Structural, electrochemical and photophysicalproperties of an exocyclic di-ruthenium complexand its application as a photosensitizer†
Sahan R. Salpage,a Avishek Paul,a Bozumeh Som,a Tanmay Banerjee,b
Kenneth Hanson,b Mark D. Smith,a Aaron K. Vannuccia and Linda S. Shimizu*a
The reaction of cis-bis(2,2’-bipyridine)dichlororuthenium(II) hydrate with a conformationally mobile bipyri-
dyl macrocycle afforded [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O, a bridged di-Ru complex. Single crystal X-ray
diffraction showed the macrocyclic ligand adopting a bowl-like structure with the exo-coordinated Ru(II)
centers separated by 7.29 Å. Photophysical characterization showed that the complex absorbs in the
visible region (λmax = 451 nm) with an emission maximum at 610 nm (τ = 706 ns, ΦPL = 0.021). Electro-
chemical studies indicate the di-Ru complex undergoes three one-electron reversible reductions and a
reversible one-electron oxidation process. This electrochemical reversibility is a key characteristic for its
use as an electron transfer agents. The complex was evaluated as a photocatalyst for the electronically
mismatched Diels–Alder reaction of isoprene and trans-anethole using visible light. It afforded the
expected product in good conversion (69%) and selectivity (dr > 10 : 1) at low loadings (0.5–5.0 mol%) and
the sensitizer/catalyst was readily recycled. These results suggest that the bipyridyl macrocycle could be
widely applied as a bridging ligand for the generation of chromophore-catalyst assemblies.
Introduction
Coordination complexes that contain macrocyclic ligands,such as naturally occurring magnesium-porphyrins and iron-porphyrins, play a vital role in biological systems.1,2 The chela-tion effects of a macrocyclic ligand, known as the macrocycliceffect, affords thermodynamically and kinetically stable com-plexes and offers an easily modulated ligand environment.3,4
In addition, macrocycles that contain multiple N-donorbinding sites, such as bipyridine, allow the macrocycle to actas a ‘bridging ligand’ between multiple transition metalcenters, as illustrated in Fig. 1. These bridged, multiple tran-sition metal complexes have shown great promise for use inphotocatalysis.5–8 Additionally, the photosensitizing and elec-tron transport properties of ruthenium complexes are ofparticular interest as functional materials for use in light
harvesting,9 solar conversion,10 catalysis,11 molecular recog-nition,12 and in supramolecular devices.13
Studies have been conducted in order to investigate andunderstand the photophysical and electrochemical behavior of
Fig. 1 A conformationally mobile bipyridyl macrocycle was used asbridging ligand to complex two ruthenium bis(2,2’-bipyridine) units. (a)The structure of the bipyridyl bis-urea macrocycle (L) used in the studyas the bridging ligand. (b) The crystal structure of the macrocyclic ligand(L) highlighting freely rotating bonds. (c) The reaction of ligand (L) withRu(bpy)2Cl2·2H2O generates the doubly exo-coordinated rutheniumcomplex shown schematically.
†Electronic supplementary information (ESI) available: The X-ray crystal data forcomplex 1, .CIF file, 1H NMR and 13C NMR data, additional electrochemical andphotocatalytic data. CCDC 1472718. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c6dt01377e
aDepartment of Chemistry and Biochemistry, University of South Carolina,
Columbia, SC 29208, USA. E-mail: [email protected] of Chemistry and Biochemistry, Florida State University, Tallahassee,
macrocyclic ruthenium complexes.14–19 These individualmetal-macrocycle unit can be used as starting building blocksto construct photo- and redox-active supramolecular materialsthat bridge two or more metal centers. For example, transitionmetal complexes of osmium and ruthenium assembledthrough macrocyclic bridging ligands showed that the chemi-cal structure and nature of the macrocycle plays a significantrole in (a) determining the photophysical and electrochemicalproperties of the bound metals, (b) regulating the electro-chemical communication between multiple metal centers, and(c) determining the overall structure and properties of the finalassembly.
Our group has reported a bipyridyl bis-urea macrocycle(L) (Fig. 1a), which displays conformational mobility andcan be used as a ligand to chelate metals in its interior(endo), or through rotation position the binding sites onthe exterior (exo), which allows for the bridging of twometal centers20,21 This manuscript reports the use of thismacrocycle as a bridging ligand (µ-L) to synthesize a di-ruthenium complex for use as a photosensitizer (Fig. 1c).The new diruthenium complex was characterized by NMR,HRMS, X-ray diffraction, photophysical, and electrochemicalmethods. Further, we investigated the electronically mis-matched Diels–Alder reaction of isoprene and trans-anetholein the presence of complex and visible light as a modelsystem to test the ability of the complex to act as aphotosensitizer.
Results and discussion
The bipyridyl bis-urea macrocycle, L, offers the advantage ofconformational mobility and can rotate to afford either aninterior or exterior metal binding site. A ruthenium salt withtwo additional bipyridine units was employed to provide thesteric bulk to force L into the exo conformation, the confor-mation capable of bridging two metals (µ-L). The di-nuclearruthenium complex 1 (Fig. 2) was synthesized using asolvothermal method. The bipyridyl ligand (L, 10.0 mg,0.021 mmol) and Ru(bpy)2Cl2·2H2O (21.85 mg, 0.042 mmol)were placed in ethanol (12.5 mL). A 1 : 2 L: metal salt ratio wasused to ensure the saturation of the two bipyridine bindingsites of L. The reagents and the solvent were added to apressure tube (∼25 mL) and sonicated for 10 minutes.The pressure tube was secured in a steel tube and thesample temperature/time was control according to theramp cycle illustrated in the Fig. 2a in a programmable crystal-lization oven. At the end of the reaction orange block likecrystals were obtained with the molecular formula of[(Ru(C10H8N2)2)2C26H24N8O2)](Cl)4(H2O)6 as confirmed byNMR, HRMS, and single crystal XRD analysis.
Solid state structure of [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O (1)
The compound crystallizes in the space group C2/c andobtained in the homochiral form (Fig. 2b) with the unitcell containing equal numbers of DD and LL isomers.
Fig. 2 Synthesis and the structure of [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O (1). (a) Ligand L (0.021 mmol) and Ru(bpy)2Cl2·2H2O (0.042 mmol) wereheated in ethanol (12.5 mL) as indicated to afford orange block crystals. (b) Top view of the bowl-like structure of complex 1 cation with the ruthe-nium centers 7.29 Å apart. (c) Side view of complex 1 cation comparing the Ru coordination geometry. (d) Part of the hydrogen bonding network(red dashed lines) formed from the urea groups, water molecules and chloride anions.
The asymmetric unit consists of half of one[(Ru(C10H8N2)2)2C26H24N8O2)]
4+ complex, which is situatedabout a two-fold axis of rotation, three chloride anions, andthree water molecules. The coordination of the Ru to themacrocycle resulted in the formation of a bowl-like structure.As can be seen from Fig. 2b, the ruthenium centers are 7.29 Åapart and the macrocyclic carbonyl oxygen atoms are directedinward of the macrocycle whilst the [Ru(bpy)2]
2+ units aredirected toward the outside of the macrocycle. The macrocyclicligand (L), depicted in Fig. 2c, has a wide top and narrow base.This conformation helps alleviate repulsion between the bipyr-idine units of the octahedral coordinated Ru centers (Fig. 2c).The Ru–N bond lengths range from 2.05 to 2.08 Å, which arecomparable to those reported for other di-exo-Ru complexes.17
However, the Ru–N interatomic distances were different foreach bipyridine unit, which is probably due to steric inter-actions. One of the free bipyridine units has the shortest Ru–Ninteratomic distances (Ru1–N5 and Ru1–N6). The macrocyclicN3–Ru1–N4 bite angle (78.96°) is almost identical with that ofthe free bipyridine units (79.31°), which is less than the idealoctahedral angle of 90°. On the other hand, N–Ru–N anglesbetween bipyridine units are higher averaging 93.86° indicat-ing a response to congestion around the Ru center. Fig. 2dshows a hydrogen bonding network surrounding complex 1cation formed by the urea group hydrogens, water moleculesand chloride ions (see Fig. S11† for more details).
Photophysical measurements
The photophysical properties of 1 measured in N2 deaeratedacetonitrile are presented in Fig. 3 and Table S1.† The absorp-tion spectrum of 1 exhibits strong ligand centered π–π* tran-sitions from 260–320 nm and dRu(II) → π* MLCT transitionfrom 370–550 nm that are typical for ruthenium polypyridylcomplexes.22 Upon excitation at 450 nm 1 exhibits broad3MLCT emission with a peak maximum at 610 nm, an excitedstate lifetime of 706 ns and an emission quantum yield of
0.021 (Table S1†). At sufficiently high excitation intensities onemight expect to observe excited state quenching in 1 due totriplet–triplet annihilation between adjacent Ru(bpy)3
2+ moi-eties of the dimer, but this was not observed under the rela-tively weak excitation intensities used here (<1 mW cm−2).
Electrochemical measurements
The electrochemical properties of 1 were measured and theresults are presented in Table 1 and Fig. 4. Cyclic voltammetrydata in DMF indicates all redox couples of 1 are quasi-revers-ible (ΔE = 80–100 mV and peak currents vary linearly with thesquare root of scan rate from 10 to 500 mV s−1 (Fig. S4). Theanodic wave at E1/2 = 1.52 V vs. NHE is attributed to the Ru(II)Ru(III)/Ru(II)Ru(II) redox couple and is similar to the Ru(III)/Ru(II) couple for the related [Ru(bpy)3](PF6)2 compound. Threecathodic waves are also observed for 1 as shown in Fig. 4.These cathodic waves are assigned to ligand-based reductionsas is observed in analogous compounds.22 In acetonitrile, thethird reduction of the complex during CV measurements at Ep≈ −1.6 V vs. NHE became irreversible, possibly indicating bpyligand dissociation and coordination of acetonitrile solvent.
All observed redox couples are characteristic for one elec-tron processes. This one-electron assignment is supported bydifferential pulse voltammetry (DPV) in Fig. 4, and by a com-parison to the one-electron oxidation couple of ferrocene(Fig. S5†). The integrated area under all of the redox peak for 1as well as the peak for ferrocene (1 equivalent) are similar.Since peak currents in DPVs are directly proportional to the
Fig. 3 Normalized absorption and emission spectra of 1 in N2 deaer-ated acetonitrile at room temperature (λex = 450 nm).
Table 1 Electrochemical data for 1 in 0.1 M TBAPF6/DMF, GC asworking electrode, Pt as counter electrode and scan rate of 100 mV s−1.Potentials reported versus the normal hydrogen electrode
1st oxidation(E1/2)/ΔE (mV)
1st reduction(E1/2)/ΔE (mV)
2nd reduction(E1/2)/ΔE (mV)
3rd reduction(E1/2)/ΔE (mV)
1 1.52/90 −1.02/80 −1.26/90 −1.50/80
Fig. 4 DPV (top) and CV (below) of 1 complex in 0.1 M TBAPF6 in DMF.GC as working electrode; Pt as counter electrode; scan rate = 100 mV s−1.DPV parameters are as follows; pulse amplitude = 10 mV, pulse width =100 ms, pulse period = 1000 ms and step increment = 1.5 mV, sampleperiod = 3 ms.
number of electrons transferred, each redox peak of 1 is asingle electron transfer event. Therefore, only a single oxi-dation event for 1 was observed within the potential window ofDMF despite there being two Ru centers in 1. The second oxi-dation is assumed to occur at higher potentials due to electro-static interactions. Compound 1 in the ground state is a cationwith +4 charge, and an one-electron oxidation generates acation with +5 charge. Further oxidation to a cation with +6charge is a large buildup of positive charge and is likelyaccompanied by a large solvent reorganization energy, whichresults in the second oxidation potential being beyond thesolvent window.
Spectroelectrochemistry of complex 1 was studied in MeCNwith UV-Visible absorbance changes observed over timeduring a controlled potential electrolysis. The absorptionspectra change versus time under 1.55 V vs. NHE of appliedpotential is shown in Fig. 5 and S6.† Decreases in the metal-to-ligand charge transfer (MLCT) absorption at 455 nm and theπ–π* transition at 290 nm are accompanied by increases in theabs. at 257 nm, 305 nm and 315 nm. A slight increase inabsorptivity was observed at 690 nm (inset Fig. 5). These spec-tral changes are indicative of the formation of a Ru(III)Ru(II) 1+
species. The analogous [Ru(II)(bpy)3](PF6)2 molecule, upon oxi-dation to Ru(III), also exhibits a decrease in absorptivity near290 and 450 nm and an increase in absorptivity near680 nm.23 After electrolysis, the complex regains its originalspectral features and intensity in just one minute. The CVs ofthe complex after and before electrolysis also remain almostidentical with no such decrease in peak current intensity(Fig. S7 and S8†). These results demonstrate that complex 1 iselectrochemically reversible and chemically stable during theone-electron oxidation process in acetonitrile, which is animportant characteristic for photosensitizers and electrontransfer agents.
Photocatalytic experiments
To evaluate the photoinduced electron transfer of complex 1,we sought to employ it as a photosensitizer in photocatalysis.
Ruthenium based photosensitizers have been widely used invisible light catalysis.24–28 These reactions rely on the ability ofa photosensitizer to undergo single electron transfers (SETs)upon visible light excitation. The unfavorable Diels–Alder reac-tion between isoprene and trans-anethole, two electron richsubstrates was used to test the efficiency of 1 as an SET cata-lyst. Such inverse-demand Diels–Alder reactions require SETsto generate organic radicals for the reaction to efficiently takeplace.29–31,32 Indeed, no reaction is observed upon prolongedheating at 200 °C for 24 h. However, Yoon and coworkersdemonstrated that the reaction proceed under mild conditionsusing an SET catalyst and co-oxidant under visible lightirradiation.33 Thus, this model reaction was used to (1) assessthe ability of 1 to act as an SET catalyst and (2) to monitor thestability and recyclability of 1.
Diels–Alder reactions were carried out in dram vials inwhich the dienophile trans-anethole, the diene isoprene,complex 1, and the co-oxidant methyl viologen were stirred inCH3NO2 at room temperature. The reaction was subjected tovisible light irradiation for 1 h using a commercial 13 W CFLbulb, compared to a 23 W CFL bulb in previous studies.33
Adapting our prior electrochemical studies with Yoon’s earliermechanistic work gives the mechanistic picture illustrated inFig. 6. First, upon excitation with visible light photosensitizer1 undergoes a metal to ligand charge transfer (MLCT) result-ing in photoexited 1*, which is oxidatively quenched by methylviologen, generating 1+. The oxidation potential for the 1+/1couple of 1.52 V vs. NHE (Table 1) is sufficient to generate thetrans-anethole radical cation (2•+) as illustrated in Fig. 6. Thetrans-anethole radical cation then undergoes a subsequentreaction with the electron rich isoprene to produce [4 + 2]cycloadduct (4) via radical cation intermediate 3•+.
Table 2 summarizes the photocatalytic studies. In theabsence of 1, no production of the cycloadduct was observed(entry 1). However, the cycloadduct product was obtained in69% yield (entry 2) upon a 5 mol% loading of 1 (10 mol% withrespect to Ru) demonstrating that the reported complex can beemployed as an photoinduced SET catalyst. Complex 1 and theco-oxidant were then recovered from this reaction mixture andused for a second consecutive reaction in the presence of freshdiene and dienophile. The recycled catalyst afforded the Diels–Alder adduct in similar conversion (61%) suggesting that thecatalyst is robust (entry 3). Decreased catalyst loading 0.5 mol%loading (1 mol% Ru) still afforded reasonable product for-mation of 40–66% (entries 4–6). For comparison, a comparable
Fig. 5 Absorption spectrum of complex 1 during controlled potential(1.55 V vs. NHE) electrolysis over the period of 58 minutes. Inset: mag-nification of spectral changes that occur between 550 and 900 nm.
Fig. 6 Proposed mechanism for the photocatalytic Diels–Alder reac-tion between trans-anethole and isoprene.
loading of Ru(bpy)3(PF6)2, 1 mol%, afforded slightly loweryield of the adduct than obtained for an equivalent mol% Rufrom 1 (31% vs. 40%). These results indicate that 1 is not onlyrobust, but exhibits equal to slightly greater reactivity for thisDiels–Alder reaction compared to Ru(bpy)3(PF6)2. We are cur-rently examining photocatalytic processes where the SET cata-lyst 1 may also interact with substrates via its urea moieties.
Conclusions
These results demonstrate that complex 1 is an SET catalystand facilitates the radical cation mediated Diels–Alder reactionupon visible light excitation. The bimetallic complex 1 couldbe used at low catalyst loading and was recyclable. Photo-physical investigations show that 1 strongly absorbs light inthe visible spectrum and has a relatively long-lived excitedstate. Electrochemically reversible one-electron oxidation andthree reversible one-electron reductions were also observed. Asreversible redox behavior is important for electron transferagents and photosensitizers, complex 1 was tested as a photo-catalyst for the radical cation Diels–Alder reaction betweentrans-anethole and isoprene. The catalyst facilitated the reac-tion in good conversion and high selectivity. Following thereaction, the catalyst was recovered and reused, suggesting ithas good stability. Strong visible light absorbtion, powerfulreductive driving force and the exceptional stability of thecomplex will broaden its applicability as a photosensitizer forvariety of organic transformations. In addition, the ability tobridge two distinct metal centers also makes this macrocycle acandidate as a bridging ligand in chromophore-catalyst assem-blies.34 Studies are underway on the synthesis of mixed metalcomplexes containing µ-L for use in photoredox catalysis.
Experimental sectionGeneral methods and materials
Unless otherwise specified, reagents were used as receivedwithout further purification. The bipyridine bis-urea macro-
cycle (L) was synthesized according to previous procedures.35
All catalytic reactions were conducted in the presence of mole-cular sieves. 1H NMR and 13C NMR spectra were recorded onVarian Mercury/VX 300 NMR.
Synthesis of [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O (1). Singlecrystals of complex 1 were synthesized via the solvothermalreaction of bipyridine bis-urea macrocycle (10.0 mg,0.021 mmol) and Ru(bpy)2Cl2·2H2O (21.85 mg, 0.042 mmol) inethanol (12.5 mL). The reagents and the solvent were added toa ∼25 mL pressure tube and sonicated for 10 minutes. Thenthe tube was secured in a steel tube and placed in a programm-able crystallization oven. The sample was heated (4 °C h−1)to 90 °C for 48 h and cooled (0.1 °C min−1) to room tempera-ture. At the end of the reaction, orange block like crystalswere obtained in 92.5% yield with the molecular formula of[(Ru(C10H8N2)2)2C26H24N8O2)](Cl)4(H2O)6 as confirmed by singlecrystal XRD analysis.
X-ray crystallography
X-ray intensity data were collected at 100(2) K using a BrukerSMART APEX diffractometer (Mo Kα radiation, λ =0.71073 Å).36 The raw area detector data frames were reducedwith the SAINT+ program.36 Final unit cell parameters weredetermined by least-squares refinement of 3647 reflectionsfrom the data set. Direct methods structure solution, differ-ence Fourier calculations and full-matrix least-squares refine-ment against F2 were performed with SHELXS/L37 asimplemented in OLEX2.38
The compound [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O (1) crystal-lizes in the space group C2/c as determined by the pattern ofsystematic absences in the intensity data and by the success-ful solution and refinement of the structure. Two chlorideanions are located on special positions: Cl2 is on an inver-sion center and Cl3 is on a two-fold axis of rotation. Non-hydrogen atoms were refined with anisotropic displacementparameters. Hydrogen atoms bonded to carbon were placedin geometrically idealized positions and included as ridingatoms. The two urea group hydrogen atoms H1 and H2 werelocated in difference maps and refined freely. The waterhydrogen atoms were also located in difference maps butcould not be refined freely. Their located positions wereadjusted to give d(O–H) = 0.85 Å and they were subsequentlyrefined as riding atoms. The largest residual electron densitypeak in the final difference map is located 0.9 Å from theruthenium atom.
Photophysical experiments
Steady-state and time-resolved emission data were collected atroom temperature using an Edinburgh FLS980 spectrometer.For steady-state emission, samples were excited using lightoutput from a housed 450 W Xe lamp passed through a singlegrating (1800 l mm−1, 250 nm blaze) Czerny–Turner mono-chromator and finally a 1 nm bandwidth slit. Emission fromthe sample was passed through a single grating (1800 l mm−1,500 nm blaze) Czerny–Turner monochromator (1.5 nm band-width) and finally detected by a peltier-cooled Hamamatsu
Table 2 Summary of photocatalytic reaction conditionsa
a trans-Anethole, (0.11 mmol), isoprene, (1 mmol). b Crude mixtures werepassed through a silica column (EtOAc eluent) to remove the catalyst andexcess trans-anethole. The product was isolated as a clear oil and charac-terized by NMR and GC/MS. c The catalyst and co-oxidants were collectedoff the silica together and directly reused (see ESI).
R928 photomultiplier tube. The dynamics of emission decaywere monitored by using the FLS980’s time-correlated single-photon counting capability (1024 channels; 10 μs window)with data collection for 5000 counts. Excitation was providedby an Edinburgh EPL-445 picosecond pulsed diode laser (445± 10 nm, pulse width – 100.0 ps) operated at 0.1 MHz. Kineticswere fit with a single exponential function by using Edinburghsoftware package. Absolute Emission quantum yields wereacquired using an integrating sphere incorporated into aspectrofluorimeter (FLS980, Edinburgh Instruments). Thesamples were placed in the sphere and a movable mirror wasused for direct or indirect excitation, making it possible tomeasure absolute emission quantum efficiency following theDe Mello method.39 No filters were used during quantum yieldmeasurements.
Electrochemical experiments
Cyclic voltammetry (CV) and differential pulse voltammetry(DPV) were carried out using a WaveDriver 20 Bipotentiostat/Galvanometer (Pine Research Instrumentation). The workingelectrode was a 3 mm diameter glassy-carbon electrode (CHInstruments). A Pt wire (99.99%) was used as the counter elec-trode. The reference electrode was a saturated calomel elec-trode (SCE) (CH Instruments). The potential of the referenceelectrode was adjusted by 0.24 V for the reported potentialsversus the normal hydrogen electrode (NHE). The glassy-carbon electrode was prepared by manually polishing with0.05 µm Alumina suspension (DE agglomerated, Allied HighTech Product, iNC).
All solutions used for electrochemical measurementscontained 0.1 M tetrabutylammonium hexafluorophosphate(TBAPF6, Acros Organics) further purified by recrystallizationfrom ethanol and dried under vacuum at 80 °C for24 hours. Solution of dimethylformamide (DMF) (Acros,extra dry, water ≤50 ppm) and acetonitrile (EMD ChemicalsDriSolv®, 99.8%, water ≤50 ppm) were used without furtherdrying, but were purged with N2 for five minutes beforemeasurements were performed. Spectroelectrochemicalexperiments were performed using a platinum honeycombspectroelectrochemical cell-kit (Pine Research Instru-mentation) with an Agilent Technologies Cary 8454 UV-Visinstrument.
Photocatalytic experiments
Starting compounds trans-anethole, isoprene, and the solventCH3NO2 were dried with molecular sieves prior to use. Thereactions were carried out as follows. trans-Anethole,(14.82 mg, 0.11 mmol) and isoprene (68.12 mg, 1 mmol) werestirred in CH3NO2 (1 mL) and calculated amounts of complex1 and the co-oxident methyl viologen were added (seeTable S2† for more details). The mixture was irradiated with a13 W CFL for 1 h. The crude mixture was passed through asilica column (EtOAc eluent) to remove the catalyst, co-oxidant,and excess trans-anethole. The Diels–Alder product was iso-lated as a clear oil. The catalyst was recovered from silica-geland reused.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgements
The authors gratefully acknowledge partial support for thiswork from the NSF (CHE-1608874 CHE-1305136) and by theUniversity of South Carolina.
References
1 Synthesis and Coordination Chemistry (Part 1), inHandbook of Porphyrin Science, ed. K. M. Kadish,K. M. Smith and R. Guilard, World Scientific Press, Singa-pore, 2010, vol. 2.
2 J. D. Harvey and C. J. Ziegler, Coord. Chem. Rev., 2003, 247,1–19.
3 P. Comba and W. Schiek, Coord. Chem. Rev., 2003, 238–239,21–29.
4 R. D. Hancock and A. E. Martell, Comments Inorg. Chem.,1988, 6, 237–284.
5 E. A. Medlycott and G. S. Hanan, Coord. Chem. Rev., 2006,250, 1763–1782.
6 V. Balzani and A. Juris, Coord. Chem. Rev., 2001, 211,97–115.
7 L. De Cola and P. Belser, Coord. Chem. Rev., 1998, 177, 301–346.
8 J. P. Sauvage, J. P. Collin, J. C. Chambron, S. Guillerez,C. Coudret, V. Balzani, F. Barigelletti, L. De Cola andL. Flamigni, Chem. Rev., 1994, 94, 993–1019.
9 M. Grätzel, Acc. Chem. Res., 2009, 42, 1788–1798.10 J.-F. Yin, M. Velayudham, D. Bhattacharya,
H.-C. Lin and K.-L. Lu, Coord. Chem. Rev., 2012, 256, 3008–3035.
11 J. D. Blakemore, R. H. Crabtree and G. W. Brudvig, Chem.Rev., 2015, 115, 12974–13005.
12 K.-C. Chang, S.-S. Sun, M. O. Odago and A. J. Lees, Coord.Chem. Rev., 2015, 284, 111–123.
13 V. Balzani, G. Bergamini and P. Ceroni, Coord. Chem. Rev.,2008, 252, 2456–2469.
14 E. C. Constable, C. E. Housecroft, M. Neuburger,P. J. Rösel, S. Schaffner and J. A. Zampese, Chem. – Eur. J.,2009, 15, 11746–11757.
15 O. Henze, D. Lentz, A. Schäfer, P. Franke andA. D. Schlüter, Chem. – Eur. J., 2002, 8, 357–365.
16 C. Kaes, M. Wais Hosseini, C. Kaes, A. De Cian andJ. Fischer, Chem. Commun., 1997, 2229–2230.
17 P. D. Beer, F. Szemes, V. Balzani, C. M. Salà, M. G. B. Drew,S. W. Dent and M. Maestri, J. Am. Chem. Soc., 1997, 119,11864–11875.
18 M. Venturi, F. Marchioni, B. Ferrer Ribera, V. Balzani,D. M. Opris and A. D. Schlüter, ChemPhysChem, 2006, 7,229–239.
19 M. Venturi, F. Marchioni, V. Balzani, D M. Opris, O. Henzeand A. D. Schlüter, Eur. J. Org. Chem., 2003, 2003, 4227–4233.
20 L. S. Shimizu, S. R. Salpage and A. A. Korous, Acc. Chem.Res., 2014, 47, 2116–2127.
21 S. Dawn, S. R. Salpage, M. D. Smith, S. K. Sharma andL. S. Shimizu, Inorg. Chem. Commun., 2012, 15, 88–92.
22 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belserand A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85–277.
23 K. Kalyanasundaram, Coord. Chem. Rev., 1982, 46, 159–244.24 D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013,
42, 97–113.25 J. W. Beatty and C. R. J. Stephenson, Acc. Chem. Res., 2015,
48, 1474–1484.26 D. M. Schultz and T. P. Yoon, Science, 2014, 343.27 C. K. Prier, D. A. Rankic and D. W. C. MacMillan, Chem.
Rev., 2013, 113, 5322–5363.28 T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2,
527–532.29 A. Gieseler, E. Steckhan, O. Wiest and F. Knoch, J. Org.
Chem., 1991, 56, 1405–1411.
30 D. Valentine, N. J. Turro and G. S. Hammond, J. Am. Chem.Soc., 1964, 86, 5202–5208.
31 B. P. Sullivan, D. J. Salmon and T. J. Meyer, Inorg. Chem.,1978, 17, 3334–3341.
32 Z. M. Wosinska, F. L. Stump, R. Ranjan, E. D. Lorance,G. N. Finley, P. P. Patel, M. A. Khawaja, K. L. Odom,W. H. Kramer and I. R. Gould, Photochem. Photobiol., 2014,90, 313–328.
33 S. Lin, M. A. Ischay, C. G. Fry and T. P. Yoon, J. Am. Chem.Soc., 2011, 133, 19350–19353.
34 S. Jasimuddin, T. Yamada, K. Fukuju, J. Otsuki andK. Sakai, Chem. Commun., 2010, 46, 8466–8468.
35 L.-l. Tian, C. Wang, S. Dawn, M. D. Smith, J. A. Krause andL. S. Shimizu, J. Am. Chem. Soc., 2009, 131, 17620–17629.
36 SMART Version 5.631, SAINT+ Version 6.45a, Bruker analyti-cal X-ray Systems, Inc., Madison, Wisconsin, USA, 2003.
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