Fullerenes, particularly the iconic C60 buckminsterfullerene, exhibit distinct spectroscopic signatures that reveal their structural features, electronic properties, and purity. Several spectroscopic techniques are employed to characterize these carbon-based nanostructures, each providing complementary information about their molecular and electronic configurations. The most commonly used methods include nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and infrared (IR) spectroscopy, alongside Raman spectroscopy, which is particularly sensitive to the vibrational modes of fullerene cages.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for elucidating the symmetry and purity of fullerenes. The 13C NMR spectrum of C60, for instance, displays a single sharp peak at approximately 143 ppm, corresponding to all 60 equivalent carbon atoms in the highly symmetric icosahedral structure. This single resonance is a direct consequence of the molecule's uniformity, where every carbon atom occupies an identical chemical environment. In contrast, higher fullerenes like C70 exhibit multiple peaks due to reduced symmetry. C70 shows five distinct 13C NMR signals between 130–150 ppm, reflecting the presence of non-equivalent carbon atoms in its elongated structure. NMR is also critical for detecting impurities, such as residual solvents or incomplete cage structures, which introduce additional peaks or broaden the resonances.

Ultraviolet-visible (UV-Vis) spectroscopy provides insights into the electronic transitions of fullerenes, which are governed by their conjugated π-electron systems. C60 exhibits characteristic absorption bands at 213 nm, 257 nm, and 329 nm, attributed to π→π* transitions within the carbon framework. The absence of strong absorption in the visible region explains its faint purple hue in solution. C70, with its lower symmetry and extended conjugation, shows a more complex UV-Vis spectrum, including additional peaks at 330 nm, 360 nm, and 470 nm, resulting in a reddish-brown color. Deviations from these expected absorption profiles can indicate impurities or structural modifications, such as functionalized fullerenes or cage defects.

Infrared (IR) spectroscopy is particularly useful for identifying vibrational modes associated with the fullerene cage. The IR spectrum of C60 features four major absorption bands at 526 cm−1, 576 cm−1, 1182 cm−1, and 1428 cm−1. These correspond to the radial and tangential vibrational modes of the carbon atoms, with the lower-frequency peaks (526 cm−1 and 576 cm−1) arising from cage breathing and pentagon-pinch motions. The higher-frequency bands (1182 cm−1 and 1428 cm−1) are attributed to C=C stretching vibrations. C70, due to its lower symmetry, exhibits a more complex IR spectrum with additional peaks, including those at 535 cm−1, 642 cm−1, and 795 cm−1. The presence of unexpected peaks or shifts in these spectra can indicate chemical modifications, such as hydrogenation or oxidation, or the presence of solvent residues.

Raman spectroscopy, though often grouped with vibrational techniques, offers unique insights into fullerene properties due to its sensitivity to electronic and vibrational coupling. The Raman spectrum of C60 is dominated by a strong peak at 1469 cm−1, assigned to the pentagon-pinch mode (Ag symmetry), which is highly sensitive to changes in the electronic environment. Additional peaks at 273 cm−1 (Hg symmetry) and 497 cm−1 (Hg symmetry) correspond to radial cage vibrations. Doping or functionalization of fullerenes often leads to shifts or splitting of these peaks, providing evidence of chemical modification. For example, the Ag mode in C60 shifts to lower wavenumbers upon reduction (e.g., in C60 anions) due to increased electron density weakening the carbon-carbon bonds.

Mass spectrometry (MS) is another critical tool for confirming the molecular weight and purity of fullerenes. Electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI) techniques typically yield strong signals corresponding to the intact molecular ion (e.g., m/z 720 for C60). Fragmentation patterns or additional peaks may indicate impurities or derivatized species. For instance, oxidized fullerenes (e.g., C60O) appear at m/z 736, while hydrogenated species (e.g., C60H36) produce a distribution of peaks corresponding to varying degrees of hydrogenation.

The combination of these spectroscopic techniques allows for comprehensive characterization of fullerenes, ensuring their structural integrity and purity. NMR confirms molecular symmetry and detects impurities, UV-Vis reveals electronic transitions, IR identifies vibrational fingerprints, Raman probes electron-phonon coupling, and MS validates molecular weight. Together, these methods provide a robust framework for analyzing fullerenes in research and industrial applications, from materials science to biomedical engineering.

Quantitative analysis of spectral data further enhances the utility of these techniques. For example, the intensity ratios of specific IR or Raman peaks can be used to assess the degree of functionalization in derivatized fullerenes. Similarly, the extinction coefficients derived from UV-Vis spectra enable precise concentration measurements in solution-phase studies. By correlating spectroscopic signatures with structural features, researchers can tailor fullerene properties for specific applications, such as organic photovoltaics, drug delivery, or catalytic systems.

In summary, the spectroscopic characterization of fullerenes is a multifaceted process that leverages the complementary strengths of NMR, UV-Vis, IR, Raman, and MS. Each technique contributes unique information about the molecular structure, electronic properties, and purity of these nanomaterials, enabling their precise manipulation and application in advanced technologies. The spectral signatures serve as definitive fingerprints, ensuring accurate identification and quality control in fullerene-based research and development.