Exploring the Threshold between Fullerenes and Nanotubes: Characterizing Isomerically Pure, Empty-Caged, and Tubular...

We report the fully fledged photophysical characterization of isomerically pure, empty-caged, tubular fullerenes D₅-C₉₀ and D₅-C₁₀₀ and compare their key properties. In particular, the focus was on cage sizes between 60 and 150 carbon atoms with D₃, D₃, and D₅ symmetry. The optical band gap of D₅-C₁₀₀ is 1.65 eV, which is larger than 1.37 eV of D₅-C₉₀. In stark contrast to the nonluminescent D₅-C₉₀, D₅-C₁₀₀ luminesces at room temperature. Transient absorption spectroscopy shows that photoexcited D₅-C₁₀₀ is subject to a slow intersystem crossing that generates a triplet excited state. In contrast, a fast, nonradiative internal conversion governs the deactivation of D₅-C₉₀: In this case, exploring the corresponding triplet excited state required triplet-triplet sensitization experiments with anthracene. Density functional theory calculations revealed the electronic structure of the fullertubes, and calculations are consistent with our experimental findings. The calculated band gap systematically decreases with the number of carbon atoms within the D₃ and D₃ series. In contrast, an oscillating behavior is noted within the series of D₅ fullertubes. Finally, photoexcited D₅-C₁₀₀ was found to undergo hole transfer with electron-donating triethylamines readily but not electron transfer with electron-accepting methyl viologens.