J. Phys. Chem. C 2009, 113, 11623–11627
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Photocyclization Reaction of Diarylethene-Perylenebisimide Dyads upon Irradiation with Visible (>500 nm) Light† Tuyoshi Fukaminato,*,‡,§ Takao Doi,| Masaaki Tanaka,| and Masahiro Irie*,⊥ Research Institute for Electronic Science, Hokkaido UniVersity, N20, W10, Kita-ku, Sapporo 001-0020, Japan, PREST, Japan Science and Technology Agency (JST), Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan, and Department of Chemistry and Research Center for Smart Molecules, Rikkyo UniVersity, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: May 6, 2009
Two types of fluorescent diarylethene-perylenebisimide dyads were synthesized and their photochromic performance was studied. The dyads showed an unexpected photochromic reactivity. The photocyclization reaction takes place upon irradiation with visible light (>500 nm), where the diarylethene unit has no absorption. Inferring from the excitation wavelength dependence and oxygen effect, the triplet states of the diarylethene unit are considered to play an essential role in the photocyclization reaction with visible (>500 nm) light. Introduction Recently, organic photochromic molecules have attracted considerable attention because of their potential application in molecular switches and memories.1 Among various photochromic molecules, diarylethene (DE) derivatives have been extensively studied because of their thermal stability in both open- and closed-ring isomers, fatigue resistant character, and photochromic reactivity even in the solid state.2-5 In particular, fluorescent photochromic diarylethenes, in which a fluorescent unit is linked to the diarylethene chromophore, have aroused increasing interest due to their potential use in optical data storage media, molecular switches, fluorescent biological markers, and extremely high resolution fluorescence images.6-9 In a previous paper, we have developed a new fluorescent diarylethene derivative which is composed of a fluorescent perylenebisimide (PBI) and a photochromic S,Sdioxidized diarylethene.10 Although the derivative showed the fluorescence quenching along with the photochromic reactions in polar solvents, the electron-transfer quenching and the photocycloreversion reactivity were inefficient. To overcome the deficiencies, we have synthesized several S,S-dioxide diarylethene-perylenebisimide (DE-PBI) dyads and examined their photochromic performance and fluorescence quenching. In this work, two types of DE-PBI dyads (1 and 2) (Scheme 1) were prepared. The dyads showed an unexpected photochromic reactivity. The photocyclization reaction was found to proceed upon irradiation with visible light (>500 nm), where the DE unit has no absorption. Experimental Section All solvents used were spectroscopic grade and purified by distillation before use. Absorption and fluorescence spectra in †
Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author: E-mail:
[email protected] (T.F.);
[email protected] (M.I.), Phone & Fax: +81-11-706-9350 (T.F.); +813-3985-2397 (M.I.). ‡ Hokkaido University. § Japan Science and Technology Agency (JST). | Kyushu University. ⊥ Rikkyo University.
solution were measured with a Hitachi U-3100 and Hitachi F-2500 spectrometers, respectively. Photoirradiation was carried out by using an USHIO 500 W xenon lamp or a diode laser (532 nm, 5 mW/cm2, Crystal Laser). Monochromatic light was obtained by passing the light through a Ritsu MV-10N monochromator. The closed-ring isomers (1b, 2b, 3b, and 4b) were isolated from the solution containing the open-ring isomers (1a, 2a, 3a, and 4a) irradiated with 450 nm light by using HPLC (column, Wako Chemical Wakosil-5SIL 10 × 250; eluent, dichloromethane:ethyl acetate ) 96:4 for 1b, dichloromethane:ethyl acetate ) 98:2 for 2b, hexane:ethyl acetate ) 40:60 for 3b, hexane:ethyl acetate ) 35:65 for 4b). The photocyclization and photocycloreversion quantum yields were determined by using the well-established procedure.11 1 H NMR spectra were recorded on a NMR spectrometer (Bruker AVANCE-400, 400 MHz). Samples were dissolved in CDCl3 and tetramethylsilane was used as an internal standard. Mass spectra were measured with mass spectrometers (Shimadzu GCMS-QP5050A and JEOL JC-mate II). Results and Discussion Dyads 1 and 2 and reference compounds (3 and 4) (Scheme 1) were newly synthesized. The details of the synthesis will be published elsewhere. The structures were identified with 1H NMR, MS spectra, and elemental analysis.12 Parts a and b of Figure 1 show the absorption spectra of component units of 1 and 2 in 1,4-dioxane; 3a, 3b, 4a, 4b and N,N′-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylbisimide (5) (PBI). Fluorescence spectra of PBI unit in 1,4-dioxane solution are also shown in Figures 1a,b. Compounds 3 and 4 underwent reversible photochromic reactions in 1,4-dioxane solution. Upon irradiation with 450 nm light, the absorption spectra of 3a and 4a showed hypsochromic shifts. The spectral shifts are ascribed to the photocyclization reactions of the diarylethene chromophores.13 The photocyclization reaction was also confirmed by NMR measurement. Upon irradiation with 365 nm light, the blue-shifted spectra again returned to the original spectra. The result indicates that the absorption maxima of the closed-ring isomers are shorter than those of the open-
10.1021/jp902880d CCC: $40.75 2009 American Chemical Society Published on Web 06/10/2009
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SCHEME 1
ring isomers. The coversion ratios from the open- to the closedring isomers were estimated to be 82% for 3 and 6% for 4, respectively. As shown in Figure 1, the fluorescence spectrum of PBI unit has no overlap with the absorption spectra of both the open- and the closed-ring isomers. This indicates that the energy levels of both isomers are higher than that of PBI unit and the singlet-singlet energy transfer from PBI to the DE unit does not take place. The spectra of component PBI and DE units suggest that, in 1a and 2a, PBI is selectively excited by irradiation with >500 nm light. Figure 2a shows the absorption spectral change of 1 in 1,4dioxane solution upon irradiation with visible (λ ) 436 nm) and UV (λ ) 365 nm) light. Before photoirradiation, the chracteristic absoption bands ascribed to PBI unit were observed at 522 nm (ε ) 7.8 × 104 M-1 cm-1), 486 nm (ε ) 4.8 × 104 M-1 cm-1), and 455 nm (ε ) 1.8 × 104 M-1 cm-1) and a broad absorption band ascribed to DE unit was observed in the 350-400 nm region. Upon irradiation with 436 nm light, a new absorption band appeared around 350 nm. The band is ascribed to the closed-ring structure. Upon UV (365 nm) light irradiation, the absorption band returned to the original one. The coversion ratio from 1a to 1b was estimated to be 69%.
Figure 1. (a) Absorption spectra of 3a (black line), 3b (blue line), PBI (5) (red line) and fluorescence spectrum of 5 (green line) in 1,4-dioxane. (b) Absorption spectra of 4a (black line), 4b (blue line), 5 (red line) and fluorescence spectrum of 5 (geren line) in 1,4-dioxane.
Figure 3a shows the absorption spectral change along with photochromic reactions of dyad 2 in 1,4-dioxane solution. The spectral change of 2 upon irradiation with visible (436 nm) and UV (365 nm) light is almost similar to 1. Before photoirradiation, the chracteristic absoption bands ascribed to PBI unit were observed at 524 nm (ε ) 8.5 × 104 M-1 cm-1), 488 nm (ε ) 5.2 × 104 M-1 cm-1), and 457 nm (ε ) 1.9 × 104 M-1 cm-1) and a broad absorption band ascribed to DE unit was observed in the 350-400 nm region. Upon irradiation with 436 nm light, a new absorption band appeared around at 350 nm. The band is ascribed to the closed-ring structure. Upon UV (365 nm) light irradiation, the absorption band returned to the original one. The coversion ratio from 2a to 2b was estimated to be 72%. The photophysical properties of 1 and 2 are summarized in Table 1. Upon excitation with 532 nm laser light, an unexpected photoreaction was observed for the dyads. The laser light can selectively excite the PBI unit in both dyads. The absorption spectrum of 1a gradually changed to that of 1b upon irradiation with 532 nm laser light as shown in Figure 4a. The NMR spectrum, mass spectrum, and HPLC indicate the formation of 1b upon 532 nm laser irradiation. The absorption spectrum
Figure 2. (a) Absorption and (b) fluorescence spectra of 1 in 1,4dioxane solution; the open-ring isomer (1a) (s), the closed-ring isomer (1b) (---), and the photostationary state under irradiation with 436 nm light (- · -).
Photocyclization of Diarylethene-Perylenebisimide Dyads
Figure 3. (a) Absorption and (b) fluorescence spectra of 2 in 1,4dioxane solution; the open-ring isomer (2a) (s), the closed-ring isomer (2b) (---), and the photostationary state under irradiation with 436 nm light (- · -).
TABLE 1: Photophysical Properties of 1a and 2a in 1,4-Dioxane Solution ε (M-1cm-1) 1a 78000 (522nm) 48000 (486nm) 18000 (455nm) 2a 85000 (524nm) 52000 (488nm) 19000 (457nm) c
Φofc (410 nm) Φofc (522 nm)
Φcfo b
Φfa
0.015
0.016
0.26
0.013
0.0012
0.014c 0.91
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Figure 4. (a) Absorption spectral change of 1 in 1,4-dioxane solution under irradiation with 532 nm light; the open-ring isomer (1a) (s), the closed-ring isomer (1b) (---), and the photostationary state under irradiation with 532 nm light (- · -). (b) Absorption spectral change of 2 in 1,4-dioxane solution under irradiation with 532 nm light; the open-ring isomer (2a) (s), the closed-ring isomer (2b) (---), and the photostationary state under irradiation with 532 nm light (- · -).
0.81
a Excitation wavelength; 500 nm. b Measured with 313 nm light. Measured with 365 nm light.
almost returned to the initial one upon irradiation with 365 nm light. The spectrum of 2a also changed to 2b upon irradiation with 532 nm laser light as shown in Figure 4b. The conversion ratio from the open- to the closed-ring isomers under irradiation with 532 nm light was estimated to be almost 100% for 1 and 93% for 2, which are relative high in comparison to that of the model compounds (3 and 4). This result suggests that efficient photocyclization and less efficient photocycloreversion reaction take place upon excitation of the PBI unit. In fact, the photocycloreversion reaction under irradiation with 532 nm light was not observed for the pure closed-ring isomer (1b) isolated by HPLC. Similar behavior was also observed for dyad 2. As shown in Figure 1, both isomers of the DE unit have no absorption longer than the 500 nm region. In fact, any photocyclization reaction was not observed for 3 and 4 upon irradiation with 532 nm laser light. These results indicate that the excited PBI unit induces the photocyclization reaction of the DE unit. As described above, the fluorescence spectrum of PBI unit has no overlap with the absorption spectra of the DE units. Therefore, simple any singlet-singlet energy transfer from the excited PBI to the DE unit can not take place. To elucidate the mechanism, we measured the excitation wavelength dependence of the quantum yield of photocyclization reaction for 1 and 2 in 1,4-dioxane solution. The quantum yield was measured between 400 and 530 nm. Clear excitation wavelength dependence was observed for dyad 2. The quantum yield is constant (Φofc ) 0.0012 ( 0.0005) between 530 and 460 nm, where only the PBI unit absorbs the light, while it increases below 460 nm. The quantum yield gradually reaches
Figure 5. Excitation wavelength dependence of the photocyclization reaction of 1 (b) and 2 (9) in 1,4-dioxane.
maximum value (Φofc ) 0.013) around 410 nm. The wavelength dependence of 2 suggests that the photocyclization reaction upon irradiation with λ > 460 nm light and the reaction upon irradiation with λ < 460 nm proceed via different reaction pathways. On the other hand, the quantum yield of 1 was constant (Φofc ) 0.015 ( 0.002) in all wavelength regions (Figure 5). The constant quantum yield of the photocyclization reaction is due to high photocyclization reaction efficiency upon excitation of PBI unit. Figures 2b and 3b show the fluorescence spectra of 1 and 2 in 1,4-dioxane solution, respectively. Characteristic fluorescence bands due to the PBI unit are observed at 531 and 571 nm for 1. Fluorescence quantum yields of both 1a and 1b upon excitation with 500 nm light were measured by utilizing the fluorescence quantum yield of 5 (Φf ∼ 1.0) as the standard.14 The fluorescence quantum yield was estimated to be 0.81 for 1a and 0.99 for 1b, respectively. The fluorescence quantum yield slightly increases along with the photocyclization reaction. Significant fluorescence intensity change along with the photochromic reactions was not observed for 2. Fluorescence quantum yields of 2a and 2b upon excitation with 500 nm light were determined to be 0.91 for 2a and 0.99 for 2b, respectively. The fluorescence quantum yields of 1a and 2a are smaller than that of standard PBI derivative. The decreases in the fluorescence
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Figure 6. (a) Formation of 1b upon irradition with 532 nm laser light (5 mW/cm2): (2) in 1,4-dioxane; (b) in ethyl acetate; (9) in dimethylformamide. (b) Formation of 1b upon irradition with 532 nm laser light (5 mW/cm2) in 1,4-dioxane: (2) argon-bubbling solution and (b) oxygen-bubbling solution.
quantum yields of 1a and 2a indicate that some part of the photoexcited energy of PBI is used for the photocyclization reaction. We also measured the solvent polarity dependence for the photocyclization reaction, because the photoinduced electrontransfer (PET) process can be one of the possible mechanisms. Any solvent dependence was not observed upon irradiation with 532 nm laser light, as shown in Figure 6a. The PET process is excluded as the mechanism of the photocyclization reaction. Recently, several groups reported the photochromic reactions of DE via triplet states.15 Upon photoexcitation into the 1MLCT (metal-to-ligand charge-transfer) state of these complexes followed by intersystem crossing to 3MLCT state, photoreactive 3 IL (triplet) states are populated by an efficient energy-transfer process. DE derivatives can undergo photochromism via not only singlet excited states but also triplet states. To confirm the contribution of the triplet state for the photocyclization reaction upon visible (>500 nm) light irradiation, we measured the effect of oxygen. Figure 6b shows the formation rate of 1b upon irradiation with 532 nm light under the conditions of argon bubbling and oxygen bubbling. Appreciable oxygen quenching was observed for the photocyclization reaction. The results described above (wavelength dependence of the photocyclization quantum yields, fluorescence quantum yields decreasing of the open-ring isomer, and oxygen effect) indicate that the triplet states of the DE unit play an essential role in the photocyclization reactions upon irradiation with visible (>500 nm) light, as shown in Scheme 2. Upon irradiation with light shorter than 500 nm for 1 (or 460 nm for 2), the DE unit can be excited to its S1 state and undergo the photocyclization reaction from the excited state of DE unit. On the other hand, upon irradiation with light longer than 500 nm for 1 (or 460 nm for 2), only the PBI unit absorbs the light and the photoexcited energy of the PBI unit transfers to the T1 state of the DE unit and leads to the photocyclization reaction of the DE unit. In conclusion, two types of fluorescent diaryletheneperylenebisimide dyads were synthesized and their photochromic performances were studied. These dyads showed an unexpected photochromic reactivity. The photocyclization reactions take
place upon irradiation with visible (>500 nm) light, where the diarylethene unit is transparent. From the excitation wavelength dependence and oxygen effect studies, the triplet state of the diarylethene unit is considered to play an essential role in the photocyclization reaction with visible (>500 nm) light. Acknowledgment. We are grateful to Dr. Shinichiro Nakamura, Dr. Satoshi Yokojima, Dr. Keiko Shinoda, Prof. Hiroshi Miyasaka, and Dr. Yukihide Ishibashi for fruitful discussion. This work was partly supported by Grant-in-Aids for Scientific Research on Priority Areas “New Frontiers in Photochromism (471)” (No. 19050008) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government. T.F. acknowledges support from PREST, JST, and Grant-in-Aids for Young Scientist (B) (No. 18750119). References and Notes (1) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001. (b) Brown, G. H. Phoochromism; Wiley-Interscience: New York, 1971. (c) Durr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2003. (d) Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777. (2) (a) Irie, M. Chem. ReV. 2000, 100, 1685. (b) Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985. (c) Matsuda, K.; Irie, M. J. Photochem. Photobiol. C 2004, 5, 169. (d) Tian, H.; Yang, S. Chem. Soc. ReV. 2004, 33, 85. (e) Tian, H.; Wang, S. Chem. Commun. 2007, 781. (3) (a) Nakamura, S.; Irie, M. J. Org. Chem. 1998, 53, 6136. (b) Uchida, K.; Nakayama, M.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311. (c) Takami, S.; Kobatake, S.; Kawai, T.; Irie, M. Chem. Lett. 2003, 32, 892. (4) (a) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc., Chem. Commun. 1992, 206. (b) Irie, M.; Lifka, T.; Uchida, K.; Kobatake, S.; Shindo, Y. Chem. Commun. 1999, 744. (c) Jeong, Y.-C.; Park, D. G.; Kim, E.; Ahn, K.-H.; Yang, S. I. Chem. Commun. 2006, 1881. (5) (a) Irie, M.; Uchida, K.; Eriguchi, T.; Tsuzuki, H. Chem. Lett. 1995, 899. (b) Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am. Chem. Soc. 1999, 121, 2380. (c) Irie, M.; Kobatake, S.; Horichi, M. Science 2001, 291, 1769. (d) Kobatake, S.; Irie, M. Bull. Chem. Soc. Jpn. 2004, 77, 945. (e) Morimoto, M.; Irie, M. Chem. Commun. 2005, 3895. (f) Kobatake, S.; Takami, S.; Muto, H.; Irie, M. Nature 2007, 446, 778. (g) Colombier, I.; Spagnoli, S.; Corval, A.; Baldeck, P. L.; Giraud, M.; Leaustic, A.; Yu, P.; Irie, M. J. Chem. Phys. 2007, 126, 011101. (h) Irie, M. Bull. Chem. Soc. Jpn. 2008, 81, 917. (6) (a) Fukaminato, T.; Kawai, T.; Kobatake, S.; Irie, M. Proc. Jpn. Acad. 2001, 77, 30. (b) Corredor, C. C.; Huang, Z.-L.; Belfield, K. D. AdV. Mater. 2006, 18, 2910. (c) Lim, S.-J.; Seo, J.; Park, S. Y. J. Am. Chem. Soc. 2006, 128, 14542. (d) Corredor, C. C.; Huang, Z.-L.; Belfield, K. D.; Morales, A. R.; Bondar, M. V. Chem. Mater. 2007, 19, 5165. (7) (a) Endtner, J. M.; Effenberger, F.; Hartschuh, A.; Port, H. J. Am. Chem. Soc. 2000, 122, 3037. (b) Kawai, T.; Sasaki, T.; Irie, M. Chem. Commun. 2001, 711. (c) Giordano, L.; Jovin, T. M.; Irie, M.; Jares-Erijman, E. A. J. Am. Chem. Soc. 2002, 124, 7481. (d) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature 2002, 420, 759. (e) Lim, S.-J.; An, B.-K.; Jung, S. D.; Chung, M.-A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346. (f) Raymo, F. M.; Tomasulo, M. Chem. Soc. ReV. 2005, 34, 327. (8) (a) Jares-Erijman, E. A.; Jovin, T. M. Nat. Biotechnol. 2003, 21, 1387. (b) Soh, N.; Yoshida, K.; Nakajima, H.; Nakano, K.; Imato, T.; Fukaminato, T.; Irie, M. Chem. Commun 2007, 5206. (c) Zou, Y.; Yi, T.; Xiao, S.; Li, F.; Li, C.; Gao, X.; Wu, J.; Mengxiao Yu, M.; Huang, C. J. Am. Chem. Soc. 2008, 130, 15750.
Photocyclization of Diarylethene-Perylenebisimide Dyads (9) (a) Dedecker, P.; Hotta, J.; Flors, C.; Sliwa, M.; Uji-i, H.; Roeffaers, M. B. J.; Ando, R.; Mizuno, H.; Miyawaki, A.; Johan Hofkens, J. J. Am. Chem. Soc. 2007, 129, 6132. (b) Dedecker, P.; Flors, C.; Hotta, J.; Uji-I, H.; Hofkens, J. Angew Chem. Int. Ed. 2007, 46, 8330. (c) Hell, S. W. Science 2007, 316, 1153. (d) Fo¨lling, J.; Polyakova, S.; Belov, V.; van Blaaderen, A.; Bossi, M. L.; Hell, S. W. Small 2008, 4, 134. (10) Odo, Y.; Fukaminato, T.; Irie, M. Chem. Lett. 2007, 36, 240. (11) (a) Yokoyama, Y.; Kurita, Y. J. Synth. Org. Chem. Jpn. 1991, 49, 364. (b) Heller, H. G.; Langan, J. R. J. Chem. Soc., Perkin Trans. 2 1981, 341. (12) 1a: 1H-NMR (400 MHz CDCl3) δ 1.95 (s, 3H), 2.02 (s, 6H), 2.40 (s, 3H), 5.91 (s, 2H), 6.98 (s, 1H), 7.16-7.19 (m, 1H), 7.29-7.36 (m, 3H), 7.42-7.45 (m, 2H), 7.57-7.60 (m, 2H); MS (FAB+) m/z ) 1129 [M]+. Anal. Calcd for C62H50F6N2O8S2: C, 65.95; H, 4.46; N, 2.48. Found: C, 65.78; H, 4.61; N, 2.45. 2a: 1H-NMR (400 MHz CDCl3) δ 0.83 (t, J ) 6.8 Hz, 6H), 1.22-1.37 (m, 16H), 1.84-1.92 (m, 2H), 2.13 (s, 3H), 2.202.28 (m, 2H), 2.31 (s, 3H), 5.13-5.21 (m, 1H), 6.76 (s, 1H), 7.39-7.45 (m, 4H), 7.63 (dd, J ) 8.4 Hz, 2.0 Hz, 1H), 7.68-7.71 (m, 2H), 7.83 (d, J ) 2.0 Hz, 1H), 8.54-8.67 (m, 8H); MS (FAB+) m/z ) 1130 [M + 1]+. Anal. Calcd for C62H50F6N2O8S2: C, 65.95; H, 4.46; N, 2.48. Found: C, 66.04; H, 4.49; N, 2.62. 3a: 1H-NMR (400 MHz CDCl3) δ 1.92 (s, 3H), 2.38 (s, 3H), 6.99 (s, 1H), 7.30-7.38 (m, 2H), 7.43-7.47 (m, 2H), 7.51-
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11627 7.59 (m, 3H), 7.73-7.82 (m, 3H), 7.93-7.98 (m, 2H); MS (FAB+) m/z ) 704 [M + 1]+. Anal. Calcd. for C33H19F6N2O6S2: C, 56.33; H, 2.72; N, 1.99. Found: C, 56.42; H, 2.82; N, 1.98. 4a: 1H-NMR (400 MHz CDCl3) δ 2.11 (s, 3H), 2.27 (s, 3H), 6.74 (s, 1H), 7.35-7.46 (m, 4H), 7.64-7.69 (m, 2H), 7.77-7.85 (m, 3H), 7.93-7.99 (m, 2H), 8.01 (d, J ) 1.6 Hz, 1H); FAB-MS m/z ) 704 [M + 1]+. Anal. Calcd for C33H19F6N2O6S2: C, 56.33; H, 2.72; N, 1.99. Found: C, 56.18; H, 2.76; N, 2.07. (13) Fukaminato, T.; Tanaka, M.; Kuroki, L.; Irie, M. Chem. Commun. 2008, 3924. (14) Langhals, H. HelV. Chim. Acta 2005, 88, 1309. (15) (a) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Inorg. Chem. 2004, 43, 2779. (b) Yam, V. W. W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2004, 126, 12734. (c) Ko, C.-C.; Kwok, W. M.; Yam, V. W. W.; Phillips, D. L. Chem.sEur. J. 2006, 12, 5840. (d) Lee, P. H. M.; Ko, C.C.; Zhu, N.; Yam, V. W. W. J. Am. Chem. Soc. 2007, 129, 6058. (e) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Coord. Chem. ReV. 2005, 249, 1327. (f) Indelli, M. T.; Carli, S.; Ghirotti, M.; Chiorboli, C.; Ravaglia, M.; Garavelli, M.; Scandola, F. J. Am. Chem. Soc. 2008, 130, 7286. (g) Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O. Inorg. Chem. 2009, 48, 19.
JP902880D
