Highly Photoluminescent Nanocrystals Based on a Gold(I) Complex

Jun 28, 2011 - Graduate School of Materials and Science, Nara Institute of Science and ... University of California, Davis, California 95616, United S...
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Highly Photoluminescent Nanocrystals Based on a Gold(I) Complex and Their Electrophoretic Patterning Masashi Saitoh,† Alan L. Balch,‡ Junpei Yuasa,† Kazuya Tada,§ Mitsuyoshi Onoda,§ Takuya Nakashima,† and Tsuyoshi Kawai*,† †

Graduate School of Materials and Science, Nara Institute of Science and Technology, NAIST, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan ‡ Department of Chemistry, University of California, Davis, California 95616, United States § Department of Electrical Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan

bS Supporting Information ABSTRACT: The fabrication of nanocrystals (NCs) composed of the cationic Au(I) complex was demonstrated by the reprecipitation method in which the colloidal solution of the NCs showed brilliant green phosphorescence with a quantum yield of 83% in n-hexane. Characterization of the prepared NCs was performed by transmission electron microscopy observation and elemental analysis with energy-dispersive X-ray spectroscopy. The obtained Au(I) NCs were particles of random shapes with a diameter of 200 400 nm. The selected-area electron diffraction and X-ray diffraction measurements showed the characteristic diffraction patterns attributable to the crystal structure of the bulk crystal of the Au(I) complex. A similar method was performed with a different counteranion, leading to a colloidal solution of the microcrystals (MCs) with brilliant yellow phosphorescence and a quantum yield of 26% in n-hexane. Luminescence patterning of the NCs and MCs was also achieved successfully by electrophoretic deposition onto an indium tin oxide (ITO)-coated glass substrate, resulting in characteristic luminescence patterns on the ITO substrates with relatively high photoluminescence quantum yields.

1. INTRODUCTION The fabrication of nanometer-scale structures based on organic and inorganic molecules has been considered to be an efficient approach for future materials in various molecular-based devices and systems.1 6 The application of noncovalent intermolecular interactions between molecular units has been widely regarded as an efficient strategy in designing and programming molecular assembly systems with well-ordered structures.7 10 Some specific intermolecular attractive forces, such as hydrogen bonding, π π stacking, and electrostatic and van der Waals interactions have been utilized for such fabrication.11 17 The use of aurophilic interactions in designing elaborate architectures has attracted considerable attention from researchers in supramolecular chemistry in recent years.18 23 In particular, some Au(I) complexes have been known to self-assemble and form 1-D and quasi-1-D structures such as nanotubes,21 nanowires,22 and sol gel systems.23 The 1-D Au(I) structures show intense, long-lived photoluminescence resulting from the aurophilic interactions both in solution and in the crystalline state.24 26 This unique property of the Au(I) complexes has been manipulated by changing the aurophilic interactions through, for example, doping/dedoping in organogels27 and changing solvents.28 We have recently reported that some Au(I) complexes with coplanar carbene ligands show tunable photoluminescent colors depending on the type of counteranions in the crystalline state.29 r 2011 American Chemical Society

In our system, the Au(I) complexes are found to form networklike structures via aurophilic, hydrogen bonding, and other weak noncovalent interactions. We report herein the fabrication of highly photoluminescent nanocrystals (NCs) composed of one of the Au(I) complexes that exhibits green phosphorescence with a significantly high quantum yield. A patterned bright luminescent layer of Au(I) nanocrystals is formed on an indium tin oxide-coated glass substrate by means of the electrophoretic deposition method.30,31

2. EXPERIMENTAL SECTION Materials. All reagents and starting materials for the preparation of Au(I) X (X = PF6 and I ) complexes were purchased from Tokyo Chemical Industry Co., Ltd. and were used without further purification. The synthesis procedure of the Au(I) X complexes used in this work has already been reported elsewhere.29 Their 1H NMR and MALDITOF mass and elemental analyses were demonstrated before the fabrication of Au(I) PF6 NCs and Au(I) I MCs. Au(I) PF6 : 1H NMR (300 MHz, CDCl3): 8.25 (brs, 2H), 4.29 (s, 6H), 2.94 (d, J = 5.0 Hz, 6H). MALDI-TOF mass calcd for C6H14AuF6N2O2P: m/z 488.1. Found: m/z 343.3 [M PF6]+. Anal. Calcd for Received: December 1, 2010 Revised: June 23, 2011 Published: June 28, 2011 10947

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Scheme 1. Schematic Illustrations of (a) the Molecular Structure of the Au(I) PF6 Complex and (b) the Nanocrystallization of the Au(I) PF6 Complex by the Reprecipitation Method

C6H14AuF6N2O2P: C, 14.76; H, 2.89; N, 5.74. Found: C, 14.88; H, 2.69; N, 5.89. Au(I) I : 1H NMR (300 MHz, CDCl3): 9.90 (brs, 2H), 4.31 (s, 6H), 2.89 (d, J = 4.6 Hz, 6H). MALDI-TOF mass calcd for C6H14N2O2IAu: m/z 470.0. Found: m/z 343.3 [M I]+. Anal. Calcd for C6H14N2O2IAu: C, 15.33; H, 3.00; N, 5.96. Found: C, 15.24; H, 2.65; N, 6.35. Fabrication of Au(I) X NCs and MCs. In a typical procedure for Au(I) PF6 nanocrystals (NCs) and Au(I) I microcrystals (MCs), 0.20 mL of a solution of Au(I) X (X = PF6 and I ) complexes in dichloromethane (1.0 mM) was rapidly added using a microsyringe to 20 mL of n-hexane under vigorous stirring, and ultrasonication was carried out for 5 min, which resulted in a photoluminescent solution under UV (365 nm) irradiation. Apparatus. The prepared Au(I) X (X = PF6 and I ) NCs and MCs were characterized by transmission electron microscopy (JEOL, JEM-3100FEF). The size distribution of the Au(I) PF6 NCs and Au(I) I MCs dispersed in n-hexane was obtained by dynamic light scattering spectroscopy (Otsuka Electronic Co., DLS-6000). The crystal structures of the NCs and MCs were analyzed by powder X-ray diffraction spectroscopy (λ = 1.54 Å (Cu KR1), Rigaku, RINTTTRIII/NM). The stationary photoluminescence spectra of Au(I) PF6 NCs and Au(I) I MCs in n-hexane were measured on a spectrofluorometer (JASCO, F-6500). Photoluminescence lifetime measurements were performed using a picosecond luminescence measurement system (Hamamatsu, C4780) with a streak camera scope (Hamamatsu, C4334). The excitation source was generated by a Nd: YVO4 laser (Coherent, Verdi)-pumped Ti:sapphire laser system (Coherent, Mira-900) equipped with a cavity dumper (Coherent, PulseSwitch). This system delivers 100 fs pulse trains at 800 nm and runs at a repetition rate of 10 kHz for the time-resolved photoluminescence measurements. After the frequency was doubled with a LiB3O5 crystal, the incident pulse was less than 100 nW and had a value that was optimized to avoid thermal degradation of the samples. Photoluminescence quantum yields of the Au(I) PF6 NCs and Au(I) I MCs dispersed in n-hexane and deposited onto indium tin oxide (ITO)-coated glass substrates (100 mm  20 mm  1.0 mm, 10 Ω/cm2) were determined by an absolute method using a spectrofluorometer (JASCO, F-6500) with an integrating sphere (JASCO, ILF-533). Field-emission scanning electron microscopy (JEOL, JSM-7400F) was employed to examine the morphologies of the Au(I) PF6 NCs and Au(I) I MCs on ITO-coated glass substrates.

Figure 1. (a) Photographs of the n-hexane solution of Au(I) PF6 NCs. The left and right images were obtained under normal light and UV (365 nm) irradiation, respectively. (b) UV visible spectrum of the n-hexane solution of Au(I) PF6 NCs. (c) Emission ( ) and excitation ( 3 3 3 ) spectra of the n-hexane solution of Au(I) PF6 NCs.

Electrophoretic Deposition. The selective deposition of Au(I) X (X = PF6 and I ) NCs and MCs by the electrophoretic deposition method onto the ITO-coated glass substrate was carried out by applying a dc voltage of 100 V with a power source (KENWOOD, PA 250 0.25A) for 1 min. The ITO-coated glass substrate was carefully cleaned by ultrasonication in acetone for 15 min before use. Measurements of emission quantum yields and scanning electron microscopy analyses were performed after rinsing the patterned ITO-coated glass substrate with n-hexane several times.

3. RESULTS AND DISCUSSION Preparation of Au(I) NCs and Their Photoluminescence Property. Au(I) complex Au(I) PF6 (Scheme 1a) was synthe-

sized as described in a previous report.29 Au(I) PF6 nanocrystals (NCs) were prepared as follows by the reprecipitation method (Scheme 1b).32 36 The solution of the Au(I) PF6 complex dissolved in dichloromethane was rapidly injected from a microsyringe into n-hexane under vigorous stirring and then ultrasonicated to afford a colloidal solution of Au(I) PF6 NCs (details in Experimental Section). The Au(I) PF6 complex formed a colorless transparent solution as shown in Figure 1a, and the transparency was maintained during the spectroscopic measurements without any surfactant, which means that 10948

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Figure 2. Size distribution of Au(I) PF6 NCs dispersed in n-hexane obtained by DLS spectroscopy.

Au(I) PF6 NCs are expected to have excess surface charge in n-hexane. The absorption spectrum of the solution presented in Figure 1b showed a clear peak at 380 nm, which can be assigned to the transition between the filled valence band composed of 5dz2σ* states and the conduction band composed of 6pzσ states. Under UV-light illumination, as shown in Figure 1a, the solution showed characteristic green photoluminescence, and its emission peak was observed at 524 nm as shown in Figure 1c. The excitation peak wavelength roughly corresponds to the absorption peak wavelength. The emission quantum yield (Φem) of the solution was as high as 83%, Φem = 0.83, which was slightly smaller than that of a bulk single crystal of the Au(I) PF6 complex, Φem = 0.99. These emission properties were similar to those of the single crystal of the Au(I) PF6 complex.37 Because the bulk crystal of the Au(I) PF6 complex showed no solubility in n-hexane, the rapid dispersion of dichloromethane solution into n-hexane seemed to result in a colloidal solution of small particles composed of the Au(I) PF6 complex. The dichloromethane solution of the Au(I) PF6 complex showed neither the absorption band at 380 nm nor emission in the visible range, and thus these optical features of the obtained colloidal solution are characteristic of the structures with specific aurophilic interactions.29 We also evaluated the emission lifetime of the NCs. Time-resolved emission spectroscopy under a pulsed excitation light source at 400 nm showed a single-exponential decay in a time range of 0 to 6 μs, providing an excited-state lifetime of 2.7 μs (Supporting Information, Figure S1a). The relatively long lifetime indicates that this emission is phosphorescence derived from the excited dimer of Au(I) complexes.29 Apparent values of radiative (kr) and nonradiative (knr) rate constants were estimated from the emission lifetime and the Φem.38 The kr of the NCs (3.1  105 s 1) was similar to the kr of the bulk crystal of Au(I) PF6 complex (krbulk = 3.8  105 s 1) but a larger knr (6.3  104 s 1) compared with that of the bulk crystal (knrbulk = 3.8  103 s 1) was evaluated; that is, the Φem as well as its kr is almost maintained in the NCs. In the NCs composed of organic molecules, complexes, and inorganic semiconductors, usually the generated excitons are supposed to be depressed by the trapping at the defect sites and/or the exciton recombination at their surface, resulting in their decreased emission efficiency. In the case of Au(I) PF6 NCs, however, the exciton would be efficiently confined because of the characteristic excited-state dimerization in the Au(I) 1-D systems.39,40 The slight decrease in Φem might be ascribed to

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Figure 3. (a) TEM image of Au(I) PF6 NCs and energy-dispersive spectroscopic analysis of Au(I) PF6 NCs with elemental mappings for (b) Au and (c) P. The inset in image a shows the electron diffraction pattern for Au(I) PF6 NCs.

Figure 4. (a) XRD pattern of Au(I) PF6 NCs and (b) simulated powder pattern from the published crystal structure data of the Au(I) PF6 complex.

the space charge at the surface of the NCs because the excess surface charge is expected with respect to their significant stability in n-hexane (vide supra). Characterization of Au(I) NCs. In order to determine the size and morphology of the Au(I) PF6 NCs, dynamic light scattering (DLS) spectroscopy and transmission electron microscopy (TEM) observations were performed. A size histogram of the NCs obtained from a DLS measurement is shown in Figure 2. The size of the obtained NCs in n-hexane was 300 500 nm with a relatively narrow size distribution. From the TEM observation as shown in Figure 3a, the obtained Au(I) PF6 colloidal solution contained NCs of random shapes with a diameter of 200 400 nm which roughly coincides with a DLS measurement. The elemental analysis was performed with energy-dispersive X-ray spectroscopy, and the 2-D distribution images of Au and P atoms are presented in Figure 3b,c, respectively. The Au and P atoms were distributed over the same area, which strongly suggests that the NCs observed in the TEM are composed of the Au(I) PF6 complex. High-resolution scanning TEM observation failed because of the decomposition of the samples. Thus, the selected-area electron diffraction and X-ray diffraction (XRD) patterns were measured to evaluate the crystallinity of the 10949

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Figure 5. Electrophoretic deposition of Au(I) PF6 NCs and Au(I) I MCs. (a) Schematic representation of the electrophoretic deposition. Photographs of the patterned ITO electrode (2 cm  2 cm) for (b) Au(I) PF6 NCs and (c) Au(I) I MCs after the electrophoretic deposition. The left and right images were obtained under normal light and UV (365 nm) irradiation, respectively. Scale bars: 0.5 cm.

NCs. As presented in the inset of Figure 3a, the obtained selected-area electron diffraction pattern showed sharp diffraction spots, which means that the NCs of Au(I) PF6 are singlecrystalline. XRD experiments were also performed to investigate the crystal structure of the NCs further. The comparison between the XRD profiles of Au(I) PF6 NCs and the simulated powder diffraction patterns is shown in Figure 4. The simulated XRD pattern of the Au(I) PF6 complex was obtained from the data based on the single-crystal structural data reported previously.41 The observed diffraction peaks for the Au(I) PF6 NCs were assigned to the single-crystal data of the Au(I) PF6 complex, which strongly indicates that the prepared NCs have the same crystal structure as that of the bulk single crystal. The authors tried to prepare similar NCs of the Au(I) complexes with different counteranions such as CF3CO2 , ClO4 , Br , and I . However, all of these attempts did not afford stable NCs but rather opaque and unstable colloidal solutions. The absorption spectrum of a colloidal solution of the Au(I) I complex showed a marked scattering profile over the whole visible wavelength range, and the solution also showed characteristic yellow photoluminescence under UV irradiation (Supporting Information, Figure S2). From TEM, DLS, and XRD observations of the obtained Au(I) I colloidal solution (Supporting Information, Figures S3 S5), the solution contained microcrystals (MCs) composed of the Au(I) I complex with the same crystal structure as the Au(I) I bulk crystal. The obtained Au(I) I MCs dispersed in n-hexane showed yellow photoluminescence at 568 nm with an emission quantum yield of 26%, Φem = 0.26.42 We also measured an emission lifetime of the colloidal solution (Supporting Information, Figure S1b), and apparent values of kr and knr were estimated from the emission lifetime and the emission quantum yield to be 2.3  105 and 6.5  105 s 1, respectively. The knr that is about 4 times larger than that of the bulk single crystal (knrbulk = 1.7  105 s 1) seems to be responsible for the decreased quantum yield. Although TEM observation indicated well-defined crystal particles of the Au(I) I complex, their size could not be controlled down to less than 400 nm. The reprecipitation method is a complex process where the mixing of the two solutions, nucleous formation, and crystal growth occur simultaneously. The kinetics at an earlier stage in this process have been discussed in detail on the basis of classical nucleation formation

theory.34 The subsequent crystal growth is regarded to proceed with the mechanism of Ostwald ripening.43 In our system, both Au(I) PF6 and Au(I) I complexes as well as other organic compounds in an earlier stage of the reprecipitation process would nucleate. The difference in the crystal size between Au(I) PF6 NCs and Au(I) I MCs could thus be caused by the different crystal growth process, which sensitively depends on the adsorption/desorption equilibrium at the surfaces of the particles. In the case of the Au(I) PF6 complex, there are two hydrogen bonds between two hydrogen atoms of N H groups and two fluorine atoms of PF6 and three specific CH F interactions, which form the network structure via PF6 (Supporting Information, Figure S6). However, in the case of the Au(I) I complex, there are only two hydrogen bonds between two hydrogen atoms of N H groups and an iodine atom. As a result of the specific network structure supported by the multipoint interactions in the Au(I) PF6 crystal, the adsorption/desorption equilibrium at the surfaces of Au(I) PF6 NCs and thus the Ostwald ripening of the NCs would be suppressed, which can be responsible for the efficient stabilization of Au(I) PF6 NCs. Although we tried to fabricate the nanocrystals composed of the Au(I) complex with different types of counteranions (X = CF3CO2 , ClO4 , Br , and I ) under the same conditions in which Au(I) PF6 NCs was fabricated, the nanocrystals could not be obtained (vide supra). These results indicate that the specific interactions via PF6 anions in the crystal lattice play key roles in stabilizing the NCs. Electrophoretic Deposition of Au(I) NCs onto the ITO Substrate. Because the prepared Au(I) X (X = PF6 and I ) NCs and MCs were stably dispersed in n-hexane without any surfactant, the Au(I) X NCs and MCs were expected to have relatively high ζ potentials. Expecting their excess surface charge, we deposited the NCs and MCs by means of the electrophoretic method. The electrophoretic deposition of Au(I) PF6 NCs and Au(I) I MCs was carried out using the patterned indium tin oxide (ITO)-coated glass substrates as schematically illustrated in Figure 5a. After the application of a dc voltage to the ITO electrode for 1 min, both Au(I) PF6 NCs and Au(I) I MCs were efficiently deposited onto the positively biased ITO electrode, which suggested negatively charged surfaces of NCs that may originate from the adsorption of excess anions with CH F interactions at the surfaces of the NCs. As shown in 10950

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Langmuir Figure 5b,c, area-selective deposition onto the ITO electrode was successfully accomplished. Each of the obtained electrodes was transparent and almost colorless and showed very intense luminescence under UV light irradiation. These optical properties suggest that they have the potential to be applied for invisible marking. Field emission scanning electron microscopy (FE-SEM) analyses were performed to examine the morphologies of the Au(I) PF6 NCs and Au(I) I MCs on the ITO-coated glass substrate (Supporting Information, Figure S7). No morphological deformation of the NCs and MCs was seen in the FESEM images after electrophoretic deposition. Photoluminescence quantum yields on the ITO electrode were evaluated to be 72%, Φem = 0.72, for Au(I) PF6 NCs and 22%, Φem = 0.22, for Au(I) I MCs. The significantly high Φem of Au(I) PF6 NCs was almost maintained in the condensed film of Au(I) PF6 NCs. Generally, condensed NCs of various materials such as organic molecules, complexes, and inorganic semiconductors tend to exhibit a suppressed emission efficiency in comparison to those of dispersed NCs. This quenching usually originates from the interparticle energy migration and/or electron transfer at their surfaces. In the present NCs and MCs, however, these quenching effects in the condensed system seem to be less effective because of the partial confinement of excitons in the present systems.

4. CONCLUSIONS We successfully demonstrated the fabrication of Au(I) PF6 nanocrystals (NCs) and Au(I) I microcrystals (MCs) depending on the counteranions. The prepared NCs and MCs were found to have different sizes and morphologies. Their colloidal solutions showed green phosphorescence with a quantum yield of 83% for Au(I) PF6 NCs and yellow phosphorescence with a quantum yield of 26% for Au(I) I MCs. The electrophoretic deposition of the obtained NCs and MCs was also successfully performed to produce the characteristic luminescent patterns on indium tin oxide-coated substrates with relatively high photoluminescence quantum yields. ’ ASSOCIATED CONTENT

bS

Supporting Information. Photoluminescence decay profiles of Au(I) PF6 nanocrystals (NCs) and Au(I) I microcrystals (MCs) in n-hexane, absorption and photoluminescence spectra of Au(I) I MCs in n-hexane, a transmission electron microscopy image and energy dispersive spectroscopic analysis with an elemental mappings of Au(I) I MCs, the size distribution of Au(I) I MCs, the XRD pattern of Au(I) I MCs, a molecular packing diagram of the Au(I) PF6 complex in the single crystal, and SEM images of Au(I) PF6 NCs and Au(I) I MCs on ITO substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-743-72-6170. Fax: +81-743-72-6179. E-mail: tkawai@ ms.naist.jp.

’ ACKNOWLEDGMENT We thank Ms. S. Fujita and Ms. Y. Kawai of NAIST for their technical support with transmission electron microscopic

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analyses. M.S. also thanks Ms. Y. Fujisawa and Mr. L. McDowell for correcting the manuscript. Part of this work was supported by a grant-in-aid for scientific research on innovative area “Coordination Programming” (no. 2107) from the Ministry of Education, Culture, Sports, Science and Technology, MEXT, Japan. We are also grateful for financial support under “The Green Photonics Project” of NAIST.

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