Communication pubs.acs.org/IC
An Electropolymerized Crystalline Film Incorporating Axially-Bound Metalloporphycenes: Remarkable Reversibility, Reproducibility, and Coloration Efficiency of Ruthenium(II/III)-Based Electrochromism Masaaki Abe,*,†,§ Hiroki Futagawa,† Toshikazu Ono,†,‡ Teppei Yamada,†,‡ Nobuo Kimizuka,†,‡ and Yoshio Hisaeda*,†,‡ †
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744, Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Center for Molecular Systems (CMS), Kyushu University, 744, Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ITO electrode surfaces to give 1D polymeric chains as a crystalline film. The polymeric film shows a well-defined electrochromism with high coloration efficiency through reversible control of oxidation levels of the ruthenium center, RuII/RuIII, with excellent stability and reproducibility against multiple electrochromic operations. Among recent advances on porphyrin- and phthalocyanine-based materials,5 this work explores, for the first time, a significant potential of porphyrin isomers as electrochromic building blocks. Complex 1 was synthesized by photolysis of [RuII(TPrPc) (CO) (MeOH)]6 in toluene containing a large excess of btp to allow dissociation of CO and subsequent coordination of the pyridyl moiety in btp to the axial sites.7 The remarkable utility of the ligand btp for oxidative electropolymerization of transition-metal complexes has been previously described.8 The above synthetic reaction initially gave a RuII intermediate [Ru(TPrPc)(btp)2], but it was gradually oxidized, during workup in air, to a cationic RuIII complex and finally obtained as a PF6− salt. The compound was characterized by various methods including ESI mass spectrometry (Figure S1) and UV−vis spectroscopy(Figure S2) and structurally determined by singlecrystal X-ray diffraction analysis (Figure 1). Cyclic voltammetry (CV) of 1 in 0.1 M n-Bu4NPF6−CH2Cl2 revealed a reversible redox wave due to the RuII/RuIII couple at E1/2 = −0.14 V vs Ag/AgCl (Figure S3), which was supported by UV−vis spectroelectrochemistry (Figure S4). Irreversible oxidation
ABSTRACT: Oxidative electropolymeization of an axially bound, bithiophene−pyridine complex of ruthenium(III)− porphycene [Ru(TPrPc) (btp)2]PF6 (1) gives a submicrometer-thick, polymeric film on an ITO electrode with a crystalline morphology. The polymeric film, the first example of axially linked multimetalloporphycene coordination arrays, exhibits highly stable and reproducible electrochromic response with high electrochromic efficiency upon electrochemical control over the metalcentered electron transfer process (RuII/RuIII).
The recent development of metal-containing polymers or metallopolymers has been triggered by widespread interest from both basic science and practical applications such as electronic devices and switches.1 Redox-active coordination compounds are thus eminent candidates for molecular building blocks due to their optical, electronic, magnetic, and redox properties. Multiple oxidation states formed by applied potentials lead to a color change in the UV−vis−NIR region, offering a significant potential group of coordination compounds as electrochromic motifs.2 The ability in molecular devices heavily relies not only on the selection of molecular components but also on the precise control of supramolecular organization of the building blocks in a thin film architecture, but facile construction of such organized devices yet remains an enormous challenge. Electropolymerization of organic and coordination compounds with thienyl pendants3 is a key technique to obtain polymeric thin-film materials on electrode surfaces but usually requires multiple synthetic processes to obtain desired monomolecular components. Herein, we report a novel redox-active metallopolymer film that incorporates ruthenium−porphycene complexes as electrochromic pigments. Porphycene is a constitutional isomer of porphyrin4 and due to its more intense and tunable nature of visible absorption bands, we consider that the porphycene sits on a superior and more favorable position among “porphyrin isomers” for chromic pigments. We show that a new ruthenium(III)−porphycene complex with axial bithienyl pendants [Ru(TPrPc) (btp)2]PF6 (1), where TPrPc = 2,7,12,17-tetra-n-propylporphycenato dianion and btp = 4(2,2′-bithienyl)pyridine, is readily electropolymerized onto © XXXX American Chemical Society
Figure 1. Molecular structure of 1 with thermal ellipsoids at a 50% probability level. Hydrogen atoms are omitted for clarity. Received: September 15, 2015
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DOI: 10.1021/acs.inorgchem.5b02129 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
ments and simulation indicate (Figures 2d and e) a crystalline morphology of the electroplolymerized film, in which the intrachain Ru···Ru separation and the nearest-neighbor interchain separation in the ensemble of the proposed 1D polymer chains are estimated to be 24.9 and 10.8 Å, respectively (Figure 2e). It should be noted that the facile formation of such a highly ordered electropolymerized film using molecular components is quite rare. The intermetallic separations estimated here still allow electron hopping to occur across the film via intra- and/or interchain mechanisms as shown below. CV profiles of the polymeric film in contact with 0.1 M nBu4NPF6−CH2Cl2 exhibited the RuII/RuIII redox couple at ca. 0 V, which is low enough for actual electrochromic operation, and the peak-to-peak separation (ΔEp) is highly dependent on the scan rates (Figure 3a). The current peak intensities of the
due to the thienyl pendants, which is responsible for oxidative electropolymerization, occurs at ipa = +1.50 V. Electropolymerization of 1 has been carried out by placing an ITO-coated glass plate (10 × 50 mm2) as a working electrode into a CH2Cl2 solution of 1 (1.0 mM) containing 0.1 M nBu4NPF6 as a supporting electrolyte and applying the electrode potential at +1.50 V. The film was also prepared by potential cycles (between +0.4 and +1.5 V). The film growth was confirmed by a gradual increase in the CV current, where the current intensity of the RuII/RuIII redox couple increases upon increasing the number of potential cycles (Figure S5). The resulting film showed a UV−vis absorption feature characteristic of ruthenium(III)−porphycenes with Q-bands at λmax = 614 and 543 nm together with the Soret band at λmax = 379 nm (Figure S6). The polymeric film comprises electropolymerized 1D chains containing sequentially linked metal−ligand alternate repeating units (Figure 2a). Energy dispersive X-ray (EDX) analysis gave
Figure 3. CV responses of the polymeric film in contact with 0.1 M nBu4NPF6−CH2Cl2. (a) Scan-rate dependence of the RuII/RuIII redox couple. Scan rate = 10 (a), 50 (b), 100 (c), 200 (d), 400 (e), 600 (f), 800 (g), and 1000 mV s−1 (h). (b) Linear relationships between current peak intensities of the RuII/RuIII couple and the square root of the scan rates.
RuII/RuIII couple are proportional to the square root of the scan rate (Figure 3b). These observations suggest a diffusioncontrolled redox event, which most probably is associated with the anion (PF6−) mobility across the film for charge compensation, in which the positively charged (RuIII) and uncharged (RuII) states are formed upon repetitive potential sweeps (Scheme S1). The electrochromic switching rate from RuII to RuIII and that from RuIII to RuII were 3.7 (± 0.1) and 2.3 (± 0.1) s, respectively (Figure S7), and depended on the sort of electrolyte anions X− for 0.1 M n-Bu4NX (X− = PF6−, ClO4−, Cl−, Br−, and I−; Figure S8 and Table S1). We therefore conclude that the anion mobility across the polymeric film9 plays a dominant role in determining the switching rate of electrochromism. The metal-based redox event was further confirmed by in situ spectroelectrochemistry (Figure 4a), which showed spectra characteristic of the RuII (−0.4 V; blue) or RuIII (+0.4 V; olive green) redox states of ruthenium−porphycene complexes. The electrochromic behavior was completely reversible, and the film showed excellent stability of the repetitive potential-switching operation (at least 5 × 103 cycles; Figure 4b). Significantly, the coloration efficiency (CE), defined as CE = (ΔOD)/Qd, where ΔOD = the amount of optical density change and Qd = the injected/ejected electronic charge, was calculated to be 178 cm2 C−1 at 609 nm (Q-band) of the polymeric film. This is comparable to values previously reported for organic electrochromic devices and high enough over inorganic devices.10 The high CE value is achieved by use of the ruthenium−porphycene complex where a considerably large absorbance difference in
Figure 2. Solid-state characterizations of the polymeric film prepared by 26 potential cycles (+0.4 to +1.5 V). (a) Chemical structure of the polymeric chain. (b) A tapping-mode AFM image. (c) A crosssectional SEM image. (d) An XRD profile and simulation. Dot: observed reflections. Red curve: simulation. Blue curve: residual error. (e) A proposed model for the polymer assemblies in the film.
a sulfur/ruthenium ratio of 1.3 (± 0.1) which is roughly consistent with the calculated value, 1.26. Atomic force microscopy (AFM) provided a smooth surface ranging over a micrometer-scale (Figure 2b). Cross-sectional scanning electron microscopy (SEM; Figure 2c) clearly showed a uniform film formation with a thickness of 0.40 (± 0.05) μm, which roughly estimated a film growth of ca. 0.015 μm per single potential cycle. Notably, the X-ray diffraction (XRD) measureB
DOI: 10.1021/acs.inorgchem.5b02129 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
Communication
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ACKNOWLEDGMENTS We are grateful for financial support from Grants-in-Aid for Scientific Research on Innovative Areas “Molecular Activation” (No. 25105537) and “Coordination Programming” (No. 24108730), Grants-in-Aid for Scientific Research (A; No. 21245016) and (B; No. 25288031), the Global COE Program “Science for Future Molecular Systems,” and Nanotechnology Platform (No. NPS13103) from MEXT. The authors thank Prof. Sunao Yamada and Prof. Hiroaki Yonemura (Kyushu University) for the AFM measurements.
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Figure 4. Electrochromic response. (a) UV−vis absorption spectral change from RuII (−0.40 V) to RuIII (+0.40 V) in contact with 0.1 M n-Bu4NPF6−CH2Cl2. Inset: photographs of the RuII and RuIII polymers. (b) Repetitive switching of absorption intensity between −0.40 and +0.40 V.
the visible region is achieved between two distinct redox states (e.g., RuII and RuIII) in the metalloporphycene polymer chains. In summary, we have developed a facile and convenient route to electrochromic thin-film materials on an ITO electrode by oxidative electropolymerization of a new ruthenium−porphycene complex, which also gives an unprecedented 1D multiporphycene array through axial coordination. The polymeric film described here exhibits remarkably stable and reproducible electrochromism with high CE values. For comparison and for more general interest, studies are also in progress using porphyrin complexes with btp. The present work offers a significant potential for use of metalloporphycene complexes toward more advanced, functional materials and switching devices.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02129. Experimental details, spectra, cyclic voltammograms, electrochromic response rate, switching profiles, and a summary of switching times (PDF)
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§ Graduate School of Material Science, University of Hyogo, 32-1, Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
Notes
The authors declare no competing financial interest. C
DOI: 10.1021/acs.inorgchem.5b02129 Inorg. Chem. XXXX, XXX, XXX−XXX
