PCCP
Photoinduced energy and electron transfer processes in heteropolynuclear polypyridyl complexes of Ru(II) and Fe(II)y Fre´de´ric Lafolet,a Je´roˆme Chauvin,a Marie-Noe¨lle Collomb,a Alain Deronzier,*a He´le`ne Laguitton-Pasquier,za Jean-Claude Lepreˆtre,*a Jean-Claude Vialb and Bernard Brasmec a
Laboratoire d’Electrochimie Organique et de Photochimie Re´dox, Universite´ Joseph Fourier, UMR CNRS 5630, Institut de Chimie Mole´culaire de Grenoble, FR CNRS 2607, BP 53, 38041 Grenoble, Cedex 9, France. E-mail:
[email protected]; Fax: +33 4/76514267 b Laboratoire de Spectrome´trie Physique, Universite´ Joseph Fourier, UMR CNRS 5588, BP 53, 38041 Grenoble, Cedex 9, France c Centre de Recherches du service de Sante´ des Arme´es ‘‘ Emile Parde´ ’’, Laboratoire de Biospectrome´trie, 24 Avenue des Maquis du Gre´sivaudan, BP 87, 38702 La Tronche Cedex Received 20th March 2003, Accepted 7th May 2003 First published as an Advance Article on the web 22nd May 2003
Photophysical, photochemical and electrochemical studies of a new series of heterobinuclear complexes of ruthenium and iron, [RuII(bpy)2(Ln)FeII(bpy)2]4+ (bpy ¼ 2,20 -bipyridine, n ¼ 2 ([RuII(L2)FeII]4+), n ¼ 4 ([RuII(L4)FeII]4+), n ¼ 6 ([RuII(L6)FeII]4+), issued from the linkage of the Ru(bpy)32+ and Fe(bpy)32+ subunits by covalently bridging bis-bipyridine Ln ligands have been investigated. The cyclic voltammetry of the three complexes exhibits two successive metal-centered electron reversible oxidation processes, clearly separated, in the positive region. In the negative area, three successive reversible two electron waves are observed (the second and the third being strongly distorted by adsorption phenomena) corresponding to the ligand-based reduction processes. In addition, the two oxidized forms of these complexes [RuII(Ln)FeIII]5+ and [RuIII(Ln)FeIII]6+ obtained by two successive exhaustive electrolyses exhibit high stability, which is crucial for an efficient photocatalytic operating system. Luminescence of the complexes [RuII(Ln)FeII]4+ has been observed indicating that the covalently linkage between the Ru(bpy)32+ and Fe(bpy)32+ units leads to an only partial quenching of the RuII* excited state by energy transfer to FeII. The nature of the energy transfer process involved in those heterobinuclear complexes is studied and an intermolecular electron exchange mechanism is proposed as a preferably deactivation route. On the other hand, the photoxidation of the RuII subunit into the RuIII one could be easily obtained in the presence of a diazonium salt, playing the role of sacrificial oxidant. Finally photocatalytic oxidation of the complexes has been performed by continuous photolysis experiments. For each heteronuclear complexes, the multi-step oxidation process (FeII ! FeIII and RuII ! RuIII) has been observed. The comparison with an isoconcentrated mixture of the corresponding homonuclear parent complexes has been made.
Introduction Construction of molecular assemblies effecting energy transfer, electron transfer and/or photocatalysis in view to mimic, at the molecular scale, functions performed by natural systems has been growing of interest.1–3 A peculiar interesting design is based on a bimetallic complex where the two metal coordination sites are bridged by an innocent group (e.g.: alkyl chain) to prevent alteration of the individual properties of the both sites. Most complexes contain bridged two identical diimine liganding sites (2,20 -bipyridine (bpy) derivatives for instance) in which a [Ru(bpy)3]2+ chromophore moiety is connected to another metallic bipyridine complex acting as another chromophore or an electron acceptor or donor.4–16 Among these systems, those based on Ru–Mn bimetallic complexes have attracted a special interest since few years with the aim to model the donor-side of the photosystem II (PSII).17–20 A series y Electronic supplementary information (ESI) available: 1H NMR spectra of ruthenium complexes. See http://www.rsc.org/suppdata/ cp/b3/b303156j/ z Present address: Laboratoire de Chimie-Physique, UMR CNRS 8000, Universite´ de Paris-Sud, Bat. 350, 91405 Orsay Cedex, France.
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of binuclear Mn–Ru complexes, in which the [Ru(bpy)3]2+ moiety played the role of the photoactive P680 chlorophylls, has been synthesized and studied. It has been reported that an intramolecular electron transfer could occur from the Mn(II) site to the photogenerated RuIII species using an external electron acceptor like viologen in acetonitrile. However, it was found that the rate constant is fairly slow and that the efficiency of the process depends on the distance between the two metallic centers. On the other hand, the system is more complicated since the MnII/MnIII redox couple is generally only poorly reversible. MnIII species reacts with residual water to lead to oxo MnIII–MnIV binuclear complexes.21–23 Moreover, in most cases, MnII quenches the excited state of the ruthenium(II) complex presumably by an energy transfer process. In view to study this kind of intramolecular electron transfer, we have investigated the photoredox behavior of a series of heterobinuclear metallic complexes of RuII and FeII, [FeII(bpy)2(Ln)RuII(bpy)2]4+, (denoted [Ru(Ln)Fe]4+), based on a bridging bis-bipyridine ligand Ln, we recently synthesized (Scheme 1).11,24 This system is expected to be simpler than the RuII–MnII one since the FeII/FeIII is a perfectly reversible redox system, the oxidation of [Fe(bpy)3]2+ occurring at a potential close to that of [Mn(bpy)3]2+. In the present study,
Phys. Chem. Chem. Phys., 2003, 5, 2520–2527 This journal is # The Owner Societies 2003
DOI: 10.1039/b303156j
voltage was set within the range 40–200 V. Complexes in solution (1 mg ml1 in CH3CN:H2O (1 : 1)) were injected using a syringe pump at a flow rate in the range 5–10 mL min1.
Electrochemistry
Scheme 1 Chemical structure of the ligands Ln and of the heterobinuclear complexes [Ru(Ln)Fe]4+.
we have investigated the possibility to photoinduce electron transfer in those bimetallic complexes using an external sacrificial electron acceptor like an aryl diazonium salt. The expected multi-step oxidation process is summarized in Scheme 2. A first step leads to the photogeneration of the [RuII(Ln)FeIII]5+ species while a second one to the two electrons oxidized [RuIII(Ln)FeIII]6+ species. It has been demonstrated by continuous irradiation experiments that these processes occur with a great efficiency although the presence of the concurrent energy transfer process between the [Ru(bpy)3]2+ and [Fe(bpy)3]2+ moieties. Indeed, some previous studies have demonstrated that the ground state of [Fe(bpy)3]2+ quenches the luminescence of [Ru(bpy)3]2+*.25–28 Comparison of the resulting data issued from the [RuII(Ln)FeII]4+ complexes to those obtained with an isoconcentrated mixture of the non-covalently linked parent compounds, i.e. [Fe(bpy)3]2+ and [Ru(bpy)3]2+, seems to indicate that electron-transfer processes are mainly intermolecular processes.
All electrochemical measurements were run under an argon atmosphere in a dry-glovebox at room temperature. Cyclic voltammetry and controlled potential electrolysis experiments were performed using an EG&G PAR model 173 potentiostat/galvanostat equipped with a PAR model universal programmer and a PAR model 179 digital coulometer. The standard three-electrodes electrochemical cell was used. Potentials were referred to an Ag/10 mM AgNO3 reference electrode in CH3CN + 0.1 M Bu4NClO4 . Potentials referenced to that system can be converted to the ferrocene/ferricinium couple by adding 70 mV or to the ENH reference electrode by adding 0.52 V. The working electrodes were platinum disks polished with 2 mm diamond paste (Mecaprex Presi) that were 5 mm in diameter for cyclic voltammetry (CV; Epa , anodic peak potential; Epc , cathodic peak potential; E1/2 ¼ (Epa + Epc)/2; DEp ¼ Epa Epc) and 2 mm in diameter for rotating disk electrode experiments (RDE). Exhaustive electrolyses were carried out with a carbon felt electrode (RCV 2000, 65 mg cm3, from Le Carbone Lorraine). The auxiliary electrode was a Pt wire in CH3CN + 0.1 M Bu4NClO4 . For electrochemical experiments, electronic absorption spectra were recorded on a HewlettPackard 8452 A diode array spectrophotometer. Initial and electrolyzed solutions were transferred to a conventional cuvette 1 mm glass cell in the glovebox.
Spectroscopy Absorption. UV–Visible spectra were obtained using a Cary 1 absorption spectrophotometer on 1 cm path length quartz cells. The steady-state emission spectra were recorded on a Photon Technology International (PTI) SE-900M spectrofluorimeter. Emission quantum yield fL were determined at 25 C in deoxygenated acetonitrile solutions with a CH3CN solution of [Ru(bpy)3](PF6)2 as a standard (fref L ¼ 0.062 at 25 C26) according to eqn. (1) ref
fSL ¼
ILS ð1 10OD Þ ref f ILref ð1 10ODS Þ L
ð1Þ
where IL , the R emission intensity, was calculated from the spectrum area I( n )d n , OD represents the optical density at the excitation wavelength (450 nm). The superscripts ‘‘ S ’’ and ‘‘ ref ’’ refer respectively to the sample and to the standard. Scheme 2 Photocatalytic process for [Ru(Ln)Fe]4+ with a sacrificial oxidant (ArN2+) under irradiation.
Experimental Materials and general methods Acetonitrile (CH3CN, Rathburn, HPLC grade) was used after distillation over P2O5 and stored under an argon atmosphere in a dry-glovebox. Potassium hexafluorophosphate (Fluka), Tetra-n-butylammonium perchlorate (Bu4NClO4 , Fluka) and Ru(bpy)2Cl22H2O (Alfa) were used as received. 1H NMR spectra were recorded on a Brucker AC 250 spectrometer. The electrospray ionization mass spectrometry (ESI-MS) experiments were recorded on a Micromass Quattro mass spectrometer. The temperature was set at 80 C. The electrospray probe (capillary) voltage was optimized in the range 3.5–5 kV for positive ion electrospray. The sample cone
Luminescence. All the sample for luminescence and photoelectron transfer experiments were prepared in a dry-glovebox and contained in a quartz cell. The samples were maintained in anaerobic conditions with a Teflon cap. The luminescence lifetime of the complexes were performed after irradiation at l ¼ 337 nm with a 4 ns pulsed N2 laser (optilas VSL–337ND–S) and recorded at l ¼ 700 nm using filter and a photomultiplicator tube (Hamamatsu E2380–01) coupled with an ultra-fast oscilloscope (LeCroy 9310 AM).
Irradiation. The irradiation experiments have been performed using a mercury lamp (Oriel 66 901) (250 Watt) whose radiations were filtered with a large band filter centered around l ¼ 560 nm, P ¼ 0,15 W. The solutions were constituted by a mixture of heterobinuclear complexes (0.1 mM) and the diazonium salt (15 mM). In these conditions the concentration of diazonium salt can be considered as constant during the Phys. Chem. Chem. Phys., 2003, 5, 2520–2527
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experiments. The absorbance spectra of the solution were recorded after irradiation periods range from 10 to 30 seconds.
Results and discussion Synthesis
General synthesis 0
Synthesis of the ligands. The ligands 1,2-bis[4-(4 -methyl2,20 -bipyridinyl)]ethane (L2), 1,4-bis[4-(40 -methyl-2,20 -bipyridinyl)]butane (L4) and 1,6-bis[4-(40 -methyl-2,2 ’-bipyridinyl)]hexane (L6) were prepared as previously described.9,11 Synthesis of the diazonium salt. The synthesis of 4-bromophenyl diazonium tetrafluoroborate p-BrC6H4N2+, BF4 (ArN2+, BF4) was synthesized as previously described.29 Synthesis of [RuII(bpy)2(Ln)](PF6)2. The synthesis of [RuII(bpy)2(L2)](PF6)2 and [RuII(bpy)2(L4)](PF6)2 followed a similar procedure than that described previously for [RuII(bpy)2(L6)](PF6)2 .11 Reaction of one molar equivalent of Ln and [RuII(bpy)2(Cl)2]2H2O (100 mg, 0.19 mm) in 100 ml of ethanol at reflux during 4 h afforded an orange solution. After cooling to room temperature, addition of aqueous KPF6 solution (1 g in 100 ml H2O) allowed precipitation of the crude complex. The complexes were purified by chromatography on silica gel eluting with H2O–CH3CN (30 : 70) containing 0–2 mm KPF6 and isolated by removing CH3CN at reduced pressure and extraction by CH2Cl2 . Yields after chromatography: 60–70%. The purity of the complexes could be estimated by the observation of a single spot by thin-layer chromatography (TLC) on SiO2 (eluent H2O–CH3CN (30 : 70) containing 2 mM KPF6) and by 1H NMR. Synthesis of [RuII(bpy)2(Ln)FeII(bpy)2](PF6)4. Complexes [Ru(L2)Fe]4+ and [Ru(L4)Fe]4+ have been synthesized following a similar procedure which has been previously described for complex [Ru(L6)Fe]4+.11 To a freshly prepared 2 mM solution of [FeII(bpy)2(S)2](ClO4)2 (20 mL, 42 mM) in CH3CN at 0 C, one molar equivalent of [RuII(bpy)2(Ln)](PF6)2 was added, leading to the formation of the red-orange [RuII(bpy)2(Ln)FeII(bpy)2]4+ complex. After stirring the solution for 10 minutes, the crude heterobinuclear complex as (ClO4)2 , (PF6)2 salt was precipitated by addition of an excess of diethyl ether (100 mL), filtered and redissolved in CH2Cl2 . The resulting solution was washed three times by an aqueous KPF6 solution (0.1 M) in order to exchange the ClO4 salt with PF6 ones and then two times with water. After drying over Na2SO4 , the CH2Cl2 was removed under reduced pressure. The purity of each complex was established by 1H NMR analysis which showed the absence of signal of free bipyridine, and confirmed by the observation of a single spot by TLC (eluent H2O–CH3CN (30:70) containing 4 mM KPF6) and by electrochemistry (CV and RDE) [RuII(bpy)2(L2)FeII(bpy)2](PF6)4 ([Ru(L2)Fe]4+) (Yield 34%). Elemental analysis: C64H54N12P4F24RuFe (1728.15): calcd. C 44.49, H 3.15, N 9.73; found C 44.64, H 3.35, N 9.28. ESI-MS: m/z (relative intensity): 1582.7 (5%) [M– PF6]+, 718.9 (40%) [M–2PF6]2+, 431.1 (100%) [M–3PF6]3+, 287.2 (12%) [M–4PF6]4+. [RuII(bpy)2(L4)FeII(bpy)2](PF6)4 ([Ru(L4)Fe]4+) (Yield 37%). Elemental analysis: C67H62ON12P4F24RuFeCl2 (1756.18, [Ru(L4)Fe]H2OCH2Cl2): calcd. C 43.27, H 3.36, N 9.04; found C 43.35, H 3.20, N 8.55. ESI-MS: m/z (relative intensity): 1610.7 (1%) [M–PF6]+, 732.8 (36%) [M–2PF6]2+, 440.3 (100%) [M–3PF6]3+. [RuII(bpy)2(L6)FeII(bpy)2](PF6)4 ([Ru(L6)Fe]4+) (Yield 32%). Elemental analysis: C68H68O3N12P4F24RuFeCl2 (1784.22, [Ru(L6)Fe].3H2O): calcd. C 44.39, H 3.72, N 9.14; found C 44.29, H 3.68, N 9.32. ESI-MS: m/z (relative intensity): 746.9 (15%) [M–2PF6]2+, 449.5 (100%) [M–3PF6]3+, 301.1 (21%) [M–4PF6]4+. 2522
Phys. Chem. Chem. Phys., 2003, 5, 2520–2527
Preparation of pure samples is a key step to determine properly the photophysical properties of compounds. Indeed, the presence of by-products (uncomplexed Fe2+ and/or [RuII(bpy)2(Ln)]2+) contributes to the perturbation of the results as evidenced by Eliott et al. in the case of an in-situ formed {[(bpy)2RuII(L)3]FeII}8+ tetranuclear complex where L are various bridged bis-bipyridine ligands.27 In our case, the synthesis of the [Ru(Ln)Fe]4+ complexes has been performed following an original procedure we previously described for the synthesis of asymmetrical iron (II) polypyridine complexes.11 Complexes are synthesized by the stoechiometric reaction of [FeII(bpy)2(CH3CN)2]2 with [RuII(bpy)2(Ln)]2+ in CH3CN solution. The two labile ligands CH3CN of [FeII(bpy)2(CH3CN)2]2+ are easily replaced in the coordination sphere by the uncomplexed bipyridine moiety of Ln in [RuII(bpy)2(Ln)]2+. The resulting [Ru(Ln)Fe]4+ complexes immediately formed are isolated and purified as described in the experimental section. The purity of [Ru(Ln)Fe]4+ was displayed by TLC and 1H NMR. The 1H NMR analysis shows (Fig. S1 for L4), by the comparison of the spectra of the starting ligand [RuII(bpy)2(Ln)]2+ and that of the heterobinuclear one, the completion of iron complexation. The formation of the expected compounds is confirmed by the fact that after complexation both two CH2 and CH3 moieties, close to the coordination site, appears as single signals, whereas in the starting ruthenium complex, two signals (i.e. two singlets and two triplets) are distinguishable (Fig. S1). This reflects that no free ligand is present in the medium and that iron and ruthenium, lead to close shift of the both CH2 and CH3 moieties. Moreover, the ESI-MS spectra of [RuII(Ln)FeII]4+ exhibit only main peaks corresponding to the different cationic forms of each complexes associated with the loss of one to four PF6 anions (see experimental section). The absence of by-products is also confirmed by luminescence measurements since, for all complexes, the emission decay curves are clean single exponentials. Electrochemical behaviour The electrochemical behaviour of [Ru(Ln)Fe]4+ has been investigated in CH3CN + 0.1 M Bu4NClO4 . The cyclic voltammetry (CV) of the three complexes exhibits two successive one electron reversible oxidation waves in the positive region, (Fig. 1 for L4). Data potentials are reported in Table 1. By comparison with mononuclear complexes [Ru(bpy)3]2+ and [Fe(bpy)3]2+, the two oxidative processes have been clearly identified, the oxidation centered on the iron appearing at
Fig. 1 Cyclic voltammograms of a 0.6 mM solution of [Ru(L4)Fe]4+ in CH3CN + 0.1 M Bu4NClO4 at a platinum electrode. Scan rate 100 mVs1.
Table 1 Electrochemical data of [RuII(Ln)FeII]4+ in CH3CN + 0.1 M Bu4NClO4 using a platinum electrode at a scan rate of 100 mV s1 E1/2/V (DEp/mV) Reduction process FeII/FeIII
Complexes II
2+
[Fe (bpy)3] [RuII(bpy)3]2+ [RuII(bpy)2(L2)]2+ [RuII(bpy)2(L4)]2+ [RuII(bpy)2(L6)]2+ [RuII(L2)FeII]4+ [RuII(L4)FeII]4+ [RuII(L6)FeII]4+ a
RuII/RuIII
1
0.975 0.912 0.905 0.905 0.902 0.900 0.895
1.660 1.635 1.665 1.665 1.665 1.670 1.665 1.675
0.747 (65)
0.697 (65) 0.690 (90) 0.685 (60)
(70) (60) (60) (60) (65) (60) (60)
2 (60) (60) (50) (50) (50) (60) (50) (60)
1.855 1.835 1.855 1.862 1.865 1.865 1.865 1.862
3 (55) (70) (50) (50) (50) (70) (50) (75)
2.100 2.080 2.105 2.115 2.125
(60) (80) (50) (50) (50)
a a a
These values cannot be accurately measured since the waves are strongly distorted by adsorption phenomena (see the text).
the smaller anodic potential. Moreover, rotating disk electrode experiments in the positive region (data not shown), confirms that the three [Ru(Ln)Fe]4+ complexes are obtained in pure form since height of the wave of FeII/FeIII and RuII/RuIII are equal magnitude in agreement with the 1–1 Ru–Fe stoichiometry in the heterobinuclear complexes. In contrast since reduction of the bpy ligands in the regular Fe(bpy)32+ and Ru(bpy)32+ moieties occurs at very close potentials, the reduction pattern of the [Ru(Ln)Fe]4+ complexes involves three successive two-electron waves. However, only the first two-electron reduction wave is clearly seen. The two other subsequent waves are strongly distorted by some electroprecipitation-redissolution phenomena (Fig. 1). The length of the alkyl bridges has only a slight effect on the E1/2 values of the oxidative and reductive processes of the [Ru(Ln)Fe]4+ complexes. However, considering the metal-based oxidation processes, the general trend, which emerges from the consideration of the potential data in Table 1 is that the E1/2 values decrease upon increasing the number of methylenes in the alkane bridge. The weak magnitude of the shift in potential of [Ru(L2)Fe]4+ relative to that [Ru(L4)Fe]4+ and [Ru(L4)Fe]4+ relative to that of [Ru(L6)Fe]4+ (about 5 mV) is close to that observed for the [Ru(bpy)2(Ln)]2+ complexes (Table 1) and indicates that the decrease of potential are only due to the electron-donating properties of the methylene of the alkyl bridge. Electrostatic interactions between the two metal centers (Fe and Ru) would increase the magnitude of the shift, as previously demonstrated in the case of triply-bridged iron complexes28 (shift of about 35 mV) and can be excluded in our case. For the ligand-based reduction processes, examination of the data in Table 1 reveals that there is no significant shifts in potential for the second reduction process of the [Ru(Ln)Fe]4+ complexes, while for the first one, the smallest potential observed corresponds to [Ru(L4)Fe]4+. The stabilities of the two oxidized forms of the complexes, [RuII(Ln)FeIII]5+ and [RuIII(Ln)FeIII]6+ have been evaluated using exhaustive electrolyses since these species are expected to be produced during the photoinduced electron transfer processes. Taking into account the large difference of potential between the FeII/FeIII and RuII/RuIII redox systems (DE1/2 ¼ 210 mV for L4 and L6 and 205 mV for L2), the mixed valent [RuII(Ln)FeIII]5+ are perfectly stable. Indeed, two successive exhaustive electrolyses carried out at 0.80 and 1.0 V consumed one-electron per [RuII(Ln)FeII]4+ and allowed the bulk build up of [RuII(Ln)FeIII]5+ and [RuIII(Ln)FeIII]6+ species, respectively. This is illustrated by the evolution of the absorption spectra of the solutions after these sequential electrolyses (Fig. 2 for L4). The typical initial band of the FeII species (lmax ¼ 516 nm) has disappeared after one electron per complex has been passed. After exchange of another electron the band of the RuII species (lmax ¼ 456 nm) has been replaced by those of the RuIII ones (lmax ¼ 431 and 628 nm).
Photophysical properties Energy transfer process. All the data relative to the absorption and emission spectra of the heterobinuclear complexes [RuII(Ln)FeII]4+ and the parent [Ru(bpy)2(Ln)]2+ ones are listed in Table 2. The absorption spectra of [RuII(Ln)FeII]4+ closely results from the overlap of those of the mononuclear units Fe(bpy)32+ and Ru(bpy)32+ units. They exhibit two bands at l ¼ 516 nm and l ¼ 456 nm associated respectively to the FeII ! bpy and RuII ! bpy metal to ligand charge transfer (Fig. 2). The emission spectra are typical of the 3MLCT excited state of deactivation of the Ru(bpy)32+ parent complex. No significant spectral change on the absorption and emission spectra is observed according to the length of the alkyl linker. Only a weak red shift of the emission band with respect to that of Ru(bpy)32+ occurs, characteristic of the Ru(II) complexes bearing bi-alkyl substituted bipyridine ligand.30 Although the emission lifetime of the heterobinuclear complexes [RuII(Ln)FeII]4+ is slightly decreased versus those of the monometallic Ru parent complexes [Ru(bpy)2(Ln)]2+, their luminescence quantum yields (f) are dramatically lowered to ca. 1/3 (Table 2). This indicates that the Fe(bpy)32+ unit quenches the 3MLCT excited state in the [RuII(Ln)FeII]4+ complexes. A quenching process by electron transfer has to be ruled out since in the most favorable case this process would be endothermic by 0.33 eV.31 Thus energy transfer process, which depends on spectral overlap of that of the luminescence of the donor with the absorption spectra of the acceptor, is the main quenching interaction that should be taken into account. These kind of energy transfer phenomena have been previously investigated for a mixture of [Fe(bpy)3]2+ and [Ru(bpy)3]2+ in water, and a quenching rate constant of 109 M1s1 has been reported.25 As a first step, the energy transfer of a mixture of [Ru(bpy)2(Ln)]2+ with [Fe(bpy)3]2+ in CH3CN has been
Fig. 2 Absorption spectra of a 1 mM solution of [Ru(L4)Fe]4+ in CH3CN + 0.1 M Bu4NClO4, (a) initial solution, (b) after exhaustive oxidation at 0.80 V, (c) after exhaustive oxidation at 1.00 V (l ¼ 1 mm).
Phys. Chem. Chem. Phys., 2003, 5, 2520–2527
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Table 2
Photochemical data and energy transfer rate constant of [RuII(bpy)2(Ln)]2+ and [RuII(Ln)FeII]4+ in a deoxygenated CH3CN solution
Complexes
labs/nm (e/M1 cm1)
lemis/nm
f (25 C)
t/ms (25 C)
kEN/L mol1 s1
[FeII(bpy)3]2+ [RuII(bpy)3]2+ [RuII(bpy)2(L2)]2+ [RuII(bpy)2(L4)]2+ [RuII(bpy)2(L6)]2+ [RuII(L2)FeII]4+ [RuII(L4)FeII]4+ [RuII(L6)FeII]4+
526 450 454 454 454 456 456 456
603 611 612 612 612 612 613
0.062[26] 0.060 0.054 0.059 0.022a 0.019a 0.023a
1.06 1.11 1.12 1.13 1.10a 1.03a 1.00a
9.0109 b 1.41010 b 1.81010 b 1,81010 b 3.5108 2.2109 3.3109
a
(8700) (15 000) (12 000) (12 300) (12 700) (17 800) 516 (8600) (17 800) 516 (8400) (17 800) 516 (8400)
values determined for a 0.4 104 M solution of the binuclear complexes
studied. We observed that the quantum yield of the luminescence of [Ru(bpy)2(Ln)]2+ decreases when the concentration of [Fe(bpy)3]2+ increases, without any influence on the luminescence lifetime of [Ru(bpy)2(Ln)]2+. It would suggest that the energy transfer is mainly a trivial radiative process in the concentration range used. The luminescence of [Ru(bpy)2(Ln)]2+ being partly re-absorbed by [Fe(bpy)3]2+. The rate constant kEN of the process has been determined in CH3CN using the following Stern–Volmer equation (eqn.(2)): f0 =f ¼ 1 þ kEN t0 ½FeðbpyÞ3 2þ
ð2Þ
where f0 and f are the luminescence quantum yield of the [Ru(bpy)2(Ln)]2+ without and in the presence of a variable concentration of [Fe(bpy)3]2+ respectively, t is the luminescence lifetime of the [Ru(bpy)2(Ln)]2+. The kEN values obtained (Table 2) are higher than the diffusion limit rate constant of the molecules and are in concordance with a trivial radiative transfer, since the phenomenon does not depend on the relative diffusion of the molecules, the approach of the two complexes is not required, [Fe(bpy)3]2+ complex acting as an optical filter. These kEN values are higher to that determined for the mixture of [Fe(bpy)3]2+ and [Ru(bpy)3]2+ in water,25 this discrepancy could be attributed to the difference of the luminescence properties of the [Ru(bpy)3]2+ derivatives in water and in organic solvents.26,32,33 The energy transfer phenomenon in the heterobinuclear structures [RuII(Ln)FeII]4+ is more complicated since the lifetime of the heterobinuclear structures are now depending of the concentration of the solution. The distance between the two metallic centers is fixed and some short distance energy transfer process may also occur in the heterobinuclear structures in complement of the trivial energy transfer mentioned above. To our knowledge, only two structures containing Ru(bpy)32+ and Fe(bpy)32+-like subunits connected by an alkyl chain have been previously studied. In the first structure one iron center is complexed by three ruthenium [Ru(bpy)2(L)]2+-like moieties.27 Unfortunately the complexes have not be isolated and were prepared in situ by a simple mixing of Fe2+ and [Ru(bpy)2(Ln)]2+-like ligand in acetonitrile solution. In these experimental conditions the authors observed a decay of luminescence fitted by a biexponential function, the additive short-time component being due to the presence of residual free Fe2+ in the medium. For the main component, kEN values determined according to the length of the bridging alkyl chain, appear to be strongly dependent of the intermetallic Fe to Ru distance. These authors have also studied a rigid heterobinuclear structure within the two metallic centers complexed by three bis-bipyridine ligands.9 Although the photophysical behaviour of their complexes appears difficult to be unambiguously analyzed, the authors suggested that the energy transfer could be due to an electron exchange mechanism involving a pathway through the solvent. The emission decay of our heterobinuclear structures are in each case a single exponential, showing that no Fe2+ ions is present in the solution. The important feature is that the lumi2524
Phys. Chem. Chem. Phys., 2003, 5, 2520–2527
b
values determined in the presence of [Fe(bpy)3]2+
nescence quantum yield and the emission lifetime t of the heterobinuclear complexes both depend on the concentration of the complexes (Fig. 3). Although the variation of t is modest, from 1.12 to 0.95 ms, it remains significant. At high dilution, the lifetime (t1) of the heterobinuclear complexes becomes equal to the lifetime (t0) of the corresponding [Ru(bpy)2(Ln)]2+ complex. The variation of t vs. concentration suggests that the energy transfer can occur partly by an intermolecular process. For complexes [RuII(Ln)FeII]4+ the distance between the donor and the acceptor metals centers has been estimated ˚ , by space filling molecular models.27 At between 10 to 13 A these distances a dipole–dipole interaction may occur.27,34 The critical Fo¨rster distance (distance at which the energy transfer is as efficient as the other deactivation way of the [Ru(bpy)2(Ln)]2+* subunit) can be calculated using eqn.(3)34 R60
f ¼ 1:25 10 4 n 17
Z1
FD ð n ÞeA ð nÞ
d n n4
ð3Þ
0
where n is the refractive index of the solution, the integral term represent the overlap between the normalized donor emission and the acceptor absorption spectrum. A critical Fo¨rster dis˚ whatever the RuII–FeII distance in the heterobitance of 20 A nuclear complexes is, has been estimated, indicating that in the system, the dipole–dipole interaction mechanism is feasible. Nevertheless this dipole–dipole interaction is not consistent with the fact that t1 is equal to t0 of the corresponding [Ru(bpy)2(Ln)]2+. For these reason, the energy transfer between the two metallic centers probably follows an electron exchange mechanism.34 This transfer is a short range phenomenon, since it involves the interchange of electrons between Ru(bpy)32+* and Fe(bpy)32+ and therefore overlap of the orbital of the two metallic subunits of the binuclear complexes. This condition can be satisfied in an intramolecular or intermolecular process, the first one involving the folding up of the complex (the linker is flexible), whereas the second requires the reaction between two molecules. Considering the probable folding up of the molecule allowing an intramolecular energy exchange, this process should be ruled out since this phenomenon is
Fig. 3 Luminescence lifetime in deoxygenated CH3CN versus concentration for: [Ru(L2)Fe]4+ (;), [Ru(L4)Fe]4+ (S) [Ru(L6)Fe]4+ (L).
independent of the concentration and should lead to t1 different to the corresponding t0 value. Moreover, t1 does not depend on the nature of the heterobinuclear complex even for [RuII(L6)FeII]4+ where the two metallic centers are sufficiently distant to allow the folding up of the complex. Thus, the variation of the lifetime t, according to the [RuII(Ln)FeII]4+ concentration, reflects the fact that the energy transfer should operate, in the time range of our experimental conditions, preferably through an intermolecular process. To estimate the kEN for the binuclear structure, we used a Stern–Volmer equation based on the luminescence lifetime measurements (eqn.(4)): t0 =t ¼ 1 þ kEN t0 ½RuðLnÞFe4þ
ð4Þ
using luminescence lifetime of [Ru(bpy)2(Ln)]2+ as t0 and the dependence of t vs. [Ru(Ln)Fe]4+ as variable. The kEN values are given in Table 2. It appears from these values that kEN decreases along the closeness of the two metallic centers, the weaker value being observed for [RuII(L2)FeII]4+. This variation seems to be governed by electrostatic interaction. Indeed for [RuII(L2)FeII]4+ the approach of the two molecules should be partly inhibited, the two cationic metallic centers, which both bear a 2+ charge being close. On the other hand, for [RuII(L6)FeII]4+ where the Fe to Ru distance is large, repulsion phenomena are less important and lead to a faster energy exchange rate. However no definitive conclusion can be given since the linker is not rigid and can rotate inducing a large variation of the distance between the metal centers, especially for the [RuII(L4)FeII]4+ and [RuII(L6)FeII]4+ complexes. On the other hand an intramolecular process for the energy transfer phenomenon cannot be fully ruled out since the luminescence intensity of the [RuII(Ln)FeII]4+ is lowered to about 1/3 compared to [RuII(Ln)]2+ and the luminescence lifetime is only slightly changed. This could result from an additional short-lived event (sub-nanosecond) which evades our experimental set-up (see experimental part). Photo-induced electron transfer To try to induce an electron transfer between the Ru and the Fe subunits, an irreversible electron acceptor as the bromophenyl diazonium tetrafluoroborate (ArN2+,BF4) has been added to the solution of the binuclear complexes in CH3CN. The role of this sacrificial oxidant is to react preferentially with the excited state of the Ru(bpy)32+* subunit and then to compete with the energy transfer process seen above. Indeed, aromatic diazonium salts are known to efficiently quench (kq ¼ 2 109 M1s1 in CH3CN + 0.1 Bu4NClO4) the excited state of the regular [Ru(bpy)3]2+ to form the [Ru(bpy)3]3+ species which is quantitatively photogenerated under continuous visible irradiation with a good quantum yield (f ¼ 0.34).29,30,35 This permanent build-up of [Ru(bpy)3]3+ arises because of the following electron transfer quenching reactions (eqns. (5) and (6)): RuðbpyÞ3 2þ ! RuðbpyÞ3 2þ hn
kq
ð5Þ
RuðbpyÞ3 2þ þ ArN2 þ þ ! RuðbpyÞ3 3þ þ ArN2
ð6Þ
Fig. 4 Stern–Volmer plots in deoxygenated CH3CN for: [Ru(L2)Fe]4+ with ArN2+(L), [Ru(L4)Fe]4+ with ArN2+(S), [Ru(L6)Fe]4+ with ArN2+(;) and Ru(bpy)32+ with ArN2+().
For the [RuII(Ln)FeII]4+ complexes, it is expected a similar efficient photogeneration of the corresponding RuIII species, i.e. [RuIII(Ln)FeII]5+, followed by a rapid electron exchange leading to the [RuII(Ln)FeIII]5+ species (Scheme 2, eqns. (1)– (3)). Indeed the lifetime of the excited state of the FeII moiety is too short (
