Article pubs.acs.org/IC
Tunable Self-Assembly and Morphology-Dependent Photoconductivity of a Donor−Acceptor-Structured Diruthenium Complex Meng-Jia Sun,†,‡ Xinliang Zhang,†,‡ Yu-Wu Zhong,*,†,‡ Chuanlang Zhan,*,†,‡ and Jiannian Yao*,†,‡ †
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: A donor−acceptor-structured diruthenium complex, 1(PF6)4, that contains an electron-deficient bridging ligand and electron-rich distal diarylamines modified with long aliphatic chains has been synthesized. By varying the solvent environments and assembly conditions, we obtained three different self-assembled nanostructures of 1(PF6)4, including zero-dimensional nanospheres, one-dimensional nanofibers, and thin films with interconnected nanowire networks. These structures were investigated by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, Xray diffraction, and atomic force microscopy (AFM) analysis. Conductive AFM analysis shows that the nanowire networks exhibit a high conductivity of 0.023 S/cm and an enhanced photoconductivity of 0.59 S/cm under visible light irradiation.
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INTRODUCTION Supramolecular self-assembly1−5 provides a crucial approach to manipulating π-conjugated organic and polymeric systems into well-ordered nano/micro-sized materials6−12 and creating organic optoelectronic devices in the nano/micrometer range. These materials and devices exhibit unique and appealing optical and electrical properties that are absent in their disordered counterparts.13−15 Among various assembled materials and structures, one-dimensional (1D) nano/microstructured assemblies, such as nanowires and nanotubes with high aspect ratios, exhibit a number of advantages in device fabrication and directional signal transmission.16−19 Apart from small organic molecules and polymers, coordination metal complexes with promising optoelectronic properties have received much attention in self-assembly.20−25 In particular, various self-assembled nanostructures with fascinating electronic and light-emitting properties have been obtained from square planar Pt(II) and Pd(II) complexes.26−31 The assemblies of these complexes are mainly driven by the intermolecular π−π and metallophilic interactions. In contrast, the assembly of the classical polypyridyl Ru(II) complexes has remained a challenging task, although they possess excellent redox and photophysical properties and have been applied in a wide range of optic and electronic fields.32−34 Unlike the coordination-unsaturated Pt(II) complexes, Ru(II) complexes have an octahedron geometry and lack well-defined structural units for metal−metal or π−π interactions. In the past few © XXXX American Chemical Society
years, there have been only a few reports of the formation of vesicles or micelles from ruthenium metallosurfactants.35−37 The design of novel polypyridyl ruthenium complexes for the formation of other assembled nanostructures with potential applications beyond electrochemiluminescence remains to be explored.38,39 In addition, the conductivities of thin films of octahedral Ru(II) or Os(II) complexes are rather low (typically in the range of a few microsiemens per centimeter to ≤1 mS/ cm),40−42 which is detrimental for their electric and optoelectric applications. It would be highly interesting to test whether self-assembled nanostructures of octahedral Ru(II) complexes could display enhanced conductivity and photoconductivity like one-dimensional nanostructures made of πconjugated organic and polymeric materials.6−19 Herein, we report the formation of various self-assembled nanostructures, including nanoparticles, nanofibers, and nanowire networks, from a designed donor−acceptor-structured diruthenium complex and the conductivity and photoconductivity measurements of the nanowire thin films using a conductive atomic force microscope. Specifically, the diruthenium complex 1(PF6)4, containing an electron-deficient bridging ligand, tetra(pyrid-2-yl)pyrazine (tppz),43,44 as the electron acceptor and distal diarylamines modified with long aliphatic chains as the electron donors, was designed and Received: October 19, 2016
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DOI: 10.1021/acs.inorgchem.6b02532 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
theory (DFT) calculations of (Me-1)4+, a model complex of 1(PF6)4 with the octyl chains replaced by methyl groups, show that the LUMO and HOMO are dominated by tppz and the triarylamine segments, respectively, consistent with the interpretations mentioned above. The LUMO−HOMO gap is calculated to be 1.98 eV. The donor−acceptor structure with a narrow energy gap and rich and intense visible absorptions of 1(PF6)4 is believed to be beneficial for its optoelectronic applications. As the ICT complex 1(PF6)4 exhibits strong dipole−dipole intermolecular interactions and aliphatic chain−assistant hydrophobic interactions, we investigated the self-assembly behavior of 1(PF6)4 in both aqueous and organic environments. When 10 μL of a 4 mM solution of 1(PF6)4 in CH3CN was rapidly injected into 1 mL of water while it was being vigorously shaken at room temperature, isolated nanoparticles with diameters varying from 50 to 200 nm were formed as shown in the scanning electron microscopy (SEM) image in Figure 2a.
synthesized (Figure 1a; see the Supporting Information for the synthetic details and characterization data). The introduction of
Figure 1. (a) Chemical structure of 1(PF6)4. (b) Cyclic voltammogram of 1(PF6)4 in 0.1 M Bu4NClO4/CH3CN. (c) Absorption spectra of 1(PF6)4 and [(tpy)Ru(tppz)(tpy)](PF6)4 in CH3CN. (d) HOMO and LUMO plots of (Me-1)4+.
four long alkyl chains on the terminal ligands not only helps to enhance the solubility of the complex in organic solvents but also provides a key driving force for intermolecular interactions during self-assembly.
Figure 2. (a and b) SEM images of the nanospheres obtained by injection of 10 or 20 μL of a 4 mM solution of 1(PF6)4 in CH3CN into 1 mL of water. (c) DLS spectra of samples a and b. (d) TEM images of sample a. The scale bar is 50 nm.
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RESULTS AND DISCUSSION The cyclic voltammogram (CV) of 1(PF6)4 shows an anodic redox couple at 1.06 V and two cathodic couples at −0.38 and −0.86 V versus Ag/AgCl (Figure 1b). They are assigned to the oxidations of two amine units and the stepwise reduction of tppz,43,44 indicating the good electron donating nature of the former and the excellent electron accepting nature of the latter. According to the electrochemical data, the LUMO and HOMO levels of 1(PF6)4 were estimated to be −4.3 and −5.8 eV versus vacuum, respectively, and the frontier energy gap was estimated to be 1.5 eV. At the further positive potential, some irreversible oxidations (the N2+/•+ process) followed by two RuIII/II processes are observed.45 At further negative potentials, the reductions of the terminal 2,2′:6′,2″-terpyridine (tpy) ligands are observed. Complex 1(PF6)4 shows intensive absorptions in the visible region (Figure 1c). The main absorption band at 564 nm is distinctly red-shifted and much more intense than that of [(tpy)Ru(tppz)Ru(tpy)](PF6)4 without two triarylamine segments.45 This absorption enhancement results from the presence of additional intraligand charge-transfer and ligandto-ligand charge-transfer transitions in 1(PF6)4 apart from the metal-to-ligand charge-transfer transitions.45 The optical band gap of 1(PF6)4 estimated from the absorption onset is estimated to be 2.1 eV, which is somewhat larger than the electrochemical band gap calculated above. Density functional
When the amount of the stock solution was increased from 10 to 20 μL, aggregated particles were formed (Figure 2b). The hydrodynamic diameters of these particles were estimated to be 74 and 91 nm, respectively, as revealed by the dynamic light scattering (DLS) measurements (Figure 2c). Transmission electron microscopy (TEM) images of these samples show some degree of fusion between adjacent particles (Figure 2d). The energy-dispersive X-ray spectroscopy (EDS) data collected in element mapping mode display even distributions of the molecules on the spherical particle (Figure S1). Interestingly, nanostructures with completely different shapes were obtained when the solvents for the assembly process were changed. Specifically, when 10 μL of a 4 mM solution of 1(PF6)4 in CHCl3 was injected into 1 mL of a mixed solvent of water and MeOH (1/1, v/v), some isolated short nanorods as long as tens of nanometers to 100 nm were formed (Figure 3a). However, cross-linked fibril structures were obtained when the amount of the injected stock solution was increased to 50 μL (Figure 3b,c). The high-resolution TEM (HR-TEM) analysis of a single fiber shows longitudinal lattice fringe patterns with a spacing of 2.77 nm (Figure 3d). This distance was further confirmed by the fast Fourier transform (FFT) analysis. The wide-angle X-ray diffraction (WXRD) patterns indicate that B
DOI: 10.1021/acs.inorgchem.6b02532 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. (a and b) SEM images of the sample obtained by injection of 10 or 50 μL of a 4 mM solution of 1(PF6)4 in CHCl3 into to 1 mL of a water/MeOH mixed solvent (1/1). (c) TEM image of sample b. (d and e) HR-TEM images of a single fiber at the body and tip section. The insets are FFT images obtained from the marked area. (f) WXRD patterns of sample b and powders of 1(PF6)4. (g) GISAXS analysis of sample b. The scale bar is 100 nm. Figure 4. (a−d) Atomic force microscopy images, (e and f) GISAXS patterns, and (g and h) I−V curves of the nanowire (a, c, e, and g) and disordered (b, d, f, and h) films prepared by spin-coating of 1(PF6)4 in CHCl3 and CH3CN, respectively. The scale bar is 1 μm.
these fibers have highly ordered structures with respect to the raw powder material (Figure 3f). Furthermore, a lamellar structure is observed by HR-TEM on the tip section of the nanofiber (Figure 3e). The lamellar spacing was measured by grazing incidence small-angle X-ray scattering (GISAXS) analysis (Figure 3g), which gave a pronounced diffraction signal at qz = 0.20 Å−1 in the out-of-plane direction, corresponding to a face-on lamellar stacking with a d spacing of 3.14 nm. Alternatively, the direct spin-coating of the solution of 1(PF6)4 in CHCl3 (4.0 mM) on a silicon substrate affords thin films showing a dense tangle of nanowires 20−30 nm in width, 10 nm in height, and a few micrometers in length (Figure 4a,c). The morphology of the interconnected wire networks was also confirmed by TEM studies (Figure S2). The morphology of the film did not change distinctly with rotation speed in the range of 1000−5000 rpm. However, when the film was prepared by drop-casting of 1(PF6)4 in CHCl3 (4 mM), discrete aggregates with irregular diameters were obtained (Figure S3a). The morphology difference between the spin-coating and dropcasting samples suggests that the applied external spin forces are likely to influence the molecular assembly behavior46 and further impact the morphology of the film. When solutions with a lower concentration were used for spin-coating, discrete nanowires with shorter lengths were observed from the atomic force microscopy (AFM) images of the obtained films (Figure 5). However, the diameter did not change much, suggesting that the growth of the nanowires
remained in one dimension and was concentration-independent. In stark contrast, only isotropic disordered films were obtained when a solution of 1(PF6)4 in CH3CN (1.0−5.0 mM) was used for spin-coating (Figure 4b,d). The two-dimensional GISAXS pattern of the nanowire film shows a strong diffraction spot in the out-of-plane direction at qz = 0.22 Å−1 (Figure 4e), corresponding to a lamellar stacking with a spacing of 2.85 nm. This suggests that the nanowire film exhibits lamellar stacking similar to that of the nanofiber structures discussed above (Figure 3). In contrast, only a diffuse ring implying isotropic structure is observed in GISAXS analysis of the film obtained from spin-coating of 1(PF6)4 in CH3CN [qz = 0.22 Å−1 (Figure 4f)]. The distinguishing GISAXS patterns of the two kinds of spin-coating films revealed that complex 1(PF6)4 contains a well-aligned order in chloroform much better than that in acetonitrile. The morphology and structure of organic semiconductors are known to play an important role in influencing the conducting and photoresponse properties of assembled materials.47−50 As discussed previously, 1(PF6)4 is a donor− acceptor-structured coordination complex with a narrow energy gap and intense visible absorption. Considering the nanowire film has well-aligned structures and the relatively low roughness with respect to the nanofiber or nanoparticle, it would be highly C
DOI: 10.1021/acs.inorgchem.6b02532 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. AFM images of thin films prepared by spin-coating of 1(PF6)4 in CHCl3 at different concentrations: (a) 1, (b) 2, (c) 3, and (d) 4 mM. The scale bar is 1 μm.
ratios (the final concentration is 0.2 mM), the absorption intensity significantly decreased and the absorption band displays a distinct blue shift due to the formation of aggregates (Figure S6b). In the 0.05/5/5 CHCl3/MeOH/H2O mixed solvent, the main absorption band is located at 543 nm. The blue shift of the absorption upon aggregation suggests that the molecules take a parallel, instead of slipped, packing mode. A proposed model for the formation of various nanostructures of 1(PF6)4 is given in Figure 6. As probed by NMR
interesting to test whether thin films of 1(PF6)4 will exhibit morphology-dependent conductivity and photoresponse. Conductive atomic force microscopy (C-AFM) was selected to perform such measurements, which allows correlation of a spatial feature of the sample with its conductivity.48,51−54 Panels g and h of Figure 4 show the C-AFM-measured I−V curves of the nanowire film prepared as described above and the disordered film on a Au-coated Si substrate, respectively, in the dark or under the illumination of white light (6 μW/cm2). The use of a Si substrate or a Au-coated Si substrate does not make a difference to the morphology of the film (Figure S3b). In the dark, the nanowire film has a conductivity of 0.023 S/cm (the diameter of the tip is 25 nm; the film thickness is 30 nm).51−54 When the light was turned on, the conductivity is enhanced to 0.59 S/cm (26-fold enhancement). In stark contrast, the conductivity and photoconductivity of the disordered film are rather low (
