Photophysics and Excited-State Properties of Cyclometalated Iridium

Apr 20, 2017 - The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of t...
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Photophysics and Excited-State Properties of Cyclometalated Iridium(III)−Platinum(II) and Iridium(III)−Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine Yang-Jin Cho,† So-Yoen Kim,† Chang Min Choi,‡ Nam Joon Kim,‡ Chul Hoon Kim,† Dae Won Cho,† Ho-Jin Son,*,† Chyongjin Pac,*,† and Sang Ook Kang*,† †

Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea Department of Chemistry, Chungbuk National University, Chungbuk 28644, Korea

‡

S Supporting Information *

ABSTRACT: We investigated the electrochemical and excited-state properties of 2,3-bis(2-pyridyl)pyrazine (dpp)bridged bimetallic complexes, (L)2Ir-dpp-PtCl [1, L = 2-(4′,6′difluorophenyl)pyridinato-N,C2 (dfppy); 2, L = 2-phenylpyridinato-N,C2 (ppy)] and [(L)2Ir]2(dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes, (L)2Ir-dpp (5, L = dfppy; 6, L = ppy) and dpp-PtCl (dpp-PtIICl2; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed that 1 and 3 have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in 5 and 6 is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir−Pt and Ir−Ir bimetallic complexes (1−4) could be explained by assuming the involvement of crossing to platinum- and iridium-based d−d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d−d states from the emissive triplet metal-to-ligand charge-transfer (3MLCT) state, followed by cleavage of the dpp-Pt and (L)2Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based 3MLCT to the Ir d−d or Pt d−d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d−d or Pt d−d) with a significant dependency on the relative 3MLCT levels of the (L)2Ir-dpp component.

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based unit that can catalyze chemical processes.5,6 Bridging ligands that connect the two metal centers are typically 2,3bis(2-pyridyl)pyrazine (dpp), 2,2′-bipyrimidine, and related polyazine ligands. Extensive studies have been performed, particularly by Brewer and co-workers,7 on the electrochemical, photophysical, and photocatalytic aspects of such bimetallic complexes. For such bimetallic complexes, it can be expected that the two metal centers may have more or less electronic interactions in the excited state depending on the metals and bridging ligands, which might control their electrochemical, photophysical, and chemical properties.8 In relation to such aspects, it is of significance to use other chromophoric metal

INTRODUCTION Multimolecular systems based on photoactive and electroactive polynuclear transition-metal complexes have received considerable attention in relation to intermetallic electronic couplings1 and the creation of novel functionalities2 (e.g., enhanced light harvesting coupled with efficient catalysis of chemical processes through intermetallic electron transfer and/ or energy transfer). Among such systems, binuclear complexes with two metal centers connected by a π-conjugated bridging ligand provide a typical prototype that consists of molecular components with characteristic functions.3 A typical lightharvesting metal center that has been extensively investigated is Ru(bpy)n (bpy = 2,2′-bipyridine and related ligands) because of its excellent photophysical and electrochemical properties,4 whereas the other metal center is usually a RhIII, PtII, or PdII© 2017 American Chemical Society

Received: February 14, 2017 Published: April 20, 2017 5305

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centers in place of frequently used Ru(bpy)n or Os(bpy)n, because investigations on various complexes might provide versatile information on the structure−property relationships of bimetallic systems. Among a variety of photoactive metal complexes,2c,9 cyclometalated iridium(III) complexes are attractive because of a wide range tunability of the electronic structure via variation of the metalated C^N ligand with potential applicability to an excellent light-harvesting chromophore.10 Their photochemistry is well-studied and established. As a bridging unit, the dpp moiety is encored in this study because the steric constraints between the pyridine rings in the dpp moiety can shed some light on solving the contradiction between preserving the photophysical and redox-catalytic profiles of each component and maximizing the communication between two components by conjugation extension,7,11 such that the inefficient interaction by its constrained geometry would be a key factor for constructing an efficient communication bridge in a bifunctional supramolecular system. Also, the PtIICl2 fragment12 is used as a generic redox-catalytic site. The goal of this study is to find suitability of the iridium metal as a photosensitizer in place of the ruthenium metal, hoping for the establishment of a high-performance singlemolecule photocatalyst eventually useful for artificial photosynthesis. Scheme 1 illustrates an isoelectronic bimetallic system, Ir−Pt, that would undergo a cascaded photochemical process similar

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RESULTS

Synthesis and X-ray. Scheme 2 shows synthetic routes for 1−7 following established methods. To ensure the productivity of the final monometallic complexes, (dfppy)2Ir-dpp (5) and (ppy) 2Ir-dpp (6), were purified by means of column chromatography. All complexes obtained were characterized by 1H and 13C NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. ESI-MS spectra of 1−4 dominantly revealed the parent peak together with minor fragment peaks, as shown in Figures S8−S11. The solidstate molecular structures of (dfppy) 2 Ir-dpp-PtCl (1), [(dfppy) 2 Ir] 2 (dpp) (3), 5, and 6 were unequivocally determined by single-crystal X-ray crystallography, as shown in Figure 1. The selected bond lengths and dihedral angles of the compounds are compiled in Table S2. The crystal structures show octahedral six coordination around the iridium metal center by two sets of carbon and nitrogen atoms of C^N (L = dfppy or ppy) and by two nitrogen atoms of L′ (dpp), with parameters similar to those reported for other related complexes.14,15 The C−Ir−N and N−Ir−N bond angles subtended by the chelating phenylpyridine (L) and dpp ligands are in the range of 75.8(5)−82.1(6)° and are slightly distorted from a regular octahedral geometry because of the steric requirements for metal coordination. Each heterocyclic ring is planar within the limits of accuracy. The structure around the dpp-PtCl (7) unit of 1 is similar to that reported for [(bpy)2Ru(dpp)PtCl2]2+ (bpy = 2,2′-bipyridine),15 suggesting that the dpp structures of the metal-dpp-PtCl2 bimetallic complexes would be almost independent of the other metal center. Table 1 shows the dihedral angles between the pyrazine ring (A) and each of the pyridine groups (B or C) extracted from the X-ray data. In the cases of 5 and 6, the angles were small for the iridium-coordinated pyridine ring B (24.9° for 5 and 19.9° for 6), whereas the noncoordinated pyridine ring C was considerably twisted with respect to the pyrazine ring (128.9° for 5 and 125.7° for 6) due to substantial steric repulsion between the two pyridine rings. In contrast, both pyridine rings (B and C) of 1 and 3 were less tilted with respect to the pyrazine ring (28.3°/18.9° and 24.7°/34.7° for 1 and 3, respectively) as a result of the secondary coordination of iridium(III) or platinum(II) metal to the A−C sides of dpp. Bimetallic coordination to the dpp unit resulted in a planar structural arrangement, which, in turn, exerted steric constraint at each metal center that eventually causes photochemical instability. It is not unreasonable to assume that the other bimetallic complexes, (ppy) 2 Ir-dpp-PtCl (2) and [(ppy)2Ir]2(dpp) (4), might have similar stereochemical situations around the dpp unit, presumably enhancing the magnitude of electron transfer toward the d−d state. Electrochemical Properties. The electrochemical behavior was investigated by cyclic voltammetry (CV) for acetonitrile (ACN) solution. The CV traces are shown in Figure 2 for 1−7, from which their oxidation and reduction potentials versus Fc+/ Fc were estimated (Table 2). All of the complexes exhibited quasi-reversible reduction waves from −0.9 to −2.31 V, among which the first and second waves can be attributed to the dpp0/− and dpp−/2− couples, respectively, as reported for related Ru(dpp) complexes.5,6a,7,16 The third reduction wave of the bimetallic complexes should be due to the C^N0/− couple,5,6a,7,16 whereas the monometallic

Scheme 1. Isoelectronic dpp-Bridged Bimetallic Complexes Ruthenium(II)−Platinum(II) and Iridium(III)− Platinum(II)

to that found in Ru−Pt: Upon irradiation, the iridium chromophore is imagined to undergo metal-to-ligand-chargetransfer (MLCT) excitation, placing a metal-based dπ electron from one of its t2g orbitals into a ligand-based π* orbital. The photoexcited electrons are to be injected into the platinum center, preparing it for the reductive event. To demonstrate this concept experimentally, we attempted to use the (L) 2 Ir-dpp complexes [5, L = 2-(4′,6′difluorophenyl)pyridinato-N,C2 (dfppy) and 6, L = 2-phenylpyridinato-N,C2 (ppy)] as the main chromophoric building blocks. With secondary coordination of PtCl2 or [(L)2Ir] through dpp unit, the bimetallic complexes, (L)2Ir-dpp-PtCl (1, L = dfppy; 2, L = ppy) and [(L)2Ir]2(dpp) (3, L = dfppy; 4, L = ppy), are prepared in this study (see Scheme 2). Herein, we report the details of the electrochemical, photophysical, and photochemical properties of the hetero- and homobimetallic complexes with those of the mononuclear counterpart dppPtCl (7, dpp-PtIICl2) and discuss the magnitude of the communication between the metal centers under various structural modulations in bimetallic systems. 5306

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Inorganic Chemistry Scheme 2. Synthesis of Heterobimetallic (1 and 2), Homobimetallic (3 and 4), and Monometallic (5−7) Complexes

Figure 1. ORTEP13 drawings of (a) 1, (b) 3, (c) 5, and (d) 6 after removal of hydrogen atoms, solvent, and PF6− for clarity (30% probability for thermal ellipsoids).

second wave of 5 and 6. In the case of 7, the third wave at the very negative potential (−2.31 V) should be attributable to the reduction of the PtCl2 unit.6a Likewise, the fourth reduction wave of the Ir−Pt heterobinuclear complexes (1 and 2), which is absent in the case of the Ir−Ir binuclear complexes, was tentatively assigned as the PtII/I couple. Electrochemical oxidation of 5 and 6 gave quasi-reversible waves at 1.28 and 0.96 V, respectively, attributable to the IrIII/IV couple. However, the cathodic peak was considerably weaker than the anodic peak, suggesting the involvement of significant irreversible nature. On the other hand, the electrochemical oxidation of 7 showed highly irreversible behavior with a very weak anodic peak at 1.62 V, which was significantly more positive than the oxidation potentials of 5 and 6, being expected to occur on the PtCl2 unit. Similarly, the bimetallic complexes

Table 1. Dihedral Angles between the Pyrazine (A) and Pyridine (B or C) Rings

counterparts (5 and 6) showed only two waves. Presumably, the dpp−/2− and C^N0/− couples might be overlapped in the 5307

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Figure 2. Cyclic voltammograms of a 1 mM solution of 1−7 in ACN containing 0.1 M tetrabutylammonium perchlorate (scan rate = 0.1 V s−1).

revealed irreversible oxidation behaviors with different features. The oxidation of 1 gave a weak anodic peak at 1.36 V with no discernible cathodic peak, whereas 2 showed a couple of anodic peaks accompanied by a very weak cathodic peak at 0.93 V. These irreversible oxidation features were different from the reversible oxidation behaviors reported for related Ru−Pt and Ru−Rh complexes.16,17 In the cases of the Ir−Ir homobimetallic systems, positive scans gave a high-current anodic peak at 1.51 V for 3 or a couple of anodic peaks in the range of 1.0−1.4 V for 4, but no cathodic peak was observed upon reverse scanning. The irreversible behavior again contrasted with the reversible electrochemical oxidation of the Ru−Ru18 and Os− Os homobimetallic systems,19 indicating that the characteristics of the iridium complexes were different from those of the ruthenium and osmium analogues. Absorption and Emission Spectra and Time-Resolved Emission Analysis. The absorption spectra in ACN at room temperature are shown in Figure 3 for 1−6 and in Figure S13 for 7; the absorption characteristics are listed in Table 3. The intense absorptions at 400 nm commonly resulted in bleaching of the characteristic absorption bands at >300 nm with an isosbestic point at 300−310 nm. The spectral changes reached a steady state after 3−5 h of irradiation, giving spectra apparently similar to that of 5 or 6 with some minor differences. As shown in Figures S8−S11, ESI-MS spectra of the samples irradiated for 5 h revealed strong fragment peaks attributable to [(ppy)2Ir(dpp)]+, [(ppy)2Ir(dpp)(X)n]+ (X = Cl, n = 1; X = ACN, n = 1 and 2), and [(ppy)2Ir(ACN)n]+ (n = 0, 1, and 2) in all cases, accompanied by extensive diminution of the parent peak of the initial compound. In the cases of 1 and 2, the peaks of dpp-PtCln+ (n = 1 and 2) indicating dissociation pathways of the bimetallic system were also detected. In all cases, moreover, it was confirmed that other fragment peaks due to the loss of chlorine and C^N ligand from the parent molecules are absent. Tracing the dissociation pattern observed with visible-light irradiation suggested that the photoreactions of the bimetallic complexes mainly proceeded via splitting of the metal-dpp linkages (eqs 1−1, 1−2, and 2), with little participation of the liberation of L (C^N) or chlorine from the complexes.

other complexes, only 3 is emissive at room temperature, showing an emission maximum at 700 nm (Φem = 0.017 and τem = 35 ns), which is 10 times weaker than that of the monometallic counterpart 5. On the other hand, no emission was detected for the other bimetallic complexes and 7 at room temperature. However, in ACN/butyronitrile (BCN; 1−4) and 2-methyltetrahydrofuran (2-MeTHF; 5−7) at 77 K, all of the complexes are moderately emissive, with lifetimes in a microsecond domain typical of MLCT phosphorescence, as shown in Figure 5b,c. It should be noted that the emission maxima of 5 and 6 at 77 K are blue-shifted by 68 nm (2200 cm−1) and 98 nm (2600 cm−1), respectively, compared to those of a fluid solution at room temperature. The temperature-dependent shifts of the emission are typical of MLCT phosphorescence,24 arising from different environments surrounding the molecule associated with stabilization of the polar MLCT emissive state, i.e., substantial stabilization by extensive solvation in a polar solvent at room temperature versus little stabilization due to the freezing of solvent reorganization at 77 K. The spacing between the 0−0 and 0−1 peaks was 1254 cm−1, a value typical of MLCT phosphorescence reported for various iridium complexes.25 Such a vibrational structure was also observed in the emission spectrum of 7 taken in 2-MeTHF at 77 K, giving a peak separation of 1360 or 1339 cm−1. On the basis of these observations, the luminescence states of all complexes were attributed to phosphorescence from a triplet state with MLCT character. Figure 6 shows the relative energies of the low-lying excited states associated with 1−7. The energies E*0−0 are based on each low-temperature emission spectrum. The emissive chargetransfer states can be tuned by varying like L (C^N) ligands. Ir−Pt and Ir−Ir counterparts with the same L, for example, had emission maxima at very similar wavelengths (595 nm for 1 vs 592 nm for 3 and 670 nm for 2 vs 655 nm for 4), indicating that the (dfppy or ppy)2Ir-dpp chromophore should dominate MLCT phosphorescence. In the monometallic complexes (5− 7), the 0−0 emission band energy of 7 in 2-MeTHF at 77 K was lower by 1044 cm−1 (0.13 eV) than that of 5 and higher only by Ir−Ir (homobinuclear) > Ir (mononuclear) with the formation of a bimetallic complex. Such decay acceleration in a bimetallic complex shows good agreement with our above estimation, meaning movement of the excited state (3MLCT) toward a thermally accessible nearby iridium-based or platinum-based d−d state for homobinuclear (Ir-dpp-Ir) and heterobinuclear (Ir-dpp-Pt) complexes, respectively. 5311

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Inorganic Chemistry Scheme 3. Possible Photolabilization Process in Heterobinuclear Complexes (a) 1 and (b) 2

is reasonable to estimate that the 2, 4, and 6 complexes also have a similar structure-quenching behavior relationship because of their similar geometrical situation around the dpp unit. Zysman-Colman et al. reported the similar tilting effect of the ligand on the emission quantum yield for monometallic iridium(III) complexes.32 This speculation seems to be in line with the photochemical behavior of 1−4, provided that iridium- and platinum-based d− d states would be responsible for the splitting of the corresponding metal-dpp linkages. As analyzed in the above ESI-MS study (see Figures S8−S11), the photoreaction of 1 indicated that both the Ir-dpp and dpp-Pt linkages were split with comparable efficiencies, whereas 2 underwent dominant splitting (dissociation) of Ir-dpp accompanied by less efficient cleavage of dpp-Pt. Such a biased dissociation pattern could be explained by comparing the triplet excited state (0−0 emission band energy) derived from low-temperature emission spectra. Although it was difficult to clearly estimate an independent emissive state for each component [the (L)2Ir-dpp and 7 parts in a bimetallic complex] due to the mixing of electronic structures (partially assigned as LML′CT) derived from stabilization of the dpp(π*) bridging ligand in bimetallic complexes, each emissive state could be quite clearly discriminated given the fact that the platinum-based M′L′CTs were positioned in a relatively higher position with respect to the iridium-based LML′CT in UV−vis absorption spectra (see Figure 4) and the iridium-based emissive state is common to be the lowest-lying triplet state of heterobinuclear (Ir-dpp-Pt) and homobinuclear (Ir-dpp-Ir) complexes. On the basis of this estimation, the platinum-based E*0−0 of 3(dfppy or ppy)2IrIII[dpp•−-PtIIICl]* (3M′L′CT) could be reasonably estimated to be at least higher than those of iridium-based 3LML′CT (2.08 eV for 3 [(dfppy) 2 Ir IV -dpp •− ]*-Pt II Cl and 1.85 eV for 3 [(ppy)2IrIV-dpp•−]*-PtIICl), as shown in Figure 6. As shown in the energy diagram based on this estimation (Scheme 3), we can understand that electron transfer from the LML′CT state to the M′L′CT (Pt-to-dpp charge transfer) state might be slightly exergonic in the case of 1 to proceed in competition with crossing to an iridium-based d−d state, thus leading to comparable generation of the M′L′CT state, followed by crossing to a platinum-based d−d state. On the other hand, in the case of 2, the energy-transfer process would be weakly endergonic so that the M′L′CT state generation might be considerably less efficient than the crossing from the LML′CT state to an iridium-based d−d state, predominantly generating [(L)2Ir]+ and 7 fragments (A).

shorter-lived emissions of ruthenium(II)-based bimetallic complexes compared to their monometallic counterparts. Provided that the MM′CT state can be represented by a simple electronic structure of IrIV-dpp-PtI, its energy level would be roughly equal to E(IrIII/IV) − E(PtII/I) above the ground state. Calculations using the oxidation potential and fourth reduction wave in Table 2 give a value of ∼3.4 eV for 1 or ∼3.2 eV for 2, which is higher by ∼1 eV than the emissivestate level (∼2.4 eV for 1 or ∼2.1 eV for 2 estimated from the onset wavelength of the emission spectra in Figure 5c). Accordingly, the crossing to such a simply represented MM′CT state was unlikely to occur as a dominant nonradiative process. Apart from energy transfer, the participation of an iridiumbased nonemissive state should be taken into consideration31 because the homobinuclear complexes are very weakly emissive or nonemissive at room temperature and photochemically reactive. The coordination of either PtCl2 or Ir(L)2 to the dpp position of (L)2Ir-dpp should open up thermal-activated nonradiative decay channels. Presumably, the “coplanar” strained conformation of dpp would have an impact on the platinum- or iridium-based d−d state to be thermally accessible from the emissive state with ease, with a clear temperature dependence. Different from almost no emission at room temperature conditions, the binuclear complex series (1−4), for example, show a distinctive emission at 77 K, indicating retarded electron transfer toward a d−d-type electronquenching pathway (a d−d-type nonradiative process) at a freezing temperature of the solvent (in the rigid matrix). The relative nonradiative quenching rates were measured and quantified by TA spectroscopy with higher-energy excitation of 350 nm, allowing a thermal dynamic equilibrium on both metal-based 1MLCT after vibronic relaxation. By a comparison between the decay lifetimes of the metal complex used, a crossing to a nonemissive platinum(II)-based d−d state might efficiently occur as a possible nonradiative process,26 with a crossing to a nonemissive iridium(II)-based d−d state of 1 and 2, probably following electron transfer from the Ir-MLCT state to the iridium- or platinum-based d−d state via planar dpp. Note that an efficiency toward a nonradiative decay pathway was significantly dependent on the degree of planarity of dpp (bridging ligand) derived from single X-ray analysis. It has been observed in the 1, 3, and 5 complexes that the planarity of the A−C sides of dpp (bridging ligand) pronounces the expected nonradiative passage: the less the torsion angle in the A−C side in the order of 1 (18.9°) < 3 (34.7°) < 5 (−51.1°) is exerted, the more pronounced the decay profile of TA at 600 nm, which is ascribed to the geometrically accelerated (or opened) decay channel toward an iridium- or platinum-based d−d state in a bimetallic complex with the coplanar-strained conformation. It 5312

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Notes

CONCLUSIONS Detailed investigations on the heterobinuclear (L)2Ir-dpp-PtCl and homobinuclear [(L)2Ir]2(dpp) compared with the iridium and platinum monometallic counterparts demonstrated that the Ir−Pt and Ir−Ir bimetallic complexes with the same L (C^N) ligand exhibited close similarities in their electrochemical, photophysical, photochemical, and TA decay behavior, i.e., the similarities were the potentials of the dpp0/−, dpp−/2−, and L0/− couples in CV, the absorption spectra with similar spectral shape and absorption coefficients, the emission spectra at 77 K similar in wavelength and lifetime, the complete or extensive quenching of emission at room temperature, and the selective photochemical splitting of the metal-dpp linkages. These similar properties have been attributed to the electronic states dominated by the (L)2Ir-dpp-based orbitals with little participation of metal−metal interactions; the lowest-energy absorption and emission have been discussed regarding L(π)participated MLCT transitions occurring between the (L)2Irbased orbitals and dpp-localized π* orbital. It has been suggested that both the emission quenching at room temperature and the photosplitting of the metal-dpp linkages are due to thermally activated crossing from the emissive triplet MLCT state to the iridium- and platinum-based d−d states with low barriers. The crossing to such d−d states has been assumed to be thermally accessible as a consequence of the strained planar conformation of dpp, which impact the relative energy levels of the relevant d−d states in the bimetallic complex. Also, we found the nonradiative decay pathway from the emissive charge-transfer state to either the iridium-based d−d or the platinum-based d−d state can be turned efficiently with the energy level tuning of L(π*), generating a different dissociation pattern after photoreaction. The electronic structure of the L (C^N) ligand was directly connected with our future research target, directing the creation of a highly efficient, robust supramolecular photocatalytic system for hydrogen generation and CO2 reduction while providing useful clues to understanding the chemical essence of the bimetallic systems associated with the light-harvesting ability and catalytic function.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MOTIE (Ministry of Trade, Industry & Energy; Grant 10051379) and KDRC (Korea Display Research Corporation) support programs for the development of future device technology for the display industry, the International Science and Business Belt Program through the Ministry of Science, ICT and Future Planning (Grant 2015K000287), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF2014R1A6A1030732).

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(1) (a) Balzani, V. M. L.; Scandola, F. Towards a Supramolecular Photochemistry: Assembly of Molecular Components to Obtain Photochemical Molecular Devices; NATO ASI Series; Springer: Amsterdam, The Netherlands, 1987; Vol. 214, pp 1−28. (b) Denti, G.; Campagna, S.; Sabatino, L.; Serroni, S.; Ciano, M.; Balzani, V. Luminescent and redox-reactive building blocks for the design of photochemical molecular devices: mono-, di-, tri-, and tetranuclear ruthenium(II) polypyridine complexes. Inorg. Chem. 1990, 29, 4750−4758. (c) Brunschwig, B. S.; Creutz, C.; Sutin, N. Optical transitions of symmetrical mixed-valence systems in the Class II−III transition regime. Chem. Soc. Rev. 2002, 31, 168−184. (d) Zhao, S.; Wang, S. Luminescence and reactivity of 7-azaindole derivatives and complexes. Chem. Soc. Rev. 2010, 39, 3142−3156. (e) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges. Coord. Chem. Rev. 2004, 248, 683−724. (2) (a) Balzani, V.; Barigelletti, F.; Belser, P.; Bernhard, S.; De Cola, L.; Flamigni, L. Rigid Rodlike Dinuclear Ru/Os Complexes of a Novel Bridging Ligand. Intercomponent Energy and Electron-Transfer Processes. J. Phys. Chem. 1996, 100, 16786−16788. (b) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K. Synthesis, Structure, and Magnetism of Bimetallic Manganese or Nickel Complexes of a Bridging Verdazyl Radical. Inorg. Chem. 2001, 40, 5581−5584. (c) Inagaki, A.; Akita, M. Visible-light promoted bimetallic catalysis. Coord. Chem. Rev. 2010, 254, 1220−1239. (d) Van Wallendael, S.; Paul Rillema, D. Photoinduced intramolecular energy transfer from one metal center to the other in a mixed-metal ruthenium/rhenium complex. Coord. Chem. Rev. 1991, 111, 297−318. (3) (a) Barthram, A. M.; Ward, M. D.; Gessi, A.; Armaroli, N.; Flamigni, L.; Barigelletti, F. Spectroscopic, luminescence and electrochemical studies on a pair of isomeric complexes [(bipy)2Ru(AB)PtCl2][PF6]2 and [Cl2Pt(AB)Ru(bipy)2][PF6]2, where AB is the bisbipyridyl bridging ligand 2,2′:3′,2″:6″2‴quaterpyridine. New J. Chem. 1998, 22, 913−917. (b) Arachchige, S. M.; Brewer, K. J. In Macromolecules Containing Metal and Metal-Like Elements: Supramolecular and Self-Assembled Metal-Containing Materials; Abd-ElAziz, A. S., Carraher, C. E., Jr., Pittman, C. U., Jr., Zeldin, M., Eds.; John Wiley & Sons: New York, 2009; Vol. 9, Chapter 7. (4) (a) Gafney, H. D.; Adamson, A. W. Excited state Ru(bipyr)32+ as an electron-transfer reductant. J. Am. Chem. Soc. 1972, 94, 8238−8239. (b) Kalyanasundaram, K. Photophysics, Photochemisty and Solar Energy Conversion with Tris(bipyridyl)ruthenium(II) and Its Analogues. Coord. Chem. Rev. 1982, 46, 159−244. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Ru(II) Polypyridine Complexes: Photophysics, Photochemistry, Electrochemistry, and Chemiluminescence. Coord. Chem. Rev. 1988, 84, 85−277. (d) Balzani, V.; Juris, A. Photochemistry and photophysics of Ru(II) polypyridine complexes in the Bologna group. From early studies to recent developments. Coord. Chem. Rev. 2001, 211, 97−115. (e) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. In Photochemistry and Photophysics of Coordination

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00384. Experimental section, 1H NMR, ESI-MS, absorption, normalized emission, and TA spectra, ORTEP drawings, crystallographic data, and selected bond distances and dihedral angles (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sang Ook Kang: 0000-0002-3911-7818 5313

DOI: 10.1021/acs.inorgchem.7b00384 Inorg. Chem. 2017, 56, 5305−5315

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