The sensing of fullerenes (C₆₀, C₇₀, and higher fullerenes) is gaining considerable interest because of their versatile applications in material science and nanotechnology, such as solar cells and field effect transistors (FETs). From this point of view, reversible formation of fullerodendrons is important [1–4], since the fullerodendron is known to be very soluble fullerene derivative. Meanwhile, Hirsch and co-workers reported that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) reacts with C₆₀ to give a zwitterionic complex via single electron transfer and radical recombination [5]. Recently, Nagata and coworkers have reported that this reaction is useful for large scale separation of C₆₀ from a fullerene mixture because of selective complexation of higher fullerenes with DBU [6]. This zwitterion formation is known to be reversible reaction. However, the resulting complexes are susceptible to air-oxidation; they must therefore be handled in an inert atmosphere. Although many studies on the fullerodendron having neutral C₆₀ or C₇₀ moiety at the focal point have been described [7–29], there are few examples of incorporation of unstable fullerene species into dendritic architectures. Meanwhile, several groups have reported that covalent incorporation of highly unstable subunits into the dendritic architecture is effective to obtain active-site mimics for enzymes [30] and/or highly reactive species as an isolable compound [31] because of site isolation effect of the dendrimer [32–34]. In this context, site-isolation of dendritic substituent expected to be effective to stabilize the unstable fullerene species, such as fullerene anion. These backgrounds prompted us to investigate stabilization of the zwitterionic complex between fullerene and bicyclic amidines using the site isolation effect of the dendron. Herein we report the synthesis of a new DBN-focal dendron, poly (amidoamine) dendron having 1,5-diazabicyclo [4.3.0] non-5-ene (DBN) at the focal point together with the formation of zwitterionic fullerodendrons via complexation of fullerene, either C₆₀ or C₇₀, and DBN moiety of the dendron.

DBN-focal poly(amidoamine) dendrons 1a and 1b were synthesized by the use of the divergent method shown in Scheme 1. A focal point of the dendron, 9-benzyloxycarbonyl-1,5-diazabicyclo [4.3.0] non-5-ene (2), was prepared as described by Kumagai et al. [35]. Compound 2 was allowed to react with ethylenediamine to afford DBN derivative 3. Then, treatment of 3 with methyl acrylate produced dendron 4. Subsequent reaction of 4 with methyl acrylate in the presence of triethylamine (TEA) produced dendron 1a in 40% yield. This three-step process can be repeated to prepare dendron 1b in 26% yield. The structures of dendrons 1a and 1b were confirmed by ¹H- and ¹³C-NMR spectroscopies and MALDI-TOF-MS. In the ¹H-NMR spectra of dendron 1a, multiplet peaks around 4.9 ppm are attributable to the methine proton of DBN’s 9-position of diastereomeric mixture of dendron 1a. Furthermore, a broad peak around 3.2 ppm, which represents the methine proton of DBN’s 7-position, was observed. In the ¹³C-NMR spectra of dendron 1a, the chemical shift at δ 166.7 is the imine carbon signal of the focal point. The MALDI-TOF-MS spectrum of 1a showed a molecular ion peak at m/z 469.27 (1a, C₂₂H₃₇N₄O₇ requires m/z 469.26) using positive-ion mode. Figure 1 shows that the MALDI-TOF-MS spectrum of 1b exhibits a molecular ion peak at m/z 870.04 (1b, C₄₀H₆₉N₈O₁₃ requires m/z 869.49) using positive-ion mode.

The formation of zwitterionic complex of dendrons 1a or 1b with C₆₀ was observed by Vis/NIR spectrum, as reported by Hirsch et al. (Scheme 2) [5]. In a typical experiment, C₆₀ (0.200 mg, 0.278 × 10⁻³ mmol) was dissolved in benzonitrile (3 mL), to which dendron 1b (9.49 mg, 0.0109 mmol) was added under an Ar atmosphere. Subsequently, the Vis/NIR spectrum of the solution was recorded.

Figure 2 shows the Vis/NIR spectrum of the reaction of dendron 1b with C₆₀. The decay of the absorbance of C₆₀ anion radical (λ_max = 1083 nm) is accompanied by the evolution of the zwitterion (λ_max = 930 nm) [5]. These assignments were confirmed by the reaction of C₆₀ with pristine DBN, in which radical ion pair (λ_max = 1083 nm) and zwitterionic complex 7 (λ_max = 930 nm) were also observed. Although the zwitterionic fullerodendrons 9a and 9b were stable under an Ar atmosphere for 24 h at room temperature, they slowly decomposed on exposure to air, as reported by Nagata et al. [6]. We examined the time course of the absorbance of zwitterionic complexes 7, 9a and 9b (λ_max = 930 nm) in the presence of atmospheric oxygen (Figure 3). The half-lives of the zwitterion complexes 7, 9a and 9b were estimated as 377, 397 and 463 min, respectively. Although a clear difference of half-lives between zwitterions 7 and 9a could not be found, the highest generation 9b, which might be stabilized by the site isolation effect of dendritic wedge, showed the longest half-life.

In order to obtain a more stable zwitterionic fullerodendron, we examined the respective reactions of dendrons 1a and 1b with C₇₀, which has higher electron affinity than C₆₀ (Scheme 3). In a typical experiment, C₇₀ (0.220 mg, 0.262 × 10⁻³ mmol) was dissolved in benzonitrile (3 mL), to which dendron 1b (9.49 mg, 0.0109 mmol) was added under an Ar atmosphere. Subsequently, the Vis/NIR spectrum of the solution was observed. Figure 4 shows the Vis/NIR spectrum of the reaction of dendron 1b with C₇₀. The decay of the absorbance at 1,380 nm is accompanied by the evolution of the absorbance at 795 nm. We interpreted that these absorbances were derived from a radical ion pair (λ_max = 1,380 nm) and zwitterionic complex (λ_max = 795 nm), as reported by Fukuzumi et al. [36]. These assignments were confirmed by the reaction of C₇₀ with pristine DBN, in which a radical ion pair (λ_max = 1,380 nm) and zwitterionic complex 10 (λ_max = 795 nm) were also observed. Although the zwitterionic fullerodendrons 12a and 12b were stable under an Ar atmosphere for 5 days at room temperature, they decomposed slowly on exposure to air as well as in the case of C₆₀. We examined the time course of the absorbance of zwitterionic complexes 10, 12a and 12b (λ_max = 795 nm) in the presence of atmospheric oxygen (Figure 5). The half-lives of the zwitterion complexes 10, 12a, and 12b were estimated as 1445, 4800 and 7345 min, respectively. The half-lives of zwitterions having dendritic wedge, compounds 12a and 12b, were longer than that of 10. Comparing half-lives of fullerodendrons 12a and 12b, it is obvious that the stability of zwitterions depends on the generation of the dendron unit. This result indicated that zwitterionic complexes 12a and 12b might be stabilized by the site isolation effect of the dendritic wedge.

The absorption maxima and half-lives of zwitterions 7, 9, 10 and 12 were summarized in Table 1. Comparing the half-lives of zwitterionic complexes 7, 9, 10 and 12, we can conclude that anionic fullerene moieties of zwitterionic fullerodendrons 9b and 12b are stabilized by the site isolation effect of the dendritic wedge. In particular, zwitterionic fullerodendron 12b, which has a C₇₀ moiety at the focal point, showed remarkable stability compared with 10, which does not have a dendritic wedge, and fullerodendron 9b, which contains C₆₀ moiety at the focal point. These observed results might be due to higher electron affinity of C₇₀ than C₆₀, and the difference of anion delocalization between C₆₀ and C₇₀. In marked contrast with the complete anion delocalization of C₆₀, the localized anion of C₇₀ are known to be the reason of regioselective addition reactions [37]. Furthermore, the structure of zwitterionic fullerodendron 12a, which could be isolated by reprecipitation from benzonitrile/1,2,4-trimethylbenzene, was confirmed by IR spectroscopy and APPI-MS. In the IR spectrum of fullerodendron 12a, the –C = N absorbance (1a, 1,680 cm⁻¹) of the DBN⁺ moiety is split into two bands at 1,663 and 1,674 cm⁻¹. This splitting occurs because the two nitrogens in the DBN⁺ moiety are not identical; therefore two –C = N vibrations appeared as reported by Hirsch et al. [5]. The APPI-MS showed a molecular ion peak at m/z 1,309.52 (C₉₂H₃₇N₄O₇ requires m/z 1,309.27 [MH⁺]) and fragment peaks at 1,223.49 ([MH⁺]-CH₂CH₂COOCH₃) and 1,077.52 ([MH⁺]-CH₂CH₂COOCH₃-(CH₂CH₂COO)₂) as shown in Figure 6.

NMR spectra were measured using a spectrometer (AL 300; JEOL). Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) was performed on a mass spectrometer (Autoflex; Bruker Daltonics Inc.) using dithranol (1,8-dihydroxy-9-anthrone) as a matrix. Atmospheric pressure photo ionization mass spectroscopy (APPI-MS) was performed on a BRUKER micrOTOF focus-Kci mass spectrometer equipped with an APCI ionization unit. The GPC experiments (LC-918V; Japan Analytical Industry Co.) were performed using JAIGEL 1H, 2H (eluent: chloroform). The UV/vis spectra (λ_max in nm (ε)) were measured using a spectrophotometer (UV-3150, Shimadzu Corp.). Infrared (IR) spectra were measured using a spectrophotometer (Avatar 360T2: Thermo Nicolet). The reagents were obtained from Wako Pure Chemical Industries Ltd., Tokyo Kasei Kogyo Co. Ltd. Co., Aldrich Chemical Co. Inc., and Frontier Carbon Co. The reagents used as reaction solvents were further purified using general methods.

The results described herein show the first example of a DBN-focal dendron and the formation of a zwitterionic fullerodendron, observed by UV-vis-NIR spectra, having an anionic fullerene moiety at the focal point. In particular, the C₇₀ anion was effectively stabilized by the site isolation effect of the dendritic wedge. The lifetime of zwitterionic fullerodendron 12b formed by the reaction of C₇₀ with the DBN-focal dendron 1b is approximately 20 times longer than that of zwitterionic complex between C₆₀ and DBN. It is notable that the reversible formation of zwitterionic fullerodendrons potentially applicable to sensing fullerenes, because the absorption maximum of an anionic fullerene moiety should depend on the number of the carbon atoms in a fullerene. Further work is in progress to explore the selective sensing of the fullerene family using zwitterionic fullerodendrons.