Article pubs.acs.org/JPCC
Gold and Silver Nanoparticles Functionalized by Luminescent Iridium Complexes: Synthesis and Photophysical and Electrofluorochromic Properties Fabien Miomandre,†,* Stefka Stancheva,† Jean-Frédéric Audibert,† Arnaud Brosseau,† Robert B. Pansu,† Marc Lepeltier,‡ and Cédric R. Mayer§ †
PPSMENS Cachan, CNRS UMR 8531, 61 Avenue du Président Wilson 94235 Cachan Cedex, France ILVCNRS UMR 8180, Université Versailles Saint Quentin, 45 Avenue des Etats-Unis 78035 Versailles Cedex, France § LISVUniversité Versailles Saint Quentin, 10-12 Avenue de l’Europe 78140 Velizy, France ‡
S Supporting Information *
ABSTRACT: Gold and silver nanoparticles in the 5 nm range functionalized by luminescent and electroactive iridium complexes are synthesized and characterized. Cyclometalated iridium complexes with a modified phenanthroline ligand bearing a pyridine end group are able to cap gold nanoparticles without changing their size and shape, while for silver the size slightly increases and aggregation starts to occur but without any flocculation. The luminescence of iridium is partially quenched by gold nanoparticles even when interactions with the complex do not involve surface functionalization (simple mixture). This quenching is much weaker in the case of silver, and capped nanoparticles retain the same luminescence as the free complex. Both iridium complexes display electrofluorochromism, that is, a reversible electrochemically driven luminescence switch when changing the redox state of the metal center.
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complexes in devices like LEDs,13 light electrochemical cells (LECs),14 or dye-sensitized solar cells15,16 is now well documented. Among all these examples, it can be noticed that cationic Ir complexes have been the subject of specific recent investigations with promising future in LECs.17−20 Nevertheless, apart from a recent work of our groups dealing with gold NPs capped by dithiolate Ir complexes,21 there are no data concerning the functionalization of metallic NPs by iridium luminescent complexes and the influence of the surface plasmon resonance on their photophysical and electrochemical features. In this paper, two luminescent iridium complexes are investigated (see scheme 1), one with an anchoring group (Ir-2) and one without (Ir-1). Complex Ir-1, namely Ir(piq)2(acac) (piq = phenylisoquinolinate ; acac = acetylacetonate), has electrochemical and photophysical properties referenced in the literature,22 while Ir-2 is a new compound that was designed to bear a modified 1,10-phenanthroline ligand with a pyridine end group already known as a capping agent able to stabilize metallic nanoparticles.6,23 Such cyclometalated iridium complexes bearing substituted phenanthroline ligands have also exhibited promising nonlinear optical (NLO) properties.24 In this study, gold and silver NPs in the 5−6 nm size range were synthesized in the presence of these two complexes, and their spectroscopic properties were analyzed. Changing the metal
INTRODUCTION Metallic nanoparticles coated with photoactive and electroactive metal complexes have been synthesized and studied in the past decade since localized surface plasmons are known to strongly influence the photophysical properties of interacting dyes.1,2 The pioneering work of Murray et al. highlighted the luminescence quenching of the well-known ruthenium trisbipyridine complex by small Au NPs stabilized by organic monolayers.3 The same group evidenced the essential roles of the NP size and spectral overlap between the plasmon resonance band (PRB) and the dye emission band in the quenching efficiency.4 Beyond the use of small protected metal clusters and thiol-ended spacers between the metal core and the ruthenium complex, more recent studies have shown that this quenching phenomenon was also visible on larger particles5 and for various anchoring groups.6,7 Applications of this phenomenon to biological issues like DNA detection and cell imaging have been recently developed.8,9 On the other hand iridium complexes are the subject of a growing interest from the scientific community due to their photophysical and electrochemical properties.10 As well as their ruthenium counterparts, they exhibit a strong long-lived red luminescence involving the triplet state, with significant Stokes shift and good efficiency. The emission wavelength can be tuned by changing the ligand in the iridium coordination sphere. This luminescence is also likely to be quenched when oxidizing the iridium center into the stable Ir(IV) redox state, similarly as does the Ru(II)−Ru(III) redox couple.11,12 The application of iridium © XXXX American Chemical Society
Received: December 21, 2012 Revised: May 21, 2013
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dx.doi.org/10.1021/jp312625x | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Scheme 1. Formulae of the Investigated Ir Complexes
core of the NPs enables to monitor the spectral overlap between the plasmon resonance band (PRB) and the emission of the Ir complexes, and thus the quenching efficiency.25 Finally the ability of the two complexes to have their luminescence reversibly switched by electrochemistry was tested, as a preliminary step to design new plasmonic electrofluorochromic systems.
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To a suspension of the dimer Ir(piq)2Cl2Ir(piq)2 (0.200 g, 0.157 mmol) in 2-ethoxyethanol (10 mL) was added acetylacetone (50 μL, 0.472 mmol) and sodium carbonate (0.166 g, 1.57 mmol). The reaction mixture was stirred at reflux for 15 h. Then, water (50 mL) was added and the product was filtered and washed with ethanol and diethyl ether. The complex Ir-1 was then purified by chromatography on silica gel to be isolated as a red powder (0.165 g, 75%). 1H NMR (300 MHz, CDCl3, ppm): 9.01 (d, 3J = 6.3 Hz, 2H), 8.47 (d, 3J = 6.0 Hz, 2H), 8.22 (d, 3J = 7.8 Hz, 2H), 7.96 (d, 3J = 4.5 Hz, 2H), 7.73 (m, 4H), 7.49 (d, 3J = 6.3 Hz, 2H), 6.91 (dt, 3J = 8.4 Hz, 4J = 1.5 Hz, 2H), 6.67 (dt, 3J = 7.8 Hz, 4J = 1.2 Hz, 2H), 6.40 (dd, 3J = 7.5 Hz, 4J = 0.9 Hz, 2H), 5.22 (s, 1H), 1.77 (s, 6H). MS (ESI) calcd for C35H27IrN2O2, 700.17; found, 700.1744 [M]+. Synthesis of Ligand L-2.
EXPERIMENTAL SECTION
Synthesis. The complexes Ir-1 and Ir-2 were prepared by following published procedures in two steps.26 Synthesis of the Dimeric Precursor [Ir(piq)2]2Cl2.
To a suspension of phendione (0.420 g, 2.00 mmol) and 4-(4pyridyl)-benzaldehyde (0.400 g, 2.18 mmol) in acetic acid (50 mL) was added ammonium acetate (3.40 g, 44.1 mmol). The reaction mixture was refluxed overnight. After the mixture was cooled to room temperature, the product precipitated and the solvent was half-evaporated. The product was filtered, washed with water and diethyl ether, and dried under vacuum to be isolated as a yellow powder (0.540 g, 72%). 1H NMR (300 MHz, DMSO-d6, ppm): 11.97 (br.s, 1H), 9.06 (d, 3J = 3.3 Hz, 2H), 8.99 (d, 3J = 8.4 Hz, 2H), 8.70 (d, 3J = 5.4 Hz, 2H), 8.44 (d, 3J = 8.4 Hz, 2H), 8.10 (d, 3J = 8.4 Hz, 2H), 7.90 (m, 2H), 7.86 (d, 3J = 5.4 Hz, 2H). Synthesis of Ir-2.
Iridium trichloride trihydrate IrCl3·3H2O (0.240 g, 0.697 mmol) and 1-phenylisoquinoline (0.500 g, 2.44 mmol) were dissolved in a mixture of 2-ethoxyethanol (20 mL) and water (5 mL). The mixture was then heated at reflux for 24 h. The solution was allowed to cool to room temperature, and water (50 mL) was added. The deep red precipitate was filtered, washed with water and diethyl ether, and then dried under vacuum. The dimeric precursor [Ir(piq)2]2Cl2 was isolated as a red powder (0.425 g, 96%). 1H NMR (300 MHz, CDCl3, ppm): 9.06 (d, 3J = 6.3 Hz, 4H), 8.99 (d, 3J = 8.4 Hz, 4H), 8.14 (d, 3J = 7.8 Hz, 4H), 7.86 (m, 8H), 7.78 (m, 4H), 6.83 (t, 3J = 8.1 Hz, 4H), 6.57 (d, 3J = 6.6 Hz, 4H), 6.52 (d, 3J = 6.9 Hz, 4H), 6.05 (d, 3J = 7.2 Hz, 4H). Synthesis of Ir-1.
To a suspension of the dimer Ir(piq)2Cl2Ir(piq)2 (0.200 g, 0.157 mmol) in 1,2-dichloroethane (20 mL) was added ligand L-2 (0.147 g, 0.393 mmol). The reaction mixture was refluxed overnight. After the mixture was cooled to room temperature, a saturated solution of sodium hexafluorophosphate in methanol (4 mL) was added. The mixture was stirred during 30 min. The solvent was then evaporated, and the product was extracted with B
dx.doi.org/10.1021/jp312625x | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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chloroform (50 mL). The solution was filtered, and the complex Ir-2 was precipitated by the addition of pentane to be isolated as a red powder (0.160 mg, 46%). MS (ESI) calcd for (C54H35IrN7)+, 974.2587; found, 974.2564 [M+]. Au-1 and Au-2 nanocomposites were synthesized as follows:27 Chloroauric acid trihydrate HAuCl4·3H2O (10 mg, 2.5 × 10−5 mol) and dodecylamine (200 mg, 8.3 × 10−4 mol) are dissolved in 10 mL of dichloromethane. Then the solution is diluted to 60 mL, and 400 μL of a cooled aqueous solution of NaBH4 (1 M) is slowly added. After at least 2 h stirring, the aqueous phase is removed with a micropipet or by decantation. At that stage, the mean diameter of the formed Au NP is ca. 3 nm (TEM). Finally a variable volume ranging from 140 μL to 1 mL of a 10−4 M solution of the Ir complex in dichloromethane is added to 5 mL of the previous Au colloidal solution under stirring. Ag-1 and Ag-2 are synthesized using the same method with the following amounts: silver nitrate (4 mg, 2.5 × 10−5 mol), dodecylamine (75 mg, 3.1 × 10−4 mol) are dissolved in 5 mL of dichloromethane. After the addition of 10 μL of oleic acid, the mixture is diluted to 60 mL, and 100 μL of cooled aqueous NaBH4 (1 M) is added dropwise. Finally, after removal of the aqueous phase, a variable volume ranging from 140 to 500 μL of the 10−4 M complex solution in dichloromethane is added to the Ag colloidal solution under stirring. All the nanocomposites are used without further purification to avoid NP aggregation. A UV−vis control is performed during the ligand exchange process to check that NPs do not aggregate. Characterization. TEM pictures were obtained on a JEOL JEM2010 at 200 kV. DLS was performed on a Cordouan particlesizer, after filtration with 0.22 μm Teflon membrane. Solvents (SDS, HPLC grade) and electrolyte salts (Fluka, puriss.) were used without further purification for electrochemical and spectroscopic analyses. Cyclic voltammetry was recorded in a three electrode cell with a potentiostat (CH Instruments 600). A platinum disk (1 mm diameter) was used as the working electrode, while a platinum wire and Ag+ (10−2 M in acetonitrile)/Ag were used as counter and reference electrodes, respectively. Electronic absorption spectra were recorded on a Cary 500 (Varian) spectrophotometer in 1 cm quartz cuvettes. Fluorescence spectra were recorded on a Fluorolog3 (Horiba) spectrofluorimeter, in a quartz cell at the right angle beam geometry. Lifetimes were extracted from luminescence decay curves obtained with an Edinburgh Instrument LP920 nanosecond laser flash photolysis spectrometer, with a NdYAG laser (Continuum) and a tripling crystal used to reach 355 nm excitation. The emission is recorded at 610 nm. The Levenberg−Marquardt algorithm was used for nonlinear least-squares fit (tail fit) as implemented in the L900 software (Edinburgh Instrument). To estimate the quality of the fit, the weighted residuals were calculated. Fluorescence microscopy coupled to electrochemistry was performed on a Nikon Ti−U microscope under TIRF illumination (see ref 28 for details) with either a mercury lamp or a laser as the excitation source. The excitation wavelength was selected using adapted filters (DAPI-5060C or FITC-3540C from SEMROCK). The setup allows simultaneous recording of the faradaic current and fluorescence intensity when applying a potential signal to the working electrode. The working electrode is a microscope glass slide (170 μm thin) coated with a thin Pt layer (typical OD: 0.23). An Ag wire is used as the pseudoreference and a Pt wire as the counter electrode in a three-electrode electrochemical cell. The fluorescence intensity is recorded through a side port of the microscope and collected by a photon counter coupled to a photodiode to record intensity, or dispatched on a
grating spectrometer for recording emission spectra, both under electrochemical control (Potentiostat CH Instruments CHI600).
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RESULTS AND DISCUSSION Synthesis. Au-2 and Ag-2 are synthesized by postfunctionalization according to a place exchange reaction between the long chain amine capping ligand and the complex already implemented in the case of Ru complexes.29 The concentration of the incoming Ir-2 complex is chosen low enough to have a small number of complexes per particle (typically 2 to 15 in our case) in the final state, in order to keep the colloidal stability30 and avoid any intermolecular charge transfer between the grafted complexes. Electronic Microscopy. Figure 1 shows the TEM pictures of the functionalized Au-1, Au-2, Ag-1, and Ag-2. Au-1 nanocomposites
Figure 1. TEM pictures of Au-1 (a), Au-2 (b), Ag-1(c), Ag-2(d). All nanocomposites shown were synthesized with 140 μL of complex solution 10−4 M in dichloromethane.
have a spherical shape and display an average diameter around 5 nm with a low polydispersity (
