Exploring the Threshold between Fullerenes and Nanotubes: Characterizing Isomerically Pure, Empty-Caged, and Tubular Fullerenes D-5h-C-90 and D-5d-C-100

We report the fully fledged photophysical characterization of isomerically pure, empty-caged, tubular fullerenes D-5h-C-90 and D-5d-C-100 and compare their key properties. In particular, the focus was on cage sizes between 60 and 150 carbon atoms with D-3, D-3d/h, and D-5d/h symmetry. The optical band gap of D-5d-C-100 is 1.65 eV, which is larger than 1.37 eV of D-5h-C-90. In stark contrast to the nonluminescent D-5h-C-90, D-5d-C-100 luminesces at room temperature. Transient absorption spectroscopy shows that photoexcited D-5d-C-100 is subject to a slow intersystem crossing that generates a triplet excited state. In contrast, a fast, nonradiative internal conversion governs the deactivation of D-5h-C-90: 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 D-3 and D-3(d/h), series. In contrast, an oscillating behavior is noted within the series of D-5d/h fullertubes. Finally, photoexcited D-5d-C-100 was found to undergo hole transfer with electron-donating triethylamines readily but not electron transfer with electron-accepting methyl viologens.