Electrochemical fabrication of one-dimensional porphyrinic wires on electrodes

Abstract

[5,15-Di(4-aminophenyl)-10,20-diphenylporphyrinato]zinc(II) was found to electropolymerize on electrodes such as glassy carbon (GC), indium tin oxide (ITO), and tin oxide, to form a redox-active, stable, and reproducible π-conjugated polymer. Electrochemical and UV/vis spectroscopic studies showed that the electropolymerization involves azobenzene linkage formation via radical intermediates. The electrode modified with the porphyrinic wire featured strong light absorptions in the visible region, thereby serving as a photoanode for a photocurrent generation system.

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Ryota Sakamoto

Ryota Sakamoto was born in Yamagata Village, Nagano, Japan in 1980. He graduated from The University of Tokyo (Japan) in 2002, and received his Ph.D. degree from the same university in 2007 under the supervision of Prof. Hiroshi Nishihara. Then he was appointed as an assistant professor at Tokyo University of Science (Japan), working with Prof. Takeshi Yamamura. In 2010 he moved to The University of Tokyo, joining Prof. Nishihara's group again. His current research interest includes the construction of molecule-based nanostructures, and photonic and electronic devices thereof.

Introduction

A series of valuable characteristics, such as developed π-conjugation, intense color, rigid structure, and high chemical stability, allows porphyrins to be utilized in broad application areas like electronics,^1–4 medicine,^5–7 catalysis^8–10 and so on. In particular, because of their versatility in light absorptions and redox activities, porphyrins are frequently used in optoelectronics and photovoltaics applications. With adequate electron transfer ability and capability to acquire light in wide wavelengths, porphyrins become eminent in light-induced electron donating systems, leading to applications in dye-sensitized solar cells^11–13 and bulk heterojunction solar cells.^14–16 Moreover, the redox properties of porphyrins can be tuned at will by introducing various types of substituents and metal centres.

Synthesizing porphyrinic polymers on electrode surfaces is an important technique for applications.¹⁷ Such polymer structures may be accomplished by coordination reactions between porphyrin-containing bridging ligand molecules and metal ions,¹⁸ or by oxidative radical coupling of functionalized porphyrins.¹⁹ The electropolymerizable substituents include vinyl, amino, hydroxylphenyl, pyrrolyl,²⁰ carbazolyl²¹ and thienyl²² groups. In the present work, we show a new procedure to form a porphyrinic polymer on various electrode surfaces, including glassy carbon (GC), indium tin oxide (ITO), and tin oxide (SnO₂). [5,15-Di(4-aminophenyl)-10,20-diphenylporphyrinato]zinc(II) (1, Scheme 1 and Fig. 1) is electrooxidized to form an N-radical, such that porphyrinic oligomers linked by azobenzene (P1) are attached on an electrode surface (Scheme 1). The polymer film is characterized by means of electrochemistry, Raman, and UV/vis spectroscopy. The photoelectric conversion ability of the modified electrode is also reported.

Scheme 1 Proposed mechanism of the oxidative electropolymerization of 1 to form P1.

Fig. 1 Molecular size of 1 estimated by means of DFT calculation.

Experimental section

Reagents and instruments

All chemicals were purchased from Kanto Chemical Co., Tokyo Chemical Industry Co., Ltd, or Wako Pure Chemical Industries, Ltd, were used as received, unless otherwise stated. 1 was synthesized according to previous literature studies^23–26 with a few modifications. Tetra-n-butylammonium perchlorate, Bu₄NClO₄, employed as a supporting electrolyte was purified by recrystallization from ethanol, and dried in vacuo. A Milli-Q purification system (Merck KGaA) was used to obtain pure water. ITO was treated before use upon sonication in acetone (10 min) and nonionic detergent in water (30 min × 2) and, and purified water (10 min). The cleaned substrate was stored in water, and dried by nitrogen blow just prior to use. All ¹H NMR data were recorded on a Bruker-DRX500 using CDCl₃ as the solvent and tetramethylsilane (δ_H = 0.00) as an internal standard. High-resolution fast atom bombardment mass spectrometry (HR-FAB-MS) was conducted using a JEOL JMS-700 MStation mass spectrometer. UV/vis spectra were obtained with a JASCO V-570 spectrometer. Raman spectra were recorded using a JASCO NRS-5100. AFM measurements were carried out using an Agilent Technologies 5500 scanning probe microscope in the high-amplitude mode (tapping mode) with a silicon cantilever Nano World PPP-NCL probe. FE-SEM images were recorded using JEOL JSM-7400FNT with an acceleration voltage of 1.5 kV. All experiments were carried out under an ambient condition unless otherwise stated. The molecular size of 1 was determined by DFT calculation. The DFT calculation was carried out using a Gaussian09 Revision D.01 program package. The geometry optimization was performed using B3LYP functional with the LANL2DZ basis set for Zn and 6-31g(d) basis set for the other atoms and the result was visualized using GaussView 5.0.8 software.²⁷

Photoelectric conversion measurement

The modified ITO electrode was assigned as a working electrode (photoanode) with an Ag⁺/Ag electrode (0.01 M AgClO₄ in 0.1 M Bu₄NClO₄/acetonitrile) acting as a reference electrode and a Pt wire as a counter electrode. All three electrodes were engaged in an acetonitrile solution of 0.1 M Bu₄NClO₄ containing triethanolamine (TEOA, 0.05 M) as a sacrificial donor reagent. The cell was sealed and deoxygenized by argon bubbling for 5 min.²⁸ Monochromatic light for an action spectrum (400–500 nm in every 10 nm) was extracted from a xenon lamp (MAX-302, Asahi Spectra Co., Ltd) and a monochromator (CT-10, JASCO Corporation) was utilized to monochromate the photon flux while the size of the electrode active area was approximated from a fluorocarbon rubber o-shaped ring; 0.264 cm². An electrochemical analyser (ALS 750A, BAS Inc.) was used to control the electrode potential and photocurrent acquisition of the photoelectric conversion system. A photon counter (8230E and 82311B, ADC Corporation) was engaged in quantifying the photon flux of the incident light. The insignificant dark current was observed when the potential of the photoanode fixed close to the open circuit potential (−0.22 V vs. Ag/Ag⁺). The calculation for quantum yield of photocurrent generation was made according to a previous literature.²⁹

Results and discussion

Porphyrinic polymer wire P1 was synthesized by the electrochemical oxidation method. Fig. 2 shows a cyclic voltammogram of 1 (2.0 mM) at GC in 0.1 M Bu₄NClO₄–CH₂Cl₂ in a repetitive redox process. An anodic peak appears at around 0.52 V vs. Ag⁺/Ag, corresponding to the oxidations of both amino group³⁰ and porphyrin moiety.³¹ This argument is strongly supported by the disappearance of the shoulder peaks in Fig. 2 after the electropolymerization was complete (Fig. 4). Its peak current as shown in Fig. 3 increases with the number of oxidation cycles, indicative of the formation and growth of an electroactive film on the GC surface.³² In general, the oxidation of aromatic amines is likely to afford either polyaniline or azobenzene.³³ In the present case, the azobenzene linkage is responsible to elongate porphyrinic polymer wire P1 (Scheme 1). Fig. 4 shows a cyclic voltammogram of P1 on GC in 0.1 M Bu₄NClO₄–CH₂Cl₂, displaying a quasi-reversible redox wave due to the [porphyrinato]zinc(II) moiety at ca. 0.52 V. The surface coverage of the porphyrin unit was calculated to be 2.7 × 10⁻⁹ mol cm⁻² from the area of the anodic redox wave.

Fig. 2 Cyclic voltammogram for 1 (2 mM) upon repetitive scans (from 1st to 50th cycles) in 0.1 M Bu₄NClO₄–CH₂Cl₂ at GC.

Fig. 3 Plot of anodic peak current vs. scan cycles.

Fig. 4 Cyclic voltammogram of P1 on GC in 0.1 M Bu₄NClO₄–CH₂Cl₂.

The electropolymerization of 1 to form P1 was also conducted on transparent electrodes such as indium tin oxide (ITO) and tin oxide (SnO₂) in order to allow spectroscopic studies. Practically identical electrochemical behaviour was observed (Fig. S1†).

The morphology of P1 was investigated by AFM and SEM. In order to observe the edge of the electropolymerized film, the modified electrode was scratched using a metal spatula, and the border of the scratched region was inspected. From the cross-sectional profile in AFM (Fig. 5), the average thickness of P1 was calculated to be 112 nm. The surface coverage of the [porphyrnato]zinc(II) unit was estimated from the average thickness and size of 1 (Fig. 1), giving a value of 3.6 × 10⁻⁹ mol cm⁻². This value was consistent with the calculated value from the cyclic voltammetry (Fig. 3). On the other hand, SEM also gave a concrete evidence for the polymer formation, showing distinguishable domains (Fig. 6). The bare ITO surface as a lower part was rather smooth, while the area modified with P1 as the higher part displayed a creased texture.

Fig. 5 (a, b) AFM height (3D and normal) images of an ITO electrode modified with the porphyrin polymer. (c) AFM cross-sectional profile along the blue line.

Fig. 6 SEM image an ITO electrode modified with the porphyrin polymer.

Fig. 7 shows the absorption spectra of 1 in dichloromethane and the P1 film on ITO. Note that P1 deposited on ITO exhibited a greenish yellow color (Fig. 8). The spectrum of the P1 film features absorption bands typical of porphyrin;³⁴ the Soret band (435 nm) and Q-bands (563 and 607 nm). Moreover, the spectrum of P1 (the Soret band at 424 nm and Q-bands at 554 and 588 nm) is quite similar to that of 1. This series of facts indicates that the porphyrinic skeleton was maintained in P1 in the course of the electropolymerization process.

Fig. 7 Absorption spectra of 1 in dichloromethane and P1 on ITO.

Fig. 8 Photographs of an ITO electrode before and after the modification with P1.

The Raman spectra of 1 and P1 were measured using probe light at 532 nm, and the result is depicted in Fig. 9. In addition to a common band at around 1460–1470 cm⁻¹, P1 showed a characteristic band at 1446 cm⁻¹, whereas no band was detected in 1. This band can be assigned to the stretching vibration of the azo group.^35–37 This result supports our claim that the azobenzene linkage was generated upon the electrooxidative polymerization process.

Fig. 9 Raman spectra of 1 and P1.

As a functionality of P1, photoelectric conversion ability was examined. P1 on an ITO electrode was employed as a working electrode, and the whole setup of the photoelectric conversion system is described in the Experimental section. Upon irradiation with visible light to the working electrode applied with a bias potential (−0.22 V) led to generate an anodic current as shown in Fig. 8. The action spectrum of the photocurrent generation was coincident with the UV-vis absorption spectrum of P1 thin films on an ITO modified electrode in the 400–500 nm range. This indicates that light absorption by the porphyrinic moiety is responsible for the photocurrent generation. The quantum efficiency of the system was calculated to be 0.005% (Fig. 10).³⁸

Fig. 10 Action spectrum for the photocurrent generation (black dots) and the absorption spectrum of P1 on an ITO substrate (black solid line with orange dots).

Proposed polymerization mechanism

Cyclic voltammetry disclosed that P1 is attached on electrode surfaces, Raman spectroscopy confirmed the presence of the azobenzene linkage in P1, and UV/vis absorption spectroscopy indicated that the porphyrinic moiety was preserved. These studies help us propose a possible polymerization mechanism (Scheme 1). The electrooxidation of P1 leads to the formation of a reactive anilino radical form,³⁹ which can be applied for the modification of the glassy carbon surface. In the case of ITO and SnO₂, although the detail chemical structure is unclear, an electron pathway is formed via the possible interactions such as coordination bonds and hydrogen bonds. On the other hand, the growth of P1 is enabled by the coupling between two anilino radical species: one is tethered on the electrode surface, and the other is in the solution phase. The coupling results in the formation of a hydrazine linkage, which undergoes electrochemical oxidation to the azobenzene linkage. In fact, the repetitive cyclic voltammetry scan in Fig. 2 shows that the anodic peak current for the oxidation process is greater than the cathodic one. The additional anodic current corresponds to the irreversible oxidation of the hydrazine linkage.

Conclusion

In summary, 5,15-di(4-aminophenyl)-10,20-diphenylporphyrinatozinc(II) monomer 1 was successfully electropolymerized on GC, ITO and SnO₂ electrodes to give porphyrin polymer P1 tethered with azobenzene linkages. A possible oxidative polymerization mechanism involves the dimerization of 1 through the formation of an anilino radical form. The presence of azobenzene linkage in P1 was confirmed by a series of electrochemical and spectroscopic studies. The polymer displays photocurrent generation ability.

Acknowledgements

The authors acknowledge Grants-in-Aid from MEXT of Japan (no. 26708005, 26107510, 26620039, 15H00862, 26220801, 26110505, areas 2406 [All Nippon Artificial Photosynthesis Project for Living Earth], 2506 [Science of Atomic Layers] and 2509 [Molecular Architectonics]), and JSPS fellowship for young scientists. R. S. is grateful to Ogasawara Foundation for the Promotion of Science & Engineering, The Kao Foundation for Arts and Sciences, The Asahi Glass Foundation, The Noguchi Institute, Japan Association for Chemical Innovation, The Mikiya Science and Technology Foundation, Yazaki Memorial Foundation for Science and Technology, Shorai Foundation for Science and Technology, The Hitachi Global Foundation, Iketani Science and Technology Foundation, Kumagai Foundation for Science and Technology, The Foundation for Interaction in Science & Technology, The Foundation for the Promotion of Ion Engineering, The Foundation Advanced Technology Institute, Izumi Science and Technology Foundation, LIXIL JS Foundation, and The Iwatani Naoji Foundation for financial support. S. M. is grateful to Monbukagakusho scholarship.

Notes and references

1. W. J. Park, S. H. Chae, J. Shin, D. H. Choi and S. J. Lee, Synthetic Metals, 2015, 205, 206–211.
2. X. Zhang and Y. Chen, Inorg. Chem. Commun., 2014, 39, 79–82.
3. M. Miyachi, Y. Yamanoi, K. Nakazato and H. Nishihara, Biochim. Biophys. Acta, 2013, 1837, 1567–1571.
4. M. Vasilopoulou, A. M. Douvas, D. G. Georgiadou, V. Constantoudis, D. Davazoglou, S. Kennou, L. C. Palilis, D. Daphnomili, A. G. Coutsolelos and P. Argitis, Nano Res., 2014, 7, 679–693.
5. X. Deng, Y. Liang, X. Peng, T. Su, S. Luo, J. Cao, Z. Gu and B. He, Chem. Commun., 2015, 51, 4271–4274.
6. K. Lu, C. He and W. Lin, J. Am. Chem. Soc., 2015, 137, 7600–7603.
7. T. Zhao, Y.-L. Wang, L.-N. Zhu, Y.-F. Huo, Y.-J. Wang and D.-M. Kong, RSC Adv., 2015, 5, 47709–47717.
8. G. Tseberlidis, P. Zardi, A. Caselli, D. Cancogni, M. Fusari, L. Lay and E. Gallo, Organometallics, 2015, 34, 3774–3781.
9. C.-H. Hung, B. B. Beyene and S. B. Mane, Chem. Commun., 2015, 51, 15067–15070.
10. S. Kumar, M. Y. Wani, C. T. Arranja, J. D. a. e Silva, B. Avula and A. J. F. N. Sobral, J. Mater. Chem. A, 2015, 3, 19615–19637.
11. J. Gasiorowski, N. Pootrakulchote, C. Reanprayoon, K. Jaisabuy, P. Vanalabhpatana, N. S. Sariciftci and P. Thamyongkit, RSC Adv., 2015, 5, 72900–72906.
12. C.-L. Mai, T. Moehl, C.-H. Hsieh, J.-D. Décoppet, S. M. Zakeeruddin, M. Grätzel and C.-Y. Yeh, ACS Appl. Mater. Interfaces, 2015, 7, 14975–14982.
13. J. Chen, Y. Sheng, S. Ko, L. Liu, H. Han and X. Li, New J. Chem., 2015, 39, 5231–5239.
14. C. V. Kumar, L. Cabau, E. N. Koukaras, A. Sharma, G. D. Sharma and E. J. Palomares, J. Mater. Chem. A, 2015, 3, 16287–16301.
15. K. Gao, L. Li, T. Lai, L. Xiao, Y. Huang, F. Huang, J. Peng, Y. Cao, F. Liu, T. P. Russel, R. A. J. Janssen and X. Peng, J. Am. Chem. Soc., 2015, 137, 7282–7285.
16. J. Kesters, P. Verstappen, M. Kelchtermans, L. Lutsen, D. Vanderzande and W. Maes, Adv. Energy Mater., 2015, 5, 1500218.
17. L. Zhang, K. Wang, X. Qian, H. Liu and Z. Shi, ACS Appl. Mater. Interfaces, 2013, 5, 2761–2766.
18. S. A. Ikbal, S. Brahma and S. P. Rath, Inorg. Chem., 2012, 51, 9666–9676.
19. P. A. Liddell, M. Gervaldo, J. W. Bridgewater, A. E. Keirstead, S. Lin, T. A. Moore, A. L. Moore and D. Gust, Chem. Mater., 2008, 20, 135–142.
20. F. Bedioui, J. Devynck and B.-D. Claude, Acc. Chem. Res., 1995, 28, 30–36.
21. J. Durantini, L. Otero, M. Funes, E. N. Durantini, F. Fungo and M. Gervaldo, Electrochim. Acta, 2011, 56, 4126–4134.
22. K. Takechi, N. Takahashi, T. Shiga, T. Akiyama and S. Yamada, Mol. Cryst. Liq. Cryst., 2011, 538, 10–14.
23. T. Lin, X. S. Shang, J. Adisoejoso, P. N. Liu and N. Lin, J. Am. Chem. Soc., 2013, 17, 3576–3582.
24. M. Boccalon, E. Iengo and P. Tecilla, Org. Biomol. Chem., 2013, 11, 4056–4067.
25. S. M. S. Chauhan and N. G. Giri, Supramol. Chem., 2008, 20, 743–752.
26. C. He, Q. He, C. Deng, L. Shi, D. Zhu, Y. Fu, H. Cao and J. Cheng, Chem. Commun., 2010, 46, 7536–7538.
27. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc., Wallingford, CT, 2009.
28. R. Matsuoka, R. Toyoda, R. Sakamoto, M. Tsuchiya, K. Hoshiko, T. Nagayama, Y. Nonoguchi, K. Sugimoto, E. Nishibori, T. Kawaii and H. Nishihara, Chem. Sci., 2015, 6, 2853–2858.
29. H. Yamada, H. Imahori, S. Fukuzumi, Y. Nishimura and I. Yamazaki, Chem. Commun., 2000, 1921–1922.
30. R. L. Hand and R. F. Nelson, J. Am. Chem. Soc., 1974, 96, 850–860.
31. K. M. Kadish and L. R. Shiue, Inorg. Chem., 1982, 21, 3623–3630.
32. H. Nishihara, M. Noguchi and K. Aramaki, Inorg. Chem., 1987, 26, 2862–2869.
33. D. A. Buttry, J. C. M. Peng, J.-B. Donnet and S. Rebouillat, Carbon, 1999, 37, 1929–1940.
34. M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138–163.
35. Z. Meić, G. Baranovic, V. Smrečki, P. Novak, G. Keresztury and S. Holly, J. Mol. Struct., 1997, 408–409, 399–403.
36. Y. C. Liu and R. L. McCreery, Anal. Chem., 1997, 69, 2091–2097.
37. S. Koide, Y. Udagawa, N. Mikami, K. Kaya and M. Ito, Bul. Chem. Soc. Jpn., 1972, 45, 3542–3543.
38. R. Sakamoto, K. Hoshiko, Q. Liu, T. Yagi, T. Nagayama, S. Kusaka, M. Tsuchiya, Y. Kitagawa, W.-Y. Wong and H. Nishihara, Nat. Commun., 2015, 6, 6713.
39. R. S. Deinhammer, M. Ho, J. W. Anderegg and M. D. Porter, Langmuir, 1994, 10, 1306–1313.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of 1, electrochemical polymerization of 1 on ITO and SnO₂ electrodes. See DOI: 10.1039/c5qi00239g

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