Ultrafast Energy Transfer in Dinuclear Complexes with Bridging 1,10

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Ultrafast Energy Transfer in Dinuclear Complexes with Bridging 1,10Phenanthroline-5,6-Dithiolate Elisa Erdmann,† Matthias Lütgens,‡ Stefan Lochbrunner,*,‡ and Wolfram W. Seidel*,† †

Institut für Chemie and ‡Institut für Physik, Universität Rostock, Albert-Einstein-Straße 3a, 18059 Rostock, Germany S Supporting Information *

ABSTRACT: We report herein the preparation and characterization of dinuclear complexes with the bridging ligand 1,10phenanthroline-5,6-dithiolate (phendt2−) bearing Ru(bpy)2 or Ir(ppy)2 at the diimine moiety and Ni(dppe), Ni(dppf), CoCp, RhCp*, and Ru(p-Me-iPr-benzene) at the dithiolate unit. In comparison with the mononuclear precursors used in the synthesis, all dinuclear complexes were characterized by absorption and photoluminescence spectroscopy as well as cyclic voltammetry. Because of the beneficial spectral and electrochemical properties of the Ir/Co complex for a lightdriven charge separation, this complex was investigated in detail by time-resolved luminescence {nanosecond (ns)-resolution} and transient absorption spectroscopy {femtosecond (fs)resolution}. All measurements supported by DFT calculations show that the observed effective luminescence quenching by the dithiolate coordinated metal is caused by an ultrafast singlet−singlet Dexter energy transfer.

■

INTRODUCTION The conversion of solar energy into chemical energy by using visible light-driven processes is a central topic in current chemical research.1−3 In particular, photocatalytic water splitting has been a driving force either dealing with bioinspired systems4,5 or basing on a coordination chemical6−12 approach. The most prominent complex building blocks are Ru(II) and Ir(III) centers with poly(pyridine) ligands showing an enormous potential for electron and energy transfer13−19 and being useful also in organic photocatalysis.20−23 The benefits of using visible light in photocatalysis, such as improved reaction control and higher chemoselectivity, triggered extensive studies in the field of dual catalysis combining transition metal with photoredox catalysis for a smarter way to realize cross-coupling reactions.24 Herein, the excited photocatalyst can undergo a single electron transfer (SET) to a redox mediator (Scheme 1), e.g., a transition metal center, for stabilization (see reviews: refs 25−27). The excited state decay of a photosensitizer also always involves decomposition paths, which is increasingly serious with actually desired long photochemical lifetimes. A fast electron transfer to a redox mediator is one option to stabilize longliving charge separated states in solution. The covalent link between a photoredox center and a second transition metal ion is an important tool to achieve such stabilization by intramolecular charge separation without too much loss of the original optical excitation energy. In addition, such systems are useful models in understanding the extent of intermetallic cooperativity and hence the underlying conditions for either charge separation or energy transfer. Phenanthrolines with donor substituents in 5,6-position are useful ditopic © XXXX American Chemical Society

Scheme 1. Conceptional Mechanism of Cross Coupling Reactions Following Photoredox/Ni Dual Catalysis Showing SET between a Photocatalytic Cycle and Transition Metal Catalysis

ligands to bridge two active sites. Due to the planar and rigid structure, they are well-suited for a very fast electron transfer by electronic coupling between two active sites. Following this principle, extended phenanthroline ligands (tetrapyridophenazines or pyridinoxazol phenanthrolines, Chart 1) connecting a Ru(diimine)2 photosensitizer with Pd, Pt, Rh, or cobaloxime centers have shown photocatalytic hydrogen production.28−30 Experimental pump−probe X-ray absorption spectroscopy and theoretical TDDFT calculations of intramolecular electron transfer in Ru(II)/Co(III) complexes conjugated by tetrapyridophenazine show that an electron transfer from the photoexcited Ru diimine complex to the Co(III) unit is comparatively persistent, if a spin transition at the octahedral Co(II) site is involved.31−33 With regard to charge separation, Received: November 17, 2017

A

DOI: 10.1021/acs.inorgchem.7b02840 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Fluorolog system (Horioba Scientific) with the help of a cryogenic sample holder and liquid nitrogen as refrigerant. Photoluminescence quantum yields (φPL) were extracted from corrected spectra on a wavelength scale (nm). A solution of [Ru(bpy)3]Cl2 (tris(2,2′bipyridine)dichlororuthenium(II)-hexahydrate, (Sigman-Aldrich) in air-equilibrated water (spectrophotometric-grade, Alfa Aesar) was applied as standard (φPL = 0.028).50 The different refractive indices for luminescence standard and sample solvent were corrected.51 Sample and luminescence standard were excited at 388 nm with an absorbance of about 0.1 at the excitation wavelength for both sample and standard. Photoluminescence lifetimes (τPL) were determined with a DeltaPro Fluorescence Lifetime System from Horiba Scientific. The sample was optically excited at λexc = 370 nm by a DeltaDiode. In one case, the time-resolved luminescence was measured in addition by a streak camera system (C10627, Hamamatsu Photonics) which provides a time resolution of 60 ps. Transient absorption measurements were performed by means of a pump−probe setup based on a Ti:sapphire laser system providing pulses at 775 nm with a pulse duration of 150 fs. A fraction of the fundamental was frequency doubled by a 0.7 mm thick BBO crystal to obtain pulses at 388 nm, which were used as excitation pulses. The change in the absorption was probed with a white light continuum, generated by focusing a small fraction of the laser fundamental into a CaF2 crystal. The nanosecond dynamics were monitored applying a YAG laser system, which delivered 7 ns long excitation pulses at 390 nm. For probing the dynamics, the same white light continuum was used as that in the femtosecond (fs) measurements. While the time delay in the fs-transients was generated by varying the length of the optical pump path via a motorized delay stage, the delay was controlled in the nanosecond (ns)-transient absorption experiments electronically. Computational Details. All DFT calculations were carried out using the G09w program package.52 The molecular geometries of complexes 10+ and 15 were optimized in the gas phase applying the GGA functional BP8653,54 in combination with split valence triple ζbasis sets (def2-TZVP) of the Ahlrich group.55 Quasi-relativistic effective core potentials of the Stuttgart/Cologne group were used for Ir (ECP60) in combination with a (8s7p6d2f1g)/[6s5p3d2f1g] basis set.56,57 In doing so, a reasonable match between the calculated and the experimentally determined structure of 15 was achieved. A comparison of calculated and experimentally obtained metric parameters for 15 is given in Table S1. Frequency calculations were systematically performed to confirm that the optimized geometries correspond to true stationary points. The final energies and absorption spectra (TDDFT) were computed at higher level using the hybrid functional b3lyp. The frontier Kohn−Sham orbitals (isosurface value 0.05) are based on the high level calculations. The calculated absorption spectra of the geometry optimized excited states S1 and S4 are depicted in the Supporting Information. Materials and Synthesis. All operations were carried out in an atmosphere of dry argon using Schlenk and glovebox techniques. Solvents were dried and saturated with argon by standard methods and freshly distilled prior to use. Dry DMF was obtained by Sigma-Aldrich. [(bpy)2RuCl2]·2H2O,58 [(ppy)2IrCl2]2,59 [Ni(dppe)Cl2]60 (dppe = 1,2-bis(diphenylphosphino)ethane) N,N′-phendt-(C2H4CN)2,49 1,3dithiolo[4,5-f ][1,10]phenanthroline-2-one,47 [(C5H5)Co(CO)I2],61,62 [CpCo(bdt)]63 (C5H5 = Cp = cyclopentadienyl), [Cp*RhCl2]264 (Cp* = pentamethylcyclopentadienyl), [Ni(dppf)Cl2]65 (dppf = 1,1′bis(diphenylphosphino)ferrocene), [{RuCl2{p-Me-iPr-benzene)}2],66 [(ppy)2Ir(N,N′-phendt-(C2H4CN)2)](PF6) (1-PF6)49 and [(bpy)2Ru(N,N′-1,3-dithiolo[4,5-f ][1,10]phenanthrolin-2-one)](PF 6 ) 2 (6(PF6)2)47 were prepared according to literature methods. All other chemicals (at least of reaction-grade quality) were obtained from commercial sources and used as received. Analytical thin layer chromatography was performed on silica gel (Silica 60 F254) or on aluminum oxide (aluminum oxide 150 F254) coated aluminum plates. Column chromatography was performed using silica gel 60 (pore size 0.063−0.2 mm) or aluminum oxide 90 (neutral, pore size 0.063−0.2 mm) purchased from Merck as the column stationary phase. (Synthesis and characterization of 3-, 4-, and 5-PF6, see the Supporting Information)

Chart 1. Literature-Known Ligands Used in Dinuclear Complexes

directional ligands with distinctly different coordination sites seem generally promising.34−39 Early examples presented by Pierpont and Eisenberg using phenanthroline-5,6-diolate as bridging ligand between two metals proved close intermetallic coupling.40,41 Thiolate substituents instead are attractive because of their potential use as biomimetric hydrogenase models and their general dithiolene type properties like strong absorptivity in the visible range,42 and redox variability at moderate potentials.43 Thus, the use of Pt diimine dithiolate complexes for the photogeneration of dihydrogen was reported to increase efficiency.44 Almeida,45 Hudhomme46 and Shatruk47,48 reported several mononuclear Ru(II) complexes representing phenanthroline derivatives with sulfur substitution in the 5,6-position. In 2014 we described the first dinuclear complexes with phenanthroline-5,6-dithiolate (phendt2−).49 The Ru/Ni complex [(bpy)2Ru(phendt)Ni(dppe)]2+ exhibited the most efficient luminescence quenching, which was attributed to an energy transfer. In our ongoing studies, we sought metal complex combinations, which allow thermodynamically a photoelectron transfer in connection with an easily observable color change at the reduced metal center for transient absorption spectroscopy. In addition, for photophysical benchmarking and to get access to a wide range of polynuclear complexes, the isolation of diimine photocenter-coordinated phenanthroline-5,6-dithiol would be of great value. Herein we report the preparative access to pure Ru/Ir phenanthroline-5,6dithiolate complexes as coordinating agents for surfaces and for definite complexation reactions with various metal precursors. On the basis of this, we present new polynuclear complexes with a significant thermodynamic driving force for charge separation. The real excited state behavior of the metal dyad [(ppy)2Ir(phendt)Co(C5H5)]+ is uncovered by time-resolved fluorescence and transient absorption spectroscopy in relation to the model component complexes [(ppy)2Ir(phendt)(C2H4CN)2]+ and [(C5H5)Co(bdt)] (bdt = 1,2-benzenedithiolato).

■

EXPERIMENTAL SECTION

Absorption and Luminescence Measurements. Steady-state UV−vis absorption spectra were recorded with a PerkinElmer Lambda 19 or an Agilent 8453 spectrometer and steady state emission spectra with a Fluoromax-4 (Horiba Scientific) or Agilent Cary Eclipse fluorometer. As solvent CH3CN of Uvasol-quality (Merck) was used. The samples were placed in a 1 cm or 0.2 mm path fused silica cuvette, and dissolved oxygen was removed by bubbling with argon for about 2 min. Corrected emission spectra were obtained via a calibration curve supplied with the instrument. Cryogenic steady-state emission spectra in 2-methyltetrahydrofuran (Sigma-Aldrich) were recorded by a B


Article

Inorganic Chemistry

(CH3CN), m/z (M = [(ppy)2Ir(phendt)CoCp]+, relative intensity): 867 ([M+], 100). Synthesis of [(ppy)2Ir(phendt)RhCp*](PF6) (11-PF6). A mixture of 1-PF6 (95 mg, 0.1 mmol) and tert-BuOK (25 mg, 0.22 mmol) was dissolved in DMF (10 mL), and the resulting dark red solution was stirred for an hour at ambient temperature. The solvent and the byproducts tert-BuOH and acrylonitrile were removed in vacuo. The residue was dissolved in THF (5 mL) and solid [Cp*RhCl2]2 (31 mg, 0.05 mmol) was added, and the reaction mixture was stirred overnight. Alternatively, a mixture of 5-PF6 (100 mg, 0.10 mmol) and MeONa (13.5 mg, 0.25 mmol) in THF (10 mL) was stirred for 1 h, and solid [Cp*RhCl2]2 (31 mg, 0.05 mmol) was added. The residue was purified by column chromatography on aluminum oxide (CH2Cl2/ MeOH 20:1) to afford a red solid. Yield: 46% (52 mg, 0.046 mmol). Anal. Calcd for C44H37F6IrN4PRhS2: C, 46.93; H, 3.31; N, 4.98. Found: C, 46.76; H, 3.70; N, 4.26. 1H NMR (CD2Cl2, 600 MHz, 298 K) δ/ppm: 9.31 (dd, 3J = 8.4 Hz, 4J = 1.0 Hz, 2H, 4/7-phen-H), 8.18 (dd, 3J = 5.0 Hz, 4J = 1.3 Hz, 2H, 2/9-phen-H), 7.95 (d, 3J = 8.3 Hz, 2H, Ar-H), 7.77 (dd, 3J = 7.8 Hz, 4J = 1.0 Hz, 2H, Ar-H), 7.71 (m, 4H, Ar-H, 3/8-phen-H), 7.35 (m, 2H, Ar-H), 7.11 (td, 3J = 7.7 Hz, 3J = 7.7 Hz, 4J = 1.2 Hz, 2H, Ar-H), 6.98 (td, 3J = 7.4 Hz, 3J = 7.4 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.80 (ddd, 3J = 7.3 Hz, 3J = 6.0 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.39 (dd, 3J = 7.6 Hz, 4J = 0.7 Hz, 2H, Ar-H), 2.10 (s, 15H, HCp*). 13C NMR (CD2Cl2, 151.0 MHz, 298 K) δ/ppm: 168.3 (C-Ir), 150.6 (C-6), 150.5 (Cq), 148.9 (C-1), 148.5 (C-e),145.2 (C-5), 143.9 (Cq), 138.0 (C-g), 133.6 (C-3), 132.2 (C-4), 131.8 (C-a), 130.7 (C-b), 126.3 (C-2), 124.9 (C-d), 123.0 (C-f), 122.7 (C-c), 119.8 (C-h), 2.1 (CCp*). 31P NMR (CD2Cl2, 202.4 MHz, 300 K) δ/ppm: −144.5 (sept, PF6-). ESI+ (CH3CN), m/z (M = [(ppy)2Ir(phendt)RhCp*]+, relative intensity): 981 ([M+], 100). Synthesis of [(ppy)2Ir(phendt)Ru(p-cymene)](PF6) (12-PF6). A mixture of 1-PF6 (128 mg, 0.126 mmol) and tert-BuOK (31 mg, 0.278 mmol) was dissolved in DMF (10 mL), and the resulting dark red solution was stirred for an hour at ambient temperature. The solvent and the byproducts tert-BuOH and acrylonitrile were removed in vacuo. The residue was dissolved in THF (12 mL), and solid [{RuCl2{p-cymene)}2] (46 mg, 0.075 mmol) was added. The reaction mixture was stirred overnight. The resulting brown residue was purified by column chromatography on aluminum oxide (CH2Cl2/ MeOH 20:1). Anal. Calcd for C44H36F6IrN4PRuS2: C, 47.05; H, 3.23; N, 4.99. Found: C, 44.57; H, 3.27; N, 5.18. 1H NMR (CD2Cl2, 500 MHz), δ/ppm: 9.49 (dd, 3J = 8.5 Hz, 4J = 1.3 Hz, 2H, 4/7-phen-H), 8.20 (dd, 3J = 5.0 Hz, 4J = 1.3 Hz, 2H, 2/9-phen-H), 7.95 (d, 3J = 8.2 Hz, 2H, Ar-H), 7.77 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 2H, 3/8-phen-H), 7.73 (dd, 3J = 8.6 Hz, 3J = 4.9 Hz, 2H, Ar-H), 7.70 (d, 3J = 8.0 Hz, 2H, Ar-H), 7.34 (d, 3J = 5.8 Hz, 2H, Ar-H), 7.11 (td, 3J = 7.7 Hz, 3J = 7.7 Hz, 4J = 1.2 Hz, 2H, Ar-H), 6.98 (td, 3J = 7.5 Hz, 3J = 7.5 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.78 (td, 3J = 7.4 Hz, 3J = 7.4 Hz, 4J = 1.4 Hz, 2H, ArH), 6.41 (d, 3J = 7.6 Hz, 2H, Ar-H), 5.98 (s, 4H, Ar-Hcymene), 2.69 (sept, 3J = 6.9 Hz, 1H, CHcymene) 2.34 (s, 3H, CH3), 1.32 (d, 3J = 7.0 Hz, 6H, CH3-isopropyl). 13C NMR (CD2Cl2, 125.7 MHz), δ/ppm: 168.5 (C-Ir), 154.8, 150.7, 149.7, 149.1, 145.6, 144.4, 138.6, 134.1, 132.7, 132.4, 131.3, 127.1, 125.5, 123.6, 123.3, 120.4, 82.3 (CHArcymene), 80.2 (CH-Arcymene), 32.6 (CH), 23.7 (CH3-isopropyl), 21.1 (CH3). 31P NMR (CD2Cl2, 202.4 MHz), δ/ppm: −144.7 (sept, PF6-). ESI+ (MeOH/0.1% HCOOH in H2O 90:10), m/z (M = [(ppy)2Ir(phendt)Ru(C10H14)]+, relative intensity): 979 ([M+], 100). Synthesis of [(ppy)2Ir(phendt)Ni(dppf)](PF6) (13-PF6). A brown suspension of 4-PF6 (35 mg, 0.03 mmol) and [Ni(dppf)Cl2] (20 mg, 0.03 mmol) in CH2Cl2 (7 mL) was stirred overnight accompanied by a color change to orange. The residue was dried, and [Cl2Sn(CH3)2] was removed by sublimation. Yield: 25% (11 mg, 0.008 mmol). Alternatively, a mixture of 5-PF6 (100 mg, 0.10 mmol) and MeONa (13.5 mg, 0.25 mmol) in THF (10 mL) was stirred for 1 h, and solid [Ni(dppf)Cl2] (68 mg, 0.10 mmol) was added. Yield: 56% (83 mg, 0.06 mmol). The resulting orange residue was purified by column chromatography on aluminum oxide (CH2Cl2/MeOH 10:1) to afford a red solid. Recrystallization from CH2Cl2/diethyl ether led to microcrystals of the pure product 13-PF6 . Anal. Calcd for C68H50F6FeIrN4NiP3S2: C, 54.41; H, 3.36; N, 3.73. Found: C,

Synthesis of [(ppy)2Ir(phendt)](Na/K) (Na/K-7). THF (7 mL) was added to a mixture of 5-PF6 (50 mg, 0.054 mmol) and MeONa (7.5 mg, 0.14 mmol) or tert-BuOK (16 mg, 0.14 mmol) at ambient temperature and stirred for 1 h. After a few minutes, the mixture changed from orange over brown to red. All byproducts and solvents were removed in vacuo resulting in a red residue. 1H NMR (THF, 300 MHz), δ/ppm: 9.82 (d, 3J = 8.5 Hz, 2H, 4/7-phen-H), 8.06 (d, 3J = 7.9 Hz, 2H, 2/9-phen-H), 7.79 (d, 3J = 7.9 Hz, 2H, Ar-H), 7.74 (m, 4H, Ar-H), 7.43 (d, 3J = 6.1 Hz, 2H, Ar-H), 7.40 (dd, 3J = 8.6 Hz, 3J = 4.9 Hz, 2H, 3/8-phen-H), 6.97 (td, 3J = 7.4 Hz, 3J = 7.4 Hz, 4J = 1.1 Hz, 2H, Ar-H), 6.85 (m, 4H, Ar-H), 6.41 (dd, 3J = 7.5 Hz, 4J = 1.0 Hz, 2H, Ar-H). 13C NMR (THF, 75.5 MHz), δ/ppm: 169.4, 153.6, 149.6, 145.2, 138.7, 136.3, 132.8, 130.9, 125.5, 124.3, 123.8, 122.7, 120.4. Synthesis of [(bpy)2Ru(phendt)] (8). 6-(PF6)2 (35 mg, 0.036 mmol) and tert-BuOK (10 mg, 0.090 mmol) were dissolved in CH3CN (5 mL) at ambient temperature and stirred for 10 min. The color changed instantaneously from orange to dark red and solvent and byproducts were removed. Synthesis of [(ppy)2Ir(phendt)Ni(dppe)](PF6) (9-PF6).49 A brown suspension of 4-PF6 (35 mg, 0.03 mmol) and [Ni(dppe)Cl2] (18 mg, 0.03 mmol) in acetonitrile (10 mL) was refluxed for 12 h accompanied by a color change to orange. The residue was dried and [Cl2Sn(CH3)2] was removed by sublimation. Yield: 68% (27 mg, 0.02 mmol). Alternatively, a mixture of 5-PF6 (100 mg, 0.10 mmol) and MeONa (13.5 mg, 0.25 mmol) in THF (10 mL) was stirred for 1 h, and solid [Ni(dppe)Cl2] (53 mg, 0.10 mmol) was added. The resulting orange residue was purified by column chromatography on aluminum oxide (CH2Cl2/MeOH 20:1) to afford a red solid. Recrystallization from CH2Cl2/diethyl ether led to microcrystals of the pure product 9-PF6. Yield: 60% (85 mg, 0.06 mmol). Anal. Calcd for C60H46F6IrN4NiP3S2: C, 53.58; H, 3.45; N, 4.17. Found: C, 53.01; H, 2.93; N, 4.69. 1H NMR (CD3CN, 250 MHz), δ/ppm: 8.68 (dd, 3J = 8.5 Hz, 4J = 1.4 Hz, 2H, 4/7-phen-H), 7.95 (m, 4H, 2/9-phen-H, ArH), 7.90−7.39 (m, 23H, Ar-H), 7.35 (dd, 3J = 5.9 Hz, 4J = 0.9 Hz, 3H, Ar-H), 6.98 (m, 3H, Ar-H), 6.86 (m, 3H, Ar-H), 6.73 (ddd, 3J = 7.4 Hz, 3J = 5.9 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.31 (dd, 3J = 7.4 Hz, 4J = 0.9 Hz, 2H, Ar-H), 2.59 (m, 4H, CH2). 13C NMR (CD3CN, 62.9 MHz), δ/ppm: 168.5, 151.8, 150.3, 148.6, 146.9 (t, J = 10 Hz), 145.8, 145.2, 139.2, 135.1, 134.8 (t, J = 5 Hz), 134.5 (t, J = 5 Hz), 132.8, 132.7, 132.3, 131.1, 130.6, 130.2, 130.0, 127.0, 125.8, 124.3, 123.5, 120.6, 23.9 (CH2). 31P NMR (CD3CN, 121.5 MHz), δ/ppm: 60.0 (s, dppe-P), −144.7 (sept, PF6-). ESI+ (CH3CN), m/z (M = [(ppy)2Ir(phendt)Ni(dppe)]+, relative intensity): 1199 ([M+], 100). Synthesis of [(ppy)2Ir(phendt)Co(C5H5)](PF6) (10-PF6). A mixture of 1-PF6 (100 mg, 0.10 mmol) and tert-BuOK (25 mg, 0.22 mmol) was dissolved in DMF (10 mL), and the resulting dark red solution was stirred for an hour at ambient temperature. DMF and the byproducts tert-BuOH and acrylonitrile were removed in vacuo. The residue was dissolved again in THF (6 mL) and solid [(C5H5)Co(CO)I2] (40 mg, 0.10 mmol) was added. The reaction mixture was stirred overnight. Alternatively, a mixture of 5-PF6 (100 mg, 0.10 mmol) and MeONa (13.5 mg, 0.25 mmol) in THF (10 mL) was stirred for 1 h and solid [(C5H5)Co(CO)I2] (40 mg, 0.10 mmol) was added. The residue was purified by column chromatography on aluminum oxide (CH2Cl2/MeOH 20:1) to afford a green solid. Yield: 60% (85 mg, 0.06 mmol). Anal. Calcd for C39H27CoF6IrN4PS2: C, 46.29; H, 2.69; N, 5.54. Found: C, 49.52; H, 3.42; N, 7.02. 1H NMR (CD2Cl2, 500 MHz, 300 K) δ/ppm: 9.46 (dd, 3J = 8.4 Hz, 4J = 1.3 Hz, 2H, 4/7-phen-H), 8.27 (dd, 3J = 5.0 Hz, 4J = 1.4 Hz, 2H, 2/9-phen-H), 7.95 (d, 3J = 8.0 Hz, 2H, Ar-H), 7.77 (m, 4H, 3/8-phen-H, Ar-H), 7.71 (m, 2H, Ar-H), 7.32 (m, 2H, Ar-H), 7.11 (td, 3J = 7.6 Hz, 3J = 7.6 Hz, 4 J = 1.3 Hz, 2H, Ar-H), 6.98 (td, 3J = 7.4 Hz, 3J = 7.4 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.79 (ddd, 3J = 7.4 Hz, 3J = 6.0 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.39 (dd, 3J = 7.6 Hz, 4J = 0.8 Hz, 2H, Ar-H), 5.74 (s, 5H, HCp). 13C NMR (CD2Cl2, 125.8 MHz, 300 K) δ/ppm: 168.5 (C-Ir), 159.3 (C-6), 150.3 (C-1), 150.3 (Cq), 149.0 (C-e), 145.6 (C-5), 144.4 (C-5), 138.7 (C-g), 135.0 (C-3), 132.7 (C-4), 132.3 (C-b), 131.3 (C-a), 127.7 (C2), 125.5 (C-d), 123.7 (C-f), 123.4 (C-c), 120.4 (C-h), 81.9 (CCp). 31P NMR (CD2Cl2, 202.4 MHz, 300 K) δ/ppm: −144.5 (sept, PF6-). ESI+ C


Article

Inorganic Chemistry

Scheme 2. Variation of Protective Groups and Synthetic Strategy for the Preparation of Dinuclear phendt2− Complexes

54.50; H, 6.78; N, 5.24. 1H NMR (CD2Cl2, 300 MHz), δ/ppm: 8.41 (dd, 3J = 8.5 Hz, 4J = 1.3 Hz, 2H, 4/7-phen-H), 7.96 (dd, 3J = 5.0 Hz, 4 J = 1.5 Hz, 2H, 2/9-phen-H), 7.91 (m, 10H, Ar-H), 7.72 (m, 4H, ArH), 7.54 (m, 4H, Ar-H), 7.45 (m, 10H, Ar-H), 7.26 (d, 3J = 5.9 Hz, 2H, Ar-H), 7.05 (td, 3J = 7.3 Hz, 3J = 7.3 Hz, 4J = 1.2 Hz, 2H, Ar-H), 6.93 (td, 3J = 7.5 Hz, 3J = 7.5 Hz, 4J = 1.3 Hz, 2H, Ar-H), 6.77 (ddd, 3J = 7.2 Hz, 3J = 5.9 Hz, 4J = 1.4 Hz, 2H, Ar-H), 6.37 (dd, 3J = 7.5 Hz, 4J = 0.9 Hz, 2H, Ar-H), 4.48 (s, 4H, dppf-H), 4.32 (s, 4H, dppf-H). 13C NMR (CD2Cl2, 75.5 MHz), δ/ppm: 168.4, 151.1, 149.0, 148.0, 145.3, 144.4, 138.5, 135.4 (t, J = 5.5 Hz), 135.1, 132.3, 131.8 (br), 131.5, 131.1, 128.6 (br), 126.3, 125.3, 123.5, 123.1, 120.2, 76.4 (t, J = 5 Hz, dppf-C), 74.6 (t, J = 3 Hz, dppf-C). 31P NMR (CD2Cl2, 121.5 MHz), δ/ppm: 26.7 (s, dppf-P), −144.5 (sept, PF6-). ESI+ (MeOH/0.1% HCOOH in H2O 90:10), m/z (M = [(ppy)2Ir(phendt)Ni(dppf)]+, relative intensity): 1355 ([M+], 100). Synthesis of [(bpy)2Ru(phendt)Ni(dppf)](Cl)2 (14-Cl2). A mixture of 6-(PF6)2 (120 mg, 0.11 mmol) and MeONa (15 mg, 0.28 mmol) in THF (10 mL) was stirred for 1 h, and solid [Ni(dppf)Cl2] (68 mg, 0.10 mmol) was added. The resulting red residue was purified by column chromatography on aluminum oxide (CH2Cl2/MeOH 10:1) to afford a red solid. Recrystallization from CH2Cl2/diethyl ether led to microcrystals of the pure product 14-Cl2. Yield: 56% (83 mg, 0.06 mmol). Anal. Calcd for C66H50F12FeN6NiP4RuS2: C, 50.85; H, 2.23; N, 5.39. Found: C, 50.00; H, 4.26; N, 5.65. 1H NMR (CD2Cl2, 300 MHz), δ/ppm: 9.27 (t, 3J = 8.0 Hz, 4H, Ar-H), 8.32 (dd, 3J = 8.4 Hz, 4 J = 1.0 Hz, 2H, 4/7-phen-H), 8.13 (t, 3J = 7.6 Hz, 2H, Ar-H), 8.04 (t, 3 J = 7.8 Hz, 2H, Ar-H), 7.90 (m, 8H, 2/9-phen-H, Ar-H), 7.73 (d, 3J = 5.4 Hz, 2H, Ar-H), 7.66 (dd, 3J = 5.0 Hz, 4J = 1.0 Hz, 2H, 3/8-phenH), 7.54 (m, 4H, Ar-H), 7.43 (m, 14H, Ar-H), 7.18 (t, 3J = 6.5 Hz, 2H, Ar-H), 4.46 (s, 4H, dppf-H), 4.31 (s, 4H, dppf-H). 13C NMR (CD2Cl2, 75.5 MHz), δ/ppm: 158.0, 157.9, 151.5, 151.0, 148.5, 146.1, 138.8, 138.6, 135.4 (t, J = 5.5 Hz), 132.7, 131.8, 131.2, 128.7 (t, J = 5 Hz), 128.2, 127.9, 126.6, 126.5, 125.9, 76.4 (t, J = 5 Hz, dppf-C), 74.5 (t, J = 3 Hz, dppf-C). 31P NMR (CD2Cl2, 121.5 MHz), δ/ppm: 26.6 (s, dppfP), −144.5 (sept, PF6-). ESI+ (MeOH/0.1% HCOOH in H2O 90:10),

m/z (M = [(bpy)2Ru(phendt)Ni(dppf)]2+, relative intensity): 1413 ([M2+ + PF6−], 5), 634 ([M2+], 100).

■

RESULTS Synthesis. In 2014,49 we presented a useful sequential route to dinuclear complexes bridged by 1,10-phenanthroline-5,6dithiolate (phendt2−). However, the removal of the cyanoethyl protective groups by tert-BuOK required DMF as a solvent (Scheme 2), which turned out to be incompatible with many subsequent complex syntheses. The syntheses of polynuclear complexes by changing the solvent to THF or acetonitrile were successful for particular complexes like Ir/Co complex 10-PF6 and the three-nuclear M/Ni/Fe (M = Ru/Ir) congeners 13-PF6 and 14-Cl2, which are suitable for purification via chromatography. In general, attempts to assemble homoleptic threenuclear (bisdithiolene-like) complexes on this basis were cumbersome, which prompted us to seek alternative SHprotective group strategies. In particular, we aimed for the isolation of analytically pure Ru or Ir complexes with terminal dithiol or dithiolate substitution like Na-7. Titanocenedithiolate complexes are known to undergo transmetalation with metal halides or cleavage to form [Cp2TiCl2] and the free dithiol.67,68 Hence, dinuclear Ir/titanocene complex 3-PF6 was prepared from 1-PF6 {[(ppy)2Ir(N,N′-phendt-(C2H4CN)2)]PF6} according to Scheme 2 and purified by chromatography. Subsequent treatment of 3-PF6 in dichloromethane with ethereal HCl resulted in an immediate color change from green to red and precipitation of a dark-yellow solid. Unfortunately, an unequivocal characterization of presumed H2-[7-PF6] was hampered by the complete insolubility of this solid. In addition, the efficiency of the chromatographical D


Article

Inorganic Chemistry Chart 2. Potential Valence Isoelectronic Complexes for Electron Transfer: 10+, 11+, and 12+

Table 1. Comparison of 1H NMR Signals of phen Protons in S-Protected Compounds (1+ and 5+), Deprotected Compound 7−, and Metal-Coordinated Ir Complex (9+, 10+, 11+, 12+, and 13+) δ (ppm)

a

phen position

1+a

5+

7−

9+

10+

11+

12+

13+

2/9 3/8 4/7

8.36 7.91 9.36

8.34 7.88 8.46

8.06 7.40 9.82

7.95 n.d.b 8.68

8.27 7.77 9.46

8.18 7.71 9.31

8.20 7.77 9.49

7.96 n.d. 8.41

See ref 49. bn.d.: The signal could not be assigned unequivocally because of overlap with the dppe phenyl signals.

Figure 1. UV−vis absorption spectra of mononuclear complexes. Left: Ir complexes 5-PF6 (black), K-7 (red), and K-7 after addition of 18-crown-6 (blue) in THF; right: Ru complexes 6-(PF6)2 (black), 8 (red), and 8 after addition of 18-crown-6 (blue) in CH3CN.

aqueous KPF6 in the formation of desired complexes 5-PF6 and 6-(PF6)2,47 respectively. The addition of NaOMe to a THF solution of 5-PF6 caused a color change from yellow to dark red indicating the existence of free thiolate functions. The evaporation of all volatiles yielded a pure mixture of Na-7 and NaPF6. Furthermore, we present also the buildup of new di- and also trinuclear complexes by adding [Ni(dppe)Cl2], [(C5H5)Co(CO)I2], [Cp*RhCl2]2, [{RuCl2{p-Me-iPr-benzene)}2], and [Ni(dppf)Cl2] to in situ generated complexes 7− and 8. The crude products of the polynuclear compounds are subjected to chromatographical purification leading to pure complexes 10+, 11+, 12+, 13+, and 142+ (Chart 2 for 10+, 11+, and 12+). ESI mass spectrometry revealed the correct masses of the complex cations (Supporting Information). The formation of Na-7 by solvolysis of the dithiocarbonate group in 5-PF6 led to distinct changes in the 1H NMR spectra. A large low-field shift of the phen protons at 4-position (8.46 ppm in 5+ to 9.82 ppm in 7−) and a smaller high-field shift for the phen protons at 2- and 3-position (0.28 and 0.48 ppm, respectively) are indicative for the formation of Na-7. In contrast, the 1H NMR resonances of the phendiyl-pyridine at Ir are not influenced by the deprotection. An additional single 1H NMR signal for either the Cp or the Cp* ligand prove the identities of 10-PF6 and 11-PF6, respectively, as dinuclear complexes, while an unequivocal assignment of aromatic signals

purification of 3-PF6 was insufficient, which is attributed to the charged nature of the complex. Another related strategy developed by Donahue69 makes use of the SnMe2 protective group. The synthesis of 4-PF6 was straightforward by changing the solvent from DMF to THF after deprotection with tert-BuOK and adding [Cl2Sn(CH3)2]. The identity of 4-PF6 was proven by HRMS spectrometry ([M+]: 891.0384 m/z; for spectra, see the Supporting Information) and 1H NMR, which showed an additional signal at 1.09 ppm representing the methyl groups. Complex 4-PF6 could successfully be applied in transmetalation reactions in dichloromethane allowing the isolation of 9-PF6, 13-PF6, and 14-Cl2. However, the moderate yields (1-PF6−4-PF6: 56%, 4PF6−9-PF6: 68%, overall: 38%) and the necessity to reapply chromatography for the di- and trinuclear complexes led us to test a third option. Dithiocarbonate is another well-studied alternative as dithiol protective group, which can be removed under basic conditions.70,71 The phenanthroline derivative 1,3-dithiolo[4,5-f ][1,10]phenanthrolin-2-one47,72 was obtained by a literature protocol via nucleophilic substitution of 5,6dibromo-1,10-phenanthroline with K2S/CS2 in DMF and subsequent desulfurization with mercury(II) acetate.73−76 Reactions of 1,3-dithiolo[4,5-f ][1,10]phenanthroline-2-one with [(ppy)2IrCl]2 in a dichloromethane/ethanol mixture or [(bpy)2RuCl2] in refluxing ethanol resulted after addition of E


Article

Inorganic Chemistry

Figure 2. Left: UV−vis absorption spectra of mono- and dinuclear Ir complexes 1-PF6 (black), 5-PF6 (olive, dashed), and valence isoelectronic dinuclear complexes 10-PF6 (blue), 11-PF6 (magenta, dashed), and 15 (red); right: absorption spectra of 2-(PF6)2 (black), 6-(PF6)2 (red), and 14Cl2 (blue); all complexes are dissolved in CH3CN.

Figure 3. Left: Normalized emission spectra {I(λmax) = 1} for 10-PF6 (black), 11-PF6 (red), and 12-PF6 (blue); right: emission spectra of 5-PF6 (black), 6-(PF6)2 (blue), 13-PF6 (red), and 14-Cl2 (magenta); measurement at 300 K in degassed CH3CN (lines) and at 77 K in 2methyltetrahydrofuran (dotted lines) (λexc = 388 nm).

caused a color change from red to green, which is reflected in the spectra by the appearance of specific features (maximum at 600 nm for 7− and 560 nm for 8). This observation can be attributed to a separation of the K+ cation from the dithiolate moiety, which in turn shed light on the nature of formal neutral 8 as [K-8]+ or even [K2-8]2+ in solution. Unfortunately, we were not able to solve this problem unequivocally (Figure 1). In comparison to the mononuclear Ir complexes the second coordinated metal has a significant influence on the spectral properties of complexes 10+ and 11+ (Figure 2) as well as 12+ and 13+ (Figure S31). The UV−vis spectrum of 5-PF6 is typical for [(ppy)2Ir(diimine)]+ complexes exhibiting a featureless increase of absorptivity with rising energy up to the ligand centered (1LC) π,π* transitions between 260 and 275 nm. Three shoulders at 300, 340, and 390 nm are assigned to spinallowed 1MLCT transitions with varying amount of π,π* mixing and a weak absorption between 430 and 520 nm is attributed to spin-forbidden 3MLCT transitions.78,79 Most apparent, the spectrum of dinuclear Ir/Co complex 10+ exhibits a maximum in the visible range at 590 nm, which is typical for such CpCo dithiolate complexes.80,81 For comparison, the UV−vis spectrum of 15 is also shown in Figure 2. The absorption at 570 nm is assigned to dithiolate-S2 to CoCp transitions (vide infra).80,82−85 In addition, in the range of 300−400 nm an

is based on 2D NMR methods. For better comparison, the chemical shifts of the phen protons are collected in Table 1. The close resemblance of the compounds 10-, 11-, and 12PF6 is also disclosed by the NMR shifts of the related phen protons showing nearly identical shift effects. Crystallization was exclusively successful for the trinuclear Ru/Ni/Fe complex 14-Cl2, which was subjected to X-ray diffraction analysis. However, the best data set obtained after several attempts did not allow a full structural characterization (crystal quality, sine(Θmax)/λ = 0.41), but the result can at least serve as a proof of identity (Figure S22). As model complex for the photophysical experiments [(C5H5)Co(bdt)] (15) was synthesized by mixing benzol-1,2-dithiol, triethylamine, and [(C5H5)Co(CO)I2] in tetrahydrofuran.63,77 UV−Vis Absorption Spectroscopy. Generally, comparison of the UV−vis absorption spectra of the S-unprotected Ir or Ru phendt2− complexes with the protected ones discloses a significant increase of absorptivity particularly in the visible range of the spectra (Figure 1). Obviously, these additional visible absorptions are due to thiolate to phen-π* transitions which are of lower energy because of the raised S lone pair energy. The competitive transitions 1MLCT and 1SLCT into the phen-π* states are illustrated by the buried standard 1 MLCT in Ru complex 8. Consistently, addition of 18-crown-6 F


Article

Inorganic Chemistry

Table 2. Photoluminescence Maxima λmax, Quantum Yields φPL, Lifetimes τPL, and Quenching Rate kQU = 1/τPL(air-equ) − 1/ τPL(deg) for Mononuclear Complexes 1+, 5+, and 62+ and Dinuclear Complexes 9+, 10+, 11+, 12+, 13+ ,and 142+a λmax (300 K) 1-PF649 5-PF6 6-(PF6)2 9-PF649 10-PF6 11-PF6 12-PF6 13-PF6 14-Cl2 a

625 645 640 620 605 600 605 610 615

nm nm nm nm nm nm nm nm nm

λmax (77 K)

540 535 545 550 540

nm nm nm nm nm

φPL(deg) 0.091 0.0392 0.12 0.0064 0.0022 0.0006 0.0009 0.0015 0.0002

(±0.014) (±0.005) (±0.015) (±0.0007) (±0.0005) (±0.0001) (±0.0002) (±0.0003) (±0.00005)

φPL(air-equ) 0.015 0.008 0.011 0.0009 0.0002 0.0001 0.0003 0.0002 0.0001

(±0.002) (±0.001) (±0.001) (±0.0002) (±0.00007) (±0.00004) (±0.00008) (±0.00008) (±0.00005)

τPL(deg) 490 200 1.73 440 536 679 653 740 1.17

ns ns μs ns ns ns ns ns μs

τPL(air-equ)

(±50 ns) (±1 ns) (±0.02 μs) (±40 ns) (±9 ns) (±8 ns) (±5 ns) (±9 ns) (±0.03 μs)

80 62 213 65 73 77 84 83 155

ns ns ns ns ns ns ns ns ns

(±10 ns) (±0.3 ns) (±1 ns) (±10 ns) (±2 ns) (±2 ns) (±1 ns) (±2 ns) (±6 ns)

kQU 1.0 1.1 4.4 1.3 1.2 1.2 1.0 1.1 5.6

× × × × × × × × ×

107 s−1 107 s−1 106 s−1 107 s−1 107 s−1 107s−1 107s−1 107s−1 106s−1

λexc = 388 nm for emission and λexc = 370 nm for lifetime measurements.

Table 3. Redox Potentials (V vs Fc/Fc+) Measured in CH3CNa E1/2 (ground state) 1-PF6 2-(PF6)2 5-PF6 9-PF6 10-PF6 11-PF6 12-PF6 13-PF6 14-Cl2

E* (excited state)

reduction

oxidation

(−1.52) (−1.49) (−1.64) −1.72 −0.80, −2.01 −1.48, −2.08 −1.59, −2.06 −1.15 −1.47, −1.83

+0.87 +0.99 +0.83 (+0.45) (+0.71, +0.92) (+0.45, +0.94) (+0.93) (+0.87, +1.32) (+0.45, +0.90, +1.01)

reduction

b

oxidationc −1.46 −1.27 −1.45d

+0.61 +0.32 +0.25 +0.27 +1.18 +0.79

Values in brackets indicate irreversible signals and represent peak potentials at 100 mV·s−1 scan rate. bE*(IrIII*/L‑) = E1/2(IrIII/L‑) + E00. cE*(IrIV/III*) = E1/2(IrIV/III) − E00. dE00 determined from the low temperature spectra or by interception method.

a

complex 142+ is more effective than the strongest quenching in Ir complex 11-PF6 by a factor of about 10. The quantum yield φPL of 0.12 for 6-(PF6)2 is reduced to 0.0002 for 14-Cl2 due to the coordination of the [Ni(dppf)] moiety. However, the so far determined quenching effect in Ir(ppy)2 complexes by [Ni(dppe)] (9-PF6) resulting in φPL of 0.006449 could be increased by a further factor of 10 to 0.0006 in degassed acetonitrile by coordinating the [Cp*Rh] moiety. Hence, only 0.9% of the original luminescence is observed in the dinuclear Ir/Rh complex. The evident quenching effect of the emission by the coordination of a second metal moiety to the dithiolate group can be either attributed to a desired electron transfer or an efficient energy transfer. As the sole exception, mononuclear [CpCo(bdt)] complex 15 did not show any photoluminescence. The photoluminescence lifetimes follow in the case of the mononuclear complexes roughly the quantum yields (see Table 2 and the Supporting Information). In the air-equilibrated solutions, pronounced lifetime shortening compared to that of the degassed solutions is observed indicating that the luminescence results from a triplet state which is sensitive to quenching by oxygen. The emission lifetime of mononuclear Ir complex 1-PF6 amounts to 490 ns in degassed acetonitrile,49 which is comparable to those of other Ir(ppy)2 complexes.86,87 Interestingly, the emission lifetime does not change significantly by coordination of a second metal center (τPL ≈ 536 ns in CH3CN for 10-PF6). This result was also confirmed by measurements applying a streak camera performed in addition to the experiments with the DeltaPro system. The low intensity emission observed for 10-PF6 could not be attributed to dragged along impurities, because the emission wavelength of 10-PF6 differs significantly from the related values of the

increase of the absorptivity of about 40% is observed for 10PF6, which leads to a real absorption maximum at 390 nm. In analogy, Ir/Rh complex 11-PF6 exhibits two broad bands in the visible region at 350 nm and at 515 nm (Figure 2). The latter is related to the singlet ligand-to-metal charge transfer (1LMCT) transition and is slightly red-shifted compared to that of the mononuclear [Cp*Rh(bdt)] complex.83,84 Ir/Ru complex 12-PF6 is rather featureless compared to 10+ and 11+ and exhibits shoulders at 360 and 450 nm followed by a smooth tailing until 650 nm (Figure S31, right). The addition of a Ni(dppf) moiety results in the first trinuclear complex 14Cl2 with bridging phendt2−. The absorption bands of trinuclear 142+ are analogous to those of mononuclear Ru complexes 2(PF6)2 and 6-(PF6)2. The entire broad absorption of the MLCT between 300 and 400 nm is increased. In summary, the spectra of the di- and trinuclear complexes can generally be described as additions of the component spectra. However, some electronic coupling of the metals in the complex moieties is evident by changes in the absorptivity and small changes of the transition energy. The latter can be explained by gradual changes in the phendt-π* frontier orbital energies and is not essentially based on direct MM coupling. Luminescence Behavior. Steady-state emission spectroscopy in solution at 300 K disclosed for all complexes either with Ir(ppy)2 or Ru(bpy)2 moieties a red photoluminescence (PL) around 615 nm in acetonitrile (Figure 3). At 77 K in frozen 2methyltetrahydrofuran, a typical blueshift and a splitting of the band in transition sublevels were observed. Compared to mononuclear complexes like 1+ and 5+ as well as 22+and 62+ showing comparable high quantum yields up to 0.13,49 the polynuclear complexes exhibit substantial luminescence quenching (Table 2). Remarkably, the PL quenching in Ru G


Article

Inorganic Chemistry

Figure 4. Cyclic voltammetry. Left: reductive side of 10-PF6 (black), 11-PF6 (red), and 12-PF6 (blue); right: 5-PF6 (black), 13-PF6 (blue), and 14Cl2 (red). All were measured in CH3CN.

Figure 5. Transformation of 10-PF6 into [10] (left) and 11-PF6 into [11] (right) under spectroelectochemical conditions in a CH3CN/0.15 M nBu4NPF6 system. The spectra were recorded using an optically transparent thin-layer electrode under bulk electrolysis conditions in spectroelectrochemical cell.

starting materials, 5-PF6, whereas free dithiolate ligand 7− does not show any luminescence at all. Apparently, the residual emission of the dinuclear complexes is still caused by an Irbased 3MLCT luminescence, while the PL quenching is associated with much faster deactivation channels like electron or energy transfer starting from energy levels above the relaxed 3 MLCT state. Electrochemistry. Electrochemical characterization by cyclic voltammetry (CV) was performed in order to determine the thermodynamic driving force for an intramolecular electron transfer in the polynuclear complexes. The oxidation and reduction potentials are given in Table 3. In general, four different redox processes have to be considered in these systems (Figure 4). First, the Ir(III)/Ir(IV) redox couple is observed at about +1 V (all potentials given vs Fc/Fc+), and the processes are reversible or at least quasi-reversible on the CV time scale. Second, above and beyond −2 V, reductions of the diimine ligand π systems are evident, which are reversible exclusively in the dinuclear complexes. The E1/2 values determined for the reversible cases can be used for the calculation of the excited state potentials (Table 3). In addition, dinuclear complexes 10-PF6, 11-PF6, 13-PF6, and 14-Cl2 exhibit an always irreversible oxidation feature in the oxidative range of about +0.3 to +0.9 V, which is attributed to the oxidation of the dithiolate moiety due to the observability of these signals only

in the polynuclear species, as described in former report.49 These additional oxidation processes render the following original oxidation of either Ru or Ir irreversible as well. Finally, of crucial importance for the aimed electron transfer are redox couples of the dithiolate coordinated metal. In trinuclear complexes 13-PF6 and 14-Cl2, the oxidation of the Nidithiolene moiety and of the dppf ligand falls in the same range. In dinuclear complexes 10-PF6 and 11-PF6, the M(II)/ M(III) redox couple is detected at −0.80 V for M = Co and at −1.48 V for M = Rh reflecting the electron-donating methyl substitution in Cp*. The excited state potentials of 1-PF6 and 3-PF6 calculated from ground state E1/2 values, and the PL energies are almost equal and amount to −1.46 V. Consequently, quenching of the photoexcited state by intramolecular electron transfer is exergonic for the Ir(ppy)2/CpCo combination in 10-PF6. On the contrary, for Ni(dithiolene)complexes 9-PF649 and 13-PF6, the reducing power of the photoexcited Ir(IV)-phen− is not strong enough to reach the redox potentials of the related [Ni(dppe)(bdt)]0/−1 complexes (E ≈ − 2.1 V (dppe)88 and −1.8 V (dppf))89 and render an electron transfer to the Ni center impossible. For these systems, the PL quenching is attributed to a reductive process with regard to the Ru/Ir photocenter. In summary, out of the investigated compounds, 10-PF6 is the most promising candidate for a photo induced electron transfer between two H


Article

Inorganic Chemistry

Figure 6. Transient absorption spectra and time traces of 1-PF6 (left) and 10-PF6 (right) with magic angle of the probe polarization with respect to the excitation at 388 nm. The first row shows time dependent spectra; the second row kinetics at different probe wavelengths together with multiexponential fits. The last row exhibits the decay associated spectra of the fits.

a narrow TA band at 420 nm and a broad band in the visible range. Small changes of the TA with time can be described by two exponential components with time constants of ∼0.7 and 45−100 ps. They are usually assigned to vibrational relaxation and interligand transfer processes (ppy to phen).92,93 By far, the strongest contribution in the TA spectrum is a long living feature whose lifetime cannot be derived from the ultrafast absorption data. However, additional ns-measurements allowed the determination of a lifetime of 560 ns (Figure S34). The decay-associated spectrum obtained from the ns-TA fits nicely to the long living transient of the fs-measurements. The lifetime is very similar to the decay time determined by the photoluminescence measurements of 1-PF6. This indicates that the long-living absorption component is due to the relaxed 3 MLCT state of the Ir complex. The ns-transient absorption measurements on dinuclear 10PF6 reveal a very weak, however, long-living signal with a lifetime above 300 ns and a shape similar to the ns-TA of 1-PF6. This is in line with the low photoluminescence quantum yield of 10-PF6 of 0.2%, and it can be concluded that in 10-PF6 contrary to 1-PF6 only a small fraction of less than 2% of the optically excited molecules reaches the relaxed 3MLCT state of the Ir moiety. This fraction is responsible for the ns-TA and photoluminescence (Figure S35) which are both very weak but otherwise similar to those of 1-PF6. The fs-TA spectra of 10-PF6 excited at 388 nm (Figure 6, right) differ substantially from those of 1-PF6. A very fast decrease of the Co-based absorption at 590 nm within the time resolution of 0.3 ps is evident from the spectra. On the contrary, typical features of the Ir-based 3MLCT state are not detected, which could indicate an electron transfer to the CpCo moiety shortly after excitation. An analysis of the spectra disclosed three different photophysical processes contributing with time constants of 0.6, 5, and 20 ps (see decay-associated spectra, Figure 6). To our surprise, selective excitation of the

metal centers. Therefore, detailed measurements using timeresolved spectroscopy were performed with 10-PF6 and its model components 1-PF6 and 15. Spectroelectrochemistry. Spectroelectrochemistry with 10-PF6 and 11-PF6 was performed to identify the spectroscopic characteristics of the desired charge separated state in transient absorption measurements. 10-PF6 and 11-PF6 were chosen due to the matching electrochemical potential of −0.80 and −1.48 V and the intense absorption band between 500 and 600 nm caused by a LMCT transition in the Co or Rh complex moiety, respectively. In Figure 5, the absorption changes under stoichiometric reduction are depicted. Both spectra exhibit a substantial decrease of the absorption bands at 590 and 520 nm, respectively, while isosbestic points indicate a definitive stoichiometric 1e reduction. Ultrafast Transient Absorption Spectroscopy. Due to the matching redox potentials of dinuclear Ir/Co complex 10PF6, which allow for an oxidation of the photoexcited Ir center, and the clear spectroscopic signatures of the reduced state at the Co moiety, fs- and ns-transient absorption (TA) spectroscopy on this complex were performed. Its transient absorption is compared to those of mononuclear Ir complex 1-PF6 and the Co complex model 15. Irradiation at 388 nm promotes 1-PF6 into the typical 1MLCT state, followed by ultrafast intersystem crossing into the triplet manifold due to strong spin orbit coupling.90,91 This initial dynamic is beyond the time resolution of the experiment and is not discussed here in more detail. Figure 6 (left) depicts fs-TA spectra of 1-PF6 with the polarization of pump and probe pulses set to magic angle. Measurements with parallel and perpendicular polarization shown in Figures S36 and S37 depict no significant differences. It indicates that the transition dipoles of the dominant TA bands are tilted with respect to the excitation dipole which is often encountered in metal complexes.92 The transient spectra reproduce comparable features of related Ir complexes showing I


Article

Inorganic Chemistry

Figure 7. Left: Comparison of decay associated spectra for excitation at 388 nm (a) and 590 nm (b) of 10-PF6. Right: Calculated absorption spectra of singlet ground state (blue) and singlet excited state (green) (c) as well as decay-associated spectra of 15 for excitation at 575 nm (d).

Figure 8. Representative frontier Kohn−Sham orbitals of Ir/Co complex 10+; orbital energies [eV] in brackets. For an orbital scheme and a comparison with complex 15, see the Supporting Information.

of both complex constituents are energetically interlocked, in which the HOMO is well-separated from the lower orbitals of comparable energy. While the HOMO and HOMO−1 in 10+ are Ir-based t2g orbitals mixed with phenyl substituent contributions, the LUMOs of both complexes, 10+ and 15, are related Co(d)S2(p) π* orbitals. Hence, the phen π* acceptor orbitals of the Ir-based MLCT (LUMO+1/+2) are well above the LUMO of the Co moiety. TDDFT calculations for [CpCo(bdt)] yielded a strong absorption at 497 nm {1A(4)}, which is mainly assigned to a HOMO−1 to LUMO transition (Table 4). The calculated lower lying singlet transitions show dd character with some ligand to metal mixing associated with very small oscillator strengths. The experimental absorption spectrum (λmax = 570 nm) is thereby reproduced with reasonable agreement (about 0.3 eV difference). For dinuclear complex 10+, the usual Ir(t2g) to phen π* transitions were calculated between 426 and 362 nm. Comparison with Me1+ (Table 4) reveal similar transitions in this range, while the distinctively higher intensity calculated for the 426 nm transition of 10+ reflects the experimental observation. The intense Co complex moiety based transition calculated at 505 nm (the red shift compared to 15 is

CpCoS2 moiety at 590 nm disclosed a very similar behavior with related time constants and the shortest component being absent (Figure 7, b). These observations prompted us to repeat related fs-TA measurements with model component complex 15. We found not only a similar behavior with a fast-appearing bleaching effect here at 565 nm but also highly similar time constants (Figure 7d). The derived time constants for the CpCoS2 moiety in complex 15 (modeling also 10-PF6) are assigned to the population of a low-lying electronically excited state (4 ps) and the final relaxation back to the ground state (19 ps). Electronic Structure. To elucidate the extent of intermetallic coupling and the type of transitions at the Co moiety, DFT calculations with 10+, Me1+ (a truncated model complex of 1+ bearing Me sulfide groups), and 15 have been performed. For any interpretation of the results for 10+ the exclusive use of single determinant methods for the gas phase has to be considered (b3lyp/def2TZVP). The frontier orbital schemes of both complexes (10+ and 15) are compared in Figure S38, and essential orbitals of 10+ are depicted in Figure 8. With respect to the orbital composition, a restricted metal− metal orbital mixing can be concluded. However, the schemes J


Article

Inorganic Chemistry Table 4. Selected Vertical Singlet−Singlet Transitions for 10+, Transitions within the Kohn−Sham Orbital Scheme transition

λ (nm) ( fosc)a

1

1005 (0.003) 497 (0.134)

A(1) 1 A(4) 1 1

1

1

A(1) A(6)

912 (0.0008) 505 (0.164)

A(7)

477 (0.012)

A(14) {1A(3)} 1 A(17) {1A(7)} 1 A(24) {1A(11)} 1 A(27) {1A(14)} a

426 {434 417 {415 384 {395 374 {375

(0.067) (0.027)} (0.039) (0.039)} (0.035) (0.033)} (0.011) (0.033)}

Me +

1 and 15 as Calculated in Vacuo and Assignment of the

dominant component(s) 15 L ← H (68%) L ← H−1 (93%) 10+ {Me1+}b L ← H−4 (70%) L ← H−2 (57%) L ← H−1 (36%) L ← H−1 (60%) L ← H−2 (31%) L+1 ← H−1 (69%) {L ← H−1 (91%)} L+4 ← H (94%) {L+3 ← H (97%)} L+1 ← H−5 (82%) {L ← H−4 (72%)} L+2 ← H−1 (60%) {L+1 ← H−7 (47%) L ← H−6 (31%)}

assignment CpCod ← bdt CpCod ← bdt CpCod ← S2π/Cod CpCod ← phen-C2π CpCod ← Ir(t2g) CpCod ← Ir(t2g) CpCod ← phen-C2π phenπ* ← Ir(t2g) ppy-pyπ* ← Ir(t2g) phenπ* ← Ir(t2g) phenπ* ← Ir(t2g)/ppy-phπ phenπ* ← S2π phenπ* ← Ir(t2g)

Calculated oscillator strength, limit ≥ 0.01 with exception of 1A(1). bCalculated for [(ppy)2Ir(phendtMe2)]+.

emissive 3MLCT states. The effective quenching of this luminescence in dinuclear complexes is paralleled by the absence of any typical fs-TA features of an Ir complex based 3 MLCT state in 10-PF6. Rather, a substantial decrease of the CpCo-based absorption at 590 nm within 0.3 ps is observed in the transient absorption measurements. As shown by spectroelectrochemistry, the observation of this absorption drop would be compatible with the reduction of Co(III) and hence with an intramolecular electron transfer. Rather unexpected, the spectroscopic signature in the TA measurements show high similarity irrespective whether dinuclear 10PF6 was irradiated at 388 nm or at the absorption maximum of the Co complex moiety at 590 nm or whether mononuclear [CpCo(bdt)] complex 15 was irradiated at 575 nm. Even the derived time constants coincide, verifying a dominating role of the CpCo moiety in the excited state dynamics even in dinuclear complex 10-PF6. These observations disprove a reasonable long-living charge separated state. There are different photodynamic processes that potentially can serve as an explanation for the highly related TA behavior of dinuclear 10-PF6 and mononuclear 15. First, an ultrafast Dexter energy transfer (Figure 9) can be singlet−singlet or triplet−triplet in character depending on the relative ISC rate compared to the energy transfer rate. Second, irradiation of 10-

reproduced) shows an interesting admixture of a direct Ir(t2g) to Co complex transition, which is also evident in a weak transition calculated at 477 nm ({1A(7)}, Table 4). However, for the pure transition of this type, an oscillator strength of zero was obtained. In addition, an open shell calculation with 10+ yielded that also the lowest lying triplet state is exclusively located at the Co complex unit. (See the Supporting Information for spin density distribution.) Finally, for small model complex 15, geometry optimizations of the electronically excited states S1 and S4 were performed, and the expected absorptions for these excited states were obtained by respective TDDFT. The calculated excited singlet state transitions (Figure 7c) are bathochromically shifted by 1940 cm−1 (S1, 551 nm, fosc = 0.11) and 5040 cm−1 (S4, 663 nm, fosc = 0.06) relative to the main visible band of the ground state (497 nm). The respective triplet state transitions were calculated at 496 and 647 nm with comparable oscillator strength ( fosc ∼ 0.02, see the Supporting Information for calculated spectra).

■

DISCUSSION In the considered dinuclear complexes, the metal centers bridged by 1,10-phenantholine-5,6-dithiolate are held apart at a fixed distance of 8.3 Å, which should generally allow an ultrafast e− transfer. In previous studies we found that the degree of photoluminescence quenching by Ni2+ coordination at [L2M(phendt)]0/− depends on the type of the photosensitizer with L2M = [Ru(bpy)2]2+ being more effective than [Ir(ppy)2]+. However, we were not able to discern between Dexter energy transfer and oxidative electron transfer quenching, while reduction of the Ni(dithiolate) moiety was ruled out by thermodynamical reasons.49 On the contrary, a sufficient potential bias for Co reduction in related CpCo complex 10PF6 by a photoinduced electron transfer was proven by cyclic voltammetry. In 11-PF6, the exergonic electron transfer is debatable because of the slightly higher reduction potential of the Cp*Rh-dithiolene moiety with −1.48 V. Mononuclear Ir/ Ru complexes 1-PF6 and 2-(PF6)2 show the normal behavior of such photosensitizers with the formation of long-living and

Figure 9. Qualitative energy diagram for effective singlet−singlet energy transfer in 10+ upon excitation. K


Article

Inorganic Chemistry

complex 15 support the existence of a barrier for the triplet− triplet Dexter energy transfer.

PF6 could lead instantaneously to CpCo-based excited states either by a direct Co-centered transition or by a metal−metal charge transfer transition (MMCT). The last two options can be ruled out by comparisons of the absorption spectra of dinuclear complex 10-PF6 with mononuclear component complexes 1-PF6 and 15 supported by results of TDDFT calculations. Co complex 15 exhibits a broad absorption minimum between 350 and 450 nm excluding a direct Co centered absorption in 10-PF6 at 388 nm. With respect to a potential MMCT, the shoulder at 380 nm in the absorption spectrum of 1-PF6 is broadened and bathochromically shifted. In addition, going from 1-PF6 to 10-PF6 the absorptivity increases from 10 to 14 × 103 L·(mol·cm)−1. The calculated absorption spectra reflect this difference as well, while the increase can be attributed to additional Ir(t2g) to phen-π* transitions. Exclusively, the 1A(7) transition (calcd 477 nm) exhibits a MMCT contribution, but this is mixed with the Cobased main transition (calcd 505 nm) and of low oscillator strength. The calculated oscillator strength of the explicit MMCT transition is zero. Hence, while some contribution by direct MMCT cannot be excluded, the general photodynamic behavior is not explained that way. Finally, an ultrafast triplet− triplet energy transfer following an ISC at the Ir complex unit cannot entirely be ruled out. However, with regard to the related behavior of mononuclear CpCo complex 15, the original state for the population of the lowest lying state assigned to the time component of about 5 ps must be a Cobased state as well. Assuming that this state would be a triplet state, the ISC has to be substantially faster than the lowest reported ISC rates for Co complexes in the literature, which span a time scale of single to double digit ps values.94−98 In addition, the fast spin state changes known are generally associated with an oxidation state change at Co. This argument and the complete absence of the spectroscopic features of the Ir complex based triplet state in the fs-TA measurements (as observed in 1-PF6) led us to consider a triplet state scenario less likely. The time constant of 0.7 ps for the formation of the relaxed 3MLCT state at Ir (determined for 1-PF6) falls in the same range as 0.3 ps for the decrease at the absorption maximum of the CpCo moiety in 10-PF6, indicating that the Dexter energy transfer can apparently compete with the ISC at the Ir chromophore. Hence, a singlet−singlet Dexter energy transfer as displayed in the devised Jablonsky diagram in Figure 9 seems the most consistent interpretation. Additional support for this conclusion is provided by the calculated transition energies (those of the highest oscillator strength) by TDDFT. The difference between the calculated ground state and the first excited state absorption amounts to 1970 cm−1, which is in excellent agreement with the experimental value of 1940 cm−1 determined by the TA measurements (Figure 7, right). In comparison, the calculated triplet state absorption spectrum (Figure S40) does not match the experimentally observed one. In this connection, the photoluminescence lifetime of 10-PF6 falls in the same range as that of 1-PF6, which is noticeable in light of the different quantum yields. Apparently, an effective channel from the emissive thermally relaxed Ir-based 3MLCT state to a Co-based triplet state does not exist, while the latter has been shown to be lowest in energy by the calculations (see Figure S39). According to Figure 9, the relaxation of the Franck−Condon state branches out into two separated deactivation pathways. The long luminescence lifetime of 10-PF6 in combination with the lack of any observed luminescence for mononuclear

■

CONCLUSION Synthetic strategies for the synthesis of dinuclear complexes with bridging 1,10-phenanthroline-5,6-dithiolate have been tested and substantially improved. Usage of the dithiocarbonate protective group in complex 5-PF6 and 6-(PF6)2 proved most beneficial and allowed the investigation of zwitterionic dithiolato complexes Na-7 and 8 as well as the straightforward synthesis of a number of dinuclear complexes combining Ru or Ir at the diimine and Ni, Co, Ru, and Rh at the dithiolato site. All di- and trinuclear complexes show effective photoluminescence quenching compared to the mononuclear complexes. Ir(III)/Co(III) complex 10-PF6 exhibits a redox potential that enables the photoactivated [(ppy)2Ir]-moiety to reduce the Co(III) to Co(II) by an exergonic electron transfer. Accordingly, fs-transient absorption spectroscopy with Ir(ppy)2 complex 10-PF6 revealed the absence of usual features of a 3 MLCT state in the dinuclear compound compared to the mononuclear precursor complexes. In addition, spectroelectrochemistry proved the expected bleaching of the CpCo(dithiolene) absorption band by reduction of Co(III) to Co(II). The fs-transient absorption measurements with mononuclear model complex [CpCo(bdt)] 15 uncovered very similar time contributions and lifetimes no matter if 15 is excited at the absorption maximum of 575 nm or if 10-PF6 is excited selectively at the Ir complex moiety (388 nm) or at the Co complex unit (590 nm). Hence, an ultrafast energy transfer from the Ir to the Co complex unit is an effect, which fully explains the high similarity of the fs-spectral features of 10-PF6 and 15. Because the fs-absorption measurements with 10-PF6 and the experimental absorption spectra of 15 supported by TDDFT calculations did not reveal any signs for triplet states, a singlet−singlet Dexter energy transfer, which effectively competes with the ISC at the Ir complex moiety, is reasonable to assume. With respect to applications in photocatalysis the dinuclear complexes presented herein are subjected to molecular deactivation processes, which are too fast for SETbased reactions. The aimed charge separation {e− transfer from the cationic Ir(III) complex moiety to the neutral CpCo(III) moiety} is most likely hampered by the intrinsic charge polarization of the complex. In conclusion, the planar and rigid 1,10-phenanthroline-5,6-dithiolate ligand causes a highly effective coupling of linked metal centers.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02840. 1 H, 13C NMR spectra, COSY, HSQC, HMBC spectra; XRD data for 14-Cl2, absorption spectra; normalized photoluminescence excitation spectra, time-resolved photoluminescence spectra, CV spectra, fs-transient absorption spectra, computational details, MO schemes, calculated absorption spectra (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

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


Article

Inorganic Chemistry ORCID

(12) Meyer, T. J. Chemical Approaches to Artificial Photosynthesis. Acc. Chem. Res. 1989, 22, 163−170. (13) Staniszewski, A.; Heuer, W. B.; Meyer, G. J. High-Extinction Ruthenium Compounds for Sunlight Harvesting and Hole Transport. Inorg. Chem. 2008, 47, 7062−7064. (14) Lainé, P. P.; Campagna, S.; Loiseau, F. Conformationally gated photoinduced processes within photosensitizer−acceptor dyads based on ruthenium(II) and osmium(II) polypyridyl complexes with an appended pyridinium group. Coord. Chem. Rev. 2008, 252, 2552− 2571. (15) Walther, M. E.; Wenger, O. S. Tuning the Rates of Long-Range Charge Transfer across Phenylene Wires. ChemPhysChem 2009, 10, 1203−1206. (16) Hammarström, L.; Johansson, O. Expanded bite angles in tridentate ligands. Improving the photophysical properties in bistridentate RuII polypyridine complexes. Coord. Chem. Rev. 2010, 254, 2546−2559. (17) Manbeck, G. F.; Brewer, K. J. Photoinitiated electron collection in polyazine chromophores coupled to water reduction catalysts for solar H2 production. Coord. Chem. Rev. 2013, 257, 1660−1675. (18) Dietrich, J.; Wünsche von Leupoldt, A.; Grabolle, M.; ReschGenger, U.; Heinze, K. Thermal and Photoinduced Electron Transfer in Directional Bis(terpyridine)ruthenium(II)-(Bipyridine)platinum(II) Complexes. Eur. J. Inorg. Chem. 2013, 2013, 3009−3019. (19) Neubauer, A.; Grell, G.; Friedrich, A.; Bokarev, S. I.; Schwarzbach, P.; Gärtner, F.; Surkus, A.-E.; Junge, H.; Beller, M.; Kühn, O.; Lochbrunner, S. Electron- and Energy-Transfer Processes in a Photocatalytic System Based on an Ir(III)-Photosensitizer and an Iron Catalyst. J. Phys. Chem. Lett. 2014, 5, 1355−1360. (20) Inagaki, A.; Edure, S.; Yatsuda, S.; Akita, M. Highly selective photo-catalytic dimerization of α-methylstyrene by a novel palladium complex with photosensitizing ruthenium(II) polypyridyl moiety. Chem. Commun. 2005, 5468−5470. (21) Koike, T.; Akita, M. Visible-light radical reaction designed by Ru- and Ir-based photoredox catalysis. Inorg. Chem. Front. 2014, 1, 562−576. (22) Schultz, D. M.; Yoon, T. P. Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014, 343, 1239176. (23) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898−6926. (24) Zhang, X.; MacMillan, D. W. C. Alcohols as Latent Coupling Fragments for Metallaphotoredox Catalysis: sp3-sp2 Cross-Coupling of Oxalates with Aryl Halides. J. Am. Chem. Soc. 2016, 138, 13862− 13865. (25) Tellis, J. C.; Kelly, C. B.; Primer, D. N.; Jouffroy, M.; Patel, N. R.; Molander, G. A. Single-Electron Transmetalation via Photoredox/ Nickel Dual Catalysis: Unlocking a New Paradigm for sp3-sp2 CrossCoupling. Acc. Chem. Res. 2016, 49, 1429−1439. (26) Fabry, D. C.; Rueping, M. Merging Visible Light Photoredox Catalysis with Metal Catalyzed C-H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants. Acc. Chem. Res. 2016, 49, 1969−1979. (27) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Dual Catalysis Strategies in Photochemical Synthesis. Chem. Rev. 2016, 116, 10035−10074. (28) Rau, S.; Schäfer, B.; Gleich, D.; Anders, E.; Rudolph, M.; Friedrich, M.; Görls, H.; Henry, W.; Vos, J. G. A Supramolecular Photocatalyst for the Production of Hydrogen and the Selective Hydrogenation of Tolane. Angew. Chem., Int. Ed. 2006, 45, 6215− 6218. (29) Fihri, A.; Artero, V.; Razavet, M.; Baffert, C.; Leibl, W.; Fontecave, M. Cobaloxime-Based Photocatalytic Devices for Hydrogen Production. Angew. Chem., Int. Ed. 2008, 47, 564−567. (30) Tschierlei, S.; Presselt, M.; Kuhnt, C.; Yartsev, A.; Pascher, T.; Sundström, V.; Karnahl, M.; Schwalbe, M.; Schäfer, B.; Rau, S.; Schmitt, M.; Dietzek, B.; Popp, J. Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst. Chem. - Eur. J. 2009, 15, 7678−7688. (31) Smolentsev, G.; Canton, S. E.; Lockard, J. V.; Sundstrom, V.; Chen, L. X. Local structure of photoexcited bimetallic complexes

Wolfram W. Seidel: 0000-0003-1891-361X Present Address

E.E.: Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany. Author Contributions

The manuscript was written through contribution of E.E. and W.W.S.; ns- and fs-transient absorption was measured by M.L. and S.L. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

E.E. thanks Landesgraduiertenförderung Mecklenburg-Vorpommern and the German Science Fundation (DFG) (GRK 1626, Chemical Photocatalysis) for graduate scholarships.

(1) Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Molecular artificial photosynthesis. Chem. Soc. Rev. 2014, 43, 7501−7519. (2) Faunce, T.; Styring, S.; Wasielewski, M. R.; Brudvig, G. W.; Rutherford, A. W.; Messinger, J.; Lee, A. F.; Hill, C. L.; deGroot, H.; Fontecave, M.; MacFarlane, D. R.; Hankamer, B.; Nocera, D. G.; Tiede, D. M.; Dau, H.; Hillier, W.; Wang, L.; Amal, R. Artificial photosynthesis as a frontier technology for energy sustainability. Energy Environ. Sci. 2013, 6, 1074−1076. (3) Wang, M.; Artero, V.; Hammarström, L.; Martinez, J.; Karlsson, J.; Gust, D.; Summers, P.; Machan, C.; Brueggeller, P.; Windle, C. D.; Kageshima, Y.; Cogdell, R.; Tolod, K. R.; Kibler, A.; Apaydin, D. H.; Fujita, E.; Ehrmaier, J.; Shima, S.; Gibson, E.; Karadas, F.; Harriman, A.; Inoue, H.; Kudo, A.; Takayama, T.; Wasielewski, M.; Cassiola, F.; Yagi, M.; Ishida, H.; Franco, F.; Kang, S. O.; Nocera, D.; Li, C.; Di Fonzo, F.; Park, H.; Sun, L.; Setoyama, T.; Kang, Y. S.; Ishitani, O.; Shen, J.-R.; Son, H.-J.; Masaoka, S. Molecular catalysts for artificial photosynthesis: general discussion. Faraday Discuss. 2017, 198, 353− 395. (4) Rauchfuss, T. B. Diiron Azadithiolates as Models for the [FeFe]Hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere. Acc. Chem. Res. 2015, 48, 2107−2116. (5) Kandemir, B.; Chakraborty, S.; Guo, Y.; Bren, K. L. Semisynthetic and Biomolecular Hydrogen Evolution Catalysts. Inorg. Chem. 2016, 55, 467−477. (6) Braumüller, M.; Schulz, M.; Sorsche, D.; Pfeffer, M.; Schaub, M.; Popp, J.; Park, B.-W.; Hagfeldt, A.; Dietzek, B.; Rau, S. Synthesis and characterization of an immobilizable photochemical molecular device for H2-generation. Dalton Trans. 2015, 44, 5577−5586. (7) Deng, X.; Tüysüz, H. Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701−3714. (8) Ding, X.; Gao, Y.; Zhang, L.; Yu, Z.; Liu, J.; Sun, L. Visible LightDriven Water Splitting in Photoelectrochemical Cells with Supramolecular Catalysts on Photoanodes. ACS Catal. 2014, 4, 2347−2350. (9) Eckenhoff, W. T.; Brennessel, W. W.; Eisenberg, R. Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts. Inorg. Chem. 2014, 53, 9860−9869. (10) Fischer, S.; Hollmann, D.; Tschierlei, S.; Karnahl, M.; Rockstroh, N.; Barsch, E.; Schwarzbach, P.; Luo, S.-P.; Junge, H.; Beller, M.; Lochbrunner, S.; Ludwig, R.; Brückner, A. Death and Rebirth: Photocatalytic Hydrogen Production by a Self-Organizing Copper-Iron System. ACS Catal. 2014, 4, 1845−1849. (11) Liu, J.; Jiang, W. Photoinduced hydrogen evolution in supramolecular devices with a rhenium photosensitizer linked to FeFe-hydrogenase model complexes. Dalton Trans. 2012, 41, 9700− 9707. M


Article

Inorganic Chemistry refined by quantitative XANES analysis. J. Electron Spectrosc. Relat. Phenom. 2011, 184, 125−128. (32) Kuhar, K.; Fredin, L. A.; Persson, P. Exploring Photoinduced Excited State Evolution in Heterobimetallic Ru(II)-Co(III) Complexes. J. Phys. Chem. B 2015, 119, 7378−7392. (33) Canton, S. E.; Zhang, X.; Zhang, J.; van Driel, T. B.; Kjaer, K. S.; Haldrup, K.; Chabera, P.; Harlang, T.; Suarez-Alcantara, K.; Liu, Y.; Pérez, J.; Bordage, A.; Pápai, M.; Vankó, G.; Jennings, G.; Kurtz, C. A.; Rovezzi, M.; Glatzel, P.; Smolentsev, G.; Uhlig, J.; Dohn, A. O.; Christensen, M.; Galler, A.; Gawelda, W.; Bressler, C.; Lemke, H. T.; Møller, K. B.; Nielsen, M. M.; Lomoth, R.; Wärnmark, K.; Sundström, V. Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts. J. Phys. Chem. Lett. 2013, 4, 1972−1976. (34) Astruc, D. From Organotransition-Metal Chemistry toward Molecular Electronics: Electronic Communication between LigandBridged Metals. Acc. Chem. Res. 1997, 30, 383−391. (35) Shukla, A. D.; Das, A. D.; Das, A. Redox responsive binuclear complexes using 5,6-dihydroxy-1,10-phenanthroline as a bridging ligand: synthesis, characterization and physicochemical studies. Polyhedron 2000, 19, 2605−2611. (36) Ishow, E.; Gourdon, A.; Launay, J.-P.; Chiorboli, C.; Scandola, F. Synthesis, Mass Spectrometry, and Spectroscopic Properties of a Dinuclear Ruthenium Complex Comprising a 20 Å Long Fully Aromatic Bridging Ligand. Inorg. Chem. 1999, 38, 1504−1510. (37) Psillakis, E.; Shonfield, P. K. A.; Jouaiti, A.-A.; Maher, J. P.; McCleverty, J. A.; Ward, M. D. Redox-mediation of electron−electron spin−spin exchange interactions, |J|, in paramagnetic trinuclear molybdenum complexes: an example of a ‘J switch’. J. Chem. Soc., Dalton Trans. 2000, 241−249. (38) McCleverty, J. A.; Ward, M. D. The Role of Bridging Ligands in Controlling Electronic and Magnetic Properties in Polynuclear Complexes. Acc. Chem. Res. 1998, 31, 842−851. (39) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Luminescent and Redox-Active Polynuclear Transition Metal Complexes. Chem. Rev. 1996, 96, 759−833. (40) Fox, G. A.; Bhattacharya, S.; Pierpont, C. G. Structural and Electrochemical Properties of Binuclear Complexes Containing 1,10Phenanthroline-5,6-diolate as a Bridging Ligand. Inorg. Chem. 1991, 30, 2895−2899. (41) Paw, W.; Eisenberg, R. Synthesis, Characterization, and Spectroscopy of Dipyridocatecholate Complexes of Platinum. Inorg. Chem. 1997, 36, 2287−2293. (42) Kirk, M. L.; McNaughton, R. L.; Helton, M. E. The Electronic Sructure and Spectroscopy of Metallo-Dithiolene Complexes. Prog. Inorg. Chem. 2004, 52, 111−212. (43) Wang, K. Electrochemical and Chemical Reactivity of Dithiolene Complexes. Prog. Inorg. Chem. 2004, 52, 267−314. (44) Zheng, B.; Sabatini, R. P.; Fu, W.-F.; Eum, M.-S.; Brennessel, W. W.; Wang, L.; McCamant, D. W.; Eisenberg, R. Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, E3987−96. (45) Rabaça, S.; Duarte, M. C.; Santos, I. C.; Pereira, L. C.J.; Fourmigué, M.; Henriques, R. T.; Almeida, M. Thio-azo proligands based on 5,6-derivatives-1,10-phenanthroline and their use for iron(II) complexes: Synthesis, characterization and crystal structures. Polyhedron 2008, 27, 1999−2006. (46) Chesneau, B.; Passelande, A.; Hudhomme, P. Efficient Access to a Versatile 5,6-Dithio-1,10-phenanthroline Building Block and Corresponding Organometallic Complexes. Org. Lett. 2009, 11, 649−652. (47) Keniley, L. K., Jr.; Dupont, N.; Ray, L.; Ding, J.; Kovnir, K.; Hoyt, J. M.; Hauser, A.; Shatruk, M. Complexes with Redox-Active Ligands: Synthesis, Structure, and Electrochemical and Photophysical Behavior of the Ru(II) Complex with TTF-Annulated Phenanthroline. Inorg. Chem. 2013, 52, 8040−8052. (48) Keniley, L. K., Jr.; Ray, L.; Kovnir, K.; Dellinger, L. A.; Hoyt, J. M.; Shatruk, M. TTF-Annulated Phenanthroline and Unexpected

Oxidative Cleavage of the C = C Bond in Its Ruthenium(II) Complex. Inorg. Chem. 2010, 49, 1307−1309. (49) Schallenberg, D.; Neubauer, A.; Erdmann, E.; Tänzler, M.; Villinger, A.; Lochbrunner, S.; Seidel, W. W. Dinuclear Ru/Ni, Ir/Ni, and Ir/Pt Complexes with Bridging Phenanthroline-5,6-dithiolate: Synthesis, Structure, and Electrochemical and Photophysical Behavior. Inorg. Chem. 2014, 53, 8859−8873. (50) Nakamaru, K. Synthesis, Luminescence Quantum Yields and Lifetimes of Trischelated Ruthenium(II) Mixed-ligand Complexes Including 3,3′-Dimethyl-2,2’-bipyridyl. Bull. Chem. Soc. Jpn. 1982, 55, 2697−2705. (51) Morris, J. V.; Mahaney, M. A.; Huber, J. R. Fluorescence Quantum Yield Determinations. 9,10-Diphenylanthracene as a Reference Standard in Different Solvents. J. Phys. Chem. 1976, 80, 969−974. (52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Peterson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V., Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wellinford CT, 2009. (53) Becke, A. D. Density functional calculations of molecular bond energies. J. Chem. Phys. 1986, 84, 4524−4529. (54) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (55) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (56) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-adjusted ab initio pseudopotentials for the second and third row transition elements. Theoret. Chim. Acta 1990, 77, 123−141. (57) Martin, J. M. L.; Sundermann, A. Correlation consistent valence basis sets for use with the Stuttgart−Dresden−Bonn relativistic effective core potentials: The atoms Ga-Kr and In-Xe. J. Chem. Phys. 2001, 114, 3408−3420. (58) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Mixed Phosphine 2,2’Bipyridine Complexes of Ruthenium. Inorg. Chem. 1978, 17, 3334− 3341. (59) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. Photophysical Effects of Metal-Carbon σ Bonds in Ortho-Metalated Complexes of Ir(III) and Rh(III). J. Am. Chem. Soc. 1984, 106, 6647− 6653. (60) Bomfim, J. A. S.; de Souza, F. P.; Filgueiras, C. A. L.; de Sousa, A. G.; Gambardella, M. T. P. Diphosphine complexes of nickel: Analogies in molecular structures and variety in crystalline arrangement. Polyhedron 2003, 22, 1567−1573. (61) Heck, R. F. Dihalocarbonylcyclopentadienylcobalt Complexes. Inorg. Chem. 1965, 4, 855−857. (62) King, R. B. Cyclopentadienylcobalt Carbonyl Diiodide. Z. Naturforsch., B: J. Chem. Sci. 1964, 19b, 1160.10.1515/znb-1964-1218 (63) Heck, R. F. Cyclopentadienylcobalt Derivatives of Chelating Aromatic Ligands. Inorg. Chem. 1968, 7, 1513−1516. (64) White, C.; Yates, A.; Maitlis, P. M.; Heinekey, D. M. (η5Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds. Inorg. Synth. 2007, 29, 228−234. N


Article

Inorganic Chemistry (65) Herich, P.; Kameníček, J.; Kuča, K.; Pohanka, M.; Olšovský, M. Planar Ni(II) 1,2-dithiolenes involving bidentate P-donor ligands. Polyhedron 2009, 28, 3565−3569. (66) Bennett, M. A.; Huang, T. N.; Matheson, T. W.; Smith, A. K.; Ittel, S.; Nickerson, W. (η6 -Hexmethylbenzene)Ruthenium Complexes. Inorg. Synth. 2007, 21, 74−78. (67) Hahn, F. E.; Seidel, W. W. Synthesis and Coordination Chemistry of ortho-Functionalized Dimercaptobenzene: Building Blocks for Tripodal Hexathiol Ligands. Angew. Chem., Int. Ed. Engl. 1996, 34, 2700−2703. (68) Seidel, W. W.; Hahn, F. E.; Lügger, T. Coordination Chemistry of N-Alkylbenzamide-2,3-dithiolates as an Approach to Poly(dithiolate) Ligands: 1,4-Bis[(2,3-dimercaptobenzamido)methyl]benzene and Its Chelate Complex with the (C5H5)Ti Fragment. Inorg. Chem. 1998, 37, 6587−6596. (69) Chandrasekaran, P.; Greene, A. F.; Lillich, K.; Capone, S.; Mague, J. T.; DeBeer, S.; Donahue, J. P. A Structural and Spectroscopic Investigation of Octahedral Platinum Bis(dithiolene)phosphine Complexes: Platinum Dithiolene Internal Redox Chemistry Induced by Phosphine Association. Inorg. Chem. 2014, 53, 9192−9205. (70) Ghosh, A. C.; Reinhardt, J. K.; Kindermann, M. K.; Schulzke, C. The ring opening reaction of 1,3-dithiol-2-one systems is fully reversible. Chem. Commun. 2014, 50, 10102−10104. (71) Cerrada, E.; García, J. F.; Laguna, M.; Terroba, R.; Villacampa, M. D. Formation of thiolene versus dithiolene species from TTBEO. Crystal structures of [Au(ddtCO2Et)(PPh3)] and [Sn(ddtCO2Et)2Me2] {TTBEO = 2,5,7,9-tetrathiabicyclo[4.3.0]non-1(6)-en-8-one; ddtCO2Et = 3-(ethoxycarbonylsulfanyl)-5,6-dihydro-1,4-dithiine-2-thiolate}. J. Chem. Soc., Dalton Trans. 1998, 3511−3516.10.1039/ a805064c (72) Qin, J.; Hu, L.; Wang, X.-S.; Xu, Y.; Zuo, J.-L.; You, X.-Z.; 3Li, G.-N. Synthesis, Characterisation, and Properties of Rhenium(I) Tricarbonyl Complexes with Tetrathiafulvalene-Fused Phenanthroline Ligands. Organometallics 2011, 30, 2173−2179. (73) Dölling, W.; Vogt, A.; Augustin, M. Synthesis of l,3-Dithiole-2one Derivatives, 2-Ethylthio-1,3-dithiolium Tetrafluoroborates, and Thieno[2,3-d]-1,3-dithiole-2-thiones. Z. Naturforsch., B: J. Chem. Sci. 1991, 46 b, 133−138.10.1515/znb-1991-0201 (74) Varma, K. S.; Bury, A.; Harris, N. J.; Underhill, A. E. Improved Synthesis of Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF): πDonor for Synthetic Metals. Synthesis 1987, 1987, 837−838. (75) Knieß, T.; Mayer, R. Synthesis and Redox Behaviour of Sulfur Substituted Naphthoquinones-1,4. Z. Naturforsch., B: J. Chem. Sci. 1996, 51b, 901−904.10.1515/znb-1996-0624 (76) Fanghäel, E.; Beye, N.; Richter, A. M. Organic Electronic Conductors and Precursors - X. Synthesis of 1,3-Tetrathiafulvalenes Based on 1,3,5,7-Tetrathia-s-Indacene-2,6-diones. Tetrahedron 1990, 46, 1553−1556. (77) King, R. B.; Eggers, C. A. Organosulfur Derivatives of the Metal Carbonyls. X. Some Transition Metal Derivatives Containing Both πCyclopentadienyl and cis-1,2-Ethylenedithiolate Ligands. Inorg. Chem. 1968, 7, 340−345. (78) Bokarev, S. I.; Bokareva, O. S.; Kühn, O. Electronic excitation spectrum of the photosensitizer [Ir(ppy)2(bpy)]+. J. Chem. Phys. 2012, 136, 214305. (79) Ladouceur, S.; Zysman-Colman, E. A Comprehensive Survey of Cationic Iridium(III) Complexes Bearing Nontraditional Ligand Chelation Motifs. Eur. J. Inorg. Chem. 2013, 2013, 2985−3007. (80) Nomura, M.; Sasao, T.; Hashimoto, T.; Sugiyama, T.; Kajitani, M. Structures and electrochemistry of monomeric and dimeric CpCo(dithiolene) complexes with substituted benzene-1,2-dithiolate ligand. Inorg. Chim. Acta 2010, 363, 3647−3653. (81) Murata, M.; Habe, S.; Araki, S.; Namiki, K.; Yamada, T.; Nakagawa, N.; Nankawa, T.; Nihei, M.; Mizutani, J.; Kurihara, M.; Nishihara, H. Synthesis of Heterometal Cluster Complexes by the Reaction of Cobaltadichalcogenolato Complexes with Groups 6 and 8 Metal Carbonyls. Inorg. Chem. 2006, 45, 1108−1116. (82) Herman, Z. S.; Kirchner, R. F.; Loew, G. H.; MuellerWesterhoff, U. T.; Nazzal, A.; Zerner, M. C. Electronic Spectra and

Structure of Bis(ethylene-1,2-dithiolato)nickel and Bis-(propene-3thione-1-thiolato)nickel. Inorg. Chem. 1982, 21, 46−56. (83) Sakamoto, R.; Kambe, T.; Tsukada, S.; Takada, K.; Hoshiko, K.; Kitagawa, Y.; Okumura, M.; Nishihara, H. π-Conjugated Trinuclear Group-9 Metalladithiolenes with a Triphenylene Backbone. Inorg. Chem. 2013, 52, 7411−7416. (84) Shibata, Y.; Zhu, B.; Kume, S.; Nishihara, H. Development of a versatile synthesis method for trinuclear Co(III), Rh(III), and Ir(III) dithiolene complexes, and their crystal structures and multi-step redox properties. Dalton Trans. 2009, 1939−1943.10.1039/b815560g (85) Akiyama, T.; Watanabe, Y.; Miyasaka, A.; Komai, T.; Ushijima, H.; Kajitani, M.; Shimizu, K.; Sugimori, A. Photochemical Reduction of (η5-Cyclopentadienyl)(1,2-disubstituated 1,2ethylenedichalcogenolato)cobalt(III) and (η5-Cyclopentadienyl)-(1,2benzenedithiolato)cobalt(III) Complexes in the Presence of Triethanolamine. Bull. Chem. Soc. Jpn. 1992, 65, 1047−1051. (86) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Photochemistry and Photophysica of Coordination Compounds: Iridium. Top. Curr. Chem. 2007, 281, 143−203. (87) Garces, F. O.; King, K. A.; Watts, R. J. Synthesis, Structure, Electrochemistry, and Photophysics of Methyl-Substituted Phenylpyridine Ortho-Metalated Iridium(III) Complexes. Inorg. Chem. 1988, 27, 3464−3471. (88) Rosenkoetter, K. E.; Ziller, J. W.; Heyduk, A. F. A Heterobimetallic W-Ni Complex Containing a Redox-Active W[SNS]2 Metalloligand. Inorg. Chem. 2016, 55, 6794−6798. (89) Gan, L.; Groy, T. L.; Tarakeshwar, P.; Mazinani, S. K. S.; Shearer, J.; Mujica, V.; Jones, A. K. A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst. J. Am. Chem. Soc. 2015, 137, 1109−1115. (90) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Very high-efficiency green organic light-emitting devices based on electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4−6. (91) Hofbeck, T.; Yersin, H. The Triplet State of fac-Ir(ppy)3. Inorg. Chem. 2010, 49, 9290−9299. (92) Tschierlei, S.; Neubauer, A.; Rockstroh, N.; Karnahl, M.; Schwarzbach, P.; Junge, H.; Beller, M.; Lochbrunner, S. Ultrafast excited state dynamics of iridium(III) complexes and their changes upon immobilisation onto titanium dioxide layers. Phys. Chem. Chem. Phys. 2016, 18, 10682−10687. (93) Li, Y.; Dandu, N.; Liu, R.; Li, Z.; Kilina, S.; Sun, W. Effects of Extended π-Conjugation in Phenanthroline (N∧N) and Phenylpyridine (C∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes. J. Phys. Chem. C 2014, 118, 6372−6384. (94) Dose, E. V.; Hoselton, M. A.; Sutin, N.; Tweedle, M. F.; Wilson, L. J. Dynamics of Intersystem Crossing Processes in Solution for SixCoordinate d5, d6, and d7 Spin-Equilibrium Metal Complexes of Iron(III), Iron(II), and Cobalt(II). J. Am. Chem. Soc. 1978, 100, 1141− 1147. (95) Heitz, M.-C.; Ribbing, C.; Daniel, C. Spin-orbit induced radiationless transitions in organometallics: Quantum simulation of the intersystem crossing processes in the photodissociation of HCo(CO)4. J. Chem. Phys. 1997, 106, 1421−1428. (96) Canton, S. E.; Kjær, K. S.; Vankó, G.; van Driel, T. B.; Adachi, S.-i.; Bordage, A.; Bressler, C.; Chabera, P.; Christensen, M.; Dohn, A. O.; et al. Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses. Nat. Commun. 2015, 6, 6359. (97) Koch, A.; Kinzel, D.; Drö ge, F.; Gräfe, S.; Kupfer, S. Photochemistry and Electron Transfer Kinetics in a Photocatalyst Model Assessed by Marcus Theory and Quantum Dynamics. J. Phys. Chem. C 2017, 121, 16066−16078. (98) Marroux, H. J. B.; Curchod, B. F. E.; Faradji, C. A.; Shuttleworth, T. A.; Sparkes, H. A.; Pringle, P. G.; Orr-Ewing, A. J. Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex. Angew. Chem., Int. Ed. 2017, 56, 13713−13716.

O


Ultrafast Energy Transfer in Dinuclear Complexes with Bridging 1,10

Recommend Documents