Published: July 20, 2011 r2011 American Chemical Society 10722 dx.doi.org/10.1021/la202231k | Langmuir 2011, 27, 10722–10729 ARTICLE pubs.acs.org/Langmuir Novel Methodology To Control the Adsorption Structure of Cationic Porphyrins on the Clay Surface Using the “Size-Matching Rule” Tsuyoshi Egawa, † Hajime Watanabe, † Takuya Fujimura, † Yohei Ishida, †,‡ Masafumi Yamato, † Dai Masui, † Tetsuya Shimada, † Hiroshi Tachibana, † Hirohisa Yoshida, † Haruo Inoue, † and Shinsuke Takagi* ,†,§ † Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397 Japan ‡ Japan Society for the Promotion of Science (DC1), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan § PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan b S Supporting Information ’ INTRODUCTION The control of molecular alignment and orientation is one of the most important techniques for developing science and technology. Especially in the field of chemistry and physics, nanotechnology attracts much attention to achieve a break- through in molecular alignment controls. Many chemists have been challenged to construct materials where the structure can be controlled on the molecular level by the bottom-up strategy. Recently, a self-assembly technique 1 has been widely utilized as the bottom-up technique. Many excellent works using the self- assembly technique have been reported in recent years. 24 In the case of the self-assembly technique, the interaction between guest molecules plays an important role to determine the molecular alignment and orientation. We have advocated the novel technique “size-matching rule”, which controls the molec- ular alignment and orientation without complicated procedures using clay minerals as host materials. 513 The host (clay miner- als)guest (molecules) interaction is crucial in our work, while the guestguest interaction is important in the typical self- assembly technique. Since typical clay minerals have anionic charges on the surfaces, cationic organic molecules can adsorb on the surface by electrostatic interactions. 1422 We have been studying synthetic saponite (SS)cationic porphyrin complexes and found that the porphyrin molecules do not aggregate on the SS surface up to about 100% adsorption versus the cation exchange capacity (CEC) of the SS. 510 Since a size-matching of the distance between anionic sites on the SS and the distance between cationic groups of the dye molecule is a crucial factor for realizing nonaggregated dye assemblies, we termed this effect the “size-matching rule”. In the case of typical synthetic clay minerals such as a saponite, the average intercharge distance is 1.2 nm on the basis of a hexagonal array. Thus, the average intermolecular distance (center to center) of tetracationic porphyrin can be fixed to 2.4 mm, when the dye loading is 100% vs CEC. 5,6 This intermolecular distance is suitable for photochemical reactions, since there is no interaction between porphyrins in the ground state which leads to the drastic decrease of the excited-state lifetime, while they can interact with each other in the excited Received: June 15, 2011 Revised: July 19, 2011 ABSTRACT: Saponite-type clays that have different cation exchange capacities were successfully synthesized by hydrother- mal synthesis. The structure and properties were analyzed by X-ray diffraction, X-ray fluorescence, 27 Al NMR, FT-IR, thermo- gravimetric and differential thermal analysis, atomic force micro- scopy, and cation exchange capacity measurement. The inter- charge distances on the synthetic saponite (SS) surfaces were calculated to be 0.81.9 nm on the basis of a hexagonal array. The complex formation behavior between SS and cationic porphyrins was examined. It turns out that the average intermolecular distance between porphyrin molecules on the SS surface can be controlled, depending on the charge density of the SS. In the case of tetrakis(1-methylpyridinium-4-yl)porphyrin (H 2 TMPyP 4+ ), the average intermolecular distances on the SS surface can be controlled from 2.3 to 3.0 nm on the basis of a hexagonal array. It was also found that absorption maxima of porphyrins depend on the charge density of the SS. The adsorption behavior of porphyrin on the SS surface can be rationally understood by the previously reported “size-matching rule”. This methodology using hostguest interaction can realize a unique adsorption structure control of the porphyrin molecule on the SS surface, where the gap distance between guest porphyrin molecules is rather large. These findings will be highly valuable to construct photochemical reaction systems such as energy transfer in the complexes.