Letter pubs.acs.org/Langmuir
Efficient Singlet−Singlet Energy Transfer in a Novel Host−Guest Assembly Composed of an Organic Cavitand, Aromatic Molecules, and a Clay Nanosheet Yohei Ishida,†,‡ Revathy Kulasekharan,§ Tetsuya Shimada,† Shinsuke Takagi,*,†,∥ and V. Ramamurthy*,§ †
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 § Department of Chemistry, University of Miami, Coral Gables, Florida 33146-0431, United States ∥ PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan S Supporting Information *
ABSTRACT: A supramolecular host−guest assembly composed of a cationic organic cavitand (host), neutral aromatic molecules (guests), and an anionic clay nanosheet has been prepared and demonstrated that in this arrangement efficient singlet−singlet energy transfer could take place. The novelty of this system is the use of a cationic organic cavitand that enabled neutral organic molecules to be placed on an anionic saponite nanosheet. Efficient singlet−singlet energy transfer between neutral pyrene and 2-acetylanthracene enclosed within a cationic organic cavitand (octa amine) arranged on a saponite nanosheet was demonstrated through steady-state and time-resolved emission studies. The high efficiency was realized from the suppression of aggregation, segregation, and self-fluorescence quenching. We believe that the studies presented here using a novel supramolecular assembly have expanded the types of molecules that could serve as candidates for efficient energy-transfer systems, such as in an artificial light-harvesting system.
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INTRODUCTION It is well-known that photochemical processes such as electron and energy transfer between an excited-state molecule and a ground-state molecule depend on the distance between the donor and acceptor molecules.1 Because a short intermolecular distance favors efficient electron and energy transfer, strategies to place molecules at such distances using well-ordered solid surfaces have been a topic of recent interest.2−11 However, energy and electron transfers between densely adsorbed dyes on solid surfaces are hampered by the aggregation and segregation of these molecules. H-type aggregation significantly decreases the lifetime of excited molecules because of selfquenching.12 However, segregation causes the decrease in transfer efficiencies because of the increased distance between donor and acceptor molecules.13 Both aggregation and segregation typically occur when the guest−guest attractive or repulsive interaction is relatively stronger than the host−guest attractive interaction.13 In general, both aggregation and segregation lead to poor energy-transfer and electron-transfer efficiencies.14−16 One of our groups (Takagi’s) has successfully developed a technique that suppresses the aggregation of organic molecules even at high concentrations by using saponite clay sheets as the host material.9,10 Saponite clay minerals are characterized by nanostructured flat sheet structures with a diameter of ∼40−100 nm possessing © 2013 American Chemical Society
negatively charged surfaces with an intercharge distance of 1.2 nm.10,17−23 Additionally these materials could be exfoliated to individual nanosheets (ca. the particle diameter of
