. At least five measurements were taken for each sample and the standard deviations were found to be less than 1%. Solid-state conductivity measurements on linked NPs poly-5 and poly-6 were carried out by electrodeposition of the samples directly onto Au/Si substrates. Au/Pd (60/40) electrodes were sputter coated on top of the films through a mask with an area between 16 and 25 mm2, and silver epoxy was used to attach a Cu wire to the Au/Pd contact. The thickness of the samples was measured using stylus profilometry (Tencor Alpha-Step 100). Conductivities (σRT) of the electrodeposited NPs were measured using linear potential sweeps and were measured from the slope (∆I/∆E) of the I-V curve between ( 500 mV using eq 5

σRT )

d∆I A∆E

(5)

where d is the film thickness and A is the area of the top contact. Voltage scans were initiated at -500 mV and scanned to 500 mV at 100 mV sec-1; the I-V curves remained linear with no evidence of hysteresis over this range.

Results and Discussion 5-Diphenylphosphino-2,2′:5′,2′′-terthiophene (3) and 5-diphenylphosphino-3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (4) were prepared via deprotonation with n-BuLi of 2,2′:5′,2′′-terthiophene (1) and 3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene (2), respectively, followed by reaction with chlorodiphenylphosphine (Scheme 1). Au NPs capped with 3 and 4 (NPs 5 and 6, respectively) were then prepared using the Weare procedure (Scheme 2).29 Characterization by transmission electron microscopy (Figure 1) on Formvar coated 300-mesh copper grids revealed that the diameters of both NPs 5 (dmean ) 1.7 nm ( 0.3 nm) and 6 (dmean ) 1.8 nm ( 0.4 nm) have narrow size distributions (Figure 1, insets). UV-vis spectroscopy supports these TEM results (Figure 2, bottom scans) showing no significant plasmon peak at ∼520 nm indicating the average size of the particles are 800 nm in the UVvis spectrum of poly-5 (Figure 6). A comparable spectrum is observed upon oxidation of sexithiophene with four peaks of similar energy.55,56 The electrical conductivities of dry films of unlinked 5 and 6 were measured on IDA electrodes. The I-V curves observed for both NPs were linear (Figure 7a), and electrical conductivities at room temperature (σRT) (50) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557. (51) Su, K. H.; Wei, Q. H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 1087. (52) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R. Opt. Commun. 2003, 220, 137. (53) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460. (54) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (55) Hill, M. G.; Penneau, J. F.; Zinger, B.; Mann, K. R.; Miller, L. L. Chem. Mater. 1992, 4, 1106. (56) Casado, J.; Miller, L. L.; Mann, K. R.; Pappenfus, T. M.; Hernandez, V.; Lopez Navarrete, J. T. J. Phys. Chem. B 2002, 106, 3597.

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Figure 7. Current-potential response of (a) 5 and 6 at room temperature on an IDA electrode and (b) poly-5 and poly-6 sandwiched between two electrodes. Table 3. Room-Temperature Electrical Conductivities for 5, 6, Poly-5, and Poly-6 sample

σRT (S cm-1)

interparticle distance (Å)

5 6 poly-5 poly-6

(6.16 ( 0.02) × 10-5 (7.20 ( 0.02) × 10-5 (3.0 ( 0.5) × 10-2 (8 ( 3) × 10-2

∼15.3a ∼15.3a 24.6b 24.7b

a Interparticle distance based on calculated 1.2 × capping ligand length. This assumes interdigitation and packing for 5 and 6 is similar to that of thiol-capped Au NPs.9,58 b Calculated using the dimerized forms of capping ligands 3 and 4.

are given in Table 3. For unlinked particles, the predominant mechanism of electrical transport is tunneling, as observed in monothiol-capped NPs, and for these materials conductivity is dependent on interparticle distance.9,21,57 The lengths of the capping ligands 3 and 4 (12.7 Å) are closest to those of decanethiolcapped NPs where the length of the ligand is calculated to be 12.7 Å.9 Assuming the same packing arrangement occurs with 5 and 6 as for thiol-capped NPs, the interparticle distance is expected to be comparable. It is then not surprising that the room-temperature conductivity of decanethiol capped NPs (10-5 S cm-1)9 is quite similar to that measured for 5 and 6. The electrical conductivities of the electrodeposited NPs 5 and 6 on a gold substrate were also measured at room temperature by sputtering an Au/Pd electrode on the film surface (Figure 7b). The electrical conductivities for undoped poly-5 and poly-6 are 3 orders of magnitude (57) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537.

greater than the conductivity for the unlinked NPs (Table 3) even though the interparticle distance is larger between NPs in poly-5 and poly-6 (24.6 and 24.7 Å, respectively, calculated using the dimerized forms of 3 and 4). The electrical conductivities reported for similarly sized Au NPs linked by similarly sized alkyl linkers are significantly lower than those for poly-5 and poly6; for example, 1,10-decanethiol-capped NPs linked by Cu2+ (2 × 10-6 S cm-1)58 or NPs linked by the dithiol linker 1,16-hexadecanedithiol (NPs 21.55 Å apart, 4.81 × 10-5 S cm-1).59 For NPs linked by saturated alkyl chains, the predominant mechanism for electrical conductivity is electron tunneling from particle to particle.58,59 Thus, the substantial increase of σRT for similar interparticle distances in poly-5 and poly-6 suggests that the main mechanism of electrical conductivity involves propagation through the π-conjugated bridging molecules as observed for partially conjugated bridges.14 A similar increase in conductivity has been observed previously with a monolayer of dodecanethiol-capped Au NPs where a few of the capping ligands have been exchanged with a terthiophene linker.15 The conductivities of poly-5 and poly-6 are similar, suggesting that the length of the conjugated bridge plays a greater role than changes in electron density on the bridge resulting from substituents. In conclusion, we have prepared films of electrodeposited Au NPs bridged by π-conjugated linkers from Au NPs 5 and 6. The NPs in poly-5 and poly-6 are linked both structurally and electronically by observed increases in conjugation, conductivity, and plasmon coupling relative to the unlinked particles. The conjugated linker results in higher conductivities for poly-5 and poly-6 relative to unlinked NPs or networks consisting of particles linked with saturated alkyl groups. The increase in electrical conductivity is the result of lowering the electronic coupling term (β) between the Au NPs by introducing another pathway for interparticle conduction, involving the π-conjugated bridge. Investigations of the electro-optical properties of the doped films and the tunability of the electrical conductivity are currently in progress. Acknowledgment. We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support of this work. Supporting Information Available: Figures S1 (NMR spectra of 5 and 6) and Figure S2 (IR spectra of 5 and poly-5). This material is available free of charge via the Internet at http://pubs.acs.org. CM035242N (58) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958. (59) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schloegl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 107, 7406.