Keywords

1 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. 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, i.e., different colors, and the process is often reversible (Fig. 11.1).

Fig. 11.1
figure 00111

Photochromic organometallics: combination of organic photochromic unit with metal species

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The color change results from a change of the electronic structure of the chromic molecule (usually π-conjugated 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 systems. One way is the combination of chromic π-conjugated organic fragments with metal species, which exhibit unique features such as redox properties, photophysical properties, and catalysis. There are many kinds of chromism, such as photo-, thermo-, iono-, halo-, electro-, solvato-, vapo-, and mechanochromism [2]. Herein photochromic system with metal complexes will be focused on, in particular, in which the photochromic unit is 1,2-dithienylethenes (DTE) [3, 4]. Over the past 10 years, photochromic metal complexes featuring other photochromes such as azobenzene, spiropyran, spirooxazine, benzopyran, and dimethyldihydropyrene derivatives have been developed from the viewpoint of modulation of photochromism or photoregulation of the redox, optical, and magnetic properties of metal units [5–10]. In addition, metal complexes themselves can also show photochromic properties usually via linkage isomerization of ligands [11]. In contrast to them, “photochromic organometallics” in this chapter refers to the DTE metal complexes, M–DTE–M or M–DTE (M: redox-active metal fragments), with a M–C bond. This molecular design of photochromic organometallics provides a promising way for creation of metal complexes with photochromic properties.

2 Photoswitchable Organometallic Molecular Wire

2.1 Introduction

Molecular electronics are regarded as one of the promising ways for miniaturization of electronic circuits leading to highly integrated electronic systems [12–25]. In order to build up molecular circuits, many molecular components such as wires, switches, transistors, and logic gates should be developed and assembled. The polyynediyldimetal complexes, M–(C≡C)n–M (M: redox-active metal fragments), with a wirelike appearance are expected to display properties applicable to molecular wires [26–30], because the d-orbitals of the metal fragments at the termini may interact with each other through the π-conjugated (C≡C)n rod. For efficient communication between the two metal centers, the properties of the metal fragments must be best tuned [31–41]. Molecular electronics, however, are still in their infancy. Other molecular components need to be synthesized and their performance should be assessed [42]. Thus, we carried out the synthesis of switching systems.

2.2 1st Generation Dinuclear Iron and Ruthenium Complexes with Dithienylethene Ligand

Dithienylethene (DTE) has been chosen as the switching mechanism. The family of DTE is one of the versatile photochromic systems developed by Prof. Irie (Fig. 11.2) [3, 4]. Their photochromism is based on the reversible photochemical cyclization–cycloreversion processes between the open 1,3,5-hexatriene skeleton and the closed cyclohexadiene skeleton in the central part, and the forward and backward processes are triggered by UV and visible light irradiation, respectively. It is notable that the double bonds in the closed isomer C are fully conjugated, whereas the open isomer O contains cross conjugation at the thiophene–cyclopentene junctions. This dramatic structural change causes the striking color change (O: pale-colored, C: deep-colored), and DTE is superior to other photochromic systems with respect to many aspects such as quick response, fatigue resistance, and facile control of the photophysical properties (e.g., λ max). If metal fragments are attached to the 5- and 5′-positions of the thiophene rings, the communication between the two metal centers through the DTE bridge may be switched.

Fig. 11.2
figure 00112

Photoswitchable organometallic molecular wire (1st generation DTE complex)

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Taking into account for the facile access to the acetylide complex-type skeleton, the DTE/Fe system with the C≡C linkers, Fe–C≡C–DTE–C≡C–Fe (Fe = (η5–C5Me5)Fe(dppe), and dppe = Ph2PCH2CH2PPh2) (1Fe*O), was designed and readily synthesized from the corresponding 1-alkyne (Fig. 11.3) [43]. For the open isomer 1Fe*O characterized by crystallographic methods, it is worthy of note that the DTE moiety adopts an antiparallel conformation suitable for photocyclization, i.e., the Fe groups bulkier than the central DTE part do not significantly affect the conformation of the central part in the ground state (Fig. 11.3a). The organometallic DTE derivative 1Fe*O underwent photocyclization upon UV irradiation, in a manner similar to organic counterparts, to be converted to the closed isomer 1Fe*C. A visible absorption at 774 nm grew progressively and reached a photostationary state with the composition of 1Fe*O/1Fe*C = 10/90 in C6D6 (Fig. 11.3b). Subsequent visible light irradiation of the equilibrated mixture regenerated 1Fe*O quantitatively. After ten photochemical cyclization–cycloreversion cycles, no noticeable deterioration was detected. Switching factor (SF) can be evaluated by comparison of wirelike performance of 1Fe*O with that of 1Fe*C. In general, wirelike performance of organometallic molecular wires is estimated on the basis of K C value (comproportionation constant) obtained by electrochemical analysis. Wirelike behavior is attributed to the stability of monocationic species, that is, the mixed-valence (MV) complex. The K C value is derived from the potential difference between the two-redox process (ΔE) according to the equation, K C = exp(ΔE × F/RT) (F: Faraday constant (9.65 × 104 C mol–1), R: gas constant (8.31 J K–1 mol–1), T: temperature (K)), and represents the thermodynamic stability of the mixed-valence monocationic species against non-mixed-valence ones, indicating that the extent of delocalization of the hole over the bridging part (Fig. 11.4).

Fig. 11.3
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(a) Molecular structure of 1Fe*O; (b) UV–vis spectra for photochromism of 1Fe*; (c) CV trace for 1Fe*O; (d) CV trace for 1Fe*C

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Fig. 11.4
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Evaluation for organometallic molecular wires based on electrochemical analysis

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The two isomers (1Fe*O and 1Fe*C) isolated by repeated recrystallization were subjected to electrochemical measurements to determine the K C values. For the open isomer 1Fe*O, as can be seen from the CV trace, a slightly broad single redox wave was observed indicating very weak interaction between the two metal centers with K C = 13 as analyzed by simulation of the CV trace (Fig. 11.3c) and deconvolution analysis of a differential pulse voltammetry (DPV) trace. In sharp contrast, the closed isomer 1Fe*C showed two separated redox waves (Fig. 11.3d). From the separation (ΔE = 160 mV), the K C value was determined to be 510, which is significantly larger than that of the open isomer. As a result, the switching factor, SF = K C (closed)/K C (open), turned out to be calculated at 39. It is clear that the present iron DTE complex 1Fe* is an excellent photoswitchable molecular wire [44].

The isoelectronic complexes, Ru–C≡C–DTE–C≡C–Ru (Ru = (η5–C5Me5)Ru(dppe)) (1Ru*O), were also prepared and tested for photochromic behavior and switching factor [45]. It should be noted that photochromic performance is significantly improved (1Ru*C was formed quantitatively under UV irradiation). Quantum yields (in toluene) were determined to be 0.0021 (1Fe*O; irradiated at 366 nm) and 0.38 (1Ru*O; irradiated at 366 nm) for the ring-closing processes and 0.00018 (1Fe*C; irradiated at λ max of 1Fe*C (768 nm)) and 0.00044 (1Ru*C; irradiated at λ max of 1Ru*C (710 nm)) for the ring-opening processes. Note that although the ring-closing quantum yield of 1Ru*O is comparable to that of the diphenyl derivative, C6H5–DTE–C6H5 5 (1,2-di(2-methyl-5-phenylthien-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene), (0.59 for ring closure and 0.013 for ring-opening in hexane) [46], the ring-opening quantum yields are significantly even smaller than the rather small quantum yield for the ring-opening process of 5. On the other hand, switching performance of 1Ru* (SF = 4) is inferior to that of 1Fe*. These results show that the structures of metal fragments strongly affect photochromic properties as well as redox properties.

3 Dual Photo- and Electrochromic Organometallics

3.1 Introduction

To gain further insight into the effect of metal fragments on photochromic behavior, we designed a new series of 2nd generation DTE complexes without acetylene linkers, M–DTE–M (M = (η5–C5R5)ML2, M = Fe, Ru; L = CO, phosphine), which redox-­active metal fragments are directly σ-bonded to the thiophene rings at the 5,5′-positions (Fig. 11.5 and Table 11.1).

Fig. 11.5
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Molecular design of 2nd generation DTE complex

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Table 11.1 Photochromic behavior of DTE complexes (1–4)
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In addition, computational analysis was conducted to investigate the photochemical process in detail. Time-dependent DFT (TDDFT) calculations suggested that the photocyclization proceeds via triplet excited state, which is a different reaction pathway from the proposed process in the photocyclization of normal organic DTE. Furthermore, these “photochromic organometallics” exhibit ring-closing reaction triggered by 2e-oxidation, i.e., the present molecular system provides dual photo- and electrochromic system. Multi-stimuli–responsive systems can be key elements for the construction of molecular devices as well as smart chemical systems, because they are expected not only to simply work as a switch but also to be applicable to logic systems. Combination of a photoresponsive DTE unit with redox-active metal fragments has potentials to produce multi-stimuli–responsive systems and multi-­chromism. We also developed the mononuclear DTE complexes, M–DTE (M = (η5–C5H5)ML2, M = Fe, Ru; L = CO, phosphine), which express dual chromism in a manner different from dinuclear complexes.

3.2 2nd Generation Dinuclear Iron and Ruthenium Complexes with Dithienylethene Ligand

A series of 2nd generation DTE complexes, M–DTE–M (M = (η5–C5H5)Fe(CO)2 (2Fe), (η5–C5H5)Ru(CO)2 (2Ru), (η5–C5H5)Fe(CO)(PPh3) (3Fe), (η5–C5H5)Ru(CO)(PPh3) (3Ru), (η5–C5H4(Me))Fe(dppe) (4Fe′), and (η5–C5Me5)Fe(dppe) (4Fe*)), was prepared by (a) metalation of the lithiated DTE using the corresponding carbonyl metal halides, (η5–C5R5)MX(CO)2 (M/X=Fe/I and M/X=Ru/Cl), and (b) subsequent photochemical ligand exchange reactions. Some of the derivatives are characterized by single-crystal X-ray crystallography, which reveals the open structure with the antiparallel conformation of the two thiophene rings being suitable for photochemical ring closure [47, 48]. The obtained organometallic DTE complexes exhibit photochromic behavior but the performance turns out to be dependent on the attached metal fragments, as expected from above-mentioned results (Sect. 11.2.2). The photochromic behavior on the basis of UV–vis spectra and NMR data was summarized in Table 11.1.

Complexes 2Fe, 2Ru, and 3Ru undergo the reversible photochemical interconversion, whereas the other complexes 3Fe, 4Fe′, and 4Fe* are virtually inert with respect to the photochromism. Isomer ratios (O/C) at the photostationary states in C6D6 are estimated to be 61/39 for 2Fe, 36/64 for 2Ru, 91/9 for 3Fe, and 30/70 for 3Ru. Dependence of the photochromic performance can be estimated by the isomer ratios at the photostationary states as follows: metal: Ru > Fe; ligand: (CO)2 > (CO)(PPh3) ≈ dppe (for the iron complexes), (CO)2 ≈ (CO)(PPh3) (for the ruthenium complexes); and linker: C≡C (1) > none (2–4). Dependence on the metal will be discussed in the next Sect. 11.3.3 on the basis of theoretical analysis. It should be noted that conversion of 2Ru and 3Ru to the closed isomers C is incomplete, whereas the acetylide derivatives 1Ru* are converted to the closed isomers almost quantitatively. The difference could be ascribed to the absorption ranges of the closed isomers. The envelope of the UV absorption of the open isomers O could be extended to the absorption ranges of 2RuC–3RuC (550–600 nm) but not to that of 1Ru*C (>700 nm) being in the far longer wavelength region. Stability (fatigue resistance) of the organometallic DTE complexes is dependent on the structure of the metal auxiliaries. For the stability, the following orders are noted: metal: Ru > Fe and ligand: (CO)(PPh3) > (CO)2. The stability appears to be limited by photochemical decarbonylation and, therefore, is dependent on the strength of back-donation to the CO ligands. The CpM(CO)(PPh3) complexes 3 are more robust than the CpM(CO)2 complexes 2, and the ruthenium complexes are more stable than the iron derivatives. In accord with this consideration, any notable deterioration was not observed for the dppe complexes without a CO ligand, 4Fe′ and 4Fe*, although they did not show photochromic behavior.

Quantum yields for the ring-closing and ring-opening processes of a representative example 2Ru in toluene were determined to be 0.22 (irradiated at 355 nm) and 0.011 (irradiated at λ max of 2RuC (549 nm)), respectively. For 2FeC, the ring-­opening quantum yield was determined to be 0.016 (irradiated at λ max of 2FeC(560 nm)), but determination of the ring-closing quantum yield was hampered by the photochemical decomposition mentioned above proceeding at a rate comparable to that of the ring-closing process. It is notable that the ring-closing quantum yield of 2RuO is in the same range of those of the organic diphenyl derivative, Ph-DTE-Ph 5, (0.59 in hexane) and the ruthenium-acetylide complex 1Ru*O (0.38) but significantly larger than that of 1Fe*O (0.0021). On the other hand, the ring-opening quantum yields of 2FeC and 2RuC are virtually the same as that of 5 (0.013 in hexane) and even much larger than those of for 1Fe*C (0.00018) and 1Ru*C (0.00044). Thus, it turns out that photochemical reactivity of the ruthenium complexes is superior to that of the corresponding iron derivatives. The 2nd generation DTE complexes exhibit the photochromic properties in a manner similar to that of organic derivatives but the performance is dependent on the metal and ancillary ligands. With respect to central metal, it has similar results as those of 1st generation DTE complexes.

Electrochemical behavior turns out to also be dependent on the attached metal fragments. Behavior of the 2- and 3-series complexes is considerably different from that of the 4-series dppe complexes. As a typical example, electrochemical behavior of 3Ru is described in detail (Fig. 11.6). The closed isomer 3RuC shows the reversible, two consecutive 1e-redox waves at −430 (A) and −174 mV (B) (vs. [FeCp2]/[FeCp2]+; Fig. 11.6a). The CV trace for the open isomer 3RuO (Fig. 11.6c) is totally different from that of 3RuC. A 2e-oxidation wave is observed at 273 mV (C) but the corresponding reduction wave is not detected and, instead, two consecutive reduction waves are observed at −495 (D) and −217 mV (E). A subsequent anodic scan gives two oxidation waves (F and G) corresponding to the two reduction processes (E and D, respectively). These two redox processes observed at E 1/2 = −430 and −174 mV (D–G) are superimposable on those of the closed isomer 3RuC (A and B) mentioned above. These results suggest that 2e-oxidation induces cyclization of the open isomer. Similar electrochemical behavior is observed for the open isomers of the ruthenium complex 2RuO and the iron complexes 2FeO and 3FeO. In contrast to 2O and 3O, the dppe complexes 4Fe′O and 4Fe*O show two consecutive reversible 1e-redox waves when scanned in the range of −1,500 to 500 mV.

Fig. 11.6
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CV traces for 3RuC(a), 3RuC 2+(b), and 3RuO(c)

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To confirm the chemical events taking place, chemical oxidation of the DTE complexes was conducted. Treatment of 3FeO, 3RuO, and 4Fe′O with [FeCp2]PF6 (2 equiv.) gave the isolable, deep-green-colored diamagnetic dicationic closed species, [3FeC 2+](PF6)2, [3RuC 2+](PF6)2, and [4Fe′C 2+](PF6)2, respectively (Fig. 11.7). Single-crystal X-ray crystallography of one of the stereoisomers of [3FeC 2+](PF6)2 confirms (a) formation of a C–C single bond between the two thiophene rings at the 2- and 2′-positions (1.52(1) Å), (b) the double bond character of the Fe═C moieties (1.858(6) and 1.872(6) Å) substantially shorter than the Fe–C single bond, and (c) a change of the pattern of the bond alternation in accordance with the canonical form C 2+ depicted in Fig. 11.7. The CV trace for the dicationic species 3RuC 2+ (Fig. 11.6b) identical to that of the neutral closed species (Fig. 11.6a) verifies that the two species contain the same closed carbon skeleton with different π-conjugated systems. It is significant that the Fe–phosphine complexes 3FeO and 4Fe′O, which do not undergo the photochemical ring closure, cyclize quantitatively upon the 2e-­oxidation. As expected from the bond alternation pattern of C 2+, the dicationic closed isomers are so stable under daylight and do not undergo photochemical cycloreversion in contrast to the neutral closed isomers C. In addition, reduction of the dicationic closed species C 2+ by cobaltocene, CoCp2, gave the corresponding neutral closed species C. For 2FeO and 2RuO, sequential in situ oxidation by CAN (cerium ammonium nitrate) and reduction with CoCp2 at −78 °C afforded the neutral closed species 2C, indicating occurrence of an analogous redox process. The cyclic voltammograms for complexes 2 similar to those of 3Ru also suggested occurrence of analogous oxidative ring closure but the cationic carbene intermediate 2C 2+ was too unstable to be isolated because of the lack of an electron-donating ligand (e.g., phosphine ligand) for stabilization of the electron-deficient Fischer-­type carbene functional group [49, 50]. The dppe complexes 4Fe′O and 4Fe*O exhibit CV features different from those of 2 and 3 as mentioned above. The Cp′ complex 4Fe′O underwent oxidative cyclization in a manner similar to 3Fe and 3Ru to give [4Fe′C 2+](PF6)2, although reduction of the resultant dicationic species with CoCp2 did not afford 4Fe′C but a complicated mixture of products. On the other hand, 2e-oxidation of the Cp* complex 4Fe* did not afford the cyclized product but the isolable, paramagnetic, open diradical species 4Fe*O 2+, which shows the same CV features as those of 4Fe*O.

Fig. 11.7
figure 00117

Oxidative cyclization of 3FeO, 3RuO, and 4Fe′O and ortep drawing of 3FeC 2+

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The redox processes of the 2nd generation DTE complexes 2–4 can be interpreted in terms of the reaction sequence summarized in Fig. 11.8. The 2e-oxidation of O should give the dicationic diradical species O 2+. The radical centers in O 2+ should be delocalized over the thiophene rings in resonance with the thienyl radical form O′ 2+, which undergoes radical coupling at the 2- and 2′-positions of the thiophene rings to give the closed diamagnetic species C 2+. Subsequent 2e-reduction gives the neutral closed isomer C. The lack of reduction waves for the oxidized, open dicationic species O 2+ (Fig. 11.6c) indicates that the ring closure (O 2+ → C 2+) proceeds at a rate faster than the time scale of the CV measurement. A further cathodic scan gives the two reduction waves for the generated, closed dicationic species C 2+, which are identical to those of the neutral closed isomer C. The ring closure of the Cp′Fe(dppe) complex 4Fe′ should follow an analogous reaction sequence but the different CV features could be ascribed to the different time scales for the CV measurement and the chemical oxidation. The electron-donating dppe ligand should stabilize the electron-deficient diradical species O 2+ to elongate its lifetime. As a result, the rate of the ring-closing process becomes slower than the CV time scale but substantial with respect to the time scale of the preparative experiment. Further introduction of the electron-donating Cp* ligand (4Fe*) finally makes the diradical intermediate 4Fe*O 2+ stable so as not to undergo the cyclization. Our studies have been followed by a few related DTE–metal complexes with ring closure induced by oxidation [51, 52].

Fig. 11.8
figure 00118

Dual photo- and electrochromic behavior

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Thus, it has been revealed that the 2nd generation DTE complexes exhibit not only photochromism but also electrochromism, i.e., dual chromic behavior. Similar electrochromism is also observed in organic DTE system [53–59], however, the feature of the present “photochromic organometallics” is the involvement of isolable C 2+. The three colors (C, O, C 2+) can be expressed by a single molecule in response to different stimuli (light and redox).

3.3 Computational Analysis on Photochromism of Dithienylethenes with Metal Fragments

To gain further insight into the photochemical processes observed for the dinuclear DTE system, time-dependent DFT (TDDFT) analysis [60–63] was performed for singlet and triplet excited states of the CpM(CO)(PMe3) complexes 3#Fe and 3#Ru (simplified PMe3-substituted analogues of 3Fe and 3Ru, respectively).

It has been established for organic DTE molecules that the ring closure occurs via the lowest singlet excited state (corresponding to S9 for 3#Fe in Fig. 11.10a) [64, 65]. By contrast, De Cola and her coworkers recently studied photophysical properties of a transition metal complex 6, where the photochemically active Ru(bpy)3 fragments were attached to DTE (Fig. 11.9), and proposed that the ring closure of 6 proceeds not only via the ligand(DTE)-centered singlet state but also via the ligand-­centered triplet state resulting from energy transfer processes by way of the metal-­based excited states [66]. Taking into account for these two extreme cases, we have carried out TDDFT analysis. To be explained is the following photocyclization tendency observed for the central metal: Ru > Fe.

Fig. 11.9
figure 00119

A DTE complex with the Ru(bpy)3 units 6

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Energy levels for Franck–Condon states for the two complexes are summarized in Fig. 11.10. Singlet and triplet states are denoted by bold lines and plain lines, respectively, and labels for the excited states of higher energies are omitted for clarity. The excited states can be divided into two categories, LMCT and IL (intraligand). The excited states of the LMCT series are metal d-orbital-based, whereas those of the IL series are DTE-based, as can be seen, for example, from the S1 and S3 states of the CpRu(CO)(PMe3) complex 3#Ru (Fig. 11.11). In addition, the metal-based LMCT series orbitals contain antibonding combinations between the metal d-orbital and the ligand orbitals (Cp, CO, and PR3). These features are common to these complexes discussed herein.

Fig. 11.10
figure 001110

Energy diagrams for Frank–Condon excited states of 3#Fe (a) and 3#Ru (b). Bold lines: singlet states; plain lines: triplet states

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Fig. 11.11
figure 001111

HOMO and the lowest metal-(S1) and ligand-based singlet excited states (S3) for 3#Ru

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Let us consider the chemical processes of 3#Ru (Fig. 11.10b). Irradiation causes excitation of a HOMO electron to singlet orbitals of higher energies. The S1 state resulting from LMCT transition lies substantially lower in energy than the S3 state resulting from IL transition, suggesting that the initial excitation may occur toward the S1 state preferentially. The oscillator strength shown in parentheses, which indicates relative transition probability, clearly demonstrates that the GS(S0)→S3 transition (I) is the most probable and major initial photochemical event; the oscillator strength for the GS→S3 transition (0.069) is substantially larger than the negligible oscillator strength for other transitions (<0.018). The energy level of S3 is in the UV region in accordance with the lack of a visible band for 3Ru. Subsequent energy transfer to a LMCT excited state (II) followed by intersystem crossing and relaxation according to the Kasha’s rule (III) leads to the lowest LMCT triplet state T2. Further back energy transfer toward the DTE part leads to the lowest excited state T1 with the significant DTE-based character (IV). Let us point out that the MO features for the DTE moiety in T1 (MO is the same as that for S3 depicted in Fig. 11.11), i.e., (a) antibonding combination of the p-orbitals of the C═C moiety in the cyclopentene ring, (b) π-bonding interactions at the thiophene–cyclopentene junctions, and (c) in-phase combination of the p-orbitals of the 2- and 2′-carbon atoms in the thiophene rings to be connected in the closed isomer, are reminiscent of the molecular orbital of a closed cyclohexadiene skeleton. These features tell us that the final ring-closing process from T1 (V) is a likely process.

Then we examined the effects of the metal centers. For the iron analogue 3#Fe, an energy diagram similar to that for 3#Ru is obtained (Fig. 11.10a) but a significant difference is noted for the relative energy gap between the excited states associated with step IV. The DTE-based T7 state lies significantly higher in energy than the lowest metal-based triplet state T1 so that the final endothermic process IV is not viable and, as a result, the iron complex 3#Fe eventually deactivates to the ground state without cyclization. The dependence of the efficiency of the photocyclization on the central metal (Ru > Fe) has been successfully interpreted in terms of the mechanism involving the crucial metal-to-DTE energy transfer step IV. The inertness of 3#Fe reveals that the ring closure of the organometallic DTE compounds may proceed via triplet excited states, because, otherwise, photocyclization should occur directly via the initial singlet state (S9) in a manner similar to organic derivatives. The inertness suggests that energy transfer from S9 to LMCT triplet states is much faster than direct ring closure from S9.

More recently, our collaborators, the group of Dr. Nakamura, reported on explanation for ancillary ligand effect on the photocyclization of 2Fe and 3#Fe [67]. However, the origin of the dependence on the metal fragment is still open to further discussion and investigation.

3.4 Mononuclear Iron and Ruthenium Complexes with Dithienylethene Ligand

A series of mononuclear DTE complexes, M–DTE (M = (η5–C5H5)Fe(CO)2 (7Fe), (η5–C5H5)Ru(CO)2 (7Ru), (η5–C5H5)Fe(CO)(PPh3) (8Fe), (η5–C5H5)Ru(CO)(PPh3) (8Ru), and (η5–C5H4(Me))Fe(dppe) (9Fe)), was prepared in a manner similar to the synthesis of dinuclear complexes.

Photochromic behavior of M–DTE is summarized in Table 11.2. It is revealed that the structure of the metal fragments dramatically affected the photochromic properties as observed in dinuclear systems. In the photochromic performance, the mononuclear complexes exhibited lower than the corresponding dinuclear complexes did (Table 11.1).

Table 11.2 Photochromic behavior of the mononuclear DTE complexes (7–9)
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The mononuclear DTE complexes are also redox-active. CV traces of 9Fe and 8Ru apparently contained single reversible redox waves at 450 and 335 mV, respectively. These results suggested that the one-electron redox processes might induce reversible color change of the mononuclear DTE complexes because transition metal fragments can serve as chromophores originated in d–d, MLCT, and LMCT transitions. In fact, radical species O +• derived from 1e-oxidation of redox-active metal fragments have a characteristic absorption band assigned to LMCT around visible light region (9Fe: 676 nm, 8Ru: 500 nm). As mentioned above, 2e-oxidation of the DTE derivatives with two redox-active organometallic fragments causes the ring closure giving the dicationic closed isomer C 2+. That 2e-redox process is not reversible as shown in Figs. 11.7 and 11.8. It is notable, in the mononuclear systems, the color of the open isomer O can be changed into two directions in reversible manners (dual chromism), i.e., photochemical ring closure/opening process of DTE moiety between O and C and 1e-redox process of the organometallic fragment between O and O +• as shown in Fig. 11.12 [68].

Fig. 11.12
figure 001112

Dual chromism of the mononuclear DTE complexes

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These results suggest that combination of a redox-active metal fragment with a DTE makes it dual chromic system (photo- and electrochromism), leading to plural colors and states. Recently, Humphrey and coworkers showed the multi-stimuli–responsive systems composed with DTE, redox-active metal fragments, and pH-responsive C≡C units [69]. They have six different states, which can be logically switched by application of the three different stimuli (light, redox, and pH) and detected by a single technique (NLO).

4 Conclusion

Photochromic organometallics with a M–C (M: redox-active metal fragments, C: carbon atom in the photochromic unit = dithienylethene (DTE)) bond show intriguing stimuli–responsive functions as described above. The functions can lead to molecular devices and sophisticated logic systems.

4.1 Molecular Devices

DTE is an excellent switch for π-conjugated systems. Organometallic molecular wires are not also exceptions. Redox properties can be switched photochemically. Switching factors and photochemical process are strongly affected by the structure of metal fragments (Fe or Ru, auxiliary ligands). The basic information on molecular design of metal complexes with photochromic properties has been collected.

4.2 Multimodal Stimuli–Responsive Systems

Attachment of a redox-active metal fragment to a photochromic system can make it dual chromic system (photo- and electrochromism). Organic photochromic ligand and redox-active organometallic fragments can serve as photochrome and electrochrome, respectively. Molecular design is important to construct such stimuli–responsive systems. In particular, the number of metal fragments changes their chromic mechanism. Further combination with other chromic systems and metal fragments should contribute to the development of sophisticated logic systems.

Photochromic organometallics are promising molecular systems for molecular devices and logic systems, although a series of systematic studies is needed for realization. Integration of these stimuli–responsive systems would lead to smart chemical systems.