Synthesis and electronic properties of fullerene derivatives substituted with oligophenylenevinylen–ferrocene conjugates

Abstract

C₆₀-bridge-Fc arrays (C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc), bearing a fulleropyrrolidine and a ferrocene (Fc) unit connected via an oligophenylenevinylene (OPV) bridge have been prepared. Suitable dyads (PV-Fc, 2PV-Fc, 3PV-Fc) to be used as references for the study of the electronic properties of the largest arrays have been also synthesized. The electrochemical properties of all the dyad and triad multicomponent arrays have been studied by cyclic voltammetry, evidencing a rich and complex electrochemical pattern due to the presence of several electroactive moieties. The first reduction is always assigned to the fullerene moiety and the first oxidation is centred on the Fc group, making the triad systems suitable candidates for photoinduced electron transfer via the interposed bridge. Photophysical studies evidence a complete quenching of the fluorescence of organic conjugated moieties in PV-Fc, 2PV-Fc, 3PV-Fc, possibly via energy transfer to the Fc unit. In the more complex C₆₀/Fc arrays the quenching of the C₆₀ moieties is ultrafast in CH₂Cl₂ solution and most likely attributable to electron transfer via the OPV wire. In toluene, the dynamic process of singlet and triplet fullerene quenching can be traced via time resolved fluorescence and transient absorption spectroscopy and the values of the rate constants are smaller with increasing donor–acceptor distance. Definitive assignment of the intercomponent quenching mechanism between the fullerene and the ferrocene moiety (energy or electron transfer) can hardly be obtained. Several repeated attempts to detect the radical anion fingerprint of fulleropyrrolidine with transient absorption failed, even in bimolecular quenching experiments. This supports the view that distance-dependent C₆₀ → Fc singlet–triplet and triplet–triplet energy transfer may compete successfully with the desired charge separation step.

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Introduction

In recent years, the use of C₆₀ as an electron and/or energy acceptor in photochemical molecular devices has been intensively investigated.¹ As part of this research, the combination of the carbon sphere with π-conjugated oligomers for the construction of donor–fullerene arrays is of particular interest.² Such hybrid systems have effectively shown excited state interactions making them excellent candidates for fundamental photophysical studies.³ In addition, covalently linked fullerene –(π-conjugated oligomer) derivatives offer interesting perspectives for optical limiting or photodynamic therapy applications.⁴ Furthermore, they can also be used as the active layer in organic photovoltaic cells.⁵ More elaborated systems in which an additional donor subunit has been connected to the C₆₀–(π-conjugated oligomer) system have also been described.^6–8 For example, a series of structurally well-defined arrays that incorporate a π-extended tetrathiafulvalene (ex-TTF) as electron donor and C₆₀ as electron acceptor, linked by oligophenylenevinylene (OPV) moieties of different lengths has been prepared.⁶ The electrochemical studies reveal a lack of significant electronic communication between the donor (ex-TTF) and the acceptor (C₆₀) moieties through the π-conjugated oligomer. However, photoinduced electron transfer leading to the radical pair ex-TTF⁺-OPV-C₆₀^– has been evidenced in all these compounds and the charge-recombination dynamics indicates a very low influence of the oligomer length on the electron transfer rate. These findings clearly reveal the nanowire behaviour of the OPV units.⁹

A variety of C₆₀-bridge-Fc triads has been also prepared and characterized photochemically. Several bridge units have been exploited such as olefins ,¹⁰ethers ,¹¹ oligothiophenes,¹² OPV dendrons,¹³ and even mixed bridges.^12,14 An extensively investigated class of photoactive arrays with C₆₀ and Fc is also that involving a photoactive porphyrin moiety as central bridge. In these systems the central porphyrin chromophore acts as primary light absorbing unit and undergoes electron transfer to the C₆₀ fragment.¹⁵ Eventually charge shift from the Fc to the porphyrin moiety leads to long-lived charge separated species, but typically with low yield due to a competitive energy transfer deactivation to the low lying Fc triplet.¹⁶

In this paper we report the preparation and the electronic properties of four novel C₆₀-bridge-Fc arrays (C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc), bearing a fulleropyrrolidine and a ferrocene (Fc) unit connected via a oligophenylenevinylene bridge, in an attempt to demonstrate the occurrence of a Fc → fullerene photoinduced electron transfer.⁸ Indeed the terminal Fc unit promotes ultrafast quenching of the fullerene moiety in CH₂Cl₂ for all of the multicomponent arrays, likely attributable to efficient photoinduced electron transfer via the OPV wire. By contrast, in less polar toluene, fullerene singlet and triplet quenching can be resolved and the rate constants show a substantial distance dependence. Combining the present experimental findings on C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc and some suitably designed reference systems with recent literature data, electron and energy transfer quenching pathways between C₆₀ and Fc are discussed.

Results and discussion

Synthesis

The preparation of compounds C₆₀-Fc, C₆₀-PV-Fc and C₆₀-2PV-Fc is shown in Scheme 1. Compound 1 was obtained in five steps from p-bromobenzaldehyde as previously reported.¹⁷ Reaction of 1 with ferrocenecarboxaldehyde under Wadsworth–Emmons conditions gave PV-Fc in 96% yield. Subsequent treatment with CF₃CO₂H afforded 2. It is worth nothing that this deprotection step must be carried out under oxygen free conditions to prevent decomposition, resulting most probably from the easy oxidation of the Fc unit under acidic conditions. Reaction of 2 with phosphonate 1 in the presence of t-BuOK afforded compound 2PV-Fc with 99% yield. Treatment with CF₃CO₂H in CH₂Cl₂/H₂O under oxygen free conditions then gave 3. The synthetic approach to prepare compounds C₆₀-PV-Fc and C₆₀-2PV-Fc relies upon the 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding aldehydes and N-benzylglycine derivative 4.¹⁸ Reaction of aldehydes 2 and 3 with 4 and C₆₀ in o-dichlorobenzene (ODCB) at 100 °C gave the corresponding pyrrolidinofullerene derivatives C₆₀-PV-Fc and C₆₀-2PV-Fc in 40 to 33% isolated yield, respectively, after column chromatography on silica gel. Compound C₆₀-Fc was prepared under similar conditions from C₆₀, 4 and ferrocenecarboxaldehyde. Owing to the presence of the 3,5-didodecyloxybenzyl subunit, the pyrrolidinofullerenes C₆₀-Fc, C₆₀-PV-Fc and C₆₀-2PV-Fc are well soluble in common organic solvents such as CH₂Cl₂, CHCl₃, or THF and complete spectroscopic characterization was easily achieved. Their structure was further confirmed by mass spectrometry. In all the cases, the expected molecular ion peak was clearly observed.

Scheme 1 Reagents and conditions: (a) ferrocenecarboxaldehyde, t-BuOK, THF, 0 °C to rt, 2 h, 96%; (b) CF₃CO₂H, H₂O, CH₂Cl₂, rt, 5 h, 80%; (c) 1, t-BuOK, THF, 0 °C to rt, 2 h, 99%; (d) CF₃CO₂H, H₂O, CH₂Cl₂, rt, 5 h, 70%; (e) ferrocenecarboxaldehyde, C₆₀, ODCB, Δ, 12 h, 32%; (f) 4, C₆₀, ODCB, 100 °C, 20 h, 40%; (g) 4, C₆₀, ODCB, 100 °C, 20 h, 33%.

The length of the OPV-Fc system was further increased (Scheme 2). In this case, phosphonate 5¹⁹ with two alkyl chain was used in order to prevent any solubility problems. Reaction of aldehyde 3 with phosphonate 5 in the presence of t-BuOK in THF afforded 3PV-Fc in 50% yield. Subsequent treatment with CF₃CO₂H in CH₂Cl₂/H₂O gave aldehyde 6 in a moderate yield (37%) which after reaction with C₆₀ and 4 in ODCB at 100 °C gave C₆₀-3PV-Fc in 26% yield.

Scheme 2 Reagents and conditions: (a) 3, t-BuOK, THF, 0 °C to rt, 2 h, 50%; (b) CF₃CO₂H, H₂O, CH₂Cl₂, rt, 5 h, 37%; (c) 4, C₆₀, ODCB, 100 °C, 20 h, 26%.

The characterization of compound C₆₀-3PV-Fc was found more difficult when compared to that of the shorter analogues C₆₀-PV-Fc and C₆₀-2PV-Fc. Indeed, two diastereoisomers (A and B) can exist in principle for compound C₆₀-3PV-Fc (Fig. 1 and 2). However, the ¹H and ¹³C NMR spectra (CDCl₃, room temperature) reveal the existence of a single C₁ symmetric compound. In addition, ¹H-NMR spectra recorded at temperatures from –50 to 120 °C are all similar and reveal no dynamic exchange between A and B. Therefore only one of the two possible conformers exists for C₆₀-3PV-Fc. As already observed for related ortho-substituted-phenyl pyrrolidinofullerenes, 2D-NOESY experiments revealed that C₆₀-3PV-Fc adopts a conformation in which the unsubstituted ortho position is located atop the C₆₀ (conformer A).¹⁸ Therefore, the reaction of 6 with C₆₀ is diastereoselective, affording only one isomer.

Fig. 1 Compounds C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc and C₆₀-3PV-Fc.

Fig. 2 Schematic representation of the two possible atropisomers of compound C₆₀-3PV-Fc.

Electrochemical properties

The electrochemical properties of C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc, PV-Fc, 2PV-Fc, 3PV-Fc, and fullerene model compound 7 were studied by cyclic voltammetry in THF/TBAPF₆ solutions. The results are presented in Table 1.

Table 1 Half-wave potentials (E_1/2 in V, vs. SCE) in THF/TBAPF₆ solutions at 298 K and, in parenthesis, at 218 K

	A	I	II	III	IV	V	VI	VII
a Electrochemical process superimposed to solvent discharge. b Chemically irreversible process; Epa value at 0.2 V s^–1. c Two-electron transfer process.
Fc	+0.46
FP		–0.47	–1.04	–1.68	–2.15	–2.96
		(–0.44)	(–0.99)	(–1.60)	(–2.10)	(–2.83)
7		–0.44	–1.00	–1.61	–2.07	–2.76
		(–0.41)	(–0.96)	(–1.60)	(–2.07)	(–2.83)
C60-Fc	+0.67	–0.45	–1.02	–1.63	–2.09	–2.8^a
	(+0.60)	(–0.44)	(–1.00)	(–1.63)	(–2.14)	(–2.92)
C60-PV-Fc	+0.58	–0.47	–1.03	–1.64	–2.10	–2.45	–2.8^a
	(+0.52)	(–0.42)	(–0.97)	(–1.60)	(–2.07)	(–2.44)	(–2.87)
PV-Fc	+0.58	–2.34
	(+0.51)	(–2.36)
C60-2PV-Fc	+0.57	–0.45	–1.02	–1.62	–2.01	–2.09	–2.31	—
	(+0.51)	(–0.43)	(–0.98)	(–1.62)	(–2.01)	(–2.15)	(–2.41)	(–2.91)
2PV-Fc	+0.57	–1.99	—
	(+0.51)	(–2.03)	(–2.29)
C60-3PV-Fc	+0.56	–0.48	–1.06	–1.67	–1.86	–2.04	–2.16	—
	(+0.51)	(–0.42)	(–0.99)	(–1.65)	(–1.81)	(–1.98)	(–2.17)	(–2.98)^bc
3PV-Fc	+0.57	–1.75	–2.39	–2.79^b
	(+0.51)	(–1.73)	(–2.39)	(–2.95)^b

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A very rich and complex electrochemical pattern has been evidenced for these compounds. Indeed, multiple redox sites are present, namely C₆₀, OPV, Fc and dialkyloxybenzene. The comparison with proper model compounds allows the assignment of the electron transfer processes to the different redox sites, as depicted in Fig. 3. In all the fullerene derivatives, we can observe five reversible one-electron reduction processes typical of the pyrrolidinofullerene core (solid squares in Fig. 3). In the case of C₆₀-3PV-Fc the last process is bielectronic and one of the two reduction is assigned to the fullerene moiety. The presence of the electron withdrawing dialkyloxybenzene group on the nitrogen atom of the pyrrolidinofullerene moiety causes a very small positive shift of the E_1/2 values, as evidenced by the comparison of N-methylpyrrolidinofullerene (FP) with 7. In the case of all the compounds containing C₆₀ and Fc, the first E_1/2 value are very similar (Table 1 and Fig. 3).

Fig. 3 Diagram showing the half-wave potential values of the investigated compounds in THF/TBAPF₆ at 218 K.

In the cathodic region, besides the fullerene -based reductions, reversible one-electron OPV centred reductions are also observed for C₆₀-PV-Fc, C₆₀-2PV-Fc, and C₆₀-3PV-Fc (open triangles in Fig. 3). In particular, peak V of C₆₀-PV-Fc, peaks V and VI of C₆₀-2PV-Fc have been assigned to the OPV moieties, on the basis of the close similarity to the reduction potentials of the corresponding model compounds. In particular, in both these fullerene derivatives a negative shift of the OPV reduction potentials has been observed, compared to the model OPV compounds (PV-Fc and 2PV-Fc, respectively). This behaviour is expected on the basis of the electrostatic repulsion with the negative charges on the fullerene cage. In the case of C₆₀-3PV-Fc (a representative cyclic voltammetric curve is shown in Fig. 4), the assignment of the redox processes is less straightforward: we tentatively assign peaks IV, VI and VII (bielectronic process visible only at 218 K) to the OPV reductions on the basis of the good correlation of the other peaks with those of FP. It is worth noticing that the second OPV reduction of 3PV-Fc is not chemically reversible, contrary to the corresponding peak VI of C₆₀-3PV-Fc. This result may explain the unexpected positive shift observed only for the second OPV reduction in the triad compared to the model 3PV-Fc. Upon increasing the length of the OPV chain (C₆₀-PV-Fc < C₆₀-2PV-Fc < C₆₀-3PV-Fc), the first reduction process of the OPV moiety occurs at less negative potentials, as expected by the extension of the π orbital conjugated system.²⁰ For compound C₆₀-3PV-Fc, the observed positive shift of the first and second OPV reductions compared to C₆₀-2PV-Fc is not only due to the extension of the π system, but also to the presence of two electron-withdrawing –OC₈H₁₇ substituents on one end of the OPV moiety.

Fig. 4 Cyclic voltammogram of compound C₆₀-3PV-Fc in THF/TBAPF₆ at scan rate of 0.2 V s^–1 and T = 298 K.

In the anodic region, only the one-electron oxidation of the Fc moiety can be observed, since the dialkyloxybenzene one is outside the available potential window. In a medium offering a wider anodic potential region such as CH₂Cl₂/TBAPF₆, a chemically irreversible electron transfer process with E_pa ∼ +1.5 V (vs. SCE) at 0.2 V s^–1 has been attributed to the dialkyloxybenzene oxidation.²¹ The constancy of the E_1/2 values corresponding to the ferrocene oxidation in compounds C₆₀-PV-Fc, C₆₀-2PV-Fc, and C₆₀-3PV-Fc compared to the corresponding model compounds (PV-Fc, 2PV-Fc, and 3PV-Fc, respectively) and to pristine Fc demonstrates that the ferrocene oxidation process is not affected by the presence of the other redox sites. On the other hand, in the case of C₆₀-Fc, the ferrocene moiety is directly connected to the electron-acceptor pyrrolidinofullerene core and a positive shift of the Fc oxidation is observed (ΔE = 100 mV), together with a decrease of the heterogeneous electron transfer rate. Simulation of the voltammetric curves in the range of scan rates between 0.1 and 2 V s^–1 yielded the following values: k_sh = 0.007 cm^–1 s^–1 and α = 0.7 much smaller than that reported for pristine ferrocene.²² On the contrary, no significant perturbation of the fullerene -centred reductions has been observed. This electrochemical behaviour would suggest that the HOMO orbital is localized not only on the Fc, but also partially on the fullerene core, conversely the LUMO is mainly localized on the fullerene .

In summary, for all the investigated C₆₀-OPV-Fc compounds the first reduction is due to the fullerene moiety and the first oxidation is centred on the Fc group.

Photophysical properties

The absorption spectrum of the C₆₀ model-compound 7 is virtually identical to similar pyrrolidinofullerene derivatives reported in the literature with the two characteristic peaks at 255 nm and 430 nm.²³ Pristine Fc in CH₂Cl₂ solution exhibits a weak absorption band centred at 443 nm (ε = 100 M^–1 cm^–1), also in line with literature data.²⁴

The ferrocene reference compounds with the spacer attached (PV-Fc, 2PV-Fc, 3PV-Fc) show the characteristic absorption band of the phenylenevinylene fragment, progressively red-shifted with the increasing conjugation: PV-Fc (λ_max = 312 nm, ε = 23 600 M^–1 cm^–1), 2PV-Fc (λ_max = 356 nm, ε = 53 200 M^–1 cm^–1, Fig. 5), 3PV-Fc (λ_max = 398 nm, ε = 71 000 M^–1 cm^–1). The potentially strong fluorescence of the organic conjugated moiety in 2PV-Fc and 3PV-Fc²⁵ is completely quenched by the ferrocene unit.

Fig. 5 Absorption spectra (CH₂Cl₂) of C₆₀-2PV-Fc (light gray), 7 (black), 2PV-Fc (gray) and the spectral trace obtained by summing the two model compounds 7 and 2PV-Fc (black dashed).

In principle, the spectral overlap between the fluorescence profiles of OPV (peaked around 380–400 nm)²⁵ and the weak lowest absorption band of the Fc acceptor attributed to d–d spin allowed transitions (ca. 2.7 eV, i.e. 460 nm)²⁴ makes it possible a singlet energy transfer via Förster mechanism. Eventually, the lowest triplet electronic state of Fc, which was recently located as low as 1.16 eV²⁶ (to be compared to ca. 1.75 eV of previous assignments),²⁷ might act as final energy sink. Unfortunately excited electronic levels of ferrocenes are known to be rather elusive from the spectroscopic point of view and their energy content has been the object of debate for decades in the photochemical community.^24,26,27 These ultrashort-lived levels do not display significant absorption or emission signatures and only indirect proof for their generation can be obtained. Quenching by electron transfer following excitation of the OPV moieties above 3.0 eV cannot be discarded for 2PV-Fc and 3PV-Fc which have, respectively, a thermodynamic driving force for charge separation of about 2.6 and 2.3 eV, as obtainable²⁸ by the one-electron oxidation and reduction potentials in Table 1.

The UV-visible absorption spectra of the four fullerene multicomponent arrays C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, and C₆₀-3PV-Fc in CH₂Cl₂ are depicted in Fig. 6. In the UV region there is overlap between fullerene and OPV absorption features, whereas in the Vis spectral window (λ > 450 nm) only the fullerene moiety absorbs, with a minor contribution of Fc until 530 nm. The characteristic fulleropyrrolidine peak at 430 nm is recorded in all cases except C₆₀-3PV-Fc, where it is masked by the band of the OPV fragment.

Fig. 6 Absorption spectra (CH₂Cl₂) of C₆₀-Fc (black), C₆₀-PV-Fc (gray), C₆₀-2PV-Fc (light gray), and C₆₀-3PV-Fc (black dashed).

The absorption spectra of the fullerene hybrid systems were compared to the spectra obtained by summing the absorption spectra of the C₆₀-model compound 7 with those of the corresponding Fc-OPV model systems (for C₆₀-2PV-Fc, see Fig. 5). Excellent match between experimental and arithmetical traces is found for C₆₀-3PV-Fc. With shorter spacers the matching is progressively lost and the largest difference is found for C₆₀-Fc in the 400–500 nm region. This trend suggests the presence of interactions between the C₆₀ and the Fc moiety, possibly mediated by the short OPV molecular wire,^9,29 progressively weaker with longer distance. This explanation is supported by the electrochemical data (Table 1) which show a substantial interaction between the two moieties in C₆₀-Fc.

Whatever the C₆₀-OPV-Fc array, it is possible to excite selectively the C₆₀ moiety at λ > 530 nm. In contrast, selective excitation of the weakly absorbing²⁴ferrocene moiety is always precluded. The occurrence of photoinduced processes in the triads has been probed via excitation of the fulleropyrrolidine moiety at 610 nm in dichloromethane and toluene solution. The fullerene fluorescence spectra of the C₆₀-OPV-Fc systems compared to the C₆₀ model-compound 7 in CH₂Cl₂ are displayed in Fig. 7 (top panel).

Fig. 7 Fullerene fluorescence spectra of 7 (black), C₆₀-Fc (gray dashed), C₆₀-PV-Fc (black dashed), C₆₀-2PV-Fc (light gray), and C₆₀-3PV-Fc (gray) in CH₂Cl₂ (top) and toluene (bottom) with λ_exc = 610 nm. Inset (top and bottom left): sensitized singlet oxygen luminescence at λ_exc = 610 nm. Inset (bottom right): fluorescence decay at λ_exc = 635 nm.

The fluorescence quantum yield and singlet lifetimes of 7 are typical for pyrrolidinofullerene derivatives in CH₂Cl₂ (φ = 3.0 × 10^–4, τ = 1.3 ns). The fullerene emission is completely suppressed for C₆₀-Fc, C₆₀-PV-Fc and reduced by 90% for C₆₀-2PV-Fc, and C₆₀-3PV-Fc. The lifetime of the quenched singlet state (<300 ps) is below our experimental resolution in all cases. However, from the steady state luminescence (Fig. 7) and according to eqn (1), a rate constant for the quenching process can be estimated for the two longest arrays C₆₀-2PV-Fc and C₆₀-3PV-Fc, k = 7 × 10⁹ s^–1

In eqn (1), Φ and Φ_ref are the fulleropyrrolidine fluorescence quantum yields of the multicomponent arrays and the reference compound 7, respectively, and τ_ref is the singlet lifetime of the latter.

Photoexcitation of the four multicomponent systems in CH₂Cl₂ does not generate fullerene triplet, as inferred by the absence of the characteristic sensitized singlet oxygen luminescence peaked at 1268 nm,³⁰ which is observed for the reference molecule 7. This indicates that the reactive excited state species is the fullerene singlet, in line with several examples of ferrocene–fullerene arrays reported to date.^11,14

Spectral data in CH₂Cl₂ suggest that there is some (small) difference in the extent of the fulleropyrrolidine quenching between the two longest (90% yield) and the two shortest (100% yield) arrays. This prompted us to make the same experiment in toluene, in order to test if a less polar environment could better resolve the behaviour of the four hybrids. Fulleropyrrolidine fluorescence spectra in toluene are reported in Fig. 7, bottom panel. As observed in CH₂Cl₂, fullerene fluorescence is almost completely quenched for the shortest systems C₆₀-Fc and C₆₀-PV-Fc. On the contrary, a quenched, but still well detectable fullerene fluorescence is recorded for C₆₀-2PV-Fc and C₆₀-3PV-Fc and the corresponding lifetimes are progressively shorter, indicating a dynamic quenching of the fullerene singlet level in toluene: 7 (1.6 ns), C₆₀-3PV-Fc (1.1 ns), C₆₀-2PV-Fc (0.6 ns), C₆₀-PV-Fc and C₆₀-Fc (<0.3 ns).

The observed fullerene singlet quenching process in the two investigated solvents might be due to photoinduced electron transfer leading to Fc⁺-OPV-C₆₀^–, in analogy with previously reported fullerene –ferrocene systems with aliphatic,¹⁰ether ,¹¹ or oligothiophene¹² linkers. Such charge separated states, according to electrochemical data, are placed at 1.12 (C₆₀-Fc), 1.05 (C₆₀-PV-Fc), 1.02 (C₆₀-2PV-Fc) and 1.04 eV (C₆₀-3PV-Fc). Repeated attempts to evidence the formation of the charge separated state in both CH₂Cl₂ and toluene through the C₆₀ radical anion fingerprint in the NIR spectral region (900–1600), down to a time resolution of 10 ns, turned out to be unsuccessful, suggesting an ultrafast charge recombination process favoured by the good electronic coupling offered by the OPV wire.^9,29 On a shorter time scale (35 ps) the available spectral window of our apparatus is 400–900 nm and the C₆₀ anion cannot be traced. No evidence of charge separated products was previously obtained with ferrocene–fullerene donor–acceptor partners connected via olefinic chains.¹⁰ On the contrary, relatively longer lived charge separated states were detected with ether (hundreds of ns)¹¹ and oligothiophene (up to 50 ns)¹² spacers.

We wish to emphasize that the lack of experimental evidence for electron transfer from the fullerene singlet might also be due to an energy transfer quenching process. Fullerene → ferrocene singlet (Förster) energy transfer is ruled out since endoergonic, whereas the singlet to triplet process via Dexter mechanism might occur (for the triplet energy of Fc, see above), as recently proposed in porphyrin–Fc systems where porphyrin fluorescence quenching is observed.¹⁶

In an attempt to evidence electron transfer quenching, we also investigated the fate of the residual fullerene triplet of C₆₀-2PV-Fc and C₆₀-3PV-Fc in toluene solution. In this solvent, for some C₆₀-bridge-Fc systems, relatively long-lived CS states have been detected, because the charge recombination process is located in a Marcus-inverted region regime with longer-lived CS states compared to polar solvents.¹⁴ Indeed, for C₆₀-2PV-Fc and C₆₀-3PV-Fc, the residual fullerene triplet state evidenced in toluene air equilibrated solution via singlet-oxygen sensitization (Fig. 7, bottom panel, left inset), is substantially quenched in oxygen free solutions, where triplet lifetimes of 0.23 and 0.62 µs are found, to be compared to 15.7 µs for the reference system 7 (Fig. 8). Again, no evidence for electron transfer was found. The same result was obtained through bimolecular quenching studies between 7 and 2PV-Fc, providing again no evidence for electron transfer products.³¹

Fig. 8 Transient absorption spectra of 7 (black), C₆₀-2PV-Fc (light gray), and C₆₀-3PV-Fc (gray) in oxygen free toluene solution immediately after the laser pulse (full line) and after 1.5 ms (dashed line). Inset: absorbance decays at 710 nm of oxygen free (top) and air-equilibrated samples. λ_exc = 532 nm.

These results strongly suggest that quenching of the fullerene triplet by the ferrocene moiety occurs via triplet energy transfer with a rate constant of 4.3 × 10⁶ and 1.5 × 10⁶ µs^–1 for, respectively, C₆₀-2PV-Fc and C₆₀-3PV-Fc. Notably, the poor (if any) photoinduced electron transfer efficiency between ferrocene derivatives and the triplet of C₆₀ has been highlighted recently by Araki et al. even in polar benzonitrile,²⁶ and our results are in line with these findings.

It is worth noticing that, in toluene solution, there is a 3.5-fold and a 3-fold decrease in the rate of the quenching of the fullerene singlet and triplet states, respectively, between C₆₀-2PV-Fc and C₆₀-3PV-Fc (Table 2). It has been demonstrated that electron transfer through OPV bridges identical to those here reported, has no D–A distance dependence up to four phenylene bridges,⁹ thus a different mechanism (i.e. energy transfer) could be responsible for the photoinduced process in toluene. On the contrary, the much weaker distance effects in more polar CH₂Cl₂ might be an indication of an electron transfer process through the OPV wire.

Table 2 Quenching rate constants of the singlet and triplet states of the fullerene moieties in toluene solution (oxygen-free for triplet measurements) calculated as k_q = 1/τ – 1/τ₀. τ is the quenched lifetime and τ₀ is the singlet and triplet lifetimes of the reference fullerene molecule 7 (1.6 ns and 15.7 µs, respectively)

	k q ^a , singlet/s^–1	k q ^b , triplet/s^–1
a Determined via fluorescence intensity quenching or lifetimes. b Determined via transient absorption measurements.
C60-Fc	>6 × 10^10	—
C60-PV-Fc	>6 × 10^10	—
C60-2PV-Fc	1.0 × 10^9	4.3 × 10^6
C60-3PV-Fc	2.8 × 10^8	1.5 × 10^6

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Conclusion

Four novel multicomponent architectures C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc along with suitable reference compounds for spectroscopic and electrochemical characterization have been prepared. The target systems are made of fulleropyrrolidine (C₆₀) and ferrocene (Fc) units directly connected (C₆₀-Fc) or separated through an oligophenylenevinylene (OPV) bridge of increasing length. Photophysical studies show a different behaviour in CH₂Cl₂ and toluene. In the former virtually complete quenching of the fullerene singlet excited states in all the multicomponent arrays is observed, tentatively attributed to ultrafast charge separation and recombination mediated by the efficient OPV molecular wire⁹ and in line with analogous triads with olefine short bridges.¹⁰ In toluene, instead, fullerene singlet and triplet quenching process are slower and can be traced via time resolved luminescence or transient absorption spectroscopy. Both singlet and triplet quenching show distance dependence along the series, differently from previous reports which demonstrate an excellent wire-like behaviour of OPV bridges with distance independent electron transfer rates up to 28 Å.⁹ This suggests that fullerene → ferrocene singlet–triplet and triplet–triplet energy transfer might be the operative quenching mechanism in apolar toluene solvent, although direct spectroscopic evidence for these processes is not obtainable. However, repeated attempts to detect transient electron transfer products (even via bimolecular quenching, which turned out to be successful with a variety of donors) failed, and no traces of the fulleropyrrolidine radical anion have been found. The present results along with the recent findings of Araki et al.,²⁶ suggest some caution in choosing Fc units as electron donors to promote efficient charge separation in multicomponent arrays, also due to its optical elusiveness as a triplet or a radical cation.³² Notably, very recent results have also evidenced the tendency of Fc to undergo energy transfer, thus leading to a substantial abatement of the long-distance charge separation in a Fc-porphyrin-C₆₀ array.¹⁶

Experimental

General methods

Reagents and solvents were purchased as reagent grade and used without further purification. THF was distilled over sodium benzophenoneketyl . Compounds 1,¹⁷4,¹⁸ and 5¹⁹ were prepared according to previously reported procedures. All reactions were performed in standard glassware under an inert Ar atmosphere. Evaporation and concentration were done at water aspirator pressure and drying in vacuo at 10^–2 Torr. Column chromatography : silica gel 60 (230–400 mesh, 0.040–0.063 mm) was purchased from E. Merck. Thin layer chromatography (TLC) was performed on glass sheets coated with silica gel 60 F254 purchased from E. Merck, visualization by UV light. NMR spectra were recorded on a Bruker AC 200 (200 MHz) or a Bruker AM 400 (400 MHz) with solvent peaks as reference. FAB-mass spectra (MS) were obtained on a ZA HF instrument with 4-nitrobenzyl alcohol as matrix. Elemental analyses were performed by the analytical service at the Institut Charles Sadron, Strasbourg.

Compound PV-Fc

t-BuOK (0.33 g, 2.9 mmol) was added to a degassed solution of ferrocenecarboxaldehyde (0.57 g, 2.6 mmol) and 1 (1.0 g, 2.9 mmol) in dry THF (50 mL) at 0 °C under argon atmosphere. The mixture was stirred for 2 h and filtered over celite. The filtrate was evaporated to dryness and was taken up in CH₂Cl₂. The organic layer was washed with H₂O, dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, hexane/CH₂Cl₂ 3 : 2) yielded PV-Fc (1.02 g, 96%) as an orange powder (mp 204 °C). ¹H NMR (300 MHz, CDCl₃): 7.45 (m, 4 H), 6.79 (AB, J = 17 Hz, 2 H), 5.39 (s, 1 H), 4.46 (t, J = 2 Hz, 2 H), 4.28 (t, J = 2 Hz, 2 H), 4.13 (s, 5 H), 3.72 (AB, J = 11 Hz, 4 H), 1.31 (s, 3 H), 0.81 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): 138.32, 127.27, 126.35, 126.04, 125.75, 101.58, 77.65, 69.71, 69.49, 67.10, 30.23, 23.05, 21.90. Anal. calcd for C₂₄H₂₆O₂Fe: C 71.65, H 6.51. Found: C 71.52, H 6.54%. UV/Vis (CH₂Cl₂): 266 (19 000), 311 (30 400), 456 (1700).

Compound 2

A 1 : 1 mixture of CF₃CO₂H/H₂O (30 mL) was added to a degassed solution of PV-Fc (1.02 g, 2.53 mmol) in CH₂Cl₂ (15 mL) under argon. The mixture was stirred at room temperature for 5 h under argon atmosphere. The reaction mixture was then washed with H₂O (until pH was near neutrality), dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, CH₂Cl₂/hexane 1 : 1) yielded 2 (0.64 g, 80%) as a dark red powder. IR (neat): 1691 (C=O), 2733, 2822 (C–H). ¹H NMR (300 MHz, CDCl₃): 9.98 (s, 1 H), 7.83 (d, J = 8 Hz, 2H), 7.56 (d, J = 8 Hz, 2H), 6.91 (AB, J = 17 Hz, 2 H), 4.51 (t, J = 2 Hz, 2 H), 4.36 (t, J = 2 Hz, 2 H), 4.16 (s, 5 H). ¹³C NMR (75 MHz, CDCl₃): 191.57, 144.00, 134.56, 131.47, 130.30, 126.02, 124.51, 82.18, 69.72, 69.33, 67.31. Anal. calcd for C₁₉H₁₆OFe: C 72.18, H 5.10. Found: C 71.99, H 5.18%.

Compound 2PV-Fc

t-BuOK (0.25 g, 2.2 mmol) was added to a degassed solution of 2 (0.64 g, 2.0 mmol) and 1 (0.76 g, 2.2 mmol) in dry THF (50 mL) at 0 °C under argon. The mixture was stirred for 2 h and filtered under celite. The filtrate was evaporated to dryness and was taken up in CH₂Cl₂. The organic layer was washed with H₂O, dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, hexane/CH₂Cl₂ 3 : 2) yielded 2PV-Fc (1.02 g, 99%) as an orange powder. ¹H NMR (300 MHz, CDCl₃): 7.55–7.41 (m, 8 H), 7.11 (s, 2 H), 6.80 (AB, J = 17 Hz, 2 H), 5.41 (s, 1 H), 4.48 (t, J = 2 Hz, 2 H), 4.30 (t, J = 2 Hz, 2 H), 4.14 (s, 5 H), 3.73 (AB, J = 11 Hz, 4 H), 1.32 (s, 3 H), 0.82 (s, 3 H). Anal. calcd for C₃₂H₃₂O₂Fe: C 76.19, H 6.39. Found: C 76.36, H 6.47%. UV/Vis (CH₂Cl₂): 231 (25 700), 356 (55 200).

Compound 3

A 1 : 1 mixture of CF₃CO₂H/H₂O (30 mL) was added to a degassed solution of 2PV-Fc (1.0 g, 1.98 mmol) in CH₂Cl₂ (100 mL) under argon. The mixture was stirred at room temperature for 5 h under argon atmosphere. The reaction mixture was then washed with H₂O (until pH was near neutrality), dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, CH₂Cl₂/hexane 1 : 1) yielded 3 (0.58 g, 70%) as a dark red powder. IR (neat): 1690 (C=O), 2719 (C–H). ¹H NMR (300 MHz, CDCl₃): 10.00 (s, 1 H), 7.77 (AB, J = 8 Hz, 4 H), 7.49 (AB, J = 8 Hz, 4 H), 7.21 (AB, J = 16 Hz, 2 H), 6.82 (AB, J = 16 Hz, 2 H), 4.50 (t, J = 2 Hz, 2 H), 4.33 (t, J = 2 Hz, 2 H), 4.16 (s, 5 H). Anal. calcd for C₂₇H₂₂OFe: C 77.52, H 5.30. Found: C 76.74, H 5.45%.

Compound 3PV-Fc

t-BuOK (0.1 g, 0.84 mmol) was added to a degassed solution of 3 (0.3 g, 0.7 mmol) and 5 (0.5 g, 0.84 mmol) in dry THF (150 mL) at 0 °C under argon. The mixture was stirred for 2 h and filtered under celite. The filtrate was evaporated to dryness and was taken up in CH₂Cl₂. The organic layer was washed with H₂O, dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, hexane/CH₂Cl₂ 3 : 2) yielded 3PV-Fc (0.3 g, 50%) as a red glassy product. ¹H NMR (300 MHz, CDCl₃): 7.54–7.40 (m, 8 H), 7.19 (s, 2 H), 7.10 (m, 4 H), 6.78 (AB, J = 17 Hz, 2 H), 5.74 (s, 1 H), 4.50 (s, 2 H), 4.32 (s, 2 H), 4.16 (s, 5 H), 4.02 (m, 4 H), 3.71 (AB, J = 11 Hz, 4 H), 1.84 (m, 4 H), 1.33-1.26 (m, 23 H), 0.88 (m, 9 H), 0.80 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): 151.12, 150.29, 137.21, 136.53, 135.89, 128.61, 128.08, 127.74, 127.32, 126.99, 126.83, 126.79, 126.69, 126.05, 125.66, 123.53, 111.49, 110.59, 97.04, 83.41, 77.85, 69.56, 69.34, 69.23, 69.11, 66.87, 31.80, 30.26, 29.66, 29.45, 29.41, 29.37, 29.33, 29.29, 26.25, 26.09, 23.20, 22.66, 21.85, 14.10. UV/Vis (CH₂Cl₂): 398 (71 000).

Compound 6

A 1 : 1 mixture of CF₃CO₂H/H₂O (10 mL) was added to a degassed solution of 3PV-Fc (0.24 g, 0.28 mmol) in CH₂Cl₂ (20 mL) under argon. The mixture was stirred at room temperature for 5 h under argon atmosphere. The reaction mixture was then washed with H₂O (until pH was near neutrality), dried over MgSO₄, filtered and evaporated to dryness. Column chromatography (SiO₂, CH₂Cl₂/hexane 1 : 1) yielded 6 (0.08 g, 37%) as a dark red powder that was used in the next step as received. ¹H NMR (300 MHz, CDCl₃): 10.38 (s, 1 H), 7.32 (m, 8H), 7.05 (AB, J = 16 Hz, 2H), 7.03 (s, 4 H), 6.72 (AB, J = 17 Hz, 2 H), 4.40 (s, 2 H), 4.22 (s, 2 H), 4.07 (s, 5 H), 4.01 (t, J = 6 Hz, 2 H), 3.95 (t, J = 6 Hz, 2 H), 1.78 (m, 4 H), 1.43–1.19 (m, 20 H), 0.83 (m, 6 H).

Compound 7

A mixture of formaldehyde (0.08 g, 0.1 mmol), C₆₀ (0.07 g, 0.1 mmol) and 4 (0.11 g, 0.2 mmol) in ODCB (25 mL) was refluxed for 12 h under argon. After cooling, the resulting solution was evaporated to dryness and column chromatography (SiO₂, hexane/CH₂Cl₂ 4 : 1) gave 7 (39%) as a brown glassy product. ¹H NMR (300 MHz, CDCl₃): 6.88 (d, J = 2 Hz, 2 H), 6.48 (t, J = 2 Hz, 1 H), 4.45 (s, 4 H), 4.25 (s, 2 H), 4.03 (t, J = 6 Hz, 4 H), 1.82 (m, 4 H), 1.44 (m, 36 H), 0.88 (t, J = 7 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): 160.53, 155.03, 147.26, 146.20, 146.09, 146.02, 145.65, 145.37, 145.24, 144.52, 143.06, 142.58, 142.21, 142.03, 141.83, 140.17, 140.10, 136.22, 107.04, 100.54, 70.78, 68.16. 67.31, 58.61, 31.92, 29.71, 29.58, 29.55, 29.47, 29.36, 29.33, 26.13, 22.69, 14.14. Anal. calcd for C₉₃H₅₉O₂N: C 91.37, H 4.86, N 1.15. Found: C 90.99, H 5.17, N 0.91%.

Compound C₆₀-Fc

A mixture of ferrocenecarboxaldehyde (0.119 g, 0.55 mmol), C₆₀ (0.4 g, 0.55 mmol) and 4 (0.59 g, 1.11 mmol) in ODCB (100 mL) was refluxed for 12 h. After cooling to room temperature, the resulting solution was evaporated to dryness and column chromatography (SiO₂, hexane/CH₂Cl₂ 4 : 1) gave C₆₀-Fc (0.25 g, 32%) as a brown glassy product. ¹H NMR (300 MHz, CDCl₃): 7.04 (d, J = 2 Hz, 2 H), 6.51 (t, J = 2 Hz, 1 H), 6.17 (d, J = 14 Hz, 1 H), 5.18 (s, 1 H), 4.86 (d, J = 9 Hz, 1 H), 4.65 (s, 1 H), 4.57 (s, 1 H), 4.31 (s, 5 H), 4.26 (t, J = 2 Hz, 2 H), 4.13 (d, J = 10 Hz, 1 H), 4.07 (m, 4 H), 3.79 (d, J = 14 Hz, 1 H), 1.84 (m, 4 H), 1.44 (m, 36 H), 0.89 (t, J = 7 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): 160.80, 156.46, 154.29, 153.97, 153.30, 147.61, 147.29, 147.25, 147.19, 146.53, 146.29, 146.25, 146.21, 146.12, 146.05, 145.92, 145.75, 145.56, 145.49, 145.25, 145.21, 145.15, 144.70, 144.65, 144.43, 144.41, 143.09, 142.99, 142.67, 142.61, 142.56, 142.24, 142.20, 142.17, 142.14, 142.07, 142.04, 141.88, 141.77, 141.59, 141.44, 140.16, 140.02, 139.47, 138.86, 136.32, 136.26, 136.05, 135.80, 106.39, 100.21, 86.58, 78.08, 75.47, 69.38, 68.45, 68.29, 68.23, 67.68, 67.46, 67.22, 57.65, 31.94, 29.72, 29.66, 29.50, 29.38, 26.17, 22.71, 14.15. Anal. calcd for C₁₀₃H₆₈O₂NFe: C 87.89, H 4.87, N 1.00. Found: C 87.30, H 4.77, N 1.04. FAB-MS: calcd. for C₁₀₃H₆₈O₂NFe 1406.46; found 1406.5 [M]⁺. UV/Vis (CH₂Cl₂): 255 (133 700), 306 (42 260), 430 (4300), 704 (360).

Compound C₆₀-PV-Fc

A mixture of 2 (0.105 g, 0.33 mmol), C₆₀ (0.24 g, 0.33 mmol) and 4 (0.35 g, 0.66 mmol) in ODCB (60 mL) was heated at 100 °C for 20 h under argon. After cooling, the resulting solution was evaporated to dryness and column chromatography (SiO₂, hexane/CH₂Cl₂ 4 : 1) gave C₆₀-PV-Fc (0.2 g, 40%) as a brown glassy product. ¹H NMR (300 MHz, CDCl₃): 7.84 (br s, 2 H), 7.50 (d, J = 8 Hz, 2 H), 6.84 (d, J = 2 Hz, 2 H), 6.80 (AB, J = 17 Hz, 2 H), 6.49 (t, J = 2 Hz, 1 H), 5.20 (s, 1 H), 4.92 (d, J = 9 Hz, 1 H), 4.51 (d, J = 13 Hz, 1 H), 4.45 (t, J = 2 Hz, 2 H), 4.28 (t, J = 2 Hz, 2 H), 4.17 (d, J = 10 Hz, 1 H), 4.14 (s, 5 H), 4.04 (t, J = 6 Hz, 4 H), 3.62 (d, J = 14 Hz, 1 H), 1.84 (m, 4 H), 1.44 (m, 36 H), 0.88 (t, J = 7 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): 160.11, 156.07, 153.68, 153.12, 153.03, 146.87, 146.42, 146.03, 145.90, 145.86, 145.77, 145.73, 145.69, 145.65, 145.51, 145.48, 145.30, 145.12, 145.08, 144.89, 144.84, 144.79, 144.70, 144.27, 144.18, 143.98, 143.94, 142.71, 142.54, 142.23, 142.14, 142.11, 141.89, 141.81, 141.70, 141.66, 141.57, 141.55, 141.51, 141.40, 141.21, 141.11, 139.72, 139.66, 139.59, 139.47, 139.11, 137.69, 136.41, 136.00, 135.53, 135.32, 134.94, 129.35, 127.16, 125.72, 125.21, 106.84, 99.83, 82.81, 80.65, 68.78, 68.65, 68.34, 67.77, 66.50, 66.46, 66.11, 56.17, 31.49, 29.27, 29.22, 29.06, 28.94, 25.72, 22.26, 13.71. Anal. calcd for C₁₁₁H₇₃O₂NFe: C 88.37, H 4.88, N 0.93. Found: C 87.91, H 4.90, N 1.11%. FAB-MS: calcd. for C₁₁₁H₇₃O₂NFe 1508.65; found 1508.3 [M]⁺. UV/Vis (CH₂Cl₂): 229 (154 600), 255 (168 000), 319 (85 900), 431 (7500), 703 (750).

Compound C₆₀-2PV-Fc

A mixture of 3 (0.23 g, 0.55 mmol), C₆₀ (0.4 g, 0.55 mmol) and 4 (0.59 g, 1.11 mmol) in ODCB (100 mL) was heated at 100 °C for 20 h under argon. After cooling, the resulting solution was evaporated to dryness and column chromatography (SiO₂, hexane/CH₂Cl₂ 4 : 1) gave C₆₀-2PV-Fc (0.3 g, 33%) as a brown glassy product. ¹H NMR (300 MHz, CDCl₃): 7.90 (br s, 2 H), 7.61 (d, J = 8 Hz, 2 H), 7.44 (AB, J = 8 Hz, 4 H), 7.12 (s, 2 H), 6.84 (d, J = 2 Hz, 2 H), 6.79 (AB, J = 17 Hz, 2 H), 6.49 (t, J = 2 Hz, 1 H), 5.22 (s, 1 H), 4.93 (d, J = 9 Hz, 1 H), 4.52 (d, J = 13 Hz, 1 H), 4.47 (t, J = 2 Hz, 2 H), 4.29 (t, J = 2 Hz, 2 H), 4.19 (d, J = 10 Hz, 1 H), 4.14 (s, 5 H), 4.04 (t, J = 6 Hz, 4 H), 3.64 (d, J = 13 Hz, 1 H), 1.84 (m, 4 H), 1.43 (m, 36 H), 0.88 (t, J = 7 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): 160.57, 156.48, 154.08, 153.47, 153.37, 147.31, 146.83, 146.46, 146.30, 146.27, 146.22, 146.18, 146.14, 146.10, 145.95, 145.74, 145.53, 145.32, 145.26, 145.15, 144.71, 144.62, 144.42, 144.38, 143.15, 142.98, 142.68, 142.55, 142.33, 142.25, 142.15, 142.10, 142.07, 142.02, 141.92, 141.85, 141.65, 141.55, 140.16, 140.11, 139.95, 139.90, 139.57, 137.70, 137.37, 136.84, 136.44, 136.28, 136.00, 135.77, 129.83, 129.78, 128.92, 127.62, 127.16, 126.88, 126.14, 125.80, 107.32, 100.30, 80.99, 69.44, 69.31, 68.77, 68.23, 66.98, 56.62, 31.94, 29.72, 29.67, 29.51, 29.38, 26.17, 22.71, 14.15. Anal. calcd for C₁₁₉H₇₉O₂NFe.H₂O: C 87.75, H 5.01, N 0.86. Found: C 87.75, H 5.03, N 1.03%. FAB-MS: calcd. for C₁₁₉H₇₉O₂NFe 1610.79; found 1610.4 [M]⁺. UV/Vis (CH₂Cl₂): 229 (154 600), 255 (168 000), 319 (859 00), 431 (7500), 703 (750).

Compound C₆₀-3PV-Fc

A mixture of 6 (0.08 g, 0.1 mmol), C₆₀ (0.07 g, 0.1 mmol) and 4 (0.11 g, 0.2 mmol) in ODCB (25 mL) was heated at 100 °C for 20 h under argon. After cooling, the resulting solution was evaporated to dryness and column chromatography (SiO₂, hexane/CH₂Cl₂ 4 : 1) gave C₆₀-3PV-Fc (0.05 g, 26%) as a brown glassy product. ¹H NMR (300 MHz, CDCl₃): 7.75 (s, 1 H), 7.49 (s, 4 H), 7.45 (d, J = 3 Hz, 1 H), 7.38 (d, J = 6 Hz, 4 H), 7.15 (s, 1 H), 7.14 (d, J = 12 Hz, 1 H), 7.09 (d, J = 2 Hz, 2 H), 6.83 (d, J = 2 Hz, 2 H), 6.74 (AB, J = 17 Hz, 2 H), 6.48 (t, J = 2 Hz, 1 H), 5.82 (s, 1 H), 4.91 (d, J = 7 Hz, 1 H), 4.56 (s, 2 H), 4.52 (d, J = 11 Hz, 1 H), 4.37 (s, 2 H), 4.25 (d, J = 7 Hz, 1 H), 4.20 (s, 5 H), 4.09 (m, 3 H), 4.02 (t, J = 5 Hz, 4 H), 3.80 (m, 1 H), 3.70 (d, J = 10 Hz, 1 H), 1.82 (m, 6 H), 1.66 (m, 2 H), 1.48–1.26 (m, 56 H), 0.88 (t, J = 5 Hz, 12 H). ¹³C NMR (75 MHz, CDCl₃): 160.53, 156.98, 155.07, 154.48, 153.88, 152.06, 150.96, 147.25, 146.79, 146.70, 146.27, 146.19, 146.14, 146.07, 146.03, 145.90, 145.66, 145.53, 145.24, 145.19, 145.10, 145.05, 144.57, 144.52, 144.42, 144.33, 142.98, 142.94, 142.61, 142.52, 142.29, 142.23, 142.13, 142.04, 141.90, 141.74, 141.67, 141.64, 141.61, 140.12, 140.04, 139.99, 139.66, 139.45, 137.22, 136.92, 136.54, 136.27, 136.20, 136.17, 136.11, 134.70, 128.77, 127.99, 127.92, 127.05, 126.95, 126.78, 126.60, 126.43, 126.33, 125.72, 123.39, 114.76, 109.53, 107.14, 100.27, 76.10, 73.11, 70.23, 69.97, 69.60, 68.96, 68.64, 68.10, 67.19, 66.26, 56.40, 31.89, 29.53, 29.68, 29.63, 29.48, 29.34, 26.89, 26.27, 26.15, 26.09, 22.74, 22.67, 14.18, 14.10. Anal. calcd for C₁₄₃H₁₁₇O₄NFe.CH₂Cl₂: C 84.29, H 5.80, N 0.68. Found: C 84.49, H 6.44, N 0.92%. FAB-MS: 1247.6 (30, [M-C₆₀]⁺, calcd. for C₈₃H₁₁₇O₄NFe 1247.83), 1968.6 (50, [MH]⁺, calcd. for C₁₄₄H₁₁₇O₄NFe 1968.8).

Electrochemistry

The electrochemical experiments were carried out in argon-purged THF (Aldrich) or CH₂Cl₂ (Romil Hi-Dry™) solutions at 298, 218 K with an EcoChemie Autolab 30 multipurpose instrument interfaced to a personal computer. THF was distilled according to a literature procedure.³³ In the cyclic voltammetry (CV) the working electrode was a glassy carbon electrode (0.08 cm², Amel) or a Pt disk ultramicroelectrode (r = 25 µm). In all cases, the counter electrode was a Pt spiral, separated from the bulk solution with a fine glass frit, and a silver wire was employed as a quasi-reference electrode (AgQRE). The potentials reported are referred to SCE by measuring the AgQRE potential with respect to ferrocene. The concentration of the compounds examined was of the order of 5 × 10^–4 M; 0.05 M tetrabuthylammonium hexafluorophosphate (TBAPF₆) was added as supporting electrolytes. Cyclic voltammograms were obtained with scan rates in the range 0.05–20 V s^–1. The CV simulations were carried out by the DIGISIM program.

Photophysics

The photophysical investigations were carried out in CH₂Cl₂ and toluene (Carlo Erba, spectrofluorimetric grade). Absorption spectra were recorded with a Perkin-Elmer λ40 spectrophotometer. Emission spectra were obtained with an Edinburgh FLS920 spectrometer (continuous 450 W Xe lamp), equipped with a Peltier-cooled Hamamatsu R928 photomultiplier tube (185–850 nm) or a Hamamatsu R5509-72 supercooled photomultiplier tube (193 K, 800–1700 nm range). Emission quantum yields were determined according to the approach described by Demas and Crosby³⁴ using [Os(phen)₃]²⁺ (Φ = 0.005) as standard.³⁵ Emission lifetimes were determined with the time correlated single photon counting technique using an Edinburgh FLS920 spectrometer equipped with a laser diode head as excitation source (1 MHz repetition rate, λ_exc = 407 or 635 nm, 200 ps time resolution upon deconvolution) and an Hamamatsu R928 PMT as detector. Transient absorption spectra in the nanosecond-microsecond time domain were obtained by using the nanosecond flash photolysis apparatus described previously.³⁶ Experimental uncertainties are estimated to be 8% for lifetime determinations, 20% for emission quantum yields, 10% for relative emission intensities in the NIR, 1 nm and 5 nm for absorption and emission peaks, respectively.

Acknowledgements

This research was supported by the CNR (Commessa PM.P04.010 “Materiali Avanzati per la Conversione di Energia Luminosa”, MACOL), MIUR (PRIN, “Sistemi supramolecolari per la conversione dell’energia luminosa”), the CNRS (UPR 8241) and the EU (contract HPRN-CT-2002-00171, FAMOUS).

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