DOI: 10.1002/cphc.200900728

An Electrochemical and Photophysical Study of aCovalently Linked Inorganic–Organic Dyad**Axel Kahnt,[a] Leo-Philipp Heiniger,[b] Shi-Xia Liu,*[b] Xiaoyan Tu,[d] Zhiping Zheng,[d]

Andreas Hauser,[c] Silvio Decurtins,[b] and Dirk M. Guldi*[a]

1. Introduction

Inspired by natural photosynthesis, the creation of multicom-ponent systems capable of undergoing directional electronand/or energy transfer reactions has attracted considerable at-tention.[1] In this context, great efforts have been directed to-wards the design and synthesis of discrete model systems con-sisting, for example, of suitable chromophores with optimalredox potentials, optimized electron donor (D) and acceptor(A) orientation, distance and chemical environment.[2, 3] Amongthe many photoactive units that have been tested successfullyas integrative building blocks for artificial photosynthetic sys-tems, porphyrins and the structurally related phthalocyaninesas well as ruthenium(II) polypyridine complexes stand out asunique components with promising absorption cross sectionsacross the solar spectrum.[4] Most of the recent work is, howev-er, concerned with combining these versatile chromophoreswith either electron-accepting fullerenes,[5] benzoquinones[6]

and perylenediimides[7] or electron-donating TTF derivatives.[8]

Similar to ruthenium(II) polypyridine complexes, hexanuclearrhenium(III) cluster complexes show unique redox and photo-luminescent properties as well as the versatility of the coordi-nation environment around the hexarhenium cores,[Re6(m3-Q)8]2 + (Q = S, Se, Te).[9] Only a few precedents to[Re6(m3-Q)8]2 + with peripherally attached porphyrin chromo-phores are known to date.[10] However, the electronic interac-tions between [Re6(m3-Q)8]2+and porphyrin units have not yetbeen investigated in detail. Moreover, no examples of electronD–A dyads consisting of [Re6(m3-Q)8]2 + and fullerene cores havebeen reported.

By taking advantage of the substitution-inert nature of[Re6(m3-Q)8]2 + , which allows controlled substitution of a termi-nal ligand in a stepwise way, a variety of supramolecular as-semblies with well-defined octahedral giant centers and fasci-nating physicochemical properties have been prepared.[11] Fol-lowing this strategy, we have obtained the dyad 1 comprisingN-pyridyl-3,4-fulleropyrrolidine and [Re6(m3-Se)8]2 + moieties

(Scheme 1). This hybrid complex with multielectron redox andphotoactive functions could be particularly interesting due totwo types of emissive chromophores directly connected toeach other via covalent association. Therefore, we reportherein on the electrochemical behavior and photophysicalproperties of this dyad. Importantly, intramolecular electroniccoupling between the individual components both in theground state and in the excited state were investigated byphotoemission and optical transient absorption measurements.

A molecular donor–acceptor dyad comprising a hexarheniumcluster core, [Re6(m3-Se)8]2 + , and a fullerene moiety which arecovalently linked through a pyridine ligand was synthesizedand fully characterized. The electrochemical and photophysicalproperties are reported. The detailed study includes cyclic vol-tammetry, steady-state absorption and fluorescence spectro-

scopy, radiation chemistry and transient absorption spectrosco-py. A light-induced electron transfer between the inorganiccluster moiety and the fullerene can be excluded. However, alight-induced energy transfer from the rhenium cluster to thefullerene is proposed.

[a] Dr. A. Kahnt,+ Prof. D. M. GuldiDepartment of Chemistry and Pharmacyand Interdisciplinary Center for Molecular MaterialsUniversit�t Erlangen-N�rnbergEgerlandstr. 3, D-91058 Erlangen (Germany)Fax: (+ 49) 9131 8528307E-mail : guldi@chemie.uni-erlangen.de

[b] L.-P. Heiniger, Dr. S.-X. Liu, Prof. S. DecurtinsDepartement f�r Chemie und BiochemieUniversit�t BernFreiestrasse 3, CH-3012 Bern (Switzerland)Fax: (+ 41) 1-6313995E-mail : liu@iac.unibe.ch

[c] Prof. A. HauserD�partement de chimie physiqueUniversit� de Gen�ve30 quai Ernest-Ansermet, CH-1211 Gen�ve 4 (Switzerland)

[d] Dr. X. Tu, Prof. Z. ZhengDepartment of ChemistryUniversity of ArizonaTucson, AZ 80721-0041 (USA)

[+] current address : Department of Chemical and BiologicalEngineering/Physical ChemistryChalmers University of TechnologySE-412 96, Gçteborg (Sweden)

[**] [Re6(m3-Se)8(PEt3)5(C60py)](SbF6)2

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2. Results and Discussion

2.1. Synthesis

Dyad 1 was prepared by reacting stoichiometric amounts of 2and 3 (Scheme 1). For this kind of ligand substitution, long re-action times and rather high temperatures are usually re-quired.[12] Therefore, a suspension of 2 and 3 in dry chloroben-zene was refluxed under argon for five days, resulting in theformation of a brown precipitate, which then was washed withtoluene and chloroform to remove traces of unreacted startingmaterials. By the addition of diethylether to its dichlorome-thane solution, pure 1 was precipitated in a good yield (78 %).

The successful replacement of MeCN of 3 by the fullerenederivative was confirmed by the disappearance of the charac-teristic CN-vibrations at 2284 and 2317 cm�1. The 1H NMR spec-trum of 1 shows no signal for the protons of acetonitrile, butinstead two doublets in the aromatic region and one singlet at5.42 ppm, corresponding to the protons of 2. As expected, theprotons ortho to the pyridine nitrogen are shifted downfieldwith respect to those of the free ligand, due to the decreasedelectron density on the nitrogen atom upon coordination tothe rhenium atom of the cluster. Compound 1 was furthercharacterised by 31P NMR, mass spectrometry and elementalanalysis. Figure 1 shows the experimental and calculated iso-tope distribution pattern of the molecular peak in the ESI massspectrum.

2.2. Electrochemistry

The redox properties of dyad 1 and reference compounds 2and 3 were studied by cyclic voltammetry (Table 1). The rheni-um cluster 3 exhibits a reversible one-electron process centredat E1/2

ox = 0.75 V, corresponding to the oxidation of the clustercore, formally involving the couple ReIII

6/ReIII5ReIV. In dyad 1,

this E1/2ox is slightly negative-shifted (30 mV). Moreover, in the

cathodic direction, three single-electron reduction waves areobserved for 1, corresponding to the consecutive reduction ofthe fullerene unit. Compared to 2, the first two reductions arereversible, while the third one is irreversible at a scan rate of0.2 V s�1. When increasing the scan rate to 1.5 V s�1, this thirdreduction wave becomes quasi-reversible. Small anodic shiftsare observed for 1 for the first (60 mV) and the second (10 mV)reductions, due to delocalisation of electron density onto therhenium cluster. In conclusion, the CV measurements confirmthat both the donor and the acceptor components essentiallypreserve their individual electro-active identities within thedyad and that they show only minor electronic interaction inthe ground state.

2.3. Photophysics and Radiation Chemistry

The absorption spectrum of the dyad molecule 1 in CH2Cl2

shows a strong absorption in the UV region of the spectrumwith a minor maximum around 330 nm. This absorption spec-trum (Figure 2) can be best described as the sum of the ab-sorption spectra of 2 and 3. Compound 3 shows an absorptionband in the UV region of the spectrum with a maximum ataround 270 nm and weaker absorption bands extending to480 nm, whereas compound 2 shows strong absorption bandsin the UV region with a characteristic minor maximum around330 nm in accordance with the literature.[13]

A first insight into possible electronic interactions betweenthe inorganic rhenium cluster and the C60 units in dyad 1comes from steady-state and time-resolved emission spectro-scopy, and in particular from time-resolved transient absorp-tion spectroscopy. When exciting compound 3 at 260 or458 nm, a broad oxygen-sensitive emission with a maximum ataround 750 nm and a lifetime of 8.6 ms is found (Figure 3). Thisobservation is in agreement with previous results reported byGray et al.[9f]

Scheme 1. A synthetic route to 1.

Figure 1. Theoretical isotopic distribution (left) and ESI mass spectrum(right) of the doubly charged species 12+ (C97H83N2P5Re6Se8).

Table 1. E1/2 redox potentials (V vs Fc+/Fc) for 1 and reference com-pounds 2 and 3 in CH2Cl2).

Compound E1/2ox E1/2

red1 E1/2red2 E1/2

red3

1 0.72 �1.02 �1.45 �2.002 – �1.08 �1.46 �1.983 0.75 – – –

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S.-X. Liu, D. M. Guldi et al.

When exciting compound 3 in CH2Cl2 with femtosecondlaser pulses at a wavelength of 258 nm, a rapid formation of along-lived transient absorption (Figure 4) with a maximum ataround 550 nm is observed. This transient shows virtually nodecay within the time detection range of the femtosecondlaser system (i.e. up to 3 ns).

In order to get more insight into the long lifetime behaviorof the excited cluster reference compound 3, correspondingns-laser photolysis experiments were performed. When excit-ing 3 with ns-laser pulses at 355 nm a transient similar to the

one obtained with fs-laser photolysis transient absorptionspectroscopy is observed. In addition, since the spectral detec-tion range of the ns-laser photolysis setup goes into the UVregion, beyond the detection range of the fs-laser setup, anadditional maximum at 325 nm is detected (Figure 5, top).Within experimental error, the decay time of the transient ab-sorption signal is the same as the phosphorescence decay(Figure 5, bottom).

As the transient absorption pattern of the oxidized and re-duced forms of the cluster reference compound 3 were un-known, these transients were generated by electron pulse radi-olysis. The oxidized form of the cluster was formed by radioly-sis of a solution of 3 in CH2Cl2 with short electron pulses gen-erating the radical cation of CH2Cl2

[14] and further oxidizing thecluster via bimolecular electron transfer according toScheme 2.

The cluster reference compound 3 was radiolytically reducedaccording to a previously documented strategy [15] by dissolv-ing a sample of it in a mixture of 80 % toluene, 10 % acetoneand 10 % isopropanol (see Scheme 3).

The pulse radiolysis transient absorption spectrum of theoxidized cluster 3 (Figure 6) shows a sharp transient absorptionband maximizing at 250 nm flanked by a minor transient ab-sorption band around 400 nm. The pulse radiolysis transientabsorption spectrum of the reduced cluster 3 (Figure 7) shows

Figure 2. Absorption spectra of 1 (c), 2 (g) and 3 (a) in dichlorome-thane.

Figure 3. Top: Steady-state emission spectrum of 3 in dichloromethane ex-cited at 458 nm. Bottom: Phosphorescence time profile at 730 nm upon ex-citation at 355 nm.

Figure 4. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon femtosecond flash photolysis (258 nm) of 3 in dichloromethane(50 � 10�6

m) with time delays of 1 ps (*) and 2892 ps (*) at room tempera-ture. Bottom: Time–absorption profile of the spectra shown above at550 nm, monitoring the excited-state deactivation.

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A Covalently Linked Inorganic–Organic Dyad

an absorption band in the 300 nm region that could not befully resolved due to the non-transparency of the solvent (tolu-ene) below 300 nm.

With regard to the fullerene reference compound 2, excita-tion at 260 nm, results in a broad and weakly structured fluo-rescence with a first maximum at 705 nm and a fluorescencequantum yield of 0.0014 (Figure 8, upper part). The fluores-cence lifetime upon 337 nm photoexcitation was measuredand typical values of 1.4 ns were obtained from a least squaresfit and deconvolution with the response function of the appa-ratus. This result is in line with previously reported results forfullerenes.[16]

When exciting the fullerene reference compound 2 withfemtosecond laser pulses at a wavelength of 258 nm, the tran-sient absorption spectrum shows a strong absorption bandwith a maximum at around 900 nm belonging to the first ex-cited singlet state of the fullerene (Figure 9).[17]

The lifetime of the transient absorption of the excited singletstate was obtained by a monoexponential fit of the 900 nmtime profile and is in very good agreement with the previouslyreported fluorescence lifetime measurement of 1.4 ns.

Corresponding with the decay of the singlet excited statefeatures, a new transient absorption with a maximum ataround 700 nm grows in. This transient is assigned to the cor-

Figure 5. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon nanosecond flash photolysis (355 nm) of 3 in dichloromethane(185 � 10�6

m) with a time delay of 2 ms at room temperature. Bottom: Time–absorption profile of the spectrum shown above at 325 nm, monitoring theexcited-state deactivation.

Scheme 2. Radiolysis of 3 to create 3+ via the formation of a CH2Cl2C+ radi-

cal.

Scheme 3. Formation of the reduced 3� radical by pulse radiolysis.

Figure 6. Differential absorption spectra (visible and near-infrared) obtainedupon pulse radiolytic oxidation (10 Gy) of 3 in dichloromethane (50 � 10�6

m)with a time delay of 2 ms at room temperature.

Figure 7. Differential absorption spectra (visible and near-infrared) obtainedupon pulse radiolytic reduction (10 Gy) of 3 in toluene, acetone and isopro-panol (80/10/10 v/v/v) [50 � 10�6

m] with a time delay of 2 ms at room tem-perature.

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S.-X. Liu, D. M. Guldi et al.

responding first excited triplet state of the fullerene populatedby intersystem crossing from the singlet state with a rate con-stant of kisc = 7.1 � 108 s�1.[17, 18] When exciting the fullerene 2with 308 nm ns-laser pulses, in order to get more insight intothe long lifetime behavior of the fullerene first excited tripletstate, the same transient absorption with a maximum ataround 700 nm is observed (Figure 10). The lifetime of the firstexcited triplet state of the fullerene reference compound 2could be determined from the decay curve of the transient ab-sorption at 700 nm. A monoexponential fit gives a typicalvalue of 10 ms.

As mentioned above, the absorption spectrum of the dyad 1is just the sum of the absorption spectra of the fullerene refer-ence compound 2 and the rhenium cluster reference com-pound 3. Exciting the dyad system in CH2Cl2 at 260 nm, thesame fluorescence pattern as for the fullerene reference com-pound 2 is observed, but the intensity is now considerablyquenched. The fluorescence quantum yield is reduced to0.00079 and the fluorescence lifetime is reduced down to0.43 ns as compared to 0.0014 and 1.4 ns, respectively, for thereference compound. No traces of the emission of the clusterwere observed in the dyad (see Figure 8).

When exciting the dyad with femtosecond laser pulses at258 nm, the transient absorption pattern of the first excitedsinglet state of the fullerene moiety with its characteristic ab-sorption around 900 nm was observed. The lifetime of thistransient absorption was fitted with a monoexponential decay,

which revealed a value of 0.41 ns with a simultaneous growthof the absorption band with the maximum at 700 nm assignedto the first excited triplet state of the fullerene (Figure 11).

Again we note that the fluorescence lifetime and the fittedlifetime of the first excited singlet state of the fullerene ob-tained from the femtosecond transient absorption measure-ments match well. Moreover, as observed in the emissionmeasurements, no trace of the excited states of the rheniumcluster moiety is found. When turning the view onto long-living transients based on our nanosecond laser photolysisstudies, again only the triplet first excited state of the fullerenewas observable and no trace of the triplet state of the clusterwas visible (Figure 12).

3. Conclusions

A first insight into the photophysical interaction between theinorganic rhenium cluster moiety and the fullerene moiety ofthe dyad comes from emission spectroscopy. When excitingthe dyad at 260 nm, only the emission of the fullerene wasmeasured but no trace of the emission of the rhenium clusterwas found. Moreover, the observed fullerene fluorescence wasquenched by a factor of two and the fluorescence lifetime wasreduced down to 0.43 ns. This observation gives a first hintthat an efficient excitation energy transfer from the photoexcit-

Figure 8. Top: Steady-state emission spectra of the 2 (*) and 1 (*) in di-chloromethane excited at 260 nm. Bottom: Time-fluorescence profiles at700 nm upon 337 nm excitation for 2 (*) and 1 (*) in dichloromethane.

Figure 9. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon femtosecond flash photolysis (258 nm) of 2 in dichloromethane(10 � 10�6

m) with time delays of 1 ps (*) and 2892 ps (*) at room tempera-ture. Bottom: Time–absorption profile of the spectra shown above at900 nm, monitoring the excited-state deactivation.

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A Covalently Linked Inorganic–Organic Dyad

ed rhenium cluster moiety to the fullerene takes place. In addi-tion, the reduced lifetime of the fullerene singlet excited stateand the reduced fullerene fluorescence quantum yield indi-cates that, even in the fullerene compound, the already effi-cient intersystem crossing is accelerated by the coupling tothe rhenium cluster moiety.

Direct proof for the proposed energy transfer comes fromtime-resolved transient absorption spectroscopy based on fem-tosecond and nanosecond laser photolysis. Upon exciting thedyad at 258 nm, where at least 15 % of the absorption comesfrom the inorganic cluster moiety, no traces of any transientsignal belonging to this cluster is observed. Since additionallyno transient signal of the oxidized or reduced form of the clus-ter or the fullerene is observed, a light-induced electron trans-fer from the rhenium cluster (i.e. 1.65 eV for the triplet excitedstate) to the fullerene (i.e. 1.76 and 1.5 eV for the singlet andtriplet excited state, respectively) or vice versa can be exclud-ed. Based on the estimated energy of the charge separatedstate of 1.75 eV from the electrochemical data, this is not sur-prising as lower-energy excited single chromophore states areavailable on either moiety. Moreover, the facts that no trace ofany transient signals related to the rhenium cluster is observedand that the intersystem crossing of the fullerene is accelerat-ed, are good reasons to propose an energy transfer from thecluster—probably from the rapidly formed first excited triplet

state of the cluster—into the first excited triplet state of thefullerene. On the other hand, energy transfer from the singletexcited state of the rhenium cluster into the singlet excitedstate of the fullerene and independently an accelerated inter-system crossing due to the heavy atom effect of the cluster inthe dyad cannot be excluded. Due to those facts we have noway to determine whether the energy transfer proceeds via aFçrster-type process[19] or a Dexter-type process[20] or via multi-ple pathways as combination thereof.

Experimental Section

General : All solvents and reagents were purchased from commer-cial suppliers and used without further purification. Reactions werecarried out under an inert argon atmosphere and were monitoredby thin-layer chromatography. N-pyridyl-3,4-fulleropyrrolidine (2)[21]

and [Re6(m3-Se)8(PEt3)5(MeCN)](SbF6)2 (3)[22] were prepared accordingto the literature procedures. 1H-, 13C- and 31P NMR spectroscopywere performed either on a Bruker Avance300 or a Bruker DRX400instrument. Chemical shifts d are reported in ppm and were cali-brated against the signal of the deuterated solvents. Infrared spec-tra were recorded on a Perkin–Elmer Spectrum One FT-IR spec-trometer. Mass spectra were obtained on a FTMS 4.7T BioAPEX IIfor the ESI. Elemental analyses were performed on a Carlo Erba EA1110 CHN instrument.

Figure 10. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon nanosecond flash photolysis (308 nm) of 2 in dichloromethane(8 � 10�6

m) with a time delay of 2 ms at room temperature. Bottom: Time–absorption profile of the spectrum shown above at 700 nm, monitoring theexcited-state deactivation.

Figure 11. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon femtosecond flash photolysis (258 nm) of 1 in dichloromethane(7 � 10�6

m) with time delays of 1 ps (*) and 2892 ps (*) at room tempera-ture. Bottom: Time–absorption profile of the spectra shown above at900 nm, monitoring the excited-state deactivation.

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S.-X. Liu, D. M. Guldi et al.

Synthesis: A suspension of compounds 2 (16.2 mg, 19.3 mmol) and3 (50 mg, 17.5 mmol) in 12 mL of dry chlorobenzene was refluxedat 140 8C for 5 days. The resulting precipitate was washed with tol-uene and chloroform, and then redissolved in CH2Cl2. By additionof diethylether to the CH2Cl2 solution, the product was precipitat-ed. After filtration, pure compound 1 was collected as a black-brown powder and dried at 50 8C (50 mg, 13.7 mmol, 78 %).1H NMR(CD2Cl2, 300 MHz): d= 8.91 (m, 2 H), 6.93 (m, 2 H), 5.42 (s, 4 H), 2.31–2.05 (m, 30 H), 1.26–1.03 ppm (m, 45 H);31P NMR (CD2Cl2, 300 MHz):d=�27.3 (4P), �30.3 ppm (1P); IR (KBr): n= 2968, 1615 (s), 1517,1456, 1413, 1378, 1224, 1034 (s), 761, 722, 657 (s), 527 cm�1; ESI-MS:m/z : 1595.80, calcd. for [M�2SbF6]2 + : 1595.80; elemental analy-sis calcd (%) for C97H83N2P5Re6Se8: C 31.90, H 2.29, N 0.77, found: C32.20, H 2.23, N 0.57.

Cyclic Voltammetry: Cyclic voltammetry was conducted on a VA-Stand 663 electrochemical analyzer. An Ag/AgCl electrode contain-ing 2 m LiCl served as reference electrode, a glassy carbon elec-trode as counter electrode, and a Pt tip as working electrode.Cyclic voltammetric measurements were performed at room tem-perature under N2 in CH2Cl2 with 0.1 m Bu4NPF6 as supporting elec-trolyte at a scan rate of 200 mV s�1. Ferrocene (Fc) was added as aninternal standard and all potentials were referenced relative to theFc+/Fc couple.

Photophysics and Radiation Chemistry: Optical absorptions weremeasured with a Lambda 2 UV/Vis spectrometer (Perkin–Elmer).Such measurements, with the exception of the femtosecond-pho-

tolysis experiments, were carried out by using quartz cells with a1 cm optical path length.Steady-state fluorescence measurements were performed by usinga Fluoromax 3 (Horiba JobinYvon). Fluorescence lifetimes weremeasured with a laser strobe fluorescence lifetime spectrometerwith 337 laser pulses from a nitrogen laser fiber coupled to a lens-based T-formal sample compartment equipped with a stroboscopicdetector (GL 3300 - PTI). The fluorescence lifetime decays werealways fitted as monoexponentials by including the instrument re-sponse function.Transient absorption spectroscopy experiments, based on nanosec-ond laser photolysis, were either performed by photolysing the sol-utions with the output of the third harmonics (i.e. 355 nm) comingfrom a Nd:YAG laser (Brilliant, Quantel). Moreover, pulse widths of<5 ns with energy up to 10 mJ were selected. The optical detec-tion is based on a pulsed Xenon lamp (XBO 450, Osram), a mono-chromator (Spectra Pro 2300i, Acton Research), R928 photomulti-plier tube (Hamamatsu Photonics) or Nano 5 InGaAs photodiode(Coherent) and a 1 GHz digital oscilloscope (WavePro7100, LeCroy).The laser power of every laser pulse was registered using a bypathwith a fast Silicon photodiode. Alternatively excitation was per-formed with the output of a XeCl Excimer laser (LPX 2000, LambdaPhysik) that is, 308 nm, selecting laser pulses with an energy of10 mJ per pulse and a pulse with of 20 ns. The optical detectionsystem was based on a pulsed Xenon lamp (XBO 450, Osram), amonochromator (Spex 270 m, Specs Industries), and a R955 photo-multiplier (Hamamatsu Photonics). The transient absorption signalswere recorded with a 1.5 GHz digital oscilloscope (LC684DXL,LeCroy). Femtosecond transient absorption measurements werecarried out with a CPA-2101 femtosecond laser (Clark MXR). Theexcitation wavelength was generated by third harmonic generation(258 nm). For these measurements, 2 mm quartz cells were em-ployed.Pulse radiolysis measurements were performed with a LINAC elec-tron accelerator (Titan Beta, TB-8/16-1S - Titan Cooperation). Elec-tron pulses with a pulse width of 8 ns and a dose of 10 Gy were se-lected. The dose per pulse was measured with thiocyanate dosime-try.[23] The transients were detected with an optical setup describedelsewhere.[24]

Acknowledgements

This work was supported by the Swiss National Science Founda-tion (grant No. 200020-116003 and 200020-115867), US NSF(CHE-0750530), EU (FUNMOLS FP7-212942-1) and the DeutscheForschungsgemeinschaft through SFB583. We like to thank theNotre Dame Radiation Lab, Notre Dame, Indiana USA for thesupport during the pulse radiolysis and XeCl Excimer laser photol-ysis measurements.

Keywords: cluster compounds · cyclic voltammetry · energytransfer · fullerenes · radiation chemistry

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Figure 12. Top: Differential absorption spectra (visible and near-infrared) ob-tained upon nanosecond flash photolysis (308 nm) of 1 in dichloromethane(14 � 10�6

m) with a time delay of 2 ms at room temperature. Bottom: Time–absorption profile of the spectrum shown above at 700 nm, monitoring theexcited-state deactivation.

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Received: September 16, 2009

Published online on January 5, 2010

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