photoinduced energy- and electron-transfer processes in dinuclear ruii–osii, ruii–osiii, and...

DOI: 10.1002/cphc.200500323

Photoinduced Energy- and Electron-TransferProcesses in Dinuclear RuII–OsII, RuII–OsIII, andRuIII–OsII Trisbipyridine Complexes Containing aShape-Persistent Macrocyclic SpacerMargherita Venturi,*[a] Filippo Marchioni,[a] Bel�n Ferrer Ribera,[a]

Vincenzo Balzani,[a] Dorina M. Opris,[b] and A. Dieter Schl!ter*[b]

Introduction

Photoinduced energy- and electron-transfer processes lie atthe heart of fundamental natural phenomena (e.g. , photosyn-thesis)[1] as well as of a variety of applications.[2] In the last15 years the research on these processes has progressivelymoved from molecular to supramolecular (multicomponent)systems[3] with the dual aim of testing current theoretical treat-ments[4] and designing nanoscale devices capable of perform-ing useful light-induced functions.[5] A large number of speciesmade of covalently linked building blocks have been investi-gated for these purposes.

RuII and OsII polypyridine-type complexes[6] have long beenknown to exhibit suitable excited-state and redox propertiesto play the role of building blocks for the construction of pho-toactive multicomponent systems. An interesting class of suchsystems are polynuclear complexes, where the metal-basedunits are linked together by bridging ligands.[7] The role playedby the bridging ligands is important for the following reasons:i) with their coordinating sites they contribute to determinethe spectroscopic and redox properties of the active metal-based units; ii) their spacers and the connections betweenspacers and coordinating sites determine the overall structureof the system; iii) their chemical nature controls the electroniccommunication between the metal-based units. Therefore, the

choice of suitable bridging ligands is crucial to obtain polynu-clear complexes capable of showing luminescence, of exhibit-ing interesting electrochemical properties, and of giving rise tophotoinduced energy- and electron-transfer processes.

A wide range of bridging ligands has been used in recentyears. Many of them contain 2,2’-bipyridine (bpy) as chelatingunits since it is well known that their complexes with RuII and

The PF6� salt of the dinuclear [(bpy)2Ru(1)Os(bpy)2]4+ complex,

where 1 is a phenylacetylene macrocycle which incorporates two2,2’-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, theRu- and Os-based units display their characteristic absorptionspectra and electrochemical properties as in the parent homo-dinuclear compounds. The luminescence spectrum, however,shows that the emission band of the RuII unit is almost complete-ly quenched with concomitant sensitization of the emission ofthe OsII unit. Electronic energy transfer from the RuII to the OsII

unit takes place by two distinct processes (ken=2.0 , 108 and2.2 , 107 s�1 at 298 K). Oxidation of the OsII unit of [(bpy)2Ru(1)Os(bpy)2]4+ by CeIV or nitric acid leads quantitatively to the[(bpy)2RuII(1)OsIII(bpy)2]5+ complex which exhibits a bpy-to-OsIII

charge-transfer band at 720 nm (emax=250m�1 cm�1). Light exci-tation of the RuII unit of [(bpy)2RuII(1)OsIII(bpy)2]5+ is followed byelectron transfer from the RuII to the OsIII unit (kel,f=1.6 , 108 and2.7 , 107 s�1), resulting in the transient formation of the[(bpy)2RuIII(1)OsII(bpy)2]5+ complex. The latter species relaxes tothe [(bpy)2RuII(1)OsIII(bpy)2]5+ one by back electron transfer (kel,b=

9.1 , 107 and 1.2 , 107 s�1). The biexponential decays of the[(bpy)2*RuII(1)OsII(bpy)2]4+ , [(bpy)2*RuII(1)OsIII(bpy)2]5+ , and[(bpy)2RuIII(1)OsII(bpy)2]5+ species are related to the presence oftwo conformers, as expected because of the steric hindrance be-tween hydrogen atoms of the pyridine and phenyl rings. Compar-ison of the results obtained with those previously reported forother Ru–Os polypyridine complexes shows that the macrocyclicligand 1 is a relatively poor conducting bridge.

[a] Prof. M. Venturi, Dr. F. Marchioni, Dr. B. Ferrer Ribera, Prof. V. BalzaniUniversit< di Bologna, Dipartimento di Chimica “G. Ciamician”via Selmi 2, I-40126 Bologna (Italy)Fax: (+39) 051-2099456E-mail : [email protected]

[b] Dr. D. M. Opris, Prof. A. D. SchlEter+

Freie UniversitGt Berlin, Institut fEr ChemieTakustrasse 3, 14195 Berlin (Germany)Fax: (+49) 308-3853357

[+] Current address:Department of MaterialsInstitute of Polymers ETH-HçnggerbergHCI J 541, CH-8093 ZErich (Switzerland)Fax: (+41) 446331395

Supporting information for this article is available on the WWW underhttp://www.chemphyschem.org or from the author.

ChemPhysChem 2006, 7, 229 – 239 8 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 229

OsII ions exhibit a long-lived luminescence and can be oxidizedand reduced reversibly.

Several types of shape-persistent macrocycles have recentlybeen studied.[8] Those containing coordinating units constitutea novel class of ligands: endo-cyclic metal–ion coordinationmay be exploited to generate nanowires,[9] whereas exo-cycliccoordination can be used to construct large arrays of polynu-clear metal complexes.[10] Besides coordinating units, shape-persistent macrocycles may bear reactive substituents that canbe used to incorporate other components capable of respond-ing to chemical, photochemical and electrochemical stimula-tion, with the aim of obtaining molecular-level devices.

We have recently synthesized shape-persistent macrocyclesincorporating 2,2’-bipyridine (bpy) ligands.[11] Compound 1(Scheme 1) is particularly interesting since it can bridge two

metal-based units. Its homodinuclear complexes[(bpy)2Ru(1)Ru(bpy)2]

4+ and [(bpy)2Os(1)Os(bpy)2]4+ (hereafter

called RuII1RuII and OsII1OsII, Scheme 1) exhibit luminescenceand reversible redox properties,[11] as expected for weakly cou-pled metal centers.

We have now prepared the hetero-dinuclear [(bpy)2RuII(1)OsII

(bpy)2]4+ complex (RuII1OsII, Scheme 1) and we have investigat-

ed its electrochemical behavior, absorption and emission spec-tra, and intercomponent energy-transfer process. This complexis particularly interesting because its OsII-based unit can bequantitatively oxidized to obtain the [(bpy)2Ru

II(1)OsIII(bpy)2]5+

species (RuII1OsIII, Scheme 1) that, upon light excitation of theRuII unit, gives rise to the transient formation of the[(bpy)2Ru

III(1)OsII(bpy)2]5+ complex (RuIII1OsII) that then relaxes

to the RuII1OsIII one. The energy- and electron-transfer process-es taking place in these systems have been carefully investigat-ed and the results obtained have been compared with thosereported for other Ru–Os polypyridine dinuclear complexes.

Experimental Section

Materials : Compound 1 and its dinuclear Ru complex[(bpy)2Ru(1)Ru(bpy)2]

4+ (RuII1RuII) were synthesized as described inthe literature.[8a,10] [Ru(bpy)2Cl2] was purchased from Aldrich and[Os(bpy)2Cl2] was prepared according to literature procedures.[12]

Model compounds [Ru(bpy)3]2+ and [Os(bpy)3]

2+ were availablefrom previous studies. The other compounds used were all high-quality commercial products.

The dinuclear complex [(bpy)2Os(1)Os(bpy)2]4+ (OsII1OsII) was syn-

thesised by reacting macrocycle 1 with a small excess of [Os-(bpy)2Cl2] in butanol for five days at reflux. The green product waspurified by column chromatography before the chlorides were ex-changed by hexafluorophosphate to increase crystallizability. Thecomplex was characterized by matrix-assisted laser desorption ioni-zation time-of-flight (MALDI-TOF) mass spectrometry (dithranol

matrix) and NMR spectroscopy. The mass spec-trum does not show the molecular ion butrather a set of characteristic fragment signalsat m/z=2758, 2613, and 2468 due to a step-wise fragmentation of three PF6

� groups (seeSupporting Information). Table 1 contains char-acteristic peaks and their assignments. The cal-culated and experimental isotopic distribu-tions for the signal at m/z=2758 are in goodagreement. The complex has two enantiomer-ic forms (L,L and D,D) and one meso form(D,L). Its 1H NMR spectrum is therefore quitecomplex. Two sets of signals are observed forthe bipy ligands and one set for the macrocy-cle. The assignment of these sets was com-pletely achieved by means of correlated spec-troscopy (COSY) and heteronuclear multiplequantum correlation (HMQC) experiments (seeSupporting Information).

The synthesis of the hetero-dinuclear complex[(bpy)2Ru(1)Os(bpy)2]

4+ (RuII1OsII) was morecomplex (see Scheme 2), specifically for purifi-cation reasons. In the first step the mononu-clear Os complex [(bpy)2Os(1)]Cl2 was preparedby reacting [Os(bpy)2Cl2] with an excess of

macrocycle 1. During this step the formation of some homodinu-clear [(bpy)2Os(1)Os(bpy)2]Cl2 was unavoidable. Repetitive and tedi-ous column chromatographic steps under specific conditions hadto be used to finally obtain the pure mononuclear complex. Itspurity was checked by MALDI-TOF mass spectrometry, NMR spec-troscopy, and thin-layer chromatography (TLC). Then, treatment of[(bpy)2Os(1)]Cl2 with [Ru(bpy)2Cl2] after column chromatographyand anion exchange gave the mixed complex [(bpy)2Os(1)Ru(bpy)2]

Table 1. Assignments of characteristic MALDI-TOF mass spectrometricsignals of complex [(bpy)2Os(1)Os(bpy)2](PF6)4.

Mass Assigned formula Fragmentation

2758 C132H124N12O4Os2P3F18 [M�PF6]+

2613 C132H124N12O4Os2P2F12 [M�2PF6]+

2468 C132H124N12O4Os2PF6 [M�3PF6]+

1966 C112H108N8O4OsPF6 [M�3PF6-2bipy-Os]+

1821 C112H108N8O4Os [M�4PF6-2bipy-Os]+

Scheme 1. Structures and abbreviations of the macrocyclic ligand 1 and its Ru and/or Os com-plexes.

230 www.chemphyschem.org 8 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2006, 7, 229 – 239

M. Venturi et al.

www.chemphyschem.org

(PF6)4. Its MALDI-TOF mass spectrum in dithranol shows two char-acteristic signals at m/z=2670 and 2525, corresponding to[M�PF6]

+ and [M�2PF6]+ , respectively (see Supporting Informa-

tion). The calculated and experimental isotope distributions for thesignal at m/z=2669.8 are in good agreement (Table 2).

The 1H NMR spectrum of the hetero-dinuclear complex is quitecomplex (Figure 1). The assignment of protons was neverthelesspossible in quite some detail using COSY and HMQC experiments.

Synthesis: [(bpy)2Os(1)Os(bpy)2](PF6)4 : A stirred solution of macro-cycle 1 (37.2 mg, 0.03 mmol) and [Os(bpy)2]Cl2·2H2O (50 mg,0.09 mmol) in butanol (15 mL) was refluxed for five days under ni-trogen. The solvent was removed and the brown–green solid waspurified by column chromatography through neutral aluminiumoxide using—as eluent—dichloromethane/methanol (95:5) toremove unreacted macrocycle and [Os(bpy)2]Cl2, followed by meth-anol to wash the complex off the column. After removal of the sol-vent, the green solid was dissolved in methanol (2 mL), and then asolution of NH4PF6 (55 mg in water 2 mL) was added. The precipi-tated solid was separated by filtration, washed with H2O (3 mL, fivetimes), and dried in vacuum to give 43.5 mg (53%) of the complexas a green solid. Rf=0.63 (methanol/2m NH4Cl/nitromethane7:2:1). 1H NMR ([D3]nitromethane, 500 MHz): d=0.84 (t, 12H, CH3),1.25–1.47 (m, 24H, g, d, e-CH2), 1.1.50–1.68 (m, 8H, b-CH2), 3.47 (t,8H, a-CH2), 4.42 (s, 8H, benzyl-CH2), 7.17 (s, 4H, phenyl-H), 7.36 (m,

4H, py-H), 7.42 (m, 4H, py-H), 7.45 (m, 2H, phenyl-H), 7.55 (m, 8H,phenyl-H), 7.86 (m, 4H, phenyl-H), 7.88 (s, 2H, phenyl-H), 7.92–7.99(m, 16H, py-H), 8.01 (s, 4H, py-H), 8.29 (d, 4H, 3J=8 Hz, py-H), 8.51(d, 4H, 3J=8 Hz, py-H), 8.58 (d, 4H, 3J=8 Hz, py-H), 8.70 ppm (d,4H, 3J=8 Hz, py-H); 13C NMR ([D3]nitromethane, 500 MHz): d=14.44, 23.70, 26.92, 30.72, 32.74, 71.87, 72.61, 90.42, 90.48, 124.59,125.30, 125.71, 125.78, 125.85, 126.75, 128.49, 129.35, 129.39,130.55, 131.22, 132.19, 132.48, 136.36, 137.20, 138.59, 140.62,142.64, 149.17, 151.96, 152.31, 152.37, 159.44, 160.43, 160.74;MALDI-TOF: 2758 [M�PF6]

+ , 2613 [M�2PF6]+ , 2468 [M�3PF6]

+ ,1966 [M�3PF6-Os-2bpy]

+ , 1821 [M�4PF6-Os-2bpy]+ ; monoisotop-

ic mass calculated for C132H124N12O4Os2P3F12: 2757.80; found2757.87.

[(bpy)2Os(1)Ru(bpy)2](PF6)4: A stirred solution of 1 (80 mg,0.06 mmol) and [Os(bpy)2Cl2] (28 mg, 0.05 mmol) in butanol (7 mL)was refluxed for three days. The solvent was removed and the re-sidual green material purified by column chromatography, firstthrough neutral aluminium oxide (dichloromethane/methanol90:10) to remove the unreacted macrocycle and the unreacted Ossource. The green fraction was collected and the solvent removed.The second step was then done through silica gel eluting first withmethanol/2m NH4Cl/nitromethane 7:2:1. Under these conditionsthe dinuclear Os complex was isolated first, followed by the mono-nuclear Os complex which was washed off the column by dichloro-methane/methanol 3:4. The solvent was removed and the precipi-tate (NH4Cl and the complex) washed with dichloromethane andfiltered to give a green solution. Removal of solvent gave a greensolid of [(bpy)2Os(1)]Cl2 (10 mg, 10%) which was partially character-ized. MS (MALDI-TOF, dithranol): m/z : 1821.04 [M�2Cl]+ , 1735.89[M�2Cl-C6H13]

+ . 1H NMR (CDCl3, 500 MHz, 20 8C): the signals arebroad; characteristic for this complex are the two different signalsfor benzylic-H at d=4.43 and 4.52. A solution of [(bpy)2Os(1)]Cl2(6 mg, 0.003 mmol) and [Ru(bpy)2Cl2]2H2O (3 mg, 0.006 mmol) inethanol (1 mL), methanol (0.5 mL), and water (0.5 mL) was refluxedfor 24 h. The solvent was removed and the residue purified bycolumn chromatography through silica gel (methanol/2m NH4Cl/ni-tromethane). The green fraction was collected and the solvent re-moved. The precipitate was washed with dichloromethane. Thegreen solution was collected and the solvent removed. The solidwas then dissolved in methanol (0.5 mL) and added to a concen-trated solution of NH4PF6 (1 mL). The green precipitate was collect-

Table 2. Calculated and experimental isotope intensities for the MALDI-TOF signal of the [(bpy)2Os(1)Ru(bpy)2](PF6)4 complex at m/z = 2669.8.

Mass Calculated Found

2664.8 38 38.32665.8 55 57.82666.8 75 752667.8 93 88.72668.8 96 95.72669.8 100 1002670.8 82 85.82671.8 60 632672.8 40 44.72673.8 21 27.7

Scheme 2. Synthesis of the hetero-dinuclear [(bpy)2Ru(1)Os(bpy)2]Cl4 complex.

ChemPhysChem 2006, 7, 229 – 239 8 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemphyschem.org 231

Photoinduced Energy- and Electron-Transfer Processes


ed and washed with water (0.5 mL, three times), and dried invacuum to give 7 mg of [(bpy)2Os(1)Ru(bpy)2](PF6)4 (83%). Rf (meth-anol/2m NH4Cl/nitromethane)=0.86. 1H NMR (CD3NO2, 500 MHz):d=0.85 (t, 12H, CH3), 1.23–1.34 (m, 24H, g-, d-, e-CH2), 1.55–1.65(m, 8H, b-CH2), 3.48 (t, 8H, a-CH2), 4.48 (s, 8H, benzyl-CH2), 7.18 (s,2H, phenyl-H), 7.20 (s, 2H, phenyl-H), 7.34 (2 dd overlapped in a t,3J=6 Hz, 2H, py-H), 7.42 (2 dd overlapped in a t, 2H, py-H), 7.43 (2dd overlapped in a t, 2H, py-H), 7.47 (d, 3J=8 Hz, 2H, phenyl-H),7.51 (2 dd overlapped in a t, 2H, py-H), 7.55–7.60 (m, 8H, phenyl-H), 7.82 (s, 2H, phenyl-H), 7.83 (s, 2H, phenyl-H), 7.85 (s, 2H,phenyl-H), 7.86 (2 dd overlapped in a t, 2H, py-H), 7.91 (d, 3J=6 Hz, 2H, py-H), 7.96–8.00 (m, 6H, py-H), 8.01 (d, 4J=2 Hz, 2H, py-H), 8.03–8.08 (m, 4H, py-H), 8.09 (d, 4J=2 Hz, 2H, py-H), 8.16 (2 ddoverlapped in a t, 2H, py-H), 8.28 (dd, 3J=8 Hz, 4J=2 Hz, 2H, py-H), 8.48 (dd, 3J=8.5 Hz, 4J=2 Hz, 2H, py-H), 8.49–8.54 (2d overlap-ped in a t, 3J=8 Hz, 4H, py-H), 8.56–8.61 (2d, overlapped in a t,3J=8 Hz, 4H, py-H), 8.71 (d, 3J=8.5, 2H, py-H), 8.73 ppm (d, 3J=8.5 Hz, 2H, py-H). MS (MALDI-TOF, dithranol): 2669.77 [M�PF6]

+ ,2524.84 [M�2PF6]

+ ; monoisotopic mass calculated forC132H124O4N12F18OsP3Ru

+ : 2669.745, found: 2669.77.

Electrochemical Measurements: The equipment and experimentalprocedures have been described previously.[11] The concentrationof the hetero-dinuclear [(bpy)2Ru

II(1)OsII(bpy)2]4+ complex was 5.0N

10�4m ; the experiments were carried out at room temperature in

argon-purged dichloromethane solution, and in the presence oftetrabutylammonium hexafluorophosphate (0.05m) as supportingelectrolyte and ferrocene as an internal standard (E1/2=+0.46 V vs.the saturated calomel electrode, SCE).

Photophysical Measurements: The equipment and experimentalprocedures have been described previously.[13] Experiments wereperformed in air-equilibrated acetonitrile solutions at 298 K and inbutyronitrile rigid matrix at 77 K.

Chemical Oxidation: The [(bpy)2RuII(1)OsII(bpy)2]

4+ complex wasoxidized in acetonitrile solution by adding a stoichiometric amount

of ammonium CeIV nitrate in aceto-nitrile solution, or by using con-centrated (65%) nitric acid inaqueous solution. Under such con-ditions, selective and quantitativeoxidation of the OsII unit can beobtained. The titration was moni-tored from the changes in absorb-ance, in the visible spectral region,and in emission upon excitation at400 nm (see below).

Results and Discussion

Electrochemical Behavior ofRuII1OsII

The cyclic voltammogram ob-tained for RuII1OsII in argon-purged dichloromethane solu-tion is displayed in Figure 2 andthe potential values are shownin Table 3 (where the potentialvalues previously reported for

the homodinuclear RuII1RuII and OsII1OsII, and mononuclear[Ru(bpy)3]

2+ and [Os(bpy)3]2+ parent compounds, and for the

free 1 and bpy ligands are also collected for comparison pur-poses). The oxidation processes of the OsII (+1.01 V vs. SCE)and RuII (+1.45 V vs. SCE) units in RuII1OsII are well separatedand occur almost at the same potentials as the OsII and RuII

units of the respective homodinuclear and mononuclear spe-cies, showing that there is no appreciable interaction betweenthe two metal centers within the limits of the electrochemicaltechnique.[14] The reduction potentials are also in line with suc-cessive independent reductions of the ligands of the twounits. Comparison with the potential values obtained for theother compounds[11] allows us to assign: i) the first process at

Figure 1. Aromatic part of the 1H NMR spectrum of [(bpy)2Ru(1)Os(bpy)2](PF6)4 with signal assignment (500 MHz,CD3NO2, 20 8C).

Figure 2. Cyclic voltammogram obtained for the RuII1OsII complex (argon-purged dichloromethane; complex concentration 5.0N10�4

m ; glassy carbonas working electrode, scan rate of 100 mVs�1).


M. Venturi et al.


�1.09 V versus SCE to the simultaneous reduction of the twobpy-based units of the ligand 1, which is easier to reduce thanbpy, and ii) the second and third processes at �1.36 and�1.42 V versus SCE to the reduction of a bpy ligand of the OsII

and RuII moieties, respectively.

Spectroscopic Properties of RuII1OsII

The absorption spectrum of RuII1OsII in acetonitrile solution isdisplayed in Figure 3 where the spectra of the homodinuclearRuII1RuII and OsII1OsII compounds are also shown for compari-son purposes. It is well known that the bands in the visiblespectral region are due to metal-to-ligand charge-transfer(MLCT) transitions.[6] It can be noticed that for the Os-basedchromophoric units, even the formally forbidden triplet–triplettransitions (550–750 nm region) display a considerable intensi-ty because of the heavy-atom effect. The absorption spectrumof RuII1OsII is equal to that of a 1:1 mixture of the homodinu-clear RuII1RuII and OsII1OsII parent compounds. A completelydifferent behavior is, however, observed as far as luminescenceis concerned (Figure 4). Upon excitation in the isosbestic point

at 475 nm, the luminescencespectrum of a 1:1 mixture of theRuII1RuII and OsII1OsII com-pounds is dominated by themuch stronger (Table 4) Ru-typeemission with maximum at645 nm, whereas in the hetero-dinuclear RuII1OsII compoundthis band is almost completelyquenched and the Os-type bandwith maximum at 780 nm pre-dominates.

Intercomponent EnergyTransfer in RuII1OsII

The results obtained showedthat 97% of the luminescence intensity of the Ru-based com-ponent is quenched by the presence of the Os-based compo-

nent. To elucidate the nature of the quenching mechanism, wehave compared the intensity (lexc at 480 nm) of the Os-basedluminescence (at 780 nm) in the mixed-metal complex withthat of the OsII1OsII compound, and we have found that, aftercorrection for the tail of the residual RuII-based luminescence,they coincide within the experimental error. This result showsthat the quenching of the Ru-based excited state takes placeby energy transfer to the Os-based unit. Since the lifetime ofthe (unquenched) Ru-based excited state is 285 ns and theconcentration of RuII1OsII was 5.0N10�5

m, intermolecularquenching can be ruled out and we must conclude that in thiscompound the quenching by energy transfer occurs intramo-lecularly (Figure 5).

As one can see from Table 4, the quenching of the lumines-cence intensity of the Ru-based component is accompanied by

Table 3. Redox potentials.[a]

Ligand-centered reductionE1/2 (DV)[b] [n][c]

Metal-centered oxidationE1/2 (DV)[b] [n][c]

Os Ru

RuII1OsII �1.42 (60) [1] �1.36 (60) [1] �1.09 (84) [2] +1.01 (60) [1] +1.45 (60) [1]RuII1RuII [d] �1.43 (80) [2] �1.13 (70) [2] +1.44 (80) [2]OsII1OsII [d] �1.35 (105) [2] �1.06 (100) [2] +1.03 (90) [2][Ru(bpy)3]

2+ [d] �1.50 (90) [1] �1.24 (70) [1] +1.43 (78) [1][Os(bpy)3]

2+ [d] �1.35 (100) [1] �1.06 (90) [1] +1.03 (90) [1]bpy[d,e] �2.69[f] �2.18 (100) [1]1[d, e] �2.36[f] �2.07 (60) [2] �1.94 (60) [2] �1.76 (66) [2]

[a] Room-temperature argon-purged dichloromethane solution, unless otherwise noted; halfwave potentialvalues in V versus SCE; tetrabutylammonium hexafluorophosphate as supporting electrolyte, glassy carbon asworking electrode. [b] Average value of the jEa�Ec j in mV. [c] Number of the exchanged electrons. [d] Data ob-tained from ref. [11] . [e] Purified tetrahydrofurane under vacuum conditions. [f] Irreversible process; potentialvalue estimated by the DPV peak.

Figure 3. Absorption spectra of RuII1OsII (c), RuII1RuII (a), and OsII1OsII

(g) complexes (acetonitrile solution at 298 K).

Figure 4. Comparison of the luminescence spectra (air-equilibrated acetoni-trile solution at 298 K, lexc=475 nm) of RuII1OsII complex (c) and of a 1:1mixture of the RuII1RuII and OsII1OsII compounds (d). The inset shows therise and the decay of the Os-based excited state (lexc=406 nm;lem=780 nm).




the quenching of the excited-state lifetime. The emission in-tensity of an excited state decays by a first-order process inthe absence of intermolecular processes. The fact that thedecay of the Ru-based emission in RuII1OsII follows two parallelfirst-order processes indicates that the compound is present astwo distinct forms. In principle such a result can be attributedto the presence of two conformers or two diastereomers. Dif-ferent conformers can indeed be expected because in 1 sterichindrance between hydrogen atoms of pyridine and phenylrings forces the RuII1OsII complex to assume a cis or transstructure, as far as the [Ru(bpy)2]

2+ and [Os(bpy)2]2+ subunits

are concerned.[8b,15] In the cis structure the two metal ions lieon the same side of the plane defined by the shape-persistentmacrocycle 1, whereas for the trans conformer the two metalions lie on opposite sides. From space-filling models, themetal–metal distance is estimated to be about 1.5 nm for thecis conformer and about 1.7 nm for the trans conformer. Thepresence of two distinct rate constants for the decay of theRu-based emission would suggest that the interconversion ofthe cis and trans conformers is slow compared to the excited-state decays.

The alternative assignment of the two observed quenchingrate constants to the presence of two different diastereomerscan be ruled out by the following considerations: 1) It is well

known that the D and L formsof RuII polypyridine-type com-plexes exhibit the same excited-state lifetime. Indeed, two differ-ent lifetimes have never beenobserved for the racemic forms,and the enantiomerically pure Dand L forms of several mono-meric,[16] dimeric,[16,17] and poly-nuclear[17] Ru-polypyridine com-plexes have been shown to ex-hibit the same excited-state life-time. 2) Two different rate con-stants for the quenching of theRu-based emission have neverbeen observed in Ru–L–Os com-plexes.[7b] 3) The small chiralityeffects observed in bimolecular

quenching processes[18] have been assigned to the differentstabilization of the encounter complexes for the different chiralforms. 4) The two diastereomeric forms of the RuII1OsII com-plex would be present in a 1:1 ratio, because the two chiralcenters are enough apart to behave independently, and there-fore they should give rise to two decay processes with 50%contribution, whereas the observed processes have relativeweights of 78% and 22% (Table 4).

As expected for an energy-transfer process, the decay of theRu-based excited state is accompanied by the rise of the Os-based excited state (Figure 4, inset). Since selective excitationof the Ru-based chromophoric unit cannot be obtained (lexc=

406 nm), a substantial fraction of Os-based excited states is al-ready present when the Os-based excited state originatingfrom energy transfer begins to accumulate. This is also thereason why it is difficult to distinguish two different compo-nents in the rise of the Os-based emission.

The rate constant for the energy-transfer processes can becalculated from the following relation [Eq. (1)]:

ken ¼ 1=t �1=to ð1Þ

where t and t8 are the luminescence lifetimes of the Ru-basedcomponent in RuII1OsII and in the RuII1RuII model compound,respectively. From the lifetimes values shown in Table 4, thevalues 2.0N108 and 2.2N107 s�1 were obtained for ken at 298 K.

Electronic energy-transfer processes can occur by two mech-anisms: the Fçrster-type mechanism,[19] based on coulombic in-teractions, and the Dexter-type mechanism,[20] based on ex-change interactions. The Fçrster-type mechanism is a long-range mechanism (its rate falls as r6), which is efficient whenthe radiative transitions, corresponding to the deactivation andexcitation of the two partners, have high oscillator strength.The Dexter-type mechanism is a short-range mechanism (itsrate falls off as er) that requires orbital overlap between donorand acceptor. When donor and acceptor are linked by chemicalbonds, the exchange interaction can be enhanced (superex-change mechanism).[21,22] The energy-transfer processes thattake place in dinuclear Ru–Os complexes involving flexible

Table 4. Luminescence properties.

298 K[a] 77 K[b]

Ru Os Ru Oslmax [nm] t [ns] Irel lmax [nm] t [ns] Irel lmax [nm] t [ns] lmax [nm] t [ns]

RuII1OsII [c] 645 4.9[d]

45[e]

3 780 31[f] 97 735 1.1[g]

RuII1RuII 645 285 100 600 5.14OsII1OsII 780 31 100 735 1.0[Ru(bpy)3]

2+ 615 170 25[h] 580 4.1[Os(bpy)3]

2+ 740 49 91[i] 710 0.8

[a] Air-equilibrated acetonitrile. [b] Butyronitrile rigid matrix. [c] Excitation was performed at 475 nm, which isan isosbestic point between the Ru- and Os-based units on equimolar solutions; for comparison purposes, theluminescence intensities of RuII1RuII at 645 nm and OsII1OsII at 780 nm were taken as 100. [d] 78% of the spe-cies. [e] 22% of the species. [f] To be compared with the rise time of the Os-based emission (7.3 ns). [g] To becompared with the rise time of the Os-based emission (50 ns). [h] The luminescence quantum yield underthese conditions is 0.016 (ref. [32]). [i] The luminescence quantum yield under these conditions is 3.5N10-3

(ref. [38]).

Figure 5. Energy-level diagram showing the photoinduced energy-transferprocesses that occur in the RuII1OsII complex. Key: (c), excitation; (a),luminescence; wavy line, radiationless decay.


M. Venturi et al.


bridges[23–26] have been interpreted as occurring via Fçrster-type mechanism or both Fçrster- and Dexter-type mecha-nisms.

The expected rate of energy transfer according to the Fçr-ster mechanism can be calculated on the basis of spectroscop-ic quantities by Equation (2):

ken ¼ 5:87� 10�25ðFD=n4tDr6ÞZ1

0

FDð�uÞeAð�uÞd�u=�u4 ð2Þ

where FD and tD are, respectively, the luminescence quantumyield and lifetime of the donor, n is the solvent refractiveindex, r is the distance (in P) between donor and acceptor, andthe integral is related to the overlap between donor emissionand acceptor absorption.

For *RuII1OsII, FD=0.014, tD=285 ns, r=15 and 17 P for thecis and trans conformer, respectively, and the value of the over-lap integral is 4.3N10�14

m�1 cm3.[27] The values obtained for ken

from Equation (2) (3.3N107 and 1.5N107 s�1, respectively, forthe cis and trans conformer) do not fully agree with the experi-mental ones (2.0N108 and 2.2N107 s�1 at 298 K). In view of theapproximations involved, it could be that the Fçrster mecha-nism accounts for the observed energy-transfer process, butthe possibility cannot be excluded that a Dexter-type mecha-nism is also involved.

If we assume that the energy-transfer process takes place bythe Dexter mechanism, the experimental rate constants can beused to obtain the electronic matrix element of the electron-exchange process. The rate constant for a Dexter-type (elec-tron exchange) energy transfer may be expressed in an abso-lute rate formalism[28,29] as [Eq. (3)]:

ken ¼ nNkexpð�DGen6¼=RTÞ ð3Þ

where nN is the average nuclear frequency factor, k is the elec-tronic transmission coefficient, and �DGen

¼6 is the free activa-tion energy. The last term can be expressed by the Marcusquadratic relationship[30] [Eq. (4)]:

DGen6¼ ¼ ðlen=4Þð1þ DGen

o=lenÞ2 ð4Þ

where len and DGen8 are, respectively, the intrinsic barrier andthe standard free energy change of the energy-transfer proc-ess. Using the reasonable[23,28, 29,31] assumptions that : i) the free-energy change can be expressed by the difference betweenthe zero–zero spectroscopic energies of the donor and accept-or state (�0.33 eV, as deduced from the emission spectra at77 K), and ii) the len parameter is equal to the spectroscopicStokes shift (i.e, about. 0.2 eV),[31,32] the process falls in theMarcus inverted region with a value of about +0.44 for theexponential term of Equation (3) at room temperature. Assum-ing nN=1N1013 s�1, the experimental values 2.0N108 and 2.2N107 s�1 for ken yield [Eq. (3)] values of about 4.5N10�5 and 5.0N10�6 for k. The processes can thus be considered nonadiabati-c[3a,33] and Equation (3) can be rewritten as [Eq. (5)]:

ken ¼ nenexpð�DGen6¼=RTÞ ð5Þ

where nen is an electronic frequency given by Equation (6):

nen ¼ ð2Hen2=hÞðp3=lenRTÞ1=2 ð6Þ

where Hen is the electronic matrix element. The resultingvalues that represent an upper limit estimation are about 4.5N108 and 5.0N107 s�1 for nen, and 0.9 and 0.3 cm�1 for Hen.

The experimental values of the energy-transfer rate constantcan thus be accounted for by small electronic interactions,comparable to those exhibited by another dinuclear polypyri-dine RuII–OsII complex linked by a rigid aliphatic bridge.[27]

Spectroscopic Properties of the Mixed-Valence RuII1OsIII

Compound

Titration of a RuII1OsII acetonitrile solution by an acetonitrilesolution of CeIV (see Experimental Section) caused strongchanges in the absorption spectrum, with isosbestic points at400 and 750 nm (Figure 6).

After addition of one equivalent of oxidant, no furtherchange was observed (Figure 7). The linear decrease in intensi-ty of the MLCT absorption bands in the spectral region be-tween 400 and 700 nm indicates that oxidation concerns onlythe Os-based unit.[34]

It can be noticed (Figure 6, inset) that such a decrease is ac-companied by the appearance of a relatively weak, broadband with maximum at 720 nm (emax=250m�1 cm�1). In princi-ple, this band could be due to either: i) a ligand-to-metalcharge-transfer band of the OsIII moiety, or ii) an “intervalence”RuII!OsIII transition in the dinuclear RuII1OsIII species. Indeed,the energy of this band (1.72 eV) is consistent with that ex-pected from the Hush relationship for an intervalence-transferband[35] [Eq. (7)]:

DEop ¼ DGo þ lel ð7Þ

Figure 6. Absorption spectral changes observed during the titration ofRuII1OsII (2.0N10�5

m) with CeIV in acetonitrile solution (optical path=5 cm).The inset shows the growth of the new band during the titration.




where DG8 is the free-energy change (+0.44 eV from the elec-trochemical data, Table 3) and lel is the reorganizationalenergy for the electron-transfer process (ca. 1.3 eV, see below).It should be noticed, however, that an intervalence-transferband was seldom observed[36] in previously investigated mixedvalence RuII–OsIII polypyridine compounds.[27,37–40]

In the assumption that the band centered at 720 nm is inter-valence-transfer in nature, following the Hush theory,[35] theelectronic-coupling matrix element, Hel, is given by Equa-tion (8):

Hel2 ¼ emax DEopD�u1=2=2380r2 ð8Þ

where emax and DEop (in cm�1), as previously seen, are the maxi-mum intensity and the energy of the band, respectively, D�u1/2(in cm�1) is the half-width of the band, and r is the intercom-ponent distance (in P). By using the experimental values emax=

250m�1 cm�1, DEop=13900 cm�1, D�u1/2=3400 cm�1, and athrough-bond distance of 2.7 nm, Hel would result to be ashigh as 80 cm�1. As we will see later, such a high value wouldbe inconsistent with the much lower Hel value obtained for theRuIII1OsII!RuII1OsIII back electron-transfer process.

In principle the intervalence-transfer nature of the 720 nmband of the RuII1OsIII species could have been checked by fur-ther oxidation of the RuII moiety, which would cause the disap-pearance of the band. Unfortunately, oxidation of the RuII

moiety cannot be obtained under our experimental conditions.Lacking this opportunity, we checked whether the alternativeassignment of the 720 nm band as a ligand-to-metal charge-transfer band of the OsIII moiety is reasonable. At first sight thisdoes not seem to be the case since the lowest energy band of[Os(bpy)3]

3+ (a doublet–doublet ligand-to-metal charge-trans-fer band) shows its maximum at 563 nm.[41] We titrated withCeIV a solution of the dinuclear OsII1OsII under the same condi-tions used for titrating RuII1OsII. The results obtained (Figure 8)showed that a broad absorption band arises above720 nm,which, after one equivalent of oxidant, has approximately thesame intensity as that found in this spectral region for

RuII1OsIII. On addition of a second equivalent of oxidant toOsII1OsII, the intensity of the band practically doubles and doesnot change on further addition of CeIV. These results clearly in-dicate that most (if not all) of the absorption shown byRuII1OsIII in the low-energy region is indeed due to a bpy-to-OsIII moiety charge-transfer band. The reason why this band isred-shifted compared to that exhibited by [Os(bpy)3]

3+

(563 nm) is related to the higher electron-accepting ability ofthe “ligand” 1Os(bpy)2

3+ compared to bpy.Oxidation of the of RuII1OsII solution also causes changes in

the emission spectrum. As we have seen above, in RuII1OsII theRuII-based emission is strongly, but not completely, quenchedby energy transfer to the OsII-based one (Figure 4). After addi-tion of one equivalent of oxidant, the OsII-based emissionband (lmax=780 nm) can no longer be observed, showing thatthe OsII unit has been quantitatively oxidized. The disappear-ance of the OsII “quenching” unit, however, does not cause acomplete recovery of the of RuII-based band (lmax=645 nm),but only a slight increase (7%) in intensity (Figure 9). In otherwords, the RuII-based emission is slightly less quenched inRuII1OsIII compared to RuII1OsII.[42]

Intercomponent ElectronTransfer in RuII1OsIII

As we have seen above, in RuII1OsIII the RuII-based emission isquenched by the OsIII-based one. The energy-level diagramschematized in Figure 10 shows that, in principle, the quench-ing of the RuII-based excited state can takes place: i) by inter-component electron transfer via direct formation of theRuIII1OsII excited state or ii) by intercomponent energy transferto form the RuII1*OsIII species that then relaxes to RuIII1OsII.Laser flash photolysis experiments have evidenced an increasein absorbance in the 500–700 nm region, showing that theRuIII1OsII species is indeed formed during the laser pulse. Theintercomponent energy transfer from *RuII1OsIII to RuII1*OsIII is

Figure 7. Normalized absorption decrease at 700 nm (*) and correspondingincrease at 800 nm (~) observed during the titration of RuII1OsII with CeIV.

Figure 8. Absorption spectral changes observed during the titration ofOsII1OsII (2.5N10�5

m) with CeIV in acetonitrile solution (optical path=1 cm).The inset shows the growth of the new band during the titration. The spec-tra reported correspond to the addition of one (g) and two (a) equiva-lents of CeIV.


M. Venturi et al.


a quite complex process becauseof the MLCT and LMCT nature ofthe RuII- and OsII-based excitedstates, respectively. Therefore,this process should exhibit anon-neglible outer and innersphere reorganizational energyand, in view of its very small ex-ergonicity (ca. 0.15 eV),[27] it canhardly be a fast process. There-fore, we assume that thequenching of the RuII-based

emission of *RuII1OsIII takes place by electron transfer withdirect formation of RuIII1OsII.

The presence of a two exponential decay (6.2 and 37 ns,that is, kel,f 1.6N108 and 2.7N107 s�1, Table 5) supports theabove-mentioned hypothesis of two conformers. Incidentally, itcan be noticed that the relative weights of the two decays aredifferent for *RuII1OsII (relative weight 78% and 22%, Table 2)and *RuII1OsIII (relative weight 50% each), suggesting that therelative abundance of the two conformers is different for thesymmetrically and unsymmetrically charged species.

Flash photolysis experiments have also allowed us to meas-ure the rate of the back electron-transfer process, RuIII1OsII!RuII1OsIII (Figure 10), (kel,b 9.1N10

7 and 1.2N107 s�1, Table 5).According to current theories,[3a,33, 43] the rate of an electron-

transfer process in the nonadiabatic limit can be expressed byEquation (9):

kel ¼ nelexpð�DGel6¼=RTÞ ð9Þ

which is the electron-transfer version of the previously seenEquation (5). Similarly [Eqs. (10)–(11)]:

DGel6¼ ¼ ðlel=4Þð1þ DGel

o=lelÞ2 ð10Þ

nel ¼ ð2Hel2=hÞðp3=lelRTÞ1=2 ð11Þ

The reorganizational energy receives contribution from thereorganization of “inner” (bond lengths and angles within the

two reaction partners) and “outer” (solvent reorientationaround the reacting pair) nuclear modes. For the electron-transfer reactions involving the excited states of this kind ofcomplexes, the inner-sphere contribution has been estimatedto be very small (�0.1 eV), and the outer-sphere contributionin the polar acetonitrile solvent has been calculated to beabout 1.3 eV.[27,31] For the back electron-transfer reaction, theinner-sphere contribution to the intrinsic barrier is close tozero[31] and the outer-sphere contribution should be the sameas above, yielding an overall value of about 1.3 eV for lel.

By using the experimental rate constants and Equations (9–11), the values of nel and Hel can be obtained for the observedelectron-transfer processes (Table 5), which appear reasonablewhen compared with those reported for a dinuclear polypyri-dine RuII-OsIII complex linked by a rigid aliphatic bridge.[27]

Conclusions

We have investigated the kinetics of three processes, namely,Equations (12)–(14):

*RuII1OsII ! RuII1*OsII energy transfer ð12Þ

*RuII1OsIII ! RuIII1OsII forward electron transfer ð13Þ

Figure 9. Emission changes observed during the titration of RuII1OsII withCeIV in acetonitrile solution (lexc=400 nm).

Figure 10. Energy-level diagram showing the photoinduced electron-transferprocesses that occur in the RuII1OsIII complex. Key: (c), excitation; (a),luminescence; wavy line, radiationless decay.

Table 5. Experimental rate constants[a] and related parameters.

k [s-1] DG8 [eV] li [eV] lo [eV] exp(�DG¼6 /RT) n [s-1] H [cm�1]

Energy transfer[b]

*RuII1OsII!RuII1*OsII 2.0N108 �0.33 0.2 0 +0.44 4.5N108[c] 0.9[c]

2.2N107 5.0N107[c] 0.3[c]

Electron transfer[d]

*RuII1OsIII!RuIII1OsII 1.6N108 �1.48 0.1 1.3 +0.95 1.7N108 0.92.7N107 2.8N107 0.4

RuIII1OsII!RuII1OsIII 9.1N107 �0.44 0 1.3 +0.004 2.3N1010 101.2N107 3.0N109 4

[a] Air-equilibrated acetonitrile solution at 298 K. [b] Classical treatment based on Equations (4)–(6). [c] Upperlimit estimation. [d] Classical treatment based on Equations (9)–(11).




RuIII1OsII ! RuII1OsIII back electron transfer ð14Þ

For each process, we have found two distinct rate constants.This result indicates that each dinuclear complex exists as amixture of two different conformers. Different conformers canindeed be expected because steric hindrance between hydro-gen atoms of pyridine and phenyl rings in the bis-chelatingligand 1 forces the dinuclear complexes to assume a cis ortrans structure.[8b,15] In the cis structure the two metal ions lieon the same side of the plane defined by the shape-persistentmacrocycle 1 and the metal–metal distance is estimated to be1.5 nm from a space-filling model, whereas for the trans con-former the two metal ions lie on opposite sides of the planeand their distance is about 1.7 nm. The through-bond distance,of course, is the same (ca. 2.7 nm) for both structures, but thisdoes not mean that the through-bond electronic coupling hasto be the same in the two conformers. Indeed, Paddon-Rowand co-workers[44] have clearly shown that the through-bondelectronic interaction between two units linked by a bridge de-pends not only on the length and chemical nature, but also onthe conformation of the bridge. The two different rates ob-tained for each of the investigated processes can thus be as-signed to the different conformers, but it is not easy to estab-lish which structure exhibits the larger electronic coupling. If,as seems possible, the energy-transfer process takes place by athrough-space Fçrster mechanism, the faster energy-transferprocess should be that occurring in the cis-like conformer.

Concerning the RuII1OsIII complex, the rate for the forwardelectron-transfer process [Eq. (13)] is relatively slow comparedwith those reported for other dinuclear RuII–OsIII com-plexes.[27,37–40] This result, that at first sight could be surprising,can be accounted for by considering that the addition of aphenyl ring between polypyridine ligand and polyalkynespacers causes a dramatic reduction in the extent of electrondelocalization.[45]

Acknowledgments

We thank Prof. F. Scandola and Dr. C. Chorboli for useful discus-sions. Financial support from the Italian MIUR (SupramolecularDevices Project), FIRB (Manipolazione molecolare per macchinenanometriche), the University of Bologna (Funds for SelectedTopics), and Deutsche Forschungsgemeinschaft (Sfb 448, TP A1) isgratefully acknowledged.

Keywords: electrochemistry · electron transfer ·luminescence · osmium · ruthenium

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Received: June 20, 2005




photoinduced energy- and electron-transfer processes in dinuclear ruii–osii, ruii–osiii, and...

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