Photochromic Organometallics, A Stimuli-Responsive System: An

Jan 4, 2011 - the photochromic system are essential components of smart chemical ... recognizes changes of the environment so as to trigger a chemical...
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Organometallics 2011, 30, 43–51 DOI: 10.1021/om100959h

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Photochromic Organometallics, A Stimuli-Responsive System: An Approach to Smart Chemical Systems† Munetaka Akita* Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received October 4, 2010

Recent progress of photochromic metal complexes is summarized with emphasis on “photochromic organometallics” with a M-C(photochromic unit) bond. Stimuli-responsive systems including the photochromic system are essential components of smart chemical systems, where the system recognizes changes of the environment so as to trigger a chemical function required for the smart chemical system. Combination with metal components, which exhibit unique properties such as redox and photophysical properties and catalysis, should lead to more sophisticated systems. Photophysical properties (e.g., luminescence) have been the major research subject of photochromic coordination compounds, whereas for “photochromic organometallics”, a limited number of studies on switching of catalytic activity of metal-catalyzed organic transformation and application to molecular devices have been reported. Intriguing switching systems (e.g., performance of organometallic molecular wire and multimodal stimuli-responsive system) have been realized by the use of dithienylethene, a representative photochromic molecule. Future prospects of “photochromic organometallics” are discussed in the final part.

Introduction Stimuli-responsive systems are essential components of smart chemical systems.1 Stimuli-responsive systems can recognize changes of the environment so as to trigger a chemical function required for the smart chemical system. Many kinds of organic and inorganic stimuli-responsive systems have been developed so far, and on their basis, switching systems have been also developed. Among them much attention has been focused on chromic systems.2 Chromism is defined as a transformation of a chemical species between two forms by application of a stimulus, where the two forms have different absorption spectra, and the process is often reversible (Scheme 1). If these changes in absorption properties occur in the visible region of the spectrum, then the stimulated transformation can result in changes in color of the material. Color plays essential roles in our daily life. Because color change can be readily recognized by the naked eye, detection of color change is one of the easiest detection methods for human beings. Chromic systems, therefore, have been widely utilized for sensing, as typically exemplified by phenolphthalein used as an indicator for acid-base titration (halochromism). The color change results from a change of the electronic structure of the chromic molecule (usually π-conjugated

Scheme 1

† Part of the special issue Future of Organometallic Chemistry. *E-mail: [email protected]. (1) Encyclopedia of Smart Materials; Schwartz, M., Ed.; Wiley: New York, 2002. (2) Chromic Phenomena, Technological Applications of Color Chemistry, 2nd ed.; Bamfield, P.; Hutchings, M. G., Eds.; Royal Society of Chemistry: Cambridge, 2010.

system), which is often associated with a change of the geometrical structure of the molecule. Combination of such chromic systems with other chemical systems should lead to the development of more sophisticated stimuli-responsive smart chemical systems. One way is combination of chromic π-conjugated organic fragments with metal species, which exhibit unique features such as redox properties, photophysical properties, and catalysis. If (1) an appropriate metal species can be attached to a chromic species, which is responsive to the desired stimulus, (2) the metal center can feel the geometrical and electronic structural changes of the chromic part induced by the environment change, and (3) the required function can be triggered at the metal center, we

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may be able to construct a stimuli-responsive chemical system based on an organometallic system, where application of the stimulus to the chromic part results in a unique function at the metal center (Scheme 1). For example, for a chromic system combined with a metal catalyst, we would be able to switch on and off or tune the catalytic activity of the metal center by application of the stimulus.3 Thus chromic systems, as they are or by combination with other systems, not only can serve as indicators of a change in state but also can trigger a chemical reaction. There are many kinds of chromism, such as photo-, thermo-, iono-, halo-, electro-, solvato-, vapo-, and mechanochromism.2 Herein discussion will be focused on photochromic systems combined with metal complexes, in particular, those in which the photochromic unit is connected to the metal center through an M-C bond, i.e., photochromic organometallics. (Throughout this article, “photochromic organometallics” refers to this type of compounds with a M-C(photochromic unit) bond. According to this definition, for example, complex 1 (Chart 1), with the N-coordinated photochromic moiety, is not photochromic organometallics, although it contains W-CO bonds and is usually regarded as an organometallic compound.) Compared to other chromic systems, photochromic systems have the following advantages: (1) photochemical processes are high-energy processes compared to other processes such as thermal processes so as to induce skeletal rearrangements under mild conditions (usually at room temperature), (2) the stimulus can be applied from a remote place so as to accomplish remote control, and (3) the chromism can be controlled by simply switching the light source (λ1/λ2) (Scheme 1). Photochromic compounds have been studied extensively as a promising optically rewritable data storage device. Information can be easily recorded by exposure to light (λ1; Scheme 1). While the recorded data can be read out by irradiation of light λ2, the irradiation also causes the reverse photochemical reaction to erase the recorded data. In the search for nondestructive readout methods, various optical properties including luminescence have been studied intensively as described below. In this article, after briefly reviewing recent progress of photochromic metal complexes with emphasis on photochromic organometallics containing dithienylethene functional groups,4 the author will discuss future prospects of photochromic organometallics. Organic Photochromic Compounds. Metal complexes themselves can show photochromic properties usually via linkage isomerization of ligands.4a For example, Nakai and Isobe recently reported the photochromic dirhodium μ-dithionite complex ( μ-O2SSO2)(RhCp*)2( μ-CH2)2.5 UV irradiation of the μ-O2SSO2 isomer causes linkage isomerization to the μO2SOSO isomer, and the reverse process is promoted thermally. While a substantial number of this type of photochromic metal complexes have been reported so far,4 rational molecular design of such molecules is still difficult. Promising molecular design of photochromic organometallics involves (3) Lorkovic, I. M.; Duff, R. R., Jr.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 3617. (4) (a) Wang, M.-S.; Xu, G.; Zhang, Z.-J.; Guo, G.-C. Chem. Commun. 2010, 46, 361. (b) Tian, H.; Yang, S. Chem. Soc. Rev. 2004, 33, 85. (c) Duerchais, V.; Ordoronneau, L.; Le Bozec, H. Coord. Chem. Rev. 2010, 254, 2533. (d) Ko, C.-C.; Yam, V. W.-W. J. Mater. Chem. 2010, 20, 2063. (e) Hasegwa, Y.; Nakagawa, T.; Kawai, T. Coord. Chem. Rev. 2010, 254, 2643. (5) Nakai, H.; Nonaka, T.; Miyano, Y.; Mizuno, M.; Ozawa, Y.; Toriumi, K.; Koga, N.; Nishioka, T.; Irie, M.; Isobe, K. J. Am. Chem. Soc. 2008, 130, 17836.

Akita

combination of an organic photochromic system with a metal fragment. Representative examples of organic photochromic molecules, which have been combined with metallic systems, are listed in Table 1.2 These molecules show the following features. (1) The forward photochemical processes involve various chemical processes such as (i) heterolytic C-O bond cleavage (spiropyran), (ii) cis-trans geometrical isomerization (azobenzene), and (iii) electrocyclic conrotatory reaction of 6π hexatriene systems, leading to cyclohexadiene skeletons (diarylethene). (2) Except azobenzene, the structural change noted in (1) causes expansion of the π-conjugated system of the colorless form localized on the separate aromatic rings over the whole structure of the open colored isomer, which shows absorptions in the visible light region. (3) The forward and backward electrocyclic reactions of diarylethene derivatives (6π systems) are promoted only by photochemical excitation, and therefore, the colored species can be kept for a substantial period of time when stored in the dark. On the other hand, spiropyran and azobenzene often undergo backward decoloration reactions under thermal conditions, and therefore, the lifetime of their colored species may not be long enough to induce a chemical reaction.2 (4) Both isomers of azobenzene and diarylethene and the closed isomer of spiropyran are neutral and nonpolar, whereas the merocyanine form, the open colored form of spiropyran, has significant contribution of the zwitterionic structure. Interaction with Metal Species. Combination of photochromic molecules with coordination compounds has been studied rather extensively, whereas the study of photochromic organometallics has attracted little attention until recently. For construction of efficient stimuli-responsive photochromic metal systems it is essential to overcome three problems. First of all, an appropriate combination of an organic photochromic component and a metal fragment should be selected (molecular design), and they should be combined into a single molecule (synthesis). As to the former point, the photochromic part should be able to induce the required changes of the coordination properties for the metal center upon light irradiation, and the metal part should be responsive to the change and initiate the required function. Second, the resultant adduct should retain the photochromic properties. Incorporation of a metal center may cause retardation or acceleration of the chromic process. Finally, the resultant adduct should show the desired stimulus-responsive function at the metal center upon application of the stimulus, light irradiation. The first two issues, the adduct formation and expression of the photochromic behavior, could be overcome by rational thought, imaginative exploitation of knowledge, and development of new synthetic methods in the cases where methods for the implementation of novel molecular motifs are not yet known. The last problem, which is closely connected to the molecular design, could be the key issue of photochromic organometallics. A common complexation method is attachment of a coordinating functional group to a photochromic compound followed by metalation. Stable and tractable ferrocene is frequently used as a redox-active organometallic attachment.

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Table 1. Representative Examples of Photochromic Molecules Used for Metal Complexation

a

Diarylethene parts are indicated by the dotted rectangles.

Photophysical properties of photochromic metal complexes thus prepared have been the major research topic relevant to rewritable memory devices mentioned above. If luminescence is induced by minimal irradiation of a certain absorption band, which does not induce the reverse decoloration process, the stored information can be read out without destroying the memory. Because of this, luminescence properties have been studied extensively. Another reason is that this field has been developed by inorganic photochemists, who are interested in photochemical processes. Photochromic Dithienylethene (DTE)-Metal Complexes. As typical examples of photochromic metal complexes, recently reported inorganic and organometallic dithienylethene (DTE) metal systems are summarized in this section. For the metal complexes with other photochromic functional groups such as spiropyran,6 azobenzene,7 and metacyclophanediene,8 see the references indicated.4 (6) See, for example: Kimura, K; Sakamoto, H.; Nakamura, M. Bull. Chem. Soc. Jpn. 2003, 76, 225. Bao, Z.; Ng, K.-Y.; Yam, V. W.-W.; Ko, C.-C.; Zhu, N.; Wu, L.-X. Inorg. Chem. 2008, 47, 8912. Jukes, R. T. F.; Bozic, B.; Belser, P.; De Cola; Hartl, F. Inorg. Chem. 2009, 48, 1711. Khairutdinov, R. F.; Giertz, K.; Hurst, J. K.; Voloshina, E. N.; Voloshin, N. A.; Minkin, V. I. J. Am. Chem. Soc. 1998, 120, 12707. Kopelman, R. A.; Snyder, S. M.; Frank, N. L. J. Am. Chem. Soc. 2003, 125, 13684. Miyashita, A.; Iwamoto, A.; Kuwayama, T.; Shitara, H.; Aoki, Y.; Hirano, M.; Nohira, H. Chem. Lett. 1997, 965. (7) Kume, S.; Nishihara, H. Dalton Trans. 2008, 3260. Kume, S.; Kanaizuka, K.; Nishihara, H. Shokubai 2008, 50, 345. Kume, S.; Nishihara, H. Struct. Bonding (Berlin) 2007, 123, 79. Nishihara, H. Bull. Chem. Soc. Jpn. 2004, 77, 407. Kurihara, M.; Nishihara, H. Coord. Chem. Rev. 2002, 226, 125. See also ref 4d. (8) Mitchell, R. H.; Brkic, Z.; Sauro, V. A.; Berg, D. J. J. Am. Chem. Soc. 2003, 125, 7581. Zhang, R.; Fan, W.; Twamley, B.; Berg, D. J.; Mitchell, R. H. Organometallics 2007, 26, 1888. Muratsugu, S.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2008, 130, 7204.

Photoisomerization of stilbene derivatives leading to dihydrophenenthrene, which has been known for a long time, has been polished up by Irie by introduction of heteroaromatic rings as well as a cyclic olefin bridge (Table 1).9 In particular, 1,2-dithienylethene (DTE) derivatives with the perfluoro- or perhydrocyclopentene bridge have been studied extensively. Compared to spiropyran and azobenzene systems, the colored form of DTE is stable when kept in the dark; that is, both the forward and backward interconversion of the electrocyclic hexatriene-cyclohexadiene interconversion system can be controlled by irradiation of light with different wavelengths. The open form with the cross π-conjugation at the thiophene-cyclopentene junctions shows absorptions only in the UV region and, upon UV irradiation, undergoes electrocyclic conrotatory ring closure to form the closed isomer with the fused tricyclic cyclohexadiene skeleton, which shows visible light (VL) absorptions owing to the π-conjugated system expanded over the molecule (Table 1). The DTE system is thus associated with advantages such as (1) the absence of thermal backward reaction, (2) quick response, (3) fatigue resistance (recyclability), and (4) facile color control by introducing appropriate substituents. While coordination compounds have been studied rather extensively, a limited number of organometallic derivatives have been reported so far. One common photophysical feature of photochromic metal systems is intramolecular sensitization by the metal (9) Irie, M. Chem. Rev. 2000, 100, 1685. Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985. See also: Brown, G. H. Photochromism; Wiley-Interscience: New York, 1971. Bouas-Laurent, H.; D€urr, H. Pure Appl. Chem. 2001, 73, 639. D€urr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2003; ref 4.

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Akita Chart 1

Scheme 2

fragment. In addition to the π-π* and n-π* transitions available for the photochromic organic part, metal-centered transitions (e.g., MLCT and LMCT) and energy transfer between the organic and metal orbitals can be utilized for the excitation; that is, the photochromism of the complex can be induced not only by UV excitation of the organic part but also by excitation of metal-centered transitions in the VL region. An example will be discussed below (Scheme 2).

i. Coordination Compounds. Representative examples of DTE complexes are shown in Chart 1. The pioneering work by Lehn’s group has been followed by many researchers, and attention has been mainly focused on luminescence switching. For example, Lehn reported the tungsten (1) and rhenium derivatives (2) coordinated through the pyridine linkers, where the luminescence properties can be switched by the photochromic processes and the switching behavior is contrasting (1 (W): ON (closed)/OFF (open); 2 (Re): ON (open)/OFF (closed)).10 Among photophysical studies of DTE-metal complexes, the M(bipy)3-connected system 3 reported by De Cola and co-workers is worthy of note with respect to intramolecular sensitization by the metal fragment (Scheme 2).11 It has been revealed, for organic DTE derivatives, that the photochemical ring closure proceeds via the singlet excited state (1IL; IL: intraligand) formed by a π-π* transition brought about by UV irradiation. By contrast, cyclization of the tris(bipyridyl)ruthenium derivative 3a is promoted both by UV irradiation leading to the 1IL state and by VL irradiation leading to the excited MLCT state. Detailed analysis of the photochemical ring-closing processes reveals the following (10) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 1439. Gilat, S. L.; Kawai, S. H. Chem.;Eur. J. 1995, 1, 275. Fernandez-Acebes, A.; Lehn, J.-M. Adv. Mater. 1998, 10, 1519. (11) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Inorg. Chem. 2004, 43, 2779. Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Coord. Chem. Rev. 2005, 249, 1327.

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Organometallics, Vol. 30, No. 1, 2011 Chart 2

sequential photophysical events (Scheme 2a): (1) MLCT excitation leading to the singlet 1MLCT state, (2) intersystem crossing (ISC) giving the triplet 3MLCT state, (3) efficient energy transfer to the ligand-centered triplet excited state (3IL), and (4) initiation of the ring closure. The π-π* excitation of the organic chromic moiety by UV light (10 ) followed by energy transfer to 1MLCT (20 ) also leads to ring closure. Thus VL, of lower energy compared to UV, can be utilized for the ring closure. The less efficient ring closure of the Os derivative 3b is explained in terms of the 3MLCT state being lower in energy than the 3IL state; that is, the 3MLCT-to-3IL energy transfer is prevented (Scheme 2b). Both of the open (O) forms of the Ru (3a) and Os complexes (3b) are emissive (3MLCT f GS), while, upon ring closure, the emission is quenched. The quenching can be also interpreted in terms of the energy levels of the excited states (Scheme 2c). Because the IL levels of the closed isomers (C) are lower in energy than the MLCT levels, efficient energy transfer from MLCT to IL quenches the emission from the 3 MLCT state. Similar luminescence switching behavior has been widely observed for complexes with DTE and modifiedDTE ligands.4e Metal complexes with the DTE units at the peripheral positions were also reported recently.12 Yam designed the unique modified DTE ligands 4-6 shown in Chart 2.13 In these examples, the cyclopentene part in Irie’s original ligand is replaced by coordinating functional groups, which would be influenced more efficiently by the photochromic change of the diarylethene moiety. The cyclization of Re(CO)3Cl-coordinated species of the phenanthroline ligand 413a-c and pyridylimidazole ligand 513d is photosensitized by the MLCT excitation in a manner analogous to 3. It is notable that the visible absorption maxima of the closed isomer of the 5-Re(CO)3Cl adduct appear in the NIR region (∼710 nm), in a lower energy region compared to the free ligand 5 (∼580 nm), and the shift is ascribed to the planarization of the pyridylimidazole moiety in the κ2-coordinated form in contrast to the twisted conformation of the free ligand 5. The NHC-type ligand 6 is quite unique, because, recently, the NHC ligand is widely employed as (12) Zhong, Y.-W.; Vila, N.; Henderson, J. C.; Flores-Torres, S.; Abru~ na, H. D. Inorg. Chem. 2007, 46, 10470. Zhong, Y.-W.; Vila, N.; Henderson, J. C.; Abru~na, H. D. Inorg. Chem. 2009, 48, 991. Zhong, Y.-W.; Vila, N.; Henderson, J. C.; Abru~na, H. D. Inorg. Chem. 2009, 48, 7080. (13) (a) Yam, V. W.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2004, 126, 12734. (b) Ko, C.-C.; Kwok, W. M.; Yam, V. W.-W.; Phillips, D. L. Chem.;Eur. J. 2006, 12, 5840. (c) Lee, P. H.-M.; Ko, C.-C.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W. Organometallics 2007, 26, 15. (d) Lee, P. H.-M.; Ko, C.-C.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 2007, 129, 6058. (e) Yam, V. W.-W.; Lee, J. K.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2009, 131, 912. (14) (a) Matsuda, K.; Takayama, K.; Irie, M. Inorg. Chem. 2004, 43, 482. (b) Matsuda, K.; Shinkai, Y.; Irie, M. Inorg. Chem. 2004, 43, 3774. (c) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T. Bull. Chem. Soc. Jpn. 1997, 70, 1727. (d) Han, J.; Konaka, H.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Isihara, H.; Munakata, M. Inorg. Chim. Acta 2006, 359, 99. (e) Giraud, M.; Leaustic, A.; Guillot, R.; Dorlet, P.; Metivier, R.; Nakatani, K. New J. Chem. 2009, 33, 1380.

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Scheme 3

supporting ligand for catalysts for various metal-catalyzed organic transformation. The precursor for 6 (imidazolium salt) behaves as a photochromic ionic liquid.13e Photochromism of coordination polymers was also reported by Irie14a,b and Munakata.14c,d For some complexes, photochromism is observed even in the single-crystalline phase. ii. Organometallic Systems. The major research topics of photochromic organometallics, i.e., catalysis and application to molecular devices, contrast with those of the above-discussed coordination compounds, i.e., photophysical properties. ii-1. Catalytic Application. Switching or tuning of performance of organometallic catalysis (e.g., activity and selectivity) is a challenge. Photochemical isomerization of a ligand incorporating the DTE unit may change the electronic and geometrical properties of organometallic intermediates to switch catalytic function at the coordinated metal center. Interesting approaches were reported by Branda, who examined the DTE molecules with phosphine (7) and oxazoline pendants (8) (Chart 1 and Scheme 3).15 The diphosphine ligand 7 can be coordinated to the gold(I) fragments (9), and the electron-donating properties at the P atoms are estimated on the basis of the 1J(77Se-31P) values of its Se adduct 10.15a The 31P NMR data (10(O) δP 22.4, 1J(77Se-31P)=744 Hz; 10(C) δP 27.0, 1J(77Se-31P) = 756 Hz) indicate that the change (C f O) is comparable in magnitude to that obtained by replacement of a phenyl group in PPh3 by an alkyl group. The change results from that of the π-conjugation in the DTE backbone; that is, the more delocalized closed form is more electron-withdrawing than the open form. We also examined changes of the electronic states at the metal centers in the M(η5-C5R5)L2-type compounds (Chart 1).16 Changes of selected spectroscopic parameters for the attached functional groups are compared in Table 2. (15) (a) Sud, D.; McDonald, R.; Branda, N. R. Inorg. Chem. 2005, 44, 5960. (b) Sud, D.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int. Ed. 2005, 44, 2019. (c) Samachetty, H. D.; Branda, N. R. Pure Appl. Chem. 2006, 78, 2351. (16) (a) Tanaka, Y.; Inagaki, A.; Akita, M. Chem. Commun. 2007, 1169. (b) Motoyama, K.; Koike, T.; Akita, M. Chem. Commun. 2008, 5812. (c) Tanaka, Y.; Ishisaka, T.; Inagaki, A.; Koike, T.; Lapinte, C.; Akita, M. Chem.;Eur. J. 2010, 16, 4762.

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Table 2. Comparison of Spectroscopic Parameters for the Open and Closed Isomers of DTE Complexes complex 1015b 13a16a,c 14a16b 14b16b 15a16b 2319 b

spectroscopic parameters 1

J(77Se-31P) ν(CtC) δ(31P) ν(CO) ν(CO) ν(CO) δ(31P) ν(CO)

Table 3. Switching of Communication Performance of Organometallic Molecular Wires As Estimeated by KC Values

differencea

complex

linker/na

KC(C)

SFb

12 Hz -53 cm-1 -1.2 ppm 5 cm-1b 5 cm-1b 7 cm-1 3.8 ppm 3 cm-1

1118a 13a16a,c 14a16b 15b16b 1616b 1719

CtC-ppc/20 CtC/12 none/8 none/8 none/8 CtC/12

12 510 4.5  103 2.2  104 7.5  104 172

∼1 39 1100 5400 880 4.6

a The value for the closed isomer - the value for the open isomer. Differences for the averaged values for the two vibrations.

The shifts of the ν(CO) vibrations to lower energies upon ring closure are in accord with the trend observed for 10 (see above), although, in general, the magnitude of the changes is small. It is of interest to note, for 13a, that the significant change of the DTE side being evident from the ν(CtC) changes do not always reach the other auxiliary ligands through the metal centers effectively, as judged by the δ(31P) change. Although catalytic application of the phosphine ligand 7 has not been reported so far, it is found that the chiral oxazoline ligand 8 causes photochromic switching of the catalytic performance for olefin cyclopropanation involving the metal-carbene intermediate (Scheme 3). When cyclopropanation of styrene with ethyl diazoacetate affording 1-methoxycabonyl-2-phenylcyclopropane is carried out in the presence of the two isomeric catalysts (Cuþ/8(O) and Cuþ/8(C)), the former catalyst composed of the 8(O) ligand shows moderate enantioselectivity, in contrast to virtually no asymmetric induction observed for the 8(C)/Cuþ catalyst.15b The difference could be ascribed to the different geometries of the two isomeric forms. The open form 8(O) can be coordinated to a Cu ion in a chelating κ2-fashion, whereas the two oxazoline groups in the closed form 8(C) directing to the opposite sides cannot be coordinated to a Cu ion. This is the first example of photochemical switching of catalytic performance of a DTE-metal complex, although the diastereoand enantioselectivity is not satisfactory at the present stage. ii-2. Application to Molecular Switching Devices. The orbital interaction between the DTE unit and a metal fragment will be maximized when the gap between the energy levels of the connected atoms is minimal. The metal-DTE interaction in organometallic derivatives with direct M-C bonds should be stronger than that in coordination compounds with M-N/O interactions. This orbital effect would be best exemplified by organometallic molecular wire,17 for which significant switching behavior is expected by using photochromic DTE bridges. Two metal centers connected at the 5-positions of the thiophene rings may interact with each other to a considerable extent through the fully conjugated π-system in the closed isomer, whereas the cross-conjugation (17) Aguirre-Etcheverry, P.; O’Hare, D. Chem. Rev. 2010, 110, 4839. Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178-180, 427. Bruce, M. I.; Low, P. J. Adv. Organomet. Chem. 2004, 50, 231. Szafert, S.; Gladysz, J. A. Chem. Rev. 2006, 106, PR1. Ren, T. Chem. Rev. 2008, 108, 4185. Akita, M.; Koike, T. Dalton Trans. 2008, 3523. Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. Rev. 1999, 99, 1863. Schwab, P. F. H.; Smith, J. R.; Michl, J. Chem. Rev. 2005, 105, 1197. (18) (a) Freysse, S.; Coudret, C.; Launay, J.-P. Eur. J. Inorg. Chem. 2000, 1581. (b) Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J.-P. J. Phys. Chem. B 2005, 109, 17445. (c) Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J.-P. J. Phys. Chem. C 2005, 111, 2770. (d) Carella, A.; Coudret, C.; Guirado, G.; Rapenne, G.; Vives, G.; Launay, J.-P. Dalton Trans. 2007, 177.

a

Number of the carbon atoms separating the two metal centers. b SF (switching factor) = KC(C)/KC(O). KC(O) = ∼4 (14a, 15a)-85 (16). c pp = pyridylphenyl.

at the thiophene-cyclopentene junctions in the open isomer should disrupt the π-conjugation. Launay first recognized the idea and prepared the dinuclear redox-active Ru(bipy)2(pyridylphenyl) derivative linked by the acetylene unit (11) (Chart 1).18a The complex 11 undergoes reversible photochromic changes (C/O=75:25 at PSS (photostationary state)). The performance of the two isomers as molecular wires is evaluated on the basis of KC (comproportionation constant) and Vab values (electronic coupling) obtained by analyses of electrochemical and nearIR data, respectively. In general, for both parameters, the larger, the better, although careful interpretation is needed for the evaluation. Switching of the communication between the two ruthenium centers is verified by the change of the Vab values obtained by analysis of the intervalence charge transfer (IVCT) bands appearing in the near-IR region (O: 0 eV vs C: 0.025 eV; Table 3), while the difference of the KC values is too small to be detected by electrochemical analysis (KC = 11 (O), 12 (C)) presumably because of the long distance between the two metal centers (separated by a 20 carbon atom chain). Shortening the carbon bridge and the use of more electrondonating metal end-caps lead to a stronger interaction, as revealed by our group.16 The diiron complex 13a, where the DTE unit and the iron fragments are connected by the CtC linkers and thus the two iron centers are separated by 12 carbon atoms (Chart 1), shows switching behavior, as indicated by the switching factor (SF = KC(C)/KC(O)) being as large as 39 (KC(C)=510, KC(O) = 13; Table 3).16a,c As to the photochromic performance of 13a, however, (1) the photochromic conversion is very slow (>1 h) and (2) the content of the closed isomer at PSS is not always so large (37-91% depending on solvents). Photochromic conversion of the Ru derivative 13b is much faster than the Fe derivative 13a, while SF of 13b (4.2) is worse than that of 13a. Furthermore 13b turns out to show electrochromic behavior, as discussed below. The SF value has been significantly improved by further shortening the bridge (Chart 1 and Table 3).16b Removal of the CtC linkers provides the eight-carbon-bridged complexes 14-16, in which the metal centers are directly connected to the DTE core through M-C σ-bonds. They are the first examples of this type of compounds and show remarkably large KC(C) values, larger than 104, while KC(O) remains small. As a result, the KC(C) value reaches 7.5  104 (16) and the SF values exceed 103; in particular, the SF of 15b reaches 5400. This work reveals that the photochromism of the DTE molecule is an excellent ON/OFF switching device of π-conjugated molecular wires. During the electrochemical assessment to determine the KC value it was also revealed for some cases (13b and 14-16)16b,c that the ring-closing process is also promoted

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Organometallics, Vol. 30, No. 1, 2011 Scheme 4

by oxidation to give the closed dicationic species C2þ (Scheme 4). 2e-Oxidation of the DTE complexes O forms a diradical species O2þ, in which the radical centers are delocalized over the thienylethynyl-metal linkages with substantial spin populations on the thiophene carbon atoms adjacent to the methyl groups (O0 2þ). Coupling of these radical centers forms the closed carbene or cumulenylidene species C2þ, which is converted to the neutral closed species C upon 2e-reduction. Electron-donating metal end groups should stabilize the radical species O2þ, and the mechanism may be akin to oxidative coupling of thiophene derivatives.20 Thus these systems are regarded as dual chromic systems responding not only to photochemical but also to electrochemical stimuli. Recently, similar dual chromic behavior was noted for the organometallic derivatives 1218c and 1719 by Launay and Rigaut, respectively, and the coordination compound with the Ru(phen)2-type pendants as well.12 While the oxidation-induced ring closing is also reported for organic derivatives,21 their oxidized intermediates are unstable, because electron donation to the electron-deficient radical species is insufficient. Combination with redox processes, therefore, leads to (1) locking the neutral closed form, otherwise reverting to the open form under daylight, and (2) a logical switch (if the stable open and closed isomers of the neutral and oxidized species can be discriminated by some physicochemical methods such as color change). Humphrey and co-workers succeeded in the synthesis of a more complicated six-state switching system by further combination of the dual light- and redox-responsive system with protonation (Scheme 5).22 2e-Oxidation of the ruthenium complex 18 produces the dication 182þ, while diprotonation at the acetylide β-carbon atoms of 18 gives the (19) Liu, Y. F.; Lagrost, C.; Costuas, K.; Touchar, N.; Le Bozec, H.; Rigaut, S. Chem. Commun. 2008, 6117. (20) Roncali, J. Chem. Rev. 1992, 92, 711. Roncali, J. Chem. Rev. 1997, 97, 173. Stott, T. L.; Wolf, M. O. Coord. Chem. Rev. 2003, 246, 89. (21) Koshido, T.; Kawai, T.; Yoshino, K. J. Phys. Chem. 1995, 99, 6110. Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404. Peters, A.; Branda, N. R. Chem. Commun. 2003, 954. Zhou, X.-H.; Zhang, F.-S.; Yuan, P.; Sun, F.; Pu, S.-Z.; Zhao, F.-Q.; Tung, C.-H. Chem. Lett. 2004, 33, 1006. Moriyama, Y.; Matsuda, K.; Tanifuji, N.; Irie, S.; Irie, M. Org. Lett. 2005, 7, 3315. See also ref 18. (22) Green, K. A.; Cifuentes, M. P.; Corkery, T. C.; Samoc, M.; Humphrey, M. G. Angew. Chem., Int. Ed. 2009, 48, 7867.

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vinylidene species 18-H22þ. The open and closed isomers of each couple can be interconverted by UV/VL irradiation. In this case the 2e-oxidation does not cause the ring closure as observed for 16 and 17 but affords the stable dication 18(O)2þ, presumably because of the long linkers preventing delocalization of the metal-centered radical centers over the thiophene rings. Thus the six species can be interconverted along seven pathways. Because the six states show distinct cubic nonlinear optical (NLO) properties, the six states generated from a single molecule by selecting an appropriate combination of the stimuli can be discriminated on the basis of the NLO properties. Recently, stepwise photoswitching of the platinum-bis(acetylide) complex, (DTE-CtC)2Pt(PBu3)2, with DTE units at the peripheral positions (24; Scheme 6), was reported by Wolff and Branda, who also revealed ON/OFF photoswitching of electronic communication over the molecule.23 The gold derivative 19, an olefin derivative (X, Y = CHdCH-RuCl(CO)(PPh3)2 (Chart 1)),24 and the η6-arene (20 and 21) and η5-Cp* derivatives (22 and 23)25 were also reported (Chart 1). Future Prospects. Photochromic metal complexes including photochromic organometallics show intriguing switching behavior as described above. Previous studies on coordination compounds are concentrated on photophysical analyses of the chromic processes and the properties of the two isomeric species, because the studies have been motivated mainly by application to data storage device. As a result, a substantial amount of knowledge on the photophysical aspects has been accumulated thanks to the extensive studies by many inorganic researchers, but applications to other research subjects (e.g., organometallic catalysis) are scattered and remain immature; therefore, systematic studies are needed for future development. In this section several important issues for future development of photochromic organometallics will be discussed. i. Molecular Design and Synthesis. Molecular design is the key issue of photochromic organometallics as described in the text. For example, in the case of catalyst design, a metal fragment readily combined with chromic molecules may not always exhibit good catalytic performance and, conversely, it may not always be easy to combine a chromic molecule with a metal fragment with good catalytic properties. Appropriate combination of the two components should be chosen so that the metal fragment feels the changes of the coordination properties induced by the photochromic change as much as possible to trigger the required function at the metal center. As to synthesis, coordination compounds can be readily synthesized by simply mixing the preformed photochromic ligand and a metal component, while, in synthesizing organometallic derivatives, highly functionalized chromic molecules frequently need to be subjected to metalation or treatment with carbanionic reagents at some stage. It is desirable that the target molecule can be prepared under mild conditions without using highly reactive reagents such as alkyllithiums. Readers may recognize that Chart 1 contains (23) Roberts, M. N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf, M. O. J. Am. Chem. Soc. 2009, 131, 16644. Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O. Inorg. Chem. 2009, 48, 19. (24) Lin, Y.; Yuan, J. J.; Hu, M.; Cheng, J.; Yin, J.; Jin, S.; Liu, S. H. Organometallics 2009, 28, 6402. Lin, Y.; Yin, J.; Yuan, J.; Hu, M.; Li, Z.; Yu, G.-A.; Liu, S. H. Organometallics 2010, 29, 2808. (25) Moriuchi, A.; Uchida, K.; Inagaki, A.; Akita, M. Organometallics 2005, 24, 6382.

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Organometallics, Vol. 30, No. 1, 2011

Akita Scheme 5

Scheme 6

several acetylide complex-type molecules. This is because the M-CtC-C[Ar] skeleton can be obtained from terminal acetylene precursors without using free carbanions such as -CtC-Li reagent; that is, the M-Ct and tC-C[Ar] bonds can be formed via a vinylidene intermediate (MdCd C(H)-)26 and Sonogashira coupling, respectively. This is an important issue common to the synthesis of functionalized organometallic compounds. Thus elaborative molecular design taking into account the required function and the synthetic route is essential. ii. Catalysis. Catalytic performance could be switched or tuned by modulating coordination properties of the photochromic ligand coordinated to the catalytic metal center by the photochromic processes. Attempts at or toward catalytic organic transformations by organometallic compounds have met with partial success, as reported by Branda (Scheme 3). The coordinating ability of a photochromic ligand can, in principle, be controlled on the basis of photochromism. But, in the case of the DTE-based systems (e.g., 7 (10) and Table 2), the magnitude of the changes of the electronic densities at ligands or the attached metal centers may not be large enough to switch the chemical reactivity thereof. The electron-donating abilities at the σ-bonded carbon atoms (5-thiophene carbon atoms), therefore, are not significantly affected by the photochromic change. More remarkable influence may be obtained by using ligands such as 6 (Chart 2) with a π-system directly interacting with the DTE moiety, while catalytic application of 6 has not been reported so far. Spiropyran-containing ligands generating the zwitterionic MC form would influence the electronic properties more. A bidentate ligand can be obtained by addition of two coordinating groups. Attachment to a single aromatic ring through σ-bonds may not be so effective owing to the same reason mentioned above. On the other hand, attachment to different aromatic rings should cause a drastic change of the geometrical arrangement. Because the two coordination sites (26) Bruce, M. I. Chem. Rev. 1991, 91, 197. Bruce, M. I. Chem. Rev. 1998, 98, 2797.

are too separated to be coordinated to a singe metal center, it may be difficult to obtain 1:1 adducts. Even if a 1:1 adduct is obtained, the resultant cyclic structure of the open form may retard the photochromism. Furthermore, as in the case of 8 (Scheme 3), even if the bidentate coordination is possible in one form, photoisomerization may cause dissociation or decomposition. Possible candidates for effective photochromic ligands involve a cis-(Th)(L)CdC(L)(Th)-type bidentate ligand (Th = 2-methythiophene derivative) analogous to 4, where photoswitching causes switching of the donor properties at L as well as a change of the twist angle L-C-C-L, and a η6arene complex-type catalyst of spiropyran (e.g., Ikariya’s Ru(η6-arene) complex-based bifunctional catalyst).27 Many other photochromic compounds not discussed herein can also be used as the photochromic component. iii. Molecular Devices. The studies described in Section ii-2 clearly indicate that DTE is an excellent switch for π-conjugated organometallic molecular wires, which is based on the different π-conjugation systems of the DTE moiety with or without cross-conjugation. Thus DTE will be assembled into molecular devices including molecular electronics together with other molecular parts. Similar approaches are possible for other photochromic compounds, and many physicochemical phenomena of metal complexes based on π-conjugation (e.g., magnetic interaction, redox properties, polarity, and optical properties)14a,b,e could also be switched photochemically. iv. Multichromophoric Systems. Transition metal fragments can be chromophores owing to d-d, MCLT, and LMCT transitions. The combination with a photochromic molecule, therefore, would make the resultant systems bichromophoric or multichromophoric. But the photochromic performance is dependent on the separation of the absorptions of the chromic and metal parts. If they are separated enough to be excited separately, we would be able to obtain multichromophoric systems by switching the light source. On the contrary, if they overlap each other even to a small extent, two or more bands will be excited to lead to incomplete isomerization at the PSS. The absorption band of the metal fragment should be taken into account at the stage of molecular design. vi. Multimodal Stimuli-Responsive System. Attachment of a redox-active metal fragment to a photochromic system makes it dual-chromic (photo- and electrochromic). By further combination with another stimulus, i.e., protonation28 at the CtC moiety (Scheme 5), Humphrey succeeded in synthesizing (27) Ikariya, T.; Gridnev, I. D. Chem. Rec. 2009, 9, 106.

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the switching system consisting of six different states, which can be interconverted by application of the three different stimuli and detected by a single technique (NLO). Further combination with other chromic systems and chemical reactions should contribute to the development of sophisticated logic systems based on multi-stimuli-responsive systems.

Photochromic organometallics are promising molecular devices for catalysis and molecular electronics, although a series of systematic studies is needed for realization. Most of the discussion in this section is common to other chromic metal systems, and integration of these stimuli-responsive systems would lead to smart chemical systems.

(28) Hirasa, M.; Inagaki, A.; Akita, M. J. Organomet. Chem. 2007, 692, 93.

Acknowledgment. The author is grateful to the reviewers for their valuable comments.