|Home | About | Journals | Submit | Contact Us | Français|
Two isomeric [5,6]-pyrrolidine-Ih-Sc3N@C80 electron donor acceptor conjugates containing triphenylamine (TPA) as the donor system were synthesized. Electrochemical and photophysical studies of the novel conjugates were made and compared with those of their C60 analogues, in order to determine i) the effect of the linkage position (N-substituted versus 2-substituted pyrrolidine) of the donor system in the formation of photoinduced charge separated states, ii) the thermal stability towards the retro-cycloaddition reaction and iii) the effect of changing C60 for Ih-Sc3N@C80 as the electron acceptor. It was found that when the donor is connected to the pyrrolidine nitrogen atom, the resulting dyad produces a significantly longer lived radical pair than the corresponding 2-substituted isomer for both the C60 and Ih-Sc3N@C80 dyads. In addition to that, the N-substituted TPA-Ih-Sc3N@C80 dyad has much better thermal stability than the 2-subtituted one. Finally, the Ih-Sc3N@C80 dyads have considerably longer lived charge separated states than their C60 analogues, thus approving the advantage of using Ih-Sc3N@C80 instead of C60 as the acceptor for the construction of fullerene based donor acceptor conjugates. These findings are important for the design and future application of Ih-Sc3N@C80 dyads as materials for the construction of plastic organic solar cells.
Fullerenes have been proposed as acceptor materials in the construction of plastic solar cell devices due to their unique structural and electron acceptor characteristics.1 Most of the fullerene based solar cells are made using the bulk hetero-junction concept, where a conjugated polymer acting as a donor is blended with a fullerene derivative with improved solubility acting as the acceptor.2 The maximum power conversion efficiencies with this type of cells have recently reached 5.5%.3 Hence higher efficiencies seem to be feasible, both theoretically and in practice.4 The molecular hetero-junction concept where a donor molecule is covalently connected to a fullerene is one of the possible alternatives to further improve the efficiency of plastic fullerene based solar cells.5 Since organic molecules allow a high degree of structural control; fine tuning of the charge separation properties, charge mobility and spatial orientation of the donor group relative to the acceptor can be achieved by using specifically engineered molecular systems.6 Following this principle, donor acceptor conjugates with the ability to efficiently generate long-lived charge separated states with lifetimes comparable to the ones observed in natural photosynthetic systems have been synthesized.7
Among the large number of fullerenes that have the potential to replace C60 or C70 as optimal electron acceptors in fullerene based solar cells, Ih-Sc3N@C80, which was discovered a few years ago by Dorn and coworkers,8 is very attractive due to its outstanding ability to stabilize charge separated states when compared to C60 as shown in our previous study.9 Recent developments have allowed its preparation10 and isomeric separation in bulk quantities by non HPLC methods11 with a higher yield than C84, the third most abundant empty fullerene.12 Due to its high symmetry, Ih-Sc3N@C80 has only two types of double bonds, described as [5,6] when they are between five and a six membered rings and [6,6] when they are between two six membered rings. These bonds have different reactivity and yield derivatives with different electrochemical behavior.13 On the other hand, triphenylamine (TPA) and its derivatives are robust molecules that have been successfully employed as donors for the construction of small molecule donor solution processable organic solar cells,14 dye sensitized solar cells15 and fullerene donor acceptor conjugates16 because of their strong donor character, their good hole transport properties,17 their propeller structure which benefits the solution processability14a and their ability to form and stabilize radical cations.17b
1,3-dipolar cycloaddition of azomethine ylides to fullerenes is a common strategy for the functionalization of fullerenes.18 However, there are very few examples of fulleropyrrolidine donor acceptor conjugates where the donor group is connected to the nitrogen atom of the pyrrolidine ring.19 Other examples of donor groups connected axially using the nitrogen atom in the pyrrolidine ring are coordinated compounds using a fullerene derivative as a ligand20 or supramolecular assemblies;21 however, attaching the donor groups to the 2-position of the pyrrolidine ring is usually the preferred choice. In our previous studies9b it was also observed that the 2-substituted Ih-Sc3N@C80 and Ih-Y3N@C80 pyrrolidine derivatives can undergo thermal 1,3-retrocycloaddition reactions.22 Importantly, there are no reports of systematic studies comparing the stability of N-substituted fulleropyrrolidines versus 2-substituted analogues and the effect of the substitution pattern on the efficiency of the charge separation process and/or thermal stability. To address these questions, we have prepared two isomeric TPA-Ih-Sc3N@C80 fulleropyrrolidine electron donor acceptor conjugates, and compared their properties with those of the corresponding C60 conjugates in order to study the effect of the pyrrolidine linkage position and the effect of changing the fullerene acceptor on the efficiency of the charge separation process.
To date, the reported chemical reactions on the Ih-Sc3N@C80 cage include Diels Alder,23 hydroxylation,24 1,3-dipolar cycloaddition of azomethine ylides, 9,13,25 disilirane addition,26 radical trifluoromethylation,27 malonate free radical addition28 and dibenzyl free radical addition.29 However, the 1,3-dipolar cycloaddition has been the most successful reaction for the functionalization of Ih-Sc3N@C80. The moderate to good yields, the high regioselectivity and availability or easy preparation of the starting materials, make this reaction attractive for the construction of donor acceptor conjugates. Thus, we decided to pursue this strategy for the synthesis of the triphenylamine based electron donor acceptor conjugates as depicted in Scheme 1.
Starting from an isomeric mixture of Ih and D5h Sc3N@C80 provided by Luna Innovations Inc. the icosahedral isomer was purified by selective chemical oxidation.11f After isolating the Ih-Sc3N@C80, compound 1 was obtained in a ~40% yield by reacting Ih-Sc3N@C80 with 50 equivalents of 4-(N-diphenylamino)benzaldehyde and 15 equivalents of sarcosine in o-dichlorobenzene (o-DCB) at 120°C. The 1H-NMR spectrum obtained in a 4:1 mixture of CS2-CD2Cl2 under selective irradiation of the residual solvent signal shows individual resonances for the protons in the pyrrolidine ring at 4.35, 3.74 and 3.07 ppm (Figure 1a). The 1.28 ppm separation of the resonances for the diastereotopic geminal protons (J = 9 Hz) in the pyrrolidine ring, attached to the same carbon atom as inferred by the HMQC spectrum – see supporting information – is consistent with the formation of the [5,6]-regioisomer, which is additionally supported by electrochemical studies – see electrochemical section. The protons of the methyl group attached to the nitrogen appear as a singlet at 2.63 ppm. The protons in the aromatic ring directly attached to the pyrrolidine appear as broad signals at 7.92 and 7.38 ppm due to a dynamic effect produced by the restricted rotation of the bulky triphenylamine group.30
The same effect is observed in the C60 conjugate,16f however, the effect is more intense in the Ih-Sc3N@C80 conjugate, which is probably a consequence of the cage size. Increasing the temperature to 40°C did not change appreciably the appearance of the spectrum. For the remaining aromatic rings, the protons in the ortho position with respect to the nitrogen atom in the TPA group are observed as a doublet centered at 7.18 ppm, the protons in the meta position are observed as a triplet centered at 7.29 ppm and the protons in the para position appear as a triplet centered at 7.03 ppm. These assignments are confirmed by the COSY spectrum – see supporting information.
Compound 4 was prepared following a similar strategy similar to the one used for the preparation of compound 1. However, paraformaldehyde was added in three portions to the mixture Ih-Sc3N@C80 and 3 in o-DCB within a half hour period to compensate the losses due to the evaporation. A higher temperature was also employed, since the formation of the product 4 was slower when compared to the synthesis of compound 1 giving satisfactory yields of the desired product. However, formation of undesired by-products also increased, which made the purification process harder. Thus the selected temperature (150°C) was a compromise between short reaction times and low production of undesirable byproducts.
In the 1H-NMR spectrum of compound 4 the pyrrolidine protons appear as two doublets centered at 4.10 and 2.96 ppm, respectively (Figure 1b). A correlation observed in the HMQC spectrum between those signals and a carbon atom at 69.0 ppm indicates the formation of the 5,6-isomer – see supporting information. The benzyl protons are observed as a sharp singlet at 3.96 ppm, which correlates with a carbon at 58.5 ppm. The aromatic protons are observed as a group of 4 signals in the region between 7.0 and 7.5 ppm. The resonances of the ring containing the benzyl group appear at 7.45 and 7.18 ppm whereas the protons on the remaining aromatic rings are observed at 7.33, 7.18 and 7.08 ppm, respectively. The well defined 1H-NMR signals are evidence that in compound 4 the fullerene does not affect the rotation dynamics of the attached addend as in the case of compound 1.
Evidence showing the higher thermal stability of compound 4 when compared to that of compound 1 was obtained upon refluxing an o-DCB solution containing both compounds in open air and following the retro-cycloaddition process by HPLC (Figure 2). It was observed that the intensity of the peak corresponding to compound 1 decreases faster than that corresponding to compound 4. It has been proved that the retro Prato reaction strongly depends on the nature of the attached addend.22a Therefore our observations can be explained by the relative thermodynamic stabilities of the corresponding azomethine ylides, which are intermediates during the decomposition process; probably the TPA substituent in the 2-position stabilizes better the corresponding azomethine ylide hence it can leave easily the surface of the fullerene.22a The MALDI-TOF mass spectra of compounds 1 and 4 show a strong peak at 1109 m/z corresponding to Ih-Sc3N@C80 resulting from fragmentation of the parent compounds – see supporting information. However, compound 4 shows a much larger relative molecular peak when compared to compound 1. Even though the fragmentation process depends on many factors, this data seems to confirm the observed trend.
The electrochemical properties of the TPA-Ih-Sc3N@C80 electron donor acceptor conjugates 1 and 4 were studied by cyclic voltammetry in o-DCB using a glassy carbon electrode and a 0.05 M solution of tetra(n-butyl)ammonium hexafluorophosphate as supporting electrolyte. The redox couple ferrocene/ferrocenium (Fc/Fc+) was used as internal standard for referencing the potentials. All the redox potentials for compounds 1, 2, 4 and 5 along with Ih-Sc3N@C80, C60 and 4-(diphenylamino)benzyl alcohol used as references are collected in Table 1.
As shown in Figure 3 trace b, 1 exhibits three reversible reductions, which is characteristic for [5,6]-Ih-M3N@C80 fulleropyrrolidine adducts.13 In the anodic scan 1 exhibits two irreversible oxidation processes. The first oxidation potential occurring at +0.39 V is probably centered on the TPA moiety. The second oxidation potential at +1.06 can be attributed to the oxidation of the pyrrolidine group, based on previous observations.22b
Compound 4 has a similar reductive behavior to that of 1, three reversible reductions based on the Ih-Sc3N@C80 cage. There is also a small wave around −2.0 V that correlates with the irreversible formation of a film over the electrode. This behavior is currently under study in our laboratories. In the anodic scan, 4 has three irreversible oxidations; the process occurring at +0.32 V was assigned to the oxidation of the TPA moiety. The second oxidation occurring at +0.63 V was assigned to the pyrrolidine group and the third at +0.99 V was assigned to the oxidation of the Ih-Sc3N@C80 cage. The first reduction potential is shifted by 50 mV towards positive potentials relative to that for the pristine Ih-Sc3N@C80 while the TPA oxidation potential is shifted towards more negative potentials, which is probably indicative of an electronic interaction between the TPA and the Ih-Sc3N@C80 moieties in the ground state.
Insight into charge transfer interactions within the TPA-fullerene conjugates came from transient absorption measurements, in which the lead compounds, that is, TPA-C60 (2 and 5) and TPA-Ih-Sc3N@C80 (1 and 4) were probed in solvents of different polarity (i.e., toluene, carbon disulfide, THF and benzonitrile). In particular, short excitation at 387 nm afforded the selective excitation of either C60 or Ih-Sc3N@C80, whose singlet excited state characteristics and lifetimes are well documented.9b,31
In line with reference experiments we monitored the singlet excited state characteristics of C60 – in 2 and 5 – and of Ih-Sc3N@C80 – in 1 and 4 – at 880 and 550/1040 nm, respectively. In contrast to what has been seen for the reference compounds, the C60 and Ih-Sc3N@C80 singlet excited state features decay rather rapidly (~1.5 ns and ~100 ps respectively). Inspecting the transient changes that develop concomitantly with these decays no spectral resemblance with any known excited state (i.e., C60 triplet, Ih-Sc3N@C80 triplet, TPA triplet, etc.) 9b,31 could be established – see Figures 4 and and5.5. Instead, transient maxima at 610 and 1020 nm suggest charge transfer activity for photoexcited 2 and 5 in THF (ε = 7.6) and benzonitrile (ε = 24.8).
These features resemble the fingerprints of the one-electron oxidized TPA radical cation and the one-electron reduced C60 radical anion, respectively.32 As a matter of fact, our experiments corroborate the successful formation of TPA•+-C60•− with rate constants of 7.6 ± 1.0 × 1010 s−1 (2) and 4.1 ± 0.5 × 1010 s−1 (5) in THF.
Turning to 1 and 4, the near-infrared region is, once again, decisive for assigning the radical ion pair state. In line with a previous investigation that focused on the photophysical, radiolytical and spectroelectrochemical generation of one-electron reduced Ih-Sc3N@C80 radical anions a transient maximum at 1060 nm confirms the reduction of this moiety in 1 and 4.9a Formation of the TPA•+-Ih-Sc3N@C80•− radical ion pair state was confirmed by detecting the 610 nm signature of TPA•+. The corresponding rate constants were 3.4 ± 0.5 × 1010 s−1 and 1.9 ± 0.5 × 1010 s−1 for 1 and 4, respectively, in THF.
In less polar solvents, on the other hand, such as toluene (ε = 2.4) or carbon disulfide (ε = 2.6) the singlet excited state features of C60 or Ih-Sc3N@C80 undergo conventional intersystem crossing – see Figures 6 and and7.7. They yield the corresponding triplet excited states (C60: 700 nm maximum; Ih-Sc3N@C80: 520 nm maximum) without, however, revealing any significant radical ion pair state formation.
The lack of charge transfer was further supported by steady-state fluorescence measurements. As Figure 8 illustrates, in toluene and carbon disulfide the C60 and Ih-Sc3N@C80 centered fluorescence is in 1 to 4 unchanged relative to the corresponding references. Quantum yields are on the order of 6.0 × 10−4 (2 and 5) and 5.3 × 10−5 (1 and 4) and, thus, identical to those of the reference systems lacking the electron donor. Notable in toluene and carbon disulfide the energies of the radical ion pair states are higher than those of the singlet excited states. Taking, for example, 2 and 5 we calculate – via the Dielectric Continuum model33 – energies of around 2.37 and 2.42 eV in toluene and carbon disulfide, respectively, while that of the C60 singlet excited state is around 1.76 eV.
Similarly, the Ih-Sc3N@C80 singlet excited state is 1.5 eV below the radical ion pair state of 1 and 4 in toluene and carbon disulfide (i.e., 1.9 and 2.0 eV). These thermodynamic considerations suggest that a competitive scenario, namely intersystem crossing versus charge separation is unlikely to play a role in toluene and carbon disulfide. Any radical ion pair state should be generated only as a minor product. THF and benzonitrile revealed, on the other hand, quenching of at least a factor greater than 50 with radical ion pair state energies of 1.67 eV in THF and 1.45 eV in benzonitrile (2 and 5) as well as 1.42 eV in THF and 1.22 eV in benzonitrile (1 and 4). Please compare these values to the singlet excited state energies of 2/5 (1.76 eV) and 1/4 (1.5 eV).
Important is the comparison of rate constants (i.e., charge separation and charge recombination) for 2 and 5 versus 1 and 4, which should shed light onto the stabilization of the radical ion pair states when employing Ih-Sc3N@C80 as a novel electron accepting building block. The fingerprints of TPA•+-C60•− and TPA•+-Ih-Sc3N@C80•− proved to be valuable assets to fit the growth and decay dynamics of the radical cation and radical anion species.
When turning to charge separation – located evidently in the normal region of the Marcus parabola – lower singlet excited state energies in 1/4 when compared to 2/5 are nearly compensated by lower radical ion pair state energies. In fact, the driving forces for charge separation are nearly identical in THF (0.08 eV in 1/4 versus 0.09 eV in 2/5) and benzonitrile (0.28 eV in 1/4 versus 0.31 eV in 2/5). In line with this thermodynamic consideration, 2 and 5 tend to charge separate only slightly faster with rate constants of around 6 × 1010 s−1 in THF and benzonitrile than 1 and 4, for which rate constants of around 5 × 1010 s−1 were determined. Increasing the donor acceptor separation, which was accomplished by linking TPA to the nitrogen of the pyrrolidine ring rather than to the carbon of the pyrrolidine ring exerts a profound impact on the charge separation dynamics, namely a notable slow down. For the charge recombination processes, a “stabilizing” trend is unambiguously given, when considering the corresponding rate constants of 6.5 ± 0.5 × 109 s−1 (2), 1.9 ± 0.5 × 109 s−1 (5), 4.5 ± 0.5 × 108 s−1 (1), < 3.3 ± 0.5 × 108 s−1 (4) in benzonitrile. A similar stabilization evolves in THF with rate constants of 8.0 ± 1.0 × 109 s−1 and 1.7 ± 0.5 × 109 s−1 for 2 and 1, respectively. Nevertheless, we note a relationship – rate constant versus solvent polarity – that suggests dynamics in the normal region of the Marcus parabola, where the rate constants increase with increasing thermodynamic driving force (−ΔG°).34 In this light, the stabilization seen for 1 and 4 – relative to 2 and 5 – is rationalized on smaller driving forces.
We have synthesized two isomeric triphenylamine-Ih-Sc3N@C80 derivatives by 1,3-dipolar cycloaddition reactions. The compound with the N-connected donor has significantly better thermal stability and longer lived photoinduced charge separated state than the corresponding properties for the 2-substituted system. The Ih-Sc3N@C80 dyads have considerably longer lived photoinduced charge separated states and lower first reduction potentials than their C60 equivalents, confirming the advantage of using Ih-Sc3N@C80 for replacing C60 as the acceptor moiety for the construction of donor acceptor conjugates.
The isomeric mixture of Ih and D5h Sc3N@C80 was provided by Luna Innovations (Nanoworks division). Pure Ih-Sc3N@C80 was obtained by eluting a solution of the isomeric mixture through a plug of silica gel containing “Magic Blue” (tris(4-bromophenyl)aminiumhexachloroantimonate), which selectively oxidized the D5h isomer. All reactions were run under an argon atmosphere and followed by TLC on silica plates. Anhydrous and deuterated solvents were purchased from Aldrich and used as received. NMR spectra were obtained using Bruker Avance 500 spectrometer using TMS or residual solvent signals as internal reference. MALDI-TOF mass spectra were obtained on a Voyager-DE STR mass spectrometer. HPLC was performed using a Varian Prostar 210 equipped with a Buckyprep column (10 × 250 mm). All electrochemical measurements were performed in o-dichlorobenzene (o-DCB) with 0.05 mol dm−3 tetra(n-butyl)ammonium hexafluorophosphate (n-Bu4NPF6) as supporting electrolyte. Voltammetric experiments were performed using a potentiostat/galvanostat Model CHI660A (CH Instruments Electrochemical Workstation) with a three-electrode cell placed in a Faraday cage. The working electrode consisted of a glassy carbon disk (Bioanalytical Systems, Inc.) with a diameter of 1 mm. The surface of the electrode was polished using 0.25 μm diamond polishing compound (Metadi II, Buehler). The electrode was then sonicated in water in order to remove traces of alumina from the metal surface, washed with water, and dried. A silver wire was used as a pseudo-reference electrode. All the potentials were calibrated against the Ferrocene/Ferrocenium (Fc/Fc+) redox couple. A platinum wire was used as counter electrode; it was cleaned by heating in a flame for approximately 30 seconds. The solution was deaerated for 20 min with argon prior to the electrochemical measurements. Femtosecond transient absorption studies were performed with 387 nm laser pulses (1 kHz, 150 fs pulse width) from an amplified Ti:Sapphire laser system (Model CPA 2101, Clark-MXR Inc.). Emission spectra were recorded with a Fluoromax 3 (Horiba) Spectrofluorometer. The experiments were performed at room temperature. Each spectrum represents an average of at least 5 individual scans, and appropriate corrections were applied whenever necessary.
9.82 mg of Ih-Sc3N@C80 (8.85 μmol, 1 eq) was poured in a 100 mL Schlenk flask along with 121.80 mg of 4-(diphenylamino)benzaldehyde (445 μmol, 50 eq) and 11.48 mg of sarcosine (128.85 μmol, 14.5 eq). The solids were dissolved in 50 mL o-dichlorobenzene and heated to 120°C under argon for 90 minutes. The solvent was finally removed under high vacuum. The remaining solid was then dissolved in CS2 and purified on a silica gel column eluting first with CS2 for removing the unreacted Ih-Sc3N@C80 followed by a 1:1 mixture of CS2 and toluene for eluting the product. After evaporating the solvent and washing with ethyl ether 5.15 mg of product was obtained. Yield 40.6%; 41.2% subtracting the recovered Ih-Sc3N@C80. This compound has retention time 26.73 min in Buckyprep column (10×250mm) - toluene 2 mL/min. 1H NMR (CS2- CD2Cl2 4:1, 500 MHz, δ ppm) 7.92 broad (s, 2H), 7.38 broad (s, 2H), 7.30 (pseudo-t, 4H, J 7.5 Hz), 7.18 (d, 4H, J 7.5 Hz), 7.04 (t, 2H, J 7.5 Hz), 4.35 (d, 1H, J 9Hz), 3.74 (s, 1H), 3.07 (d, 1H, J 9 Hz), 2.63 (s, 3H). MALDI m/Z 1411.97 (negative ionization, 9-nitroanthracene as matrix). Following the same procedure but using C60 (25.24 mg, 35 μmol, 1 eq), sarcosine (9.4 mg, 105 μmol, 3 eq) and 4-(diphenylamino)benzaldehyde (95.9 mg, 350 μmol, 10 eq). Compound 2 was obtained with a 40% yield (15.7 mg, 14 μmoles) after purification. Its NMR data matches completely the previous reported values.17f
352.0 mg of 4-(diphenylamino)benzaldehyde (1.29 mmol, 1 eq) and 326.0 mg of glycine methyl ester hydrochloride (2.59 mmol, 2 eq) were poured in a 200 mL Schlenk flask equipped with a stirring bar under argon. 100 mL of anhydrous ethanol was added, the stirring started and the mixture heated to 60°C. Once the starting materials were dissolved the solution turned to a yellowish color. At that point dropwise addition of NaBH3CN (0.705 g, 11.2 mmol, 3 eq) suspended in anhydrous ethanol was started and continued for a period of 3 hours. Finally the solvent was removed under vacuum and the residual solid treated with 5% HCl solution for destroying the excess of NaBH3CN. A saturated NaHCO3 solution was added dropwise until a neutral pH was reached. The whole mixture was extracted with CH2Cl2, dried with anhydrous Na2SO4 and the solution filtered through a silica plug. After evaporating the solvent the intermediate ester was obtained and used in the next step without further purification. 1H NMR (CD2Cl2, 500 MHz, δ ppm) 7.34-7.28 (m, 6H), 7.16-7.04 (m, 8H), 3.84 (s, 2H), 3.78 (s, 3H), 3.51 (s, 2H). 13C NMR (CD2Cl2, 125 MHz, δ ppm) 172.6 carbonyl, 147.9 q, 146.9 q, 134.1 q, 129.3 CH, 129.2 CH, 124.2 CH, 124.1 CH, 122.7 CH, 52.5 ϕ-CH2-N, 51.6 -O-CH3 and 49.7 N-CH2COO. This compound was dissolved in 20 mL of ethanol-water 8:1 and a 0.20 mL of saturated NaOH solution was added. The whole mixture was heated to 50°C and stirred overnight. The final reaction mixture was cooled and HCl 5% added dropwise until a neutral pH was reached, which crashes out the product out of the solution. The white solid obtained was washed with cold de-ionized water and dried under vacuum. Finally 98.5 mg of product was obtained (23% yield). 1H NMR (DMSO, 500 MHz, δ ppm) 9.57 broad (s, 1H), 7.42 (d, 2H, J 9 Hz), 7.32 (t, 4H, J 7.5 Hz), 7.08 (t, 2H, J 7.5 Hz), 7.02 (d, 4H, J 7.5 Hz), 6.97 (d, 2H, J 9Hz), 4.08 (s, 2H), 3.81 (s, 2H). 13C NMR (DMSO, 125 MHz, δ ppm) 167.8 carbonyl, 148.0 q, 146.9 q, 131.6 CH, 129.6 CH, 125.0 q, 124.3 CH, 123.5 CH, 122.5 CH, 49.3 ϕ-CH2-N, 46.1 N-CH2COO.
12.1 mg of Ih-Sc3N@C80 (10.9 μmol, 1 eq) was poured in a 100 mL Schlenk flask along with 34.5 mg of N-(benzyl-4-diphenylaminophenyl)glycine 3 (109.1 μmol, 10 eq) and 40 mL of anhydrous o-dichlorobenzene were added by using a cannula. This mixture was heated to 150°C under argon and slurry made with paraformaldehyde (24.5 mg, 816 μmol, 75 eq) in 10 mL of anhydrous o-DCB was added in 3 portions every 15 minutes; heating was continued while following the reaction by TLC on silica plates eluting with CS2-toluene 2:1. The reaction was stopped when the formation of bis-adducts and poly-adducts was observed in the TLC plates even though the starting Ih-Sc3N@C80 had not been consumed completely. The solvent was then removed under high vacuum and the remaining solid dissolved in CS2 and purified on a silica gel column eluting first with CS2 for removing the un-reacted Ih-Sc3N@C80 followed by a 1:1 mixture of CS2 and toluene for eluting the product. After evaporating the solvent and washing with ethyl ether 5.50 mg of product was obtained. Yield 38.5%; 43.8% subtracting the recovered Ih-Sc3N@C80. This compound has retention time 37.3 min in Buckyprep column (10×250mm) - toluene 2 mL/min. 1H NMR (CS2- CD2Cl2 4:1, 500 MHz, δ ppm) 7.46 (d, 2H, J 8.5 Hz), 7.33 (pseudo-t, 4H, J 7.5 Hz), 7.20-7.15 (m, 6H), 7.09 (t, 2H, J 7.5 Hz), 4.13 (d, 2H, J 10 Hz), 3.99 (s, 2H), 2.99 (d, 2H, J 10 Hz). MALDI m/Z 1411.68 (negative ionization, 9-nitroanthracene as matrix). Following the same procedure but using C60 (23.2 mg, 32.2 μmol, 1 eq), N-(benzyl-4-diphenylaminophenyl)glycine 3 (30.2 mg, 95.5 μmol, 3 eq) and paraformaldehyde (24.0 mg, 800 μmol, 25 eq). Compound 5 was obtained with a 32% yield (10.6 mg, 10.3 μmoles) after purification. 1H NMR (CS2- CD2Cl2 2:1, 500 MHz, δ ppm) 7.58 (d, 2H, J 8.5 Hz), 7.27 (t, 4H, J 7.5 Hz), 7.16 (d, 2H, 8.5 Hz), 7.12 (d, 4H, J 7.5 Hz), 7.02 (t, 2H, J 7.5 Hz), 4.50 (s, 4H), 4.28 (s, 2H). 13C NMR (CS2- CD2Cl2 2:1, 125 MHz, δ ppm) 155.14 q, 147.79 q, 147.40 q, 147.33 q, 146.37 q, 146.24 q, 146.18 q, 145.82 q, 145.59 q, 145.40 q, 144.69 q, 143.23 q, 142.76 q, 142.39 q, 142.21 q, 142.03 q, 140.32 q, 136.45 q, 132.21 q, 129.84 CH, 129.48 CH, 124.48 CH, 124.08 CH, 123.08 CH, 70.86 q, 67.83 Bz-N-(CH2)2, 58.67 ϕ-CH2-N. MALDI m/Z 1020.47 (positive ionization, 9-nitroanthracene as matrix).
We are grateful to Luna Innovations Inc. for providing us with the initial mixture of fullerenes. We also thank the National Science Foundation (Grants DMR 0809129 to L.E.) for support and J. Walls for assistance. This material was also based upon work supported by Luna Innovations Inc. and the Air Force Office of Scientific Research (AFOSR) under Contract No. FA9550-06-C-0010. G. B. thanks the Spanish MEC for a “Ramón y Cajal” contract. The Voyager-DE STR mass spectrometer was purchased in part with a grant from the Division of Research Resources, National Institutes of Health (RR 11966). Also the Deutsche Forschungsgemeinschaft (SFB 583), FCI and Office of Basic Energy Sciences of the U.S. Department of Energy are gratefully acknowledged. S. S. G. gratefully acknowledges the support from Alexander von Humboldt Foundation.