[60]Fullerene–perchlorotriphenylmethide anion triads. Synthesis and study of photoinduced intramolecular electron-transfer processes

Abstract

For the first time an anionic donor, the perchlorotriphenylmethide anion (PTM⁻), has been covalently bonded to C₆₀, generating the C₆₀–(PTM⁻)₂ triad that is reversibly oxidized to the corresponding C₆₀–perchlorotriphenylmethyl radical triad C₆₀–(PTM˙)₂. For the triad C₆₀–(PTM⁻)₂, photoinduced charge-separation can be confirmed to occur via the excited singlet states of the C₆₀ moiety and the PTM anion in polar and nonpolar solvents from quenching of their fluorescence intensities in the region of 700–750 nm and 560–630 nm, respectively. The charge-separation state was confirmed by the nanosecond transient absorption spectra in the visible and near-IR spectral regions. After charge-separation, back electron transfer takes place with a lifetime of about 80 ns. Steady-state concentration of the highly persistent PTM radical was observed after repeated laser light irradiation.

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Introduction

Since C₆₀ has been revealed as a very attractive electron acceptor with unique photo-physical and electrochemical properties,^1–3 considerable efforts have been devoted in recent years to develop systems in which C₆₀ is covalently linked to electron donors,^4–15 in addition to the blend systems consisting of C₆₀ and donors.^16–23 Donor–acceptor systems including C₆₀ are of particular interest, because they exhibit characteristic electronic properties in the excited states with interesting photo-physical properties attributed to the small reorganization energy of C₆₀ in electron transfer processes due to its spherical rigid molecular shape.²⁴ Thus, a lot of research has been conducted for a better understanding of photoinduced electron-transfer processes in dyads and triads including C₆₀.^4–15 These phenomena open the potential applications in the realization of new artificial photosynthetic systems, molecular electronic devices, and photovoltaic cells.^4,6,7

Among the wide variety of donor molecules that have been covalently linked to C₆₀, most donors are neutral molecules such as aromatic amines, carotene, tetrathiafulvalenes, ferrocenes, porphyrins, phthalocyanines, oligothiophenes, π conjugated phenyl oligomers, etc. These dyads generated the radical anion–radical cation pairs, some of them showing quite persistent charge-separated state in polar solvents, since the charge-recombination processes enter the inverted region of the Marcus parabola.

In the present study, for the first time an anionic donor, the perchlorotriphenylmethide anion (PTM⁻), has been covalently bonded to a C₆₀ derivative, generating the C₆₀–perchlorotriphenylmethide anion triad 1 shown in Fig. 1.

Fig. 1 Molecular structure of C₆₀–(PTM⁻)₂.

Triphenylmethide anions have been reported to be quite stable when the hydrogen atoms of the triphenyl groups are substituted by chlorine atoms.²⁵ The stability is not only due to the electron-withdrawing character of the substituents, but also to an effective steric shielding of the central carbon wrapped by the six bulky chlorine atoms in the ortho positions of the phenyl rings. In addition, PTM anions are reversibly oxidized to highly persistent perchlorotriphenylmethyl radicals (PTM˙) at low potential (around −0.55 V, versus Fc/Fc⁺) being therefore very good donors.²⁶

For the target triad C₆₀–(PTM⁻)₂ (1) we expected the occurrence of an electron transfer from the anionic part to the photoexcited C₆₀ moiety, generating the neutral perchlorotriphenylmethyl radical (PTM˙) and the radical anion of the C₆₀ moiety. This represents a completely new situation in this kind of donor–acceptor systems since the charge-recombination takes place between the C₆₀ radical anion and a neutral radical, which may show quite different behavior from the usual charge-separated states generating oppositely charged species.

Results and discussion

Synthesis of C₆₀–(PTM⁻)₂

The synthesis of the target triad 1 consisting of two donor units, the perchlorinated triphenylmethide anions, and the C₆₀ moiety as the acceptor unit, was achieved by a multi-step synthetic procedure in which the bis-acylation of the C₆₀-diol 4²⁷ using the PTM acid chloride 3 is the key step. The new acid chloride derivative 3 was obtained in good yield (80%) following the same procedure used in the synthesis of its radical counterpart²⁸ through refluxing 4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzoic acid²⁹ in an excess of thionyl chloride. We carried out the esterification reaction in refluxing CH₂Cl₂ under an argon atmosphere in the presence of dimethylaminopyridine (DMAP). Note that this reaction affords compound 5, the precursor of the target compound 1, in quite good yield (∼60%), although two bulky groups are attached to the fullerene moiety through the neighboring alcohol groups. In fact the yield is very similar to that obtained in the esterification of C₆₀-diol 4 with a TTF-acid chloride with long alkyl chains that afford high flexibility to the linkage.³⁰ As shown in Scheme 1 the deprotonation of the bis α-H precursor 5 yields quantitatively the target triad 1, by converting the non electroactive α-H triphenylmethane addends to electroactive carbanions that are very good donors. Compound 1 was isolated as the pure tetrabutylammoniun salt by washing the dark purple precipitate with hexane several times.

Scheme 1 Synthesis of C₆₀–(PTM⁻)₂(NBu₄⁺)₂. Conditions: i) SOCl₂, reflux; ii) DMAP, CH₂Cl₂, 35 °C; iii) NBu₄OH, THF, room temperature.

According to the donor characteristics of bisanion 1, when a CH₂Cl₂ solution of this salt containing an excess of AgNO₃ as oxidant is shaken for a few minutes, the bistriphenylmethyl radical 6 was obtained as a deep red solid in quantitative yield after filtration (Scheme 2). The reaction was followed by UV-Vis spectroscopy; Fig. 2 shows the evolution of the spectra over time. The disappearance of the intense band centered at 518 nm corresponding to the PTM⁻ anion^26e was accompanied by the increasing of the characteristic bands of the PTM radical at 364 and 384 nm.²⁶ The presence of two isosbestic points at 435 and 600 nm is clear evidence of the clean transformation of C₆₀–(PTM⁻)₂ (1) into C₆₀–(PTM˙)₂ (6).

Fig. 2 UV-Vis spectra obtained upon oxidation of bisanion 1 in CH₂Cl₂.

Scheme 2 Synthesis of C₆₀–(PTM˙)₂ (6).

Bisradical 6 was characterized by EPR spectroscopy. The X-band isotropic spectrum of 6 in toluene–CH₂Cl₂ at room temperature consists of a signal with a central intense line and small satellite overlapped lines corresponding to the coupling of the unpaired electron with the naturally abundant ¹³C isotope at the α and aromatic positions of the PTM˙ addends. The g_iso value, 2.0023, as well as the isotropic coupling constant values, a₁ (¹³C_α) = 30 G and a₂ (¹³C_arom) = 12 G, are usual values for the monoradicals of the PTM family.²⁶ The spectrum of bisradical 6 in frozen toluene–CH₂Cl₂ solution does not show the characteristic fine structure nor the forbidden Δm_s = 2 transition characteristic of the triplet species. All these features indicate that the two radical units in the C₆₀–(PTM˙)₂ triad are not interacting.

Electrochemical measurements

The redox behavior of all new compounds was studied by cyclic voltammetry and the PTM⁻[18-crown-6]⁺ salt and pristine C₆₀ were also studied under the same conditions as the reference compounds. As expected, triad 5, in which only the C₆₀ moiety is electroactive, presents the characteristic reversible reduction waves of the substituted C₆₀ compounds. By contrast, unexpected results were obtained in the cyclic voltammogram of the triad C₆₀–(PTM⁻)₂, 1, which contains two different electroactive moieties. In all conditions assayed, only one reversible oxidation wave appears clearly (see ESI† Fig. SI1 and SI2) whereas the waves expected for the C₆₀ moiety are quite weak. The wave corresponding to the redox characteristics of the anionic PTM moieties appears at the same potential as the reference PTM⁻[18-crown-6]⁺, indicating that there is no influence of one anionic moiety on the other and that there is no interaction with the C₆₀ moiety.

The reversibility observed in the cyclic voltammogram of compound 1 is in accordance with the stability of the free bisradical C₆₀(PTM˙)₂, 6. In Table 1, the E_1/2 values for C₆₀, C₆₀–(PTM⁻)₂1, PTM⁻[18-crown-6]⁺ and C₆₀–(PTMH)₂5 are summarized. Weak reduction peaks of the C₆₀ unit in charged triad 1 were always observed under different conditions, the phenomenon being independent of the concentration, excluding an intermolecular phenomenon, the nature of the electrode (platinum, gold and carbon vitreous), excluding a phenomenon of adsorption to the electrode, and the solvent (CH₂Cl₂ and o-dichlorobenzene (o-DCB)). The unexpected behavior of compound 1 is not understood and new studies on other negative charged compounds are underway in order to know if the negative charges present in the addends of C₆₀ can be responsible for this.

Table 1 Redox potentials of C₆₀–(PTM⁻)₂ and references (E_1/2vs. Fc/Fc⁺ in o-DCB–CH₃CN (95 : 5))

Compound	E ox/V	E ^1 red/V	E ^2 red/V	E ^3 red/V
a Very weak signal.
C60		−1.03	−1.42	−1.87
PTM^−	−0.54
C60–(PTMH)2		−1.10	−1.49
C60–(PTM^−)2	−0.52	−1.12^a	−1.50^a

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In order to calculate, from the Weller equations,³¹ the free-energy changes for charge-separation (ΔG_CS) and charge-recombination (ΔG_CR) for C₆₀–(PTM⁻)₂ the first reduction potential (E_red) value of the C₆₀ moiety and the first oxidation potential (E_ox) of the PTM⁻ moiety are needed. From these E_ox and E_red values, the ΔG_CS and ΔG_CR values were calculated using eqn (1)–(3):

−ΔG_CS = ΔE_0-0 − (−ΔG_CR) (1)

−ΔG_CR = E_ox − E_red + ΔG_S (2)

where ΔE_0-0 is the energy of the 0-0 transition of C₆₀, R⁺ the radius of the PTM radical,²⁶R⁻ the radius of the radical anion of C₆₀ and R_cc the distance between the two electroactive sites;³²e, ε₀, ε_s, and ε_r are the elementary charge, vacuum permittivity, and static permittivities of the solvents used for rate measurements and redox potential measurements, respectively. The ΔG_CS and ΔG_CR values for C₆₀–(PTM⁻)₂ in different solvents are summarized in Table 2. In polar solvent such as PhCN, charge-separation processes via excited singlet states of the C₆₀ moiety (¹C₆₀*) and the PTM⁻ (¹(PTM⁻)*) are exothermic and predicted to occur easily. In toluene, the charge-separation process is also exothermic and can occur. The charge-separation process via the excited triplet state of C₆₀ (³C₆₀*) is also exothermic in polar and nonpolar solvents.

Table 2 Free-energy changes for charge separation (−ΔG_CS) via the excited states of C₆₀ in C₆₀–(PTM⁻)₂ and charge recombination (−ΔG_CR) of C₆₀˙⁻–(PTM˙)(PTM⁻)

Solvent	−ΔG^SCS/eV^a	−ΔG^TCS/eV^a	−ΔGCR/eV^a
a Calculated from eqns (1)–(3) employing ΔE0-0 = 1.72 eV for ^1C60*, ΔE0-0 = 1.52 eV for ^3C60*, Eox = −0.52 V for PTM^−, and Ered = −1.12 V for C60vs. Fc/Fc^+ in o-DCB–CH3CN 95 : 5. R^+ = 7.4 Å for PTM^−, and R^− = 4.7 Å for C60, and RCC = 10 Å.^32 Permittivities of toluene, o-DCB, and PhCN are 2.38, 9.93, 25.2, and 37.5 respectively.
Toluene	0.92	0.70	0.82
o-DCB	1.26	1.06	0.46
PhCN	1.33	1.13	0.39
CH3CN	1.34	1.14	0.38

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Steady-state absorption measurements

Steady-state absorption spectra of C₆₀–(PTMH)₂5, C₆₀–(PTM⁻)₂1, and PTM⁻ in PhCN are shown in Fig. 3. For C₆₀–(PTMH)₂, the peaks at 700 and 430 nm bands are attributed to the characteristic bands of the functionalized C₆₀. In the case of C₆₀–(PTM⁻)₂, the broad absorption band at 518 nm is characteristic of the PTM⁻ moiety, as shown in Fig. 2. The absorption bands of the C₆₀ moiety appear at 700 nm and in the shorter wavelength than 400 nm. Since this spectrum is almost the same as the one resulting from the summed spectrum of the components PTM⁻ and C₆₀, an appreciable interaction between moieties may not be present in the ground state. For the transient absorption measurements, a 355 nm laser light was used, which excites the C₆₀ moiety about 90% and 10% of to the PTM⁻ moiety. For time-resolved fluorescence measurement, a 400 nm laser light was used, which excites both the C₆₀ moiety (75%) and the PTM⁻ moiety (25%).

Fig. 3 Steady-state absorption spectra of (a) C₆₀–(PTMH)₂ (0.1 mM) in PhCN and (b) C₆₀–(PTM⁻)₂ and PTM⁻ (0.025 mM) in PhCN.

Fluorescence measurements

Steady-state fluorescence spectra of PTM⁻, C₆₀–(PTMH)₂, C₆₀–(PTM⁻)₂ in toluene and o-DCB are shown in Fig. 4. The fluorescence peak (λ_f) for C₆₀–(PTM⁻)₂ appeared at 600 nm (Fig. 4b), which is almost the same as that of PTM⁻ (Fig. 4a). Therefore, the origin of the observed fluorescence at 600 nm of C₆₀–(PTM⁻)₂ is attributed to the PTM⁻ moiety. From the cross point of the fluorescence band and absorption band after normalizing both intensities, the lowest excited singlet energy (E_0-0) of the PTM⁻ moiety was estimated to be 2.1 eV.

Fig. 4 Steady-state fluorescence spectra of (a) PTM⁻ (0.05 mM) and C₆₀–(PTMH)₂ (0.05 mM), and (b) C₆₀–(PTM⁻)₂ (0.05 mM) in toluene and o-DCB; λ_ex = 550 nm.

The fluorescence of the C₆₀ moiety would be expected to appear at 720 nm as shown for C₆₀–(PTMH)₂ (Fig. 4a). In the case of C₆₀–(PTM⁻)₂, however, the fluorescence of the C₆₀ moiety may be hidden by the huge fluorescence tail of ¹(PTM⁻)*. Further quantitative analyses on the fluorescence properties were carried out based on fluorescence lifetime measurements.

Time-resolved fluorescence spectra and fluorescence lifetimes

The peak position and shape of time-resolved fluorescence spectra of ¹(PTM⁻)* (see ESI†) are quite similar to those of the steady-state spectra. Within the fluorescence lifetime, the spectral shape is almost the same. The time profiles of the fluorescence peak position are shown in Fig. 5. The fluorescence time profile of ¹(PTM⁻)* showed slow fluorescence decay obeying a single exponential function, yielding a fluorescence lifetime of 4100 ps in toluene and o-DCB.^20a In PhCN, a shorter lifetime (1000 ps) was evaluated, suggesting differences in the excited state of ¹(PTM⁻)* in this polar solvent from that in nonpolar and less polar solvents.

Fig. 5 Fluorescence time profiles at 560–630 nm in o-DCB and PhCN; λ_ex = 400 nm.

Also for C₆₀–(PTMH)₂, the time-resolved fluorescence spectra (see ESI†) are in good agreement with the steady-state fluorescence spectrum. Within the fluorescence lifetime of C₆₀–(PTMH)₂ the spectral shape is unchanged. Time profiles of the fluorescence intensity at the peak position in toluene and o-DCB are shown in Fig. 6. In slightly polar o-DCB, the fluorescence lifetime of the ¹C₆₀* moiety is almost the same to that in non-polar toluene within experimental errors, in which both decays are the same as that of pristine ¹C₆₀*, as expected for a compound with no donor moieties that can not present charge-separation.

Fig. 6 Fluorescence decays of C₆₀–(PTMH)₂ at 700–750 nm in toluene and o-DCB; λ_ex = 410 nm.

Time profiles of the fluorescence peak position of C₆₀–(PTM⁻)₂ in toluene and o-DCB are shown in Fig. 7. Slight blue shift of the florescence peak was observed in the time-resolved fluorescence spectra of C₆₀–(PTM⁻)₂ (see ESI†) on going from toluene to o-DCB, which is the same trend as observed in steady-state measurements. The fluorescence lifetimes of the ¹(PTM⁻)* moiety in C₆₀–(PTM⁻)₂ at 560–630 nm (Fig. 7) are quite shorter than that of ¹(PTM⁻)* in all studied solvents (Fig. 5). This finding indicates that a quenching process from the ¹(PTM⁻)* moiety to the C₆₀ moiety takes place for C₆₀–¹(PTM⁻)*(PTM⁻). Even in nonpolar solvents like toluene, the fluorescence decay rate of the ¹(PTM⁻)* moiety in C₆₀–¹(PTM⁻)*(PTM⁻) was faster than that of the isolated ¹(PTM⁻)*, it is possible to consider that both energy transfer and charge separation occur via¹(PTM⁻)*.

Fig. 7 Fluorescence time profiles at (a) 560–630 nm and (b) 710–750 nm, respectively, in toluene and o-DCB; λ_ex = 400 nm.

From the (τ_f)_sample value of the ¹(PTM⁻)* moiety in C₆₀–(PTM⁻)₂, the intramolecular quenching rate-constant (k^S_q) was evaluated using eqn (4):

k^S_q = (1/τ_f)_sample − (1/τ_f)_ref (4)

where (τ_f)_ref is the fluorescence lifetime of PTM⁻ in toluene.^20e The kinetic data are summarized in Table 3, which indicates efficient energy/electron transfer quenching processes of the PTM⁻ moiety in C₆₀–(PTM⁻)₂; thus, in the evaluated k_q^S values, both energy transfer rate constant (k_EN^S) and charge-separation rate constant (k_CS^S) may be included. For clear evidence of the energy transfer process, a rise of the fluorescence of the ¹C₆₀* moiety at 720 nm would be expected, but this was difficult to observe due to the quick decay of ¹C₆₀* even in toluene (Fig. 7b).

Table 3 Fluorescence lifetimes (τ_f) at 710–730 nm, quenching rate-constants (k^S_q) and quenching quantum yields (Φ^S_q) via C₆₀–¹(PTM⁻)*(PTM⁻), and rate constants (k^S_CS) and quantum yields (Φ^S_CS) for charge separation via¹C₆₀*–(PTM⁻)₂ at room temperature

Solvent	τ f ^b/ps via^1(PTM^−)*	k ^S q ^a/s^−1via^1(PTM^−)*	Φ ^S q ^a via ^1(PTM^−)*	τ f ^b/ps via^1C60*	k ^S CS ^a/s^−1via^1C60*	Φ ^S CS ^a via ^1C60*
a Calculated by using eqn (4) and (5). The values of (τf)ref were employed to be 4130 ps for PTM^− and 1390 for C60. b Fluorescence decays of ^1(PTM^−)* and ^1(C60)* were fitted with biexponential functions and the kq^S, Φq^S, k^SCS and Φ^SCS values were calculated from the shorter lifetimes with the major fraction. c Effect of impurity in PhCN may be included.
Toluene	150 (70%)	6.4 × 10^9	0.96	100 (85%)	9.3 × 10^9	0.93
o-DCB	79 (85%)	1.3 × 10^10	0.98	83 (94%)	1.1 × 10^10	0.94
PhCN	257 (70%)^c	(3.7 × 10^9)^c	(0.94)^c	95 (88%)	9.8 × 10^9	0.93

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The fluorescence quenching quantum yields of (Φ^S_q) via the ¹(PTM⁻)* moiety in C₆₀–(PTM⁻)₂ in polar and nonpolar solvents were evaluated from eqn (5):

Φ^S_q = [(1/τ_f)_sample − (1/τ_f)_ref]/(1/τ)_sample (5)

Thus, Φ^S_q = 0.96–0.98 was calculated for C₆₀–¹(PTM⁻)*(PTM⁻). Similarly, Φ_q^S may contain both quantum yields of energy transfer (Φ_EN^S) and quantum yields of charge separation (Φ_CS^S).

In PhCN, the fluorescence decay rate was slower than those in toluene and o-DCB, probably because of the presence of some impurity in PhCN, which was quite difficult to eliminate.

By contrast with the fluorescence decay of the C₆₀–(PTMH)₂ moiety yielding the fluorescence lifetime (τ_f) of 1390 ps in toluene (Fig. 6),²⁰ the time profiles of the fluorescence at 720 nm of the C₆₀ moiety in C₆₀–(PTM⁻)₂ (Fig. 7b) show biexponential decay in o-DCB and toluene. The major component decayed with the τ_f values in the 83 ps (94%)–100 ps (85%) region, whereas the minor slow-decay components gave τ_f values of 1300 ps. The τ_f values of the major components are summarized in Table 3.

The difference between the τ_f values evaluated from the main fluorescence decays in 710–750 nm in polar and nonpolar solvents can be attributed predominantly to charge separation via the ¹C₆₀* moiety in C₆₀–(PTM⁻)₂ generating the radical ion-pair states (C₆₀˙⁻–(PTM˙)(PTM⁻)). From the τ_f value of the ¹C₆₀* moiety in C₆₀–(PTM⁻)₂, the intramolecular charge-separation rate constants (k^S_CS) in polar and nonpolar solvents were evaluated from eqn (4) as summarized in Table 3. Thus, the k^S_CS values for the C₆₀–(PTM⁻)₂ were evaluated to be (9.3–11.0) × 10⁹ s⁻¹ which indicate that charge separation via the ¹C₆₀* moiety is an effective process in polar and nonpolar solvents. The quantum yields of charge separation (Φ^S_CS) via the ¹C₆₀* moiety in C₆₀–(PTM⁻)₂ were evaluated from eqn (5). Thus, Φ^S_CS = 0.93–0.94 were calculated for the C₆₀–(PTM⁻)₂ (Table 3) in polar and nonpolar solvents, respectively, which indicates that the charge separation via the ¹C₆₀* moiety is the main process, overwhelming the intersystem crossing (ISC) process to the ³C₆₀* moiety even in toluene.

Nanosecond transient absorption measurements

Transient absorption spectra observed upon nanosecond laser excitation (355 nm) of C₆₀–(PTMH)₂ in toluene are shown in Fig. 8. The 355 nm laser light excites the C₆₀ and PTMH moieties. The broad absorption bands were observed in the region of 600–850 nm, which is the absorption region of ³C₆₀* (peak = 700 nm).^20e The decay time profile is quite slow on the time scale of 1.5 µs. Even the PTMH moiety was excited, the energy transfer gives the ¹C₆₀* moiety, which is finally converted to ³C₆₀* by ISC.

Fig. 8 Nanosecond transient absorption spectra of 0.1 mM C₆₀-(PTMH)₂ observed by 532 nm laser irradiation at 100 ns (●) and 1000 ns (○) in PhCN. Inset: Absorption–time profiles at 720 nm.

Transient absorption spectra observed by the nanosecond laser excitation (355 nm) of C₆₀–(PTM⁻)₂ in toluene are shown in Fig. 9. Broad absorption bands were observed in the region of 600–1200 nm, which can be attributed to the absorption of the C₆₀˙⁻ moiety,^20e although the absorption bands are broader compared with the pristine C₆₀˙⁻ and other C₆₀˙⁻ derivatives. This broadening may be caused by the interaction between the C₆₀˙⁻ moiety and the (PTM˙)(PTM⁻) moiety, in which anion, radical and halogen atoms may be the candidate of the interactions. From the decay time-profile at 1020 nm, the charge-recombination rate constant (k_CR) between C₆₀˙⁻ and PTM˙ of C₆₀˙⁻–(PTM˙)(PTM⁻) was evaluated to be 1.23 × 10⁷ s⁻¹, from which the lifetime (τ_RIP) of the charge-separated state was calculated to be 81 ns at room temperature. Since the 355 nm laser light predominantly excites the C₆₀ moiety, the process photoinduced by the excitation of PTM⁻ can be neglected.

Fig. 9 Nanosecond transient absorption spectra of 0.05 mM C₆₀–(PTM⁻)₂ observed by 355 nm laser irradiation at 100 ns (●) and 1000 ns (○) in toluene. Inset: Absorption–time profiles at 1020 nm.

Similarly, transient absorption–time profiles at 1020 nm were observed in o-DCB and PhCN (see ESI†), although the transient absorption spectra were distorted in o-DCB and PhCN by laser irradiation (see ESI† Fig. SI-8). From the decay time-profiles at 1020 nm, the charge-recombination rate constants (k_CR) were evaluated to be 1.30 × 10⁷ and 1.72 × 10⁷ s⁻¹ for o-DCB and PhCN, from which the τ_RIP values of the charge-separated states were calculated to be 77 and 58 ns, respectively, at room temperature as listed in Table 4.

Table 4 Charge-recombination rate constant (k_CR) of C₆₀˙⁻–(PTM˙)(PTM⁻) at room temperature

Solvent	k CR/s^−1	τ RIP/ns
Toluene	1.23 × 10^7	81
o-DCB	1.30 × 10^7	77
PhCN	1.72 × 10^7	58

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The solvent polarity effect on the τ_RIP values was small; for polar and nonpolar solvents, almost similar lifetimes (τ_RIP) are found. However, C₆₀˙⁻–(PTM˙)(PTM⁻) in toluene gives a long lifetime.

Repeated laser irradiation

By excitation of C₆₀–(PTM⁻)₂ with the 355 nm light, the absorption band of PTM⁻ decreased; instead, a sharp peak appeared at 390 nm (Fig. 10), which is attributed to the PTM˙ moiety (see Fig. 2). This observation indicates that after the photoinduced charge-separation the stable C₆₀–(PTM˙)(PTM⁻) species was generated, probably because the C₆₀˙⁻ moiety may be quenched by some reactions such as donation of an electron to the impurity in the solution.

Fig. 10 Steady-state absorption spectra of 0.10 mM C₆₀–(PTM⁻)₂ in PhCN before and after 355 nm laser light (LS) irradiation.

Energy diagram

Fig. 11 shows an energy diagram of C₆₀–(PTM⁻)₂ when PTM⁻ and C₆₀ are excited. Energy levels of the radical ion-pairs C₆₀˙⁻–(PTM˙)(PTM⁻) are deduced from Table 2. In toluene, o-DCB and PhCN, the charge-separation takes place via C₆₀–¹(PTM⁻)*(PTM⁻) and ¹C₆₀*–(PTM⁻)₂ as indicated by the weak fluorescence intensity and the short fluorescence lifetime, since the energy levels of C₆₀˙⁻–(PTM˙)(PTM⁻) are lower than these excited singlet states even in toluene. Thus, the generation of ³C₆₀*–(PTM⁻)₂ through the ISC process from ¹C₆₀*–(PTM⁻)₂ is not possible, because the Φ^S_CS values are almost unity in all solvents. Furthermore, with excitation of PTM⁻, an energy-transfer process is also possible from the ¹(PTM⁻)* moiety to generate the ¹C₆₀* moiety.

Fig. 11 Schematic energy diagram for electron-transfer processes of C₆₀–(PTM⁻)₂.

Summary

We have succeeded, for the first time, in synthesizing a C₆₀ based triad which bears an anionic donor as addend, the perchlorotriphenylmethide anion (PTM⁻), thus generating the C₆₀–(PTM⁻)₂ triad 1. For this triad, photoinduced charge separation via both C₆₀–¹(PTM⁻)*(PTM⁻) and ¹C₆₀*–(PTM⁻)₂ were observed in polar and nonpolar solvents through photoexcitation at room temperature. The efficiency and rate of the charge-separation process are quite high. The longest lifetime for C₆₀˙⁻–(PTM˙)(PTM⁻) was 81 ns in toluene at room temperature, which is quite long and comparable with other C₆₀ based dyads and triads such as C₆₀–fluorene–diphenylamine,³³ C₆₀–bridge–dimethylaniline,⁵ C₆₀–extended TTF³⁴ or C₆₀–long flexible bridge-TTF.³⁰ Even though the properties of C₆₀ perchlorotriphenylmethide anion based triads are not so good like the best ones found in some C₆₀–porphyrins and C₆₀–chlorines based dyads,³⁶ the present study provides a new possibility for functional fullerenes based on the perchlorotriphenylmethide anion, which can be added to other known functional materials based on the parent perchlorotriphenylmethyl radical that works as an acceptor and has permitted the development of multifunctional switchable molecular systems.^26e,35 We believe that the present study gives valuable information not only about the photo-induced electron-transfer chemistry of fullerene-triad systems, but also about the materials science of perchlorotriphenylmethide anion functional compounds.

Experimental

General information

Reagents were purchased from commercial suppliers and used without further purification. Compounds 2, PTM⁻[18-crown-6]⁺, and 4 were synthesized as previously described.^25,27,28 All solvents were spectrophotometric grade and were distilled before use. Column chromatography was performed using Merck silica gel 60. The NMR spectra were recorded on a Bruker Avance DRX 500 spectrometer (500 MHz for ¹H, 125.5 MHz for ¹³C). IR spectra were run on a FT-IR spectrometer BIO-RAD FTS 155, the solid compounds being studied in KBr pellets. UV-Vis spectra were recorded on a Cary 5 E Varian Spectrometer. MALDI-TOF mass spectra were obtained on a Bruker Biflex III spectrometer, equipped with a N₂ laser (337 nm) using dithranol as a matrix. The ESI-MS spectra were obtained with a JEOL, JMS 700 B/E mass spectrometer.

Electrochemical measurements

Oxidation potentials (E_ox) and reduction potentials (E_red) were measured by a voltammetric analyzer (EGG PAR 273A) in a conventional three-electrode cell equipped with Pt working electrodes and counter electrodes. A silver wire served as a quasi-reference electrode; its potential was checked against the ferrocene/ferrocenium couple (Fc/Fc⁺) before and after each experiment, at a scan rate of 100 mV s⁻¹. In each case, the solution contained ∼0.5 mM sample and 0.1 M tetra n-butylammonium hexafluorophosphate (Bu₄NPF₆) dissolved in a mixture of o-dichlorobenzene–acetonitrile 95 : 5 or other solvents. The experiments were performed in a glove box.

Synthesis of 4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzoyl chloride (3)

A solution of 4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzoic acid (2)²⁹ (149.7 mg; 0.194 mol) in thionyl chloride (3 mL) was refluxed for 24 h. Excess SOCl₂ was removed under reduced pressure. The resulting solid was purified by chromatography (silica gel, chloroform). Compound 3 was obtained in 80% yield as a beige solid. IR (KBr): 675, 764, 810, 1780 (COCl) cm⁻¹. Anal. Calc. for C₂₀HCl₁₅O₂: C, 29.84; H, 0.12; Cl, 66.06. Found: C, 29.67; H, 0.14; Cl, 66.36%.

Synthesis of 2,2-bis{4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachlorobenzoyloxymethyl}-1,2-propano-1,2-dihydro[60]fullerene (5)

A mixture of compound 4²⁷ (68.3 mg; 0.083 mmol), compound 3 (150 mg; 0.190 mmol) and dimethylaminopyridine (35.5 mg; 0.290 mmol) in freshly distilled dichloromethane (10 mL) was stirred under argon atmosphere at 38 °C for 5 days. The solvent was evaporated and the residue was purified by column chromatography (silica gel, hexane then hexane–CH₂Cl₂ 2 ∶ 1). Compound 5 was obtained in 59% yield as a brown solid. ¹H NMR (250 MHz, CDCl₃): 3.97 (s, 4H, CH₂–C₆₀), 5.41 (s, 4H, CH₂–O–CO–), 7.06 (s, 2H, CH). IR (KBr): 527(C₆₀), 810 (C–Cl), 1749 (C=O) cm⁻¹. UV-Vis (CH₂Cl₂): λ_max (ε) = 430 (4625), 326 (32800), 305 (38900), 255 nm (167000 L mol⁻¹ cm⁻¹). MS (MALDI-TOF): m/z calcd for the maximum peak of C₁₀₅H₁₀O₄Cl₂₈ = 2327.17, observed maximum [M]⁺ = 2327.25.

Synthesis of C₆₀–(PTM⁻)₂(NBu₄)₂ (1)

Tetrabutylammonium hydroxide, 40 wt% solution in water, ∼1.5 M (31.5 µl; 47.25 µmol) was added to a solution of compound 5 (20 mg; 8.59 µmol) dissolved in freshly distilled THF (2.5 mL). This mixture immediately became garnet-red and was stirred for 2 hours in a light protected place. The compound precipitates after addition of deoxygenated distilled hexane. The resulting solid was purified by centrifugation, and washed several times with deoxygenated distilled hexane. The garnet-red compound 1 was obtained quantitatively. IR (KBr): 518 (C₆₀), 810 (C–Cl), 1738 (C=O) cm⁻¹. UV (THF), λ_max (ε) = 518 nm (75000 L mol⁻¹ cm⁻¹). MS (ESI⁻): m/z calcd for the maximum peak of C₁₀₅H₈O₄Cl₂₈ = 1163.58, observed maximum 1162.45.

Steady-state measurements

Steady-state absorption spectra in the visible and near-IR regions were measured on a Jasco V570 DS spectrophotometer. Steady-state fluorescence spectra were measured on a Shimadzu RF-5300 PC spectrofluorophotometer equipped with a photomultiplier tube having high sensitivity in the 700–800 nm region.

Time-resolved fluorescence measurements

The time-resolved fluorescence spectra were measured by single photon counting method using a streakscope (Hamamatsu Photonics, C4334-01) as a detector and the laser light (second harmonic generation (SHG), 400 nm) of a Ti ∶ sapphire laser (Spectra-Physics, Tsunami 3950-L2S, 150 fs fwhm) as an excitation source.³⁶ Lifetimes were evaluated with software attached to the equipment.

Nanosecond transient absorption measurements

Nanosecond transient absorption measurements were carried out using THG (355 nm) of a Nd ∶ YAG laser (Spectra-Physics, Quanta-Ray GCR-130, 5 ns fwhm) as an excitation source. For transient absorption spectra in the near-IR region (600–1200 nm) and the time-profiles, monitoring light from a pulsed Xe lamp was detected with a Ge-APD (Hamamatsu Photonics, B2834). For the measurements in the visible region (400–1000 nm), a Si-PIN photodiode (Hamamatsu Photonics, S1722-02) was used as a detector.^20,36

Acknowledgements

The present work was supported by a Grant-in-Aid on Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, Dirección General de Investigación, Spain (Project BQU2003-00760), DGR, Generalitat de Catalunya (Centre de Referencia CeRMAE and Project 2001SG00362), EU COST D14 and CNRS (France).

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Footnote

† Electronic supplementary information (ESI) available: CV of compounds 1 and 5, minimized geometry of compound 1, time-resolved fluorescence spectra, transient spectrum and time profiles in polar solvents. See DOI: 10.1039/b514882k

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