A fulleropyrrolidine with two oligophenylenevinylene substituents : synthesis , electrochemistry and photophysical properties

A fullerene derivative in which two oligophenylenevinylene (OPV) groups are attached to C60 through a pyrrolidine ring has been prepared and photophysical studies in CH2Cl2 solution show that photoinduced energy transfer from the OPV moieties to C60 occurs, and not electron transfer. On passing to a more polar solvent such as benzonitrile, again no evidence of electron transfer is found.


INTRODUCTION
The recent progress in the chemistry of C 60 allows the preparation of many covalent C 60 derivatives bearing electro-and/or photoactive substituents [1].Some of these systems provided entry into intramolecular processes such as electron or energy transfer, and C 60 appears to be a particularly interesting electron acceptor in photochemical molecular devices because of its symmetrical shape, large size and the properties of its πelectron system.Following the observation of electron transfer from conducting oligomers and/or polymers derived from polyphenylenevinylene or polythiophene [2], and the successful preparation of photovoltaic cells from such bulk heterojunction materials [3], a few examples of covalent fullerene derivatives bearing a conjugated oligomer substituent have been reported in the past years [4][5][6].As part of this research, we have recently shown that such hybrid compounds can be used for the preparation of plastic solar cells [5].The C 60 -oligophenylenevinylene (OPV) hybrid compound has been incorporated in a photovoltaic cell constructed by spin-casting the compound on a glass substrate covered with indium-tin oxide (ITO) and depositing an aluminum film on top.In such a device configuration, the compound is not only able to generate electrons and holes under light irradiation, but it also provides pathways for their subsequent collection at opposite electrodes, and a photocurrent is obtained.This † E-mail: armaroli@frae.bo.cnr.it‡ E-mail: niereng@ipcms.u-strasbg.frmolecular approach to photovoltaic cells appears to be promising since the bicontinuous network obtained by chemically linking the hole-conducting OPV moiety to the electron-conducting fullerene subunit prevents any problems arising from bad contacts at the junction as observed for OPV/C 60 blends.In this paper, we now report in detail the preparation and the electronic properties of fullerene derivative 1 in which two OPV groups are attached to C 60 through a pyrrolidine ring [6].Interestingly, the photophysical studies of this hybrid compound have shown that, in solution, photoinduced energy transfer from the OPV moiety to C 60 is the main deactivation pathway and not electron transfer.

RESULTS AND DISCUSSIONS
Synthesis.The preparation of Compoumd 1 is depicted in Scheme 1.The protected OPV trimer 2 was prepared in seven steps as previously reported [5].Deprotection of compound are broad at room temperature.A variable-temperature NMR study showed a clear coalescence at ca. 10 • C and the reversible narrowing of all these peaks shows that a dynamic effect occurs [5,8].This indicates restricted rotation of the phenyl substituent on the pyrrolidine ring and the activation free energy of the rotation was estimated as ∆G ‡ = 13 kcal mol −1 by following the coalescence of the aromatic C − H.This result is in good accordance with the data previously reported by F. Langa and coworkers [8].
Model fulleropyrrolidine FP was obtained in 42% yield by treatment of C 60 with 3,5-didodecyloxybenzaldehyde and sarcosine in refluxing toluene (Scheme 2).Restricted rotation of the phenyl substituent on the pyrrolidine ring was also observed for compound FP.As in the case of 1, a variable temperature NMR study showed a clear coalescence at ca. 10 • C and the activation free energy of the rotation of the phenyl group was estimated as ∆G ‡ = 13 kcal mol −1 .
Figure 1 shows the CV curves of 1, 2 and FP in the cathodic region at −65 • C. Compound 1 shows three reversible one-electron processes followed by a bielectronic peak (IV red ).The comparison of the E 1/2 potentials of 1 and FP clearly shows that peaks I red -III red correspond to fullerene-centred reductions and the slightly negative shift observed for 1 compared to FP is likely due to a small electronic interaction between the electron accepting C 60 unit and the electron donating OPV groups.However, solvation effects caused by the presence of the surrounding OPV groups could also be the source of the observed shift in potential.Peak IV red can be either fullerene-or OPV-centered since both 2 and FP shows a reduction process in the same potential region.Since peak IV red observed for 1 is bielectronic, it can likely be interpreted as the superposition of a one-electron fullerene reduction and a oneelectron OPV reduction.
In the anodic region, 1 presents two chemically ir- reversible and ill-defined peaks, corresponding to the transfer of three electrons.They can likely be attributed to the simultaneous oxidation of the two OPV groups and the bridging dialkyloxybenzene unit [9] since these groups, present in both model compounds 2 and FP, are irreversibly oxidized in the same potential region.

Photophysical properties.
The UV-VIS electronic 2 and it is superimposable to the profile obtained by summing the spectra of the reference component units FP and 2. Importantly, on each moiety of 1, a fairly good excitation selectivity can be achieved.At λ > 530 nm light is exclusively addressed to the fullerene fragment, whereas at λ = 360 nm at least 85% of the incident light is absorbed by the OPV moieties.
In CH 2 Cl 2 solution, upon selective excitation of the fullerene fragment of 1, the typical fulleropyrrolidine fluorescence and triplet-triplet transient absorption spectra are observed [7].This shows that the excited state properties of the C 60 fragment are not affected by the presence of the nearby OPV moieties.On the other hand, when excitation is addressed to the latter (e.g. at λ = 360 nm, see above), intercomponent processes are evidenced.Under these conditions, the intense fluorescence band of the OPV moiety (Φ em = 1.0, τ = 1.0 ns) is not observed (Figure 2), whereas the typical fluorescence band of the fulleropyrrolidine fragment (λ max = 710 nm, τ = 1.3 ns) is detected; in addition, the fullerene fluorescence quantum yields of 1 and FP obtained at λ exc = 360 nm are identical (Φ em = 5.5×10 −4 ), although in the former at least 85% of the incident light is absorbed by the OPV fragments.The excitation spectrum of 1, taken at λ em = 715 nm, matches the absorption profile throughout the UV-VIS, including the diagnostic band of the OPV moieties around 360 nm.These findings are consistent with quantitative occurrence of singlet-singlet energy transfer from the OPV unit to the fullerene in the multicomponent array 1.In order to monitor the fate of the lowest triplet state centred on the fullerene moiety, following excitation of the OPV unit, we performed a series of transient absorption experiments by exciting at 355 nm (Nd:YAG laser).The reference compound FP displays a triplet-triplet transient absorption spectrum with λ max = 690 nm and τ = 540 ns in air-equilibrated solution, which becomes 31000 ns in deaerated solution due to the suppression of the well known dioxygen quenching process [10,11].A quite similar behaviour is observed for 1, which gives a fullerene triplet yield formation equal to that of FP and the same triplet lifetimes.In other words, preferential excitation of the OPV moieties (> 85%) quantitatively sensitises the formation of the lowest fullerene singlet state, which then populates the lower lying triplet level (Figure 3) via intersystem crossing [7].
From the electrochemical data one can place the energy of the charge separated state [12] of 1 at about 2 eV, e.g.well below the energy of the lowest singlet excited state of the OPV moieties (3.1 eV, as derived by the 77 K fluorescence spectrum).However, even if the population of the charge separated state following photoexcitation of the OPV units is thermodynamically allowed, this process is not evidenced in CH 2 Cl 2 solution.This prompted us to set up a series of experiments in more polar benzonitrile, but the results did not change.For example, fluorescence spectra of optically matched solutions of 1 and FP in benzonitrile (λ exc = 363 nm) again suggest that in the fullerene-OPV hybrid 1 quantitative OPV → C 60 singlet-singlet energy transfer occurs (Figure 4), since quantitative sensitization of the fullerene emission is observed.By using the luminescence data of the fragment which act as energy donor (1) and the absorption spectrum of the acceptor (FP), it is possible to draw some conclusions regarding the type of energy transfer mechanism that is operative in 1.Two mechanisms, the dipole-dipole (Förster, F) [13] or the double electron exchange (Dexter, D) [14], may be involved.The possible contribution by the dipole-dipole energy transfer, which is pertinent to singlet-singlet interaction schemes, can be evaluated on the basis of the following equations: These allow to obtain estimates for (i) the energy transfer rate constant, k en F , and (ii) the critical transfer radius, R c , i.e. the distance between the fragments for which k en F equalizes the intrinsic deactivation of the donor, k d = τ −1 .In equations ( 1) and ( 2 is the overlap integral between the luminescence spectrum on an energy scale (cm −1 ) of the donor (F (υ) of 1) and the absorption spectrum of the acceptor, (ε(υ) of FP); J F was 1.4 × 10 −14 cm 6 mol −1 ; K 2 is a geometric factor (tentatively taken as 2/3), N = 6.02 × 10 23 mol −1 and n is the refractive index of the solvent.Calculations provided R c = 35.3Å, and k en F > 10 12 s −1 for d ≤ 10 Å.
It should be pointed out that these findings are appropriate for a two-center system whose donating and accepting components retain their electronic identity [15], as it happens in our case (see absorption spectra and electrochemical properties).However, one can consider also the double-electron exchange Dextertype transfer [14].This latter mechanism requires a certain amount of through-bond electronic communication (represented by the electronic coupling term H, see below), and is usually found to be important for triplet-triplet transfers.According to this approach we evaluated the pertinent spectral overlap J D , equations (equations ( 3)-( 5)), with J D = 1.4 × 10 −4 cm.Estimates of k en D were obtained for H o values ranging from 10 to 100 cm −1 , which correspond to moderately coupled moieties [16], and by assuming an attenuation factor β = 0.1 Å −1 [17].d o values ranging from 3 to 10 Å were employed, roughly corresponding to side-to-side and center-to-center geometries, respectively.
We found that the Förster mechanism is mostly effective up to d = 15 to 20 Å and that an interplay of the two energy transfer mechanisms can only be operative for longer distances or for larger H values; at any rate, energy transfer is always found to predominate over intrinsic deactivation, k en k d (k en = k en F + k en D ), and the sensitization step is expected to be quantitative (efficiency > 90%) up to d = 30 Å.These estimates suggest also that the energy transfer step is so fast that the competing charge separation path is not effective.

CONCLUSION
We have synthesized a multicomponent array containing a C 60 fulleropyrrolidine and two oligophenylenevinylene (OPV) subunits (1).The study of its electrochemical and photophysical properties, corroborated by model calculations, demonstrates that singletsinglet (OPV → C 60 ) photoinduced energy transfer is so fast that electron transfer is unable to compete even in a polar solvent like benzonitrile.It has been recently shown by Janssen et.al. [18] that in similar systems electron transfer can occur, but in that case the −OR residues on the OPV skeleton were differently located.These results show that OPVs are quite versatile moieties for the construction of photoactive multicomponent systems containing C 60 .In fact, upon a thorough choice and location of the substituents of the OPV subunits, such moieties can act either as light harvester (the present case) or as electron relay (ref.[18]) for the C 60 carbon sphere.
Spectroscopic and photophysical measurements.The solvent used for the photophysical investigations are spectrofluorimetric grade CH 2 Cl 2 (Carlo Erba) and benzonitrile (Aldrich).Absorption spectra were recorded with a Perkin-Elmer Lambda 5 spectrophotometer.Uncorrected emission spectra were obtained with a Spex Fluorolog II spectrofluorimeter eqipped with a continuous 150 W Xe lamp as excitation source and a Hamamatsu R-928 photomultiplier tube as detector.The corrected spectra were obtained via a calibration curve determined by means of a 45 W quartz-halogen tungsten filament lamp (Optronic Laboratories) calibrated in the range 400-1800 nm.Fluorescence quantum yields were measured with the method described by Demas and Crosby [21] using as standards quinine sulphate in 1 N H 2 SO 4 (Φ = 0.546) [22] and [Os(phen) 3 ] 2+ in acetonitrile (Φ em = 0.005) [23].Corrected excitation spectra were recorded with a Perkin-Elmer LS-50B spectrofluorimeter (pulsed Xe lamp).Emission lifetimes were determined with an IBH single photon counting spectrometer equipped with a thyratron gated nitrogen lamp working at 40 KHz (λ exc = 337 nm, 0.5 ns time resolution after deconvolution of the flash profile); the detector was a red-sensitive (185-850 nm) Hamamatsu R-3237-01 photomultiplier tube.The nanosecond transient absorption spectra and lifetimes were recorded by using the third harmonic (355 nm) of a Nd:YAG laser (JK Lasers) with 20 ns pulse and 2-5 mJ of energy per pulse.The details on the flash-photolysis system are reported elsewhere [24].Experimental uncertainties are estimated to be ±7% for lifetime determination, ±15% for quantum yields, and ±3 nm for emission and absorption peaks.
2 with CF 3 CO 2 H in CH 2 Cl 2 /H 2 O followed by LiAlH 4 reduction of aldehyde 3 in dry THF yielded alcohol 4. Subsequent reaction with ptoluenesulfonyl chloride (TsCl) in the presence of 4dimethylaminopyridine (DMAP) and LiCl in CH 2 Cl 2 gave 5. Treatment of 5 with 3,5-dihydroxybenzyl alcohol in refluxing acetone in the presence of K 2 CO 3 , KBr and 18-Crown-6 yielded 6. Subsequent oxidation with MnO 2 in CH 2 Cl 2 afforded benzaldehyde 7. The functionalisation of C 60 was based on the 1,3-dipolar cycloaddition [7] of the azomethine ylide generated in situ from 7.

Table 1 .
E 1/2 or E p values (V vs. SCE) determined by CV on a glassy carbon electrode at room temperature, unless otherwise noted, b of compounds 1, 2 and FP in CH 2 Cl 2 /0.1 M Bu 4 NPF 6 solutions.