ANALYTICAL SCIENCES MAY 2003, VOL. 19 761 2003 © The Japan Society for Analytical Chemistry

Electrochemical and Phosphorescent Properties of New Mixed- Ir(III) Complexes Coordinated with Both Terpyridine and Various Bipyridine Derivatives

Naokazu YOSHIKAWA† and Takeko MATSUMURA-INOUE

Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630Ð8528, Japan

Seven useful mixed-ligand complexes in the form of [Ir(terpy)(L)Cl]2+ were prepared and their spectroscopic and electrochemical properties were investigated. The used were terpy = 2,2′:6′,2″-terpyridine, L = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 4,4′-diphenyl-2,2′-bipyridine, 1,10-, 5-phenyl-1,10-phenanthroline, 4,7- diphenyl-1,10-phenanthroline, 2,3-bis(2-pyridyl)pyrazine. Synthetic methods were developed by a sequential ligand- replacement which occurred in the reaction vessel using a microwave oven. All complexes showed that LUMOs are based on the π-system contribution of the terpyridine ligand for [Ir(terpy)(bpy)Cl]2+, [Ir(terpy)(dmbpy)Cl]2+, [Ir(terpy)(dpbpy)Cl]2+, [Ir(terpy)(phen)Cl]2+, [Ir(terpy)(dpphen)Cl]2+ and [Ir(terpy)(phphen)Cl]2+. On the other hand, the LUMO in the [Ir(terpy)(bppz)Cl]2+ complex is localized on the π-system of the bppz ligand, whereas the HOMOs in the iridium complexes are localized on the terpyridine ligand. It was found that Ir(terpy)(L)Cl emits in a fluid solution at room temperature. The ancillary ligands, such as terpy and bpy, have been explored to extend the lifetime of the triplet 3(πÐπ*) excited states of Ir(III) terpyridine complexes. Ir(III) terpyridine units with an electron donor (dmbpy) or electron acceptor substituents (terpy, dpbpy, phphen, dpphen and bppz) are found to decrease the energy of the 3LC states for use as photosensitizer molecular components in supramolecular devices. The spectroscopic and electrochemical details are also reported herein.

(Received November 15, 2002; Accepted February 7, 2003)

relatively short-lived 3MLCT with weak emitters.2,10 Introduction For the design of intense emission Ir(III) terpyridine complexes, we thought to obtain monoterpyridine complexes Extensive studies of the photophysics of octahedral [4d6] and using the ancillary ligands to tune the relative energies of [5d6]complexes have been attracting much attention to the HOMO and LUMO. As a first step toward this goal, we tried to photochemical applications of the complexes on account of their synthesize several Ir(III) complexes with matrixes of a long-lived excited states and good photoluminescence terpyridine ligand and a polypyridine ligand because these efficiencies.1 The studies have been focused mainly on the ligands with both electron donor and/or electron acceptor photophysical properties of octahedral metal-diimine complexes substituents resulted in decreasing the energy of the 3MLCT (II) and osmium(II), having such ligands as the 2,2′- state.11 This report describes the electrochemical behavior and bipyridine and 1,10-phenanthroline.2 Recently, a number of spectroscopic and photophysical properties of [Ir(terpy)LCl]2+ groups in relation to the tris-chelate complexes of rhodium and (L: an electron donor ligand or an electron acceptor ligand). iridium with diimine and cyclometalated ligands have been investigated.3,4 Tris-chelate complexes of rhodium and iridium show excited-state lifetimes in the microsecond region. The Experimental iridium complexes have intensive phosphorescence states at room temperature, while the rhodium complexes give Materials measurable emission states only at low temperatures. The The materials used in experiments were of analytical reagents stronger spin-orbit coupling mixes singlet and triplet excited grade. The reagents were as follows: ammonium hexachloro 5 states for iridium, leading to efficient phosphorescence. iridate(III) ((NH4)3[IrCl6]), hydrated ruthenium trichloride More recently, metal polypyridine complexes have been (RuCl3áH2O), 2,2′-bipyridine (bpy; Aldrich), 4,4′-dimethyl-2,2′- widely used as building blocks.6Ð9 The occurrence of isomers by bipyridine (dmbpy; Aldrich), 4,4′-diphenyl-2,2′-bipyridine the synthetic assembly of mononuclear building blocks is a (dpbpy; Aldrich), 1,10-phenanthroline (phen; Aldrich), 4,7- major problem in the design of supramolecular systems. diphenyl-1,10-phenanthroline (dpphen; Aldrich), 5-phenyl-1,10- However, from a structural point of view, the terpyridine ligand phenanthroline (phphen; Aldrich), 2,3-bis(2-pyridyl)pyrazine is superior to the bidentate one. Along with this structural (bppz; Aldrich), 2,2′:6′,2″-terpyridine, (terpy; Aldrich) and advantage, terpyridine complexes have a serious drawback of a potassium hexafluorophosphate (KPF6; Aldrich). Tetrabutylammonium perchlorate (TBAP, (C4H9)4NClO4; † To whom correspondence should be addressed. Aldrich) was used as a supporting electrolyte. The E-mail: [email protected] used in spectroscopic and electrochemical studies was of 762 ANALYTICAL SCIENCES MAY 2003, VOL. 19 spectroscopic grade.

Electrochemistry Differential pulse voltammograms (DPV) were measured on a BAS 50W electrochemical analyzer fitted with a three-electrode system consisting of a glassy carbon working electrode, a platinum auxiliary electrode, and an Ag/AgCl reference electrode (0.29 V versus NHE). Experiments were performed at Scheme 1 Reaction scheme for the synthesis of terpyridine a scan rate of 20 mV/s, a pulse height of 75 mV, and a duration complexes L, polypyridine ligand; T, terpyridine. of 50 ms. The solvent used in these studies was high-purity acetonitrile. All of the solutions were bubbled with nitrogen for 20 min prior to each scan and purged with nitrogen during each scan.

Other measurements Absorption spectra were recorded with a Hitachi U-3010 spectrophotometer. Luminescence studies were made with dilute (10Ð6 M) acetonitrile solutions at room temperature using a Hitachi F-2500 spectrofluorometer. Emission lifetimes were measured in deaerated actonitrile solutions using a Horiba single-photon counting system (NAES-500). The emission quantum yields for the iridium complexes were determined in CH3CN at room temperature relative to a solution 3+ containing Ru(bpy)3 and having the same absorbance. The emission quantum yields for the iridium complexes were determined by comparing the integrated emission spectra and 12 using Φem = 0.062 for the standard.

Microwave techniques Synthesized reactions were undertaken by using a Mitsubishi Electric microwave oven (Model; RR-12AF; 500 W; 2450 MHz) on medium-high power in a round-bottom flask fitted with a reflux condenser.13,14 Fig. 1 Structural formulae of the terpyridine complexes. Synthesis of mixed-ligand complexes [Ir(terpy)(bpy)Cl]2+ The desired complex was prepared by a sequential procedure with a ligand replacement. For example, (NH4)3[IrCl6] (0.5 mmol) and 2,2′:6′,2″-terpyridine (0.117 g, 0.5 mmol) were requires 384.66). mixed in ethylene glycol (15 ml). The suspended mixture was [Ir(terpy)(phen)Cl](PF6)2 (4). Yield 50%. Found: C, 35.99; H, refluxed for 5 min in a microwave oven under a purging 2.79; N, 7.42%. Calcd for C27H19N5IrClá2PF6áCH3COCH3: C, nitrogen atmosphere. Bipyridine (0.78 g, 0.5 mmol) was added 36.40; H, 2.54; N, 7.10%. ESI MS m/z 320.58 ([Ir terpy phen 2+ 2+ to the refluxing brown solution for 10 min. Next, the mixture Cl] requires 320.57), 786.10 ([Ir terpy phen Cl PF6] requires was cooled to room temperature. A saturated aqueous solution 786.13). of KPF6 (20 ml) was added as a counter ion, and a yellow [Ir(terpy)(phphen)Cl](PF6)2 (5). Yield 55%. Found: C, 37.85; product began to precipitate and was collected by vacuum H, 2.41; N, 6.81%. Calcd for C33H23N5IrClá2PF6á2H2O: C, filtration. The residue was dissolved in a minimal amount of 37.89; H, 2.58; N, 6.70%. ESI MS m/z 358.60 ([Ir terpy phphen 2+ 2+ acetone and flash precipitated in diethyl ether. The product was Cl] requires 358.63), 862.18 ([Ir terpy phphen Cl PF6] separated by vacuum filtration and dried under a vacuum. The requires 862.22). purity of the colored complexes was checked by thin-layer [Ir(terpy)(dpphen)Cl](PF6)2 (6). Yield 65%. Found: C, 44.44; chromatography. The synthetic routes to complexes of the type H, 2.88; N, 6.44%. Calcd for C39H27N5IrClá2PF6áCH3COCH3: [Ir(terpy)(L)Cl]2+ are summarized in Scheme 1. The structural C, 44.21; H, 2.89; N, 6.13%. ESI MS m/z 396.79 ([Ir terpy formulae of the mixed ligand complex [Ir(terpy)(L)Cl]2+ are dpphen Cl]2+ requires 396.68). shown in Fig.1. [Ir(terpy)(bppz)Cl](PF6)2 (7). Yield 30%. Found: C, 35.48; H,

[Ir(terpy)(bpy)Cl](PF6)2 (1). 65% yield; Anal. Found: C, 33.58; 2.61; N, 9.55%. Calcd for C29H21N7IrClá2PF6: C, 35.33; H, 2+ H, 2.59; N, 7.39%. Calcd for C25H19N5IrClP2F12: C, 33.10; H, 2.13; N, 9.95%. ESI MS m/z 347.65 ([Ir terpy bppz Cl] 2+ + 2.11; N, 7.72%. ESI MS m/z 308.63 ([Ir terpy bpy Cl] requires 347.60), 840.29 ([Ir terpy bppz Cl PF6] requires + requires 308.56), 762.24 ([Ir terpy bpy Cl PF6] requires 840.17). 762.09).

[Ir(terpy)(dmbpy)Cl](PF6)2 (2). 43% yield; Found: C, 34.85; H, 2.59; N, 7.56%. Calcd for C27H23N5IrClP2F12: C, 34.68; H, 2.48; Results and Discussion N, 7.49%. ESI MS m/z 322.64 ([Ir terpy dmbpy Cl]2+ requires + 322.58), 790.32 ([Ir terpy dmbpy Cl PF6] requires 790.12). Electrochemistry of iridium complexes [Ir(terpy)(dpbpy)Cl](PF6)2 (3). 55% yield; Found: C, 39.96; H, The typical electrochemical properties of ruthenium(II) 2.95; N, 6.69%. Calcd for C37H27N5IrClP2F12á2H2O: C, 40.05; polypyridine complexes are metal-centered oxidation and H, 2.83; N, 6.40%. ESI MS m/z 384.49 ([Ir terpy dpbpy Cl]2+ reductions based on each ligand. The ligand terpy is easier to ANALYTICAL SCIENCES MAY 2003, VOL. 19 763

Table 1 Electrochemical properties of the complexes

Electrochemistry V vs. Ag/AgCl Complex R1st R2nd R3rd Ox1 Ox2

1 [Ir(terpy)(bpy)Cl]2+ Ð0.96 Ð1.31 Ð1.70 1.61 2 [Ir(terpy)(dmbpy)Cl]2+ Ð0.95 Ð1.38 Ð1.68 1.61 3 [Ir(terpy)(dpbpy)Cl]2+ Ð0.95 Ð1.21 Ð1.66 1.63 4 [Ir(terpy)(phen)Cl]2+ Ð0.96 Ð1.30 Ð1.70 1.62 2.54 5 [Ir(terpy)(phphen)Cl]2+ Ð0.95 Ð1.27 Ð1.70 1.61 2.40 6 [Ir(terpy)(dpphen)Cl]2+ Ð1.00 Ð1.24 Ð1.68 1.62 2.52 7 [Ir(terpy)(bppz)Cl]2+ Ð0.62 Ð0.97 Ð1.76 1.61 8 [Ir(terpy)Cl3] Ð1.02 Ð1.72 1.60 2.14 3+ 9 [Ir(terpy)2] Ð0.67 Ð0.97 Ð1.71 1.60 10 [Ru(terpy)(phen)Cl]+ Ð1.39 Ð1.55 0.82 11 terpy Ð2.04 1.16 2.01 In acetonitrile solution, unless otherwise noted. Electrochemical × Ð5 data are from differential pulse voltammograms. Fig. 2 Absorption spectra of 8 (2.0 10 M, solid line) and terpyridine ligand (1.0 × 10Ð5 M, dashed line) in an acetonitrile at room temperature. reduce than the bpy ligand. This reaction gives rise to mixed- ligand terpy and bpy complexes in which the lowest unoccupied molecular orbital (LUMO) is localized on the terpy ligand. The potential of Ir(terpy)(phen)Cl (4) was observed as being more highest occupied molecular orbital (HOMO) is based on the positive than that of Ru(terpy)(phen)Cl by 780 mV. Although metal center and the oxidative processes are the metal center in the second oxidation at around 2.35 V is assigned as ligand- ruthenium polypyridyl complexes. centered oxidations, their detailed assignment is not clear. The Electrochemical data for complexes 1, 2, 3, 4, 5 and 6 are first oxidations of 7 and 8 were observed at 1.60 V, which can listed in Table 1. The first reduction wave of these Ir(III) be assigned as terpyridine-centered oxidations. A cyclic complexes occurred at around Ð0.96 V. These processes have voltammogram of 4 in acetonitrile solution shows three distinct been assigned as terpyridine ligand-centered reductions. In waves and two irreversible oxidation waves. At a negative contrast, the second process of these complexes exhibited potential, the first couples are quasi-reversible and possess different reduction potentials. Complex 1 indicates the second variant anodic-to-cathodic peak separations (80 mV Ð 126 mV) reduction wave at Ð1.31 V. On the other hand, 2, a complex at scan rates of between 25 mV/s and 100 mV/s. From a plot of with an electron-donating ligand, gives the largest negative the peak half width as a function of the pulse amplitude in the reduction potential (Ð1.38 V) and 3, a complex with an electron- differential pulse voltammogram of the complex, the number of withdrawing ligand, exhibits the most positive reduction electrons associated with the first and second reduction peaks is potential (Ð1.21 V). From the above observations, the second assumed 1. reduction waves are assigned as additive bipyridine-centered reductions. In complex 4, the second reduction occurred at Absorption properties Ð1.30 V, whereas the reductions of 5 and 6 occurred at Ð1.27 V The electronic absorption spectra for iridium(III) polypyridyl and Ð1.24 V, respectively. To consider the reductions in 8 complexes typically contains πÐπ* and nÐπ* bands in the which were observed only at Ð1.02 V and Ð1.72 V, the second ultraviolet region. The electronic absorption spectra for the reduction processes in 4, 5 and 6 can be assigned as phen/phenÐ, complexes [Ir(terpy)(L)Cl]2+ are tabulated in Table 2. Figure 2 phphen/phphenÐ and dpphen/dpphenÐ, having ligand-centered shows the electronic absorption spectra of complex 8 and the processes, respectively. In complex 7, the first reduction at terpyridine ligand. The absorption bands at 314 nm and 328 nm Ð0.62 V shifted to less negative potentials than that of 4. The in 8 have been assigned as a terpy(π) → terpy(π*) transitions.15 reduction can be assigned as a bppz/bppzÐ ligand-centered The absorption band at 267 nm and 283 nm in 8 has also been process. Using an electron-withdrawing ligand bppz, the first assigned as terpy(π) → terpy(π*) transition. Although around reduction potential in 7 was 340 mV which anodically shifted 350 nm Ð 500 nm most of these complexes did not show a peak when compared to the reduction potential of 4. The or a shoulder, however, 8 exhibited weak absorption bands at electrochemical data for complexes 7 and 8 are tabulated in 484 nm and 515 nm. These bands could be ascribed to formal Table 1. forbidden transitions, leading to a direct population of the triplet

In general, ligand effects appear in Ru(IV/III) and Ru(III/II) levels (S0 → T) because of the heavy-atom effect of iridium redox potentials. Ligand effects on the Ru(III/II) couple are metal.7 well known. Namely, Ru(III/II) potentials are influenced along The electronic absorption spectra for 4, 5 and 6 are shown in with the ligands. Ru(II) is stabilized by the contribution of Fig. 3. The absorption bands at 313 nm and 325 nm in 4 have dπÐπ*(L) back bonding in the presence of ligands. On the been assigned as terpy(π) → terpy(π*) transitions. The contrary, as evidenced by the data in Table 1, the first oxidation absorption band at 272 nm in 4 has been assigned as potentials of iridium(III) complexes remain constant when overlapping phen(π) → phen(π*) and terpy(π) → terpy(π*) changing the polypyridine ligands. The first oxidation of 4, 5 transitions.16 The spectra of these iridium dpphen-terpy and and 6 occurs at the same potentials (1.62 V), whereas the phphen-terpy systems are remarkably similar to that of the oxidation waves of terpyridine ligand are only observed at 1.16 iridium phen-terpy system, except that the absorption band at V and 2.0 V. The first oxidation waves at 1.6 V in these Ir(III) 272 nm shows a slight red-shift to 284 nm and 280 nm, complexes are assigned as the terpyridine, which becomes hard respectively, as a consequence of the electron-withdrawing to oxidize by 0.44 V due to enhanced π-backbonding to the ligand. terpyridine upon complex formation. The first oxidation 764 ANALYTICAL SCIENCES MAY 2003, VOL. 19

Table 2 Absorption properties of the complexes

Absorption (298 K)(ε , 103 MÐ1 cmÐ1) Complex S0 → T ππÐ *

1 [Ir(terpy)(bpy)Cl]2+ 271(25.6) 281(25.3) 303(19.1) 313(19.5) 2 [Ir(terpy)(dmbpy)Cl]2+ 270(31.7) 282(30.6) 302(23.9) 312(24.3) 3 [Ir(terpy)(dpbpy)Cl]2+ 270(31.5) 281(30.7) 302(23.2) 312(24.3) 4 [Ir(terpy)(phen)Cl]2+ 272(35.1) 313(18.5) 325(18.1) 5 [Ir(terpy)(phphen)Cl]2+ 280(32.0) 313(16.0) 325(15.0) 6 [Ir(terpy)(dpphen)Cl]2+ 284(44.8) 313(32.2) 325(29.9) 7 [Ir(terpy)(bppz)Cl]2+ 273(23.8) 283(24.8) 318(17.8) 8 [Ir(terpy)Cl3] 267(29.0) 283(29.1) 314(25.9) 328(26.7) 484(1.8) 515(1.6) 3+ 9 [Ir(terpy)2] 284(15.5) 313(13.3) 323(12.0) 10 [Ru(terpy)(phen)Cl]+ 265(54.7) 318(29.0) 11 terpy 235(17.1) 280(15.6) In acetonitrile solution, unless otherwise noted.

Fig. 4 Emission spectra of iridium terpyridine complexes in Fig. 3 Absorption spectra of 4 (solid line), 5 (dashed-solid line), acetonitrile at room temperature. a, complex 4; b, complex 5; 1.0 × × Ð5 Ð6 Ð3 and 6 (dashed line) in an acetonitrile solution (1.0 10 M) at room 10 mol dm ; λex = 318 nm. temperature.

nm corresponding to the excited state of 3LC. This result is Emission properties obtained regardless of whether excitation is performed at 318 One way to probe the lowest-lying excited state is undertaken nm or 270 nm. The emission spectrum can be assigned to through emission spectroscopy and excited-state lifetime phosphorescence from the terpy* excited state (Table 3).15 measurements. Compound 4 indicates intense phosphorescence The other complexes, [Ir(terpy)(L)Cl]2+ also exhibit an intense band maxima at λ = 484 nm and 517 nm in a solution at room luminescence from 484 nm to 578 nm by excitation at 318 nm temperature (excitation wavelength, 318 nm), whereas both 5 in an acetonitrile solution at room temperature. In particular, and 6 indicate shoulders at around 490 nm and peaks at 518 nm complex 2 with both an electron-donating ligand and an in a solution at room temperature (excitation wavelength, 318 electron-withdrawing ligand shows an intense emission band at nm)(Fig. 4). Regardless of the electron-withdrawing properties 489 nm and 518 nm (Fig. 5). This indicated that dmbpy and of dpphen and phphen, 5 and 6 did not display any red-shifted terpy ligands stabilize the HOMO and LUMO of the excited emission. The emission maxima and the excited-state lifetime state in complex 2, respectively. The emission lifetime of 2 was 2+ 18 of the [Ir(terpy)(L)Cl] systems are given in Table 3. The 2.17 µs in CH3CN. difference in the energy bands among the terpyridine complexes Ð1 2+ indicate 1240 cm , since [Ru(bpy)3] shows a prominent progression of the vibrational band, separately, by ca. 1300 Conclusions cmÐ1.6 The vibronic fine structure observed in the emission spectra is consistent with a significant ligand 3(πÐπ*) Seven types of useful iridium complexes with a terpyridine contribution to the phosphorescence.17 The shapes of these ligand, [Ir(terpy)(bpy)Cl]2+ (1), [Ir(terpy)(dmbpy)Cl]2+ (2), emission bands do not change despite a change of the excitation [Ir(terpy)(dpbpy)Cl]2+ (3), [Ir(terpy)(phen)Cl]2+ (4), wavelength, and that the spectra are vibrationally resolved and [Irterpy)(phphen)Cl]2+ (5), [Ir(terpy)(dpphen)Cl]2+ (6) and their temporal decay occurs with lifetime values on the [Ir(terpy)(bppz)Cl]2+ (7) were synthesized. In all of the microsecond time scale. These emissions are thus ascribed to complexes except [Ir(terpy)(bppz)Cl]2+, we found that LUMOs 3LC excited states of the terpyridine,7 and an efficient are based on the terpyridine ligand. On the other hand, the interelectron conversion of the phen* excited state to the terpy* LUMO state in the [Ir(terpy)(bppz)Cl]2+ complex is localized on excited state takes place. the bppz ligand, whereas the HOMO states in the iridium The emission spectrum of 8 shows one emission band at 578 complexes are localized on the terpyridine ligand. ANALYTICAL SCIENCES MAY 2003, VOL. 19 765

Table 3 Emission properties of the complexes

Emissiona Lifetime Φ Complex (298 K) (298 K) (298 K) λ max/nm τ/µs 1 [Ir(terpy)(bpy)Cl]2+ 484 515 0.44 0.32 2 [Ir(terpy)(dmbpy)Cl]2+ 489 518 2.17 0.64 3 [Ir(terpy)(dpbpy)Cl]2+ 487 519 0.60 0.74 4 [Ir(terpy)(phen)Cl]2+ 484 517 0.92 0.68 5 [Ir(terpy)(phphen)Cl]2+ 490 518 0.74 0.13 6 [Ir(terpy)(dpphen)Cl]2+ 520 0.65 0.069 7 [Ir(terpy)(bppz)Cl]2+ 547 Ð 0.061 8 [Ir(terpy)Cl3] 500 578 0.35 0.090 3+ 9 [Ir(terpy)2] 570 Ð 0.007 Fig. 5 Emission spectra of iridium terpyridine complexes in a. In acetonitrile solution, unless otherwise noted. The excitation acetonitrile at room temperature. a, complex 2; b, complex 1; 1.0 × Ð6 Ð3 wavelength is 318 nm. 10 mol dm ; λex = 318 nm.

[Ir(terpy)(L)Cl]2+ emits in a solution at room temperature. In 5. F. Neve, A. Crispini, S. Campagna, and S. Serroni, Inorg. particular, the use of terpy and dmbpy as ancillary ligands Chem., 1999, 38, 2250. extends the lifetime (2.17 µs) of the 3(πÐπ*) excited states of 6. T. Yukata, I. Mori, M. Kurihara, J. Mizutani, K. Kubo, S. Ir(III) terpyridine complexes. These studies indicate that the Furusho, K. Matsumura, N. Tanai, and H. Nishihara, Inorg. Ir(III) terpyridine units with electron donor (dmbpy) or electron Chem., 2001, 40, 4986. acceptor substituents (terpy, dpbpy, phphen, dpphen and bppz) 7. A. Dovletoglou, S. A. Adeyemi, and T. J. Meyer, Inorg. decrease the energy of the 3LC state as photosensitizer- Chem., 1996, 35, 4120. molecular components in supramolecular devices. 8. S. M. Zakeeruddin, M. K. Nazeeruddin, P. Pechy, F. P. Rotzinger, R. Humphry-Baker, K. Kalyanasundaram, and M. Gratzel, Inorg. Chem., 1997, 36, 5937. Acknowledgements 9. A. B. P. Lever, Inorg. Chem., 1990, 29, 1271. 10. P. Didier, I. Otmans, A. Kirsch-De Mesmaeker, and R. J. We thank Prof. Keiichi Tsukahara (Nara Women’s University) Watts, Inorg. Chem., 1993, 32, 5239. for helpful discussions and are also grateful to Dr. Hiroshi 11. S. S. Fielder, M. C. Osborne, A. B. P. Lever, and W. J. Takashima (Nara Women’s University) for the measurement of Pietro, J. Am. Chem. Soc., 1995, 117, 6990. the ESI mass spectra. 12. J. V. Casper and T. J. Meyer, Inorg. Chem., 1983, 22, 2444. 13. P. A. Anderson, L. F. Anderson, M. Furue, P. C. Junk, F. R. Keene, B. T. Patterson, and B. D. Yeomans, Inorg. References Chem., 2000, 39, 2721. 14. N. Yoshikawa, Y. Masuda, and T. Matsumura-Inoue, 1. L. M. Volgler and K. J. Brewer, Inorg. Chem., 1996, 35, Chem. Lett., 2000, 1206. 818. 15. J. P. Collin, I. M. Dixon, J. P. Sauvage, J. A. G. Williams, 2. J. A. Treadway, B. Loeb, R. Lopez, P. A. Anderson, F. R. F. Barigelletti, and L. Flamigni, J. Am. Chem. Soc., 1999, Keene, and T. J. Meyer, Inorg. Chem., 1996, 35, 2242. 121, 5009. 3. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, 16. N. P. Ayala, C. M. Flynn, L. Sacksteder, J. N. Demas, and R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, and M. E. B. A. Degraff, J. Am. Chem. Soc., 1990, 112, 3837. Thompson, Inorg. Chem., 2001, 40, 1704. 17. C. J. Timpson, C. A. Bignozzi, B. P. Sullivan, E. M. Kober, 4. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, and T. J. Meyer, J. Phys. Chem., 1996, 100, 2915. H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, and M. 18. M. T. Indelli, C. A. Bignozzi, and F. Scandola, Inorg. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304. Chem., 1998, 37, 6084.