Porphyrins have planar structure and effectively absorb visible-light normal to the ring plane. Thus, they can be applied to artificial photosynthetic devices and photovoltaic cells when the porphyrin ring is flatly adsorbed on the solid substrate [1]. On the other hand, metal-organic frameworks (MOFs), which have a high surface area, flexibility, and designability, are one of the highly promissing materials to construct the highly oriented three-dimensional structures [2]. Thus, it is expected to construct three-dimentional MOFs with porphyrin, in which porphyrin ring is highly oriented, on the solid surfaces, in order to construct artifical photosynthetic devices.
We have succeeded to construct the three-dimensional MOFs on the solid surfaces by a layer-by-layer method [3,4] and also to construct self-assembled monolayers (SAMs) with porphyrin [5-10]. Thus, in this study, we tried to construct the three-dimensional MOFs with porphyrin group, in which porphyrin ring is oriented parallel to the solid surfaces, using various SAM modified solid surfaces, such as iso-nicotinic acid (IA) SAM on TiO2(110), mercapto-propionic acid (MPA) SAM on Au(111), acrylic acid (AA) SAM on Si(111), and amino-propyl tri-methoxy silane (APTMS) SAM on glass substrate. At each step to construct these MOFs with tetra-carboxyphenyl porphyrin (TCPP), Zn tetra-phenyl porphyrin (Zn-TPP), and bipyridine (Bipy) by a layer-by-layer fasion, the adsorbed amounts and orientation were evaluated by x-ray photoelectron spectroscopy (XPS), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), attenuated total refection infrared spectroscopy (ATR-IRS), and ultraviolet/visible absorption spectroscopy (UV-vis).
Experiment
IA SAM on TiO2(110), MPA SAM on Au(111), AA SAM on Si(111), and APTMS SAM on glass were prepared by dipping each substrate into DMF solution with saturated IA at 110 °C for 10 h, ethanol solution with 1 mM MBA, ethanol solution with saturated methoxy acrylate and then hydrolysis of methoxy group to carboxylate, and ethanol solution with saturated ATPMS at 40 °C for 10h, respectively. After these SAM preparations, two-dimentional TCPP MOF sheets, which is prepared on the water surface [11], was transferred onto IA SAM on TiO2(110) and glass substrate. On the MPA SAM on Au(111) and AA SAM on Si(111), the substrates were dipped instead of the ethanol solutions with Zn-TPP and Bipy. At each step, layered structures and orientation were evaluated XPS, PM-IRRAS, ATR-IRS, and UV-vis.
Results and discussion
As an example, it is explained that IA SAM was confirmed to construct the TiO2(110) surface. When the pre-treated TiO2(110) substrate was dipped into DMF solution with saturated IA at 110 °C for 10 h, XP peaks around 401 eV in the N1s region and 532 eV in the O1s region due to the aniline ring and calboxylate, respectively, were observed. From these XP peaks, we confirmed that the IA SAM is formed on the TiO2(110) surface. After that, two-dimensional porphyrin sheet was transferred onto this SAM modified TiO2(110) surface by a layer-by-layer fasion.
References
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