Article pubs.acs.org/JPCC
Supramolecular-Surface Photochemistry: Supramolecular Assembly Organized on a Clay Surface Facilitates Energy Transfer between an Encapsulated Donor and a Free Acceptor Yohei Ishida,†,‡,§ Revathy Kulasekharan,∥ Tetsuya Shimada,† V. Ramamurthy,*,∥ 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 (PD), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan § Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ∥ Department of Chemistry, University of Miami, Coral Gables, Florida 33146-0431, United States S Supporting Information *
ABSTRACT: We report the occurrence of efficient energy transfer reaction in a novel host−guest assembly composed of an anionic clay nanosheet, cationic porphyrin, and neutral aromatic molecule encapsulated within a cationic organic cavitand. The supramolecular assembly was prepared by the coadsorption of tetracationic Zn−porphyrin (acceptor) and 2acetylanthracene (donor) enclosed within cationic organic cavitand (octaamine in its protonated form) on anionic clay nanosheets. In this arrangement under the interguest distance of 2.4 nm, almost 100% efficiency of singlet−singlet energy transfer was achieved. Detailed time-resolved fluorescence measurements revealed that the energy transfer rate constant could be attributed to a single component (1.9 × 109 s−1). This strongly suggests that the adsorption distribution of porphyrin and cavitand is rather uniform, not segregated. This is a progress from our previous study that involves energy transfer between two encapsulated neutral molecules. The use of Zn−porphyrin as an energy acceptor in this study enables to connect this energy transfer system to charge separation processes in the same manner as natural photosynthetic systems do; moreover, the efficiency of energy transfer reaction improved to almost 100% from 85% in the previous system between two cavitands.
■
INTRODUCTION Recently in the context of solar energy utilization for artificial photosynthesis has received considerable attention.1 One of the bottlenecks of realizing artificial photosynthesis in a laboratory is low photon-flux density of sunlight. A natural photosynthetic system overcomes this photon-flux density limitation with the help of dyes that efficiently absorb sunlight and transfer the energy to a reaction center.2,3 In recent years, studies have revealed that the light-harvesting system is composed of highly organized chlorophyll molecules.4−6 This observation has prompted us to construct a regularly arranged assembly of functional dyes in the context of realization of artificial lightharvesting system. A fair amount of research for photochemical energy transfer reactions has been carried out to build artificial light-harvesting systems such as supramolecular assemblies of organic molecules, covalently linked systems, and dendrimer systems.7−24 We have focused our investigations on using clay minerals as a novel host material for guest functional dyes.7,25−28 Saponite is a group of clay materials that are characterized by (1) nanostructured flat sheets, (2) negatively charged surfaces, (3) stack ability of individual nanosheets depending on the surrounding conditions, and (4) optical transparency in the visible region in the exfoliated state, when the particle size is small (ca.
