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J. Phys. Chem. B 2006, 110, 21444-21449
Structure and Excitation Relaxation Dynamics of Dimethylanthracene Dimer in a γ-Cyclodextrin Nanocavity in Aqueous Solution Shin-ichiro Sato,* Takashi Nakamura, Satoru Nitobe, Takayuki Kiba, Kiyotada Hosokawa, Tatsuya Kasajima, Issei Otsuka, Seiji Akimoto, and Toyoji Kakuchi DiVision of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan ReceiVed: May 31, 2006; In Final Form: August 16, 2006
Dimethylanthracene (DMA), which exhibits almost no self-association in bulk organic solvents, forms a dimer and emits excimer-like fluorescence in a γ-cyclodextrin nanocavity in a dilute aqueous solution. The 1Bb and 1L electronic transitions of the DMA dimer split by 2230 and 344 cm-1, respectively, in a fluorescence a excitation spectrum obtained with the excimer-like emission. From these energy splits, the structure of dimer in relation to a dielectric constant inside γ-CD was discussed on the basis of atom-atom Lennard-Jones potential calculations including Coulombic interactions. Excitation relaxations of DMA in the presence of R-, β-, and γ-CDs in aqueous solutions were investigated by time-resolved fluorescence. The results suggest that both the hydrated and anhydrated species exist in the R- and γ-CD complexes, while only the anhydrated species exists in the β-CD complex.
1. Introduction Control of the intermolecular interactions of aromatic hydrocarbons is crucial for the exploitation of organic supramolecular semiconductors for both molecular electronics and the viable manipulation and incorporation of single molecules into nanoengineered devices.1 Aromatic hydrocarbons such as pyrene in organic solvents often emit excimer fluorescence, which indicates the presence of molecular interactions between an excited-state and a ground-state aromatic molecule. However, anthracene and anthracene derivatives emit almost no excimer emissions in dilute bulk organic solvents. The excimer fluorescence of substituted anthracenes in organic solvents is observed only under the condition of very high concentrations such as 10-1 M for a specific solvent such as CHCl3, indicating that their excimer-formation constants are very small. The excimer emissions of anthracene derivatives have been studied mainly in solid matrixes.2,3 The 9,10-dichloro- and dibromoanthracenes (DCA and DBA) were reported to form a rotated sandwich-type dimer2 in solid matrixes. Cyclodextrins (CDs) are a series of cyclic oligomers consisting of six or more R-l,4-linked D-glucopyranose units and designated by R, β, and γ for the hexamer, heptamer, and octamer, respectively.4 The smaller R- and β-CDs usually form 1:1 complexes, but γ-CD has been shown to accommodate two guest molecules in its large cavity (1:2 host-guest complexes).5 This property allows γ-CD to be used as a molecular flask in which two species can meet, as shown by the facilitated formation of dimers and charge-transfer complexes as well as by enhanced photodimerization.6 Recently, unsubstituted anthracene dimer/γ-CD in crystalline powder has been studied by means of reflection-absorption spectroscopy and ab initio molecular orbital calculations.7 According to these studies, which used ab initio MO calculations, the most preferable state of the two neutral anthracene * Corresponding author. E-mail:
[email protected].
molecules in γ-cyclodextrin is considered to be a face-to-face configuration. The drawback of using unsubstituted anthracene dimers in molecular (photonic) devices is that they are unstable under light irradiation. The two anthracenes in γ-CD easily form intermolecular bonds at the 9, 9′ and 10, 10′ positions. In contrast, no such photochemical reaction occurred for the 9,10disubstituted anthracene derivatives in γ-CD in our experiments, suggesting this host-guest system would be a potent candidate for organic molecular devices. Here, we explore the structure of the 9,10-dimethylanthracene (DMA) dimer formed in a γ-CD nanocavity in dilute aqueous solution. We also report the excitation relaxation dynamics of DMA in R-CD, β-CD, and γ-CD nanocavities. 2. Experimental Section 9,10-Dimethylanthracene (Tokyo Kasei, Tokyo) and γ-CD (Kanto Kagaku, Tokyo) were used as received. A Milli-Q system (Millipore, Bedford, MA) was used for purification of water. DMA/γ-CD aqueous solution was prepared by mixing DMA/ethanol solution (200 µL; 1 × 10-3 M) with 2.5 × 10-2 M γ-CD/water solution (20 mL; 1 × 10-3 M), stirring for 12 h, and filtering to remove the aggregates of insoluble DMA. Steady-state absorption and fluorescence spectra at room temperature were measured with a U-3010 spectrophotometer and F-4500 fluorescence spectrometer (Hitachi, Tokyo), respectively. The fluorimeter was used after the correction for wavelength dependence of the detection sensitivity. Circular dichroism (CD) spectra were measured with a J-720 spectropolarimeter (Jasco, Tokyo). Fluorescence decay curves were measured with a picosecond time-correlated single-photon counting system. The light source was a Ti:sapphire laser (Coherent MIRA 900; Coherent, Santa Clara, CA) pumped by an Ar ion laser (Coherent INNOVA 310; Coherent), and the second harmonic of the Ti:sapphire laser generated by an LBO crystal (394 nm) was used for excitation pulses. A microchannel plate photomultiplier (Hamamatsu R2809U-01; Hamamatsu Photonics, Hamamatsu, Japan) was used as a detector in
10.1021/jp063346q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/03/2006
Structure and Relaxation Dynamics of DMA Dimer
Figure 1. Absorption (broken curve) and emission (solid curves) spectra of DMA in γ-CD water solution. The thick solid emission curve was obtained with an excitation wavelength of 375 nm, while the thin solid curve was obtained at 422 nm.
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21445
Figure 2. Left panel shows fluorescence excitation spectra (solid curves) of DMA in γ-CD water solution are shown together with the absorption spectrum (broken curve) in Figure 1. The excitation spectra were recorded with an observation wavelength of 570 nm (thin solid curve) and with an observation wavelength of 450 nm (thick solid curve). Right panel shows the energy-level diagram of related states in DMA monomer and dimer.
conjunction with a monochromator (Nikon P-250; Nikon, Tokyo). The fluorescence polarization was set to the magic angle (54.7°) with respect to that of the excitation laser source. The typical time width of the instrumental response function (IRF) was ca. 30 ps. Excitation laser intensities were set to give fluorescence signals of
