LETTER pubs.acs.org/JPCL
Reversible Photogeneration of a Stable Chiral Radical-Pair from a Fast Photochromic Molecule Sayaka Hatano,† Kana Fujita,† Nobuyuki Tamaoki,‡ Takashi Kaneko,§ Takuya Nakashima,|| Masanobu Naito,|| Tsuyoshi Kawai,|| and Jiro Abe*,†,^ †
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ‡ Research Institute for Electronic Science, Hokkaido University, N20, W10, Kita-ku, Sapporo 001-0020, Japan § Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan || Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan ^ CREST, Japan Science and Technology Agency (JST), 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
bS Supporting Information ABSTRACT: The photochromic naphthalene-bridged imidazole dimer containing a naphthyl moiety that tethers two triarylimidazole units shows instantaneous coloration upon exposure to UV light and rapid fading in the dark. In this work, we demonstrate the formation of a stable chiral radical-pair that exhibits no photoracemization even by repeated photochromic cycles. The photogenerated radical-pair from the imidazole dimer exhibits the Cotton effect in the visible light region, indicating the retention of the enantiomeric conformation of the radical-pair. This result suggests that the chirality resulting from the binaphthyl moiety induces exciton coupling between the two radical chromophores by through-space interaction. SECTION: Molecular Structure, Quantum Chemistry, General Theory
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onsiderable attention has been paid to the organic photochromic materials that change their color upon light irradiation of the appropriate wavelength and return to their initial state either thermally or by subsequent irradiation with a specific wavelength of light.1,2 In particular, thermally reversible photochromic molecules are allowed to change and reset their molecular properties by simply turning a light source on and off. We recently developed a unique series of fast photochromic molecules that show instantaneous coloration upon exposure to UV light and rapid fading in the dark.3 12 The naphthalene-bridged imidazole dimers, with a naphthyl moiety that couples two triarylimidazole groups, shows fast photochromism both in solution and solid states.3,4 1-NDPI-8-TPI-naphthalene (Figure 1a) consists of two different triarylimidazole units, i.e., 2-(1-naphthyl)-4,5-diphenylimidazole (NDPI) and triphenylimidazole (TPI) units. Upon UV light irradiation, the C N bond between the two imidazole rings is homolytically cleaved to give a pair of imidazolyl radicals and the color of the solution changes from colorless to green. The green color fades very rapidly after ceasing the light irradiation. The radical radical coupling of the photogenerated radical-pair obeys first-order kinetics and the half-life of the colored species in benzene is 179 ms at 298 K.4 r 2011 American Chemical Society
Besides the fast bleaching rate, the intrinsic chirality of the binaphthyl moiety in 1-NDPI-8-TPI-naphthalene should guarantee the construction of chiral molecular switches. Axially chiral binaphthalenes show strong circular dichroism (CD) signals and large optical rotation values, which are dependent on the dihedral angle of the two naphthyl moieties. Chiral molecular switches refer to a class of chiral materials whose chiroptical properties can be reversibly modulated under external stimuli such as light irradiation, heat, pH, chemicals, and electric fields.13 22 Reversible photoregulation of chiral structures along with a photochromic reaction will lead to the switching of chiroptical properties of molecules.23 Moreover, a series of the imidazole dimers is only the photochromic molecule that results in the formation of paramagnetic imidazolyl radicals with an unpaired electron by the homolytic C N bond cleavage upon UV light irradiation. Therefore, fast optical switching of magneto-optical properties can be expected for the chiral fast photochromic molecules forming the chiral radicals. Here we show that the Received: September 22, 2011 Accepted: October 7, 2011 Published: October 07, 2011 2680
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The Journal of Physical Chemistry Letters
Figure 1. (a) Photochromism of 1-NDPI-8-TPI-naphthalene. (b) CD and UV vis absorption spectra of the first-eluted enantiomer (red) and the other enantiomer (blue) of 1-NDPI-8-TPI-naphthalene in toluene (3.5 10 5 M, light path length: 10 mm) at 293 K (left), and those of 1-NDPIR-8-TPIR-naphthalene in toluene immediately after irradiation with 365 nm of UV light at 200 K (right).
chiral radical-pair, which is stable without undergoing radical radical coupling at 200 K, can be reversibly generated by the photochromic reaction of 1-NDPI-8-TPI-naphthalene. Recrystallization from a mixture of benzene and acetonitrile under light shielding conditions yields racemic crystals of 1-NDPI-8-TPI-naphthalene. The X-ray crystallographic analysis of a single crystal of 1-NDPI-8-TPI-naphthalene affirmed the presence of a couple of enantiomers in a unit cell as described in our previous paper.4 The resolution of the racemic mixture by chiral high-performance liquid chromatography (HPLC) gave each enantiomer up to 96% ee. (Figure S2, Supporting Information). The CD and UV vis absorption spectra of each enantiomer in toluene are shown in Figure 1b. As described above, the half-life at room temperature of the colored species (1-NDPIR-8-TPIR-naphthalene) of 1-NDPI-8-TPI-naphthalene is too short to measure a CD spectrum by using a conventional CD spectrometer. Therefore the CD spectra of the colored species were measured at 200 K immediately after the irradiation with 365 nm of UV light in order to slow down the rate of thermal recombination reaction. The CD spectra of each enantiomer of 1-NDPI-8-TPI-naphthalene exhibit strong Cotton effect at wavelengths shorter than 400 nm (Figure 1b, left), which is consistent with the spectra for the axially chiral binaphthyl derivatives,24,25 whereas the CD spectra of each enantiomer of the colored species are quite different from those of the parent 1-NDPI-8-TPInaphthalene, as shown in Figure 1b (right). The broad absorption band in the visible light region can be attributed to the electronic transitions of NDPIR and TPIR chromophores. Each enantiomer of the colored species clearly shows the mirror-image CD bands in
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the visible light region where the absorption band of the colored species are located, indicating that the electronic transitions attributable to the radical chromophores have become optically active. It is known that the intensity of CD signals depends greatly on the dissymmetric spatial arrangement of interacting chromophores, particularly on the distance and the relative twisting angles between chromophores. Therefore, the Cotton effect in the visible light region of the colored species is tentatively ascribable to the chiral exciton interaction between NDPIR and TPIR chromophores.26,27 Several studies have appeared in which optically active products have been derived from cage reactions. Stowell et al. reported the stereoselectivity in the cage effect of the homolytic decomposition of an optically active azo compound.28 The radical-pairs generated closer together recombine with higher stereoselectivity, because the radical has a reactive partner initially positioned in a stereospecific manner within the solvent cage. Although the solvent cage could suppress the diffusion of the radicals generated by homolytic decomposition of the molecule, the change in orientation of one radical relative to the other radical of the caged radical-pair would decrease the stereoselectivity in the product of the radical radical coupling.29 36 We investigated the possibility of the photoracemization of 1-NDPI-8-TPI-naphthalene through the transient colored biradical at room temperature. It should be noted that the chiral HPLC analysis confirmed that no photoracemization takes place along with the photochromic reaction, suggesting the suppression of the rotation of the NDPIR moiety relative to the naphthyl unit (Figure S3, Supporting Information). Of course, 1-NDPI-8-TPI-naphthalene maintains the chiral structure in the dark because of the presence of C N bond between two imidazole rings, which inhibits the rotation of the NDPI moiety. In conclusion, we have achieved the first demonstration of reversible photogeneration of the stable chiral radical-pair from the fast photochromic molecule. The photogenerated radical species of 1-NDPI-8-TPI-naphthalene maintains the chiral conformation, and the colored radical exhibits the Cotton effect in the visible light region. This result suggests that the chirality resulting from the binaphthyl moiety induces exciton coupling between the two radical chromophores by through-space interaction. In addition, fast magneto-optical switching can be expected due to the fast switching ability between the diamagnetic colorless state and the paramagnetic colored state. This work is expected to lead us into the development of a fascinating new method for fast optical switching of chiroptical properties of materials.
’ ASSOCIATED CONTENT
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Supporting Information. Details of the measurement of CD spectra and chiral HPLC analyses. This material is available free of charge via the Internet http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (A) (22245025) from the Ministry of 2681
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The Journal of Physical Chemistry Letters Education, Culture, Sports, Science and Technology (MEXT), Japan, by the High-Tech Research Center project for private universities with a matching fund subsidy from MEXT, and by the NAIST Advanced Research Partnership Project.
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