(Invited) Exciton Generation in C60 Fullerene Clusters

Abstract

Carbon-based nanometric-size species have been proposed as valuable key nanomaterials in biomedicine, photocatalysis, photovoltaics and optoelectronics. A common strategy for the development of applications is the design of multi-adduct derivatives. Based on C60 fullerene, it depends on the desired purpose to put the appropriate substituents imparting good solubility and/or intra-/intermolecular interactions. Very promising applications are expected for giant molecules based on hexakis-adducts of C60.1-4 Recently, Martín and co-workers synthesized and characterized the structural and photophysical properties of a series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units.5 These globular multifullerene systems exhibit rather long-lived triplet excited states and, in particular, one of them, compound 13, showed an exceptional ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, which is well apart from other organic molecules or coordination compounds reported so far. It is known that molecular systems displaying triplet excited states with ultra-long lifetimes are of particular interest in applications like biomedicine, photovoltaics, nonlinear optics, or photocatalysis. For example, compound 13 might be a good choice for applications of the photodynamic treatment of hypoxic tumors.5 The structural implications were studied in reference 5, but computational studies of the photophysical properties for these tridecafullerene systems have not been unrevealed yet. In reference [5], Martín and co-workers reported the UV/Vis absorption spectra of these tridecafullerenes and compared them with their respective hexakis-adduct precursors. They also collected the fluorescence emission maxima and the lifetimes of the singlet and triplet excited states for all these systems, highlighting the long lifetimes displayed for the tridecafullerenes. It is worth noting the extraordinarily long triplet mean lifetime detected for compound 13, which also showed the highest hypochromic shift in the UV/Vis absorption spectrum, the largest blue-shifted fluorescence and the shortest-lived singlet. In this work, the experimental emission spectrum is provided for each tridecafullerene and precursor, and we emphasize the rarely appearance of the two simultaneously experimental emission peaks at around 2 eV (~620 nm) and 2.8 eV (~443 nm). The emission peak at 2 eV is usual for C60 and derivatives, but not the one at 2.8 eV. Although this abnormal peak is more intense in the precursors, it is also significantly present in the tridecafullerenes. We performed a computationally study about the photophysical properties of these tridecafullerenes compounds as well as for the C60 fullerene. 1 Ramos-Soriano, J.; Reina, J. J.; Illescas, B. M.; de la Cruz, N.; Rodríguez-Pérez, L.; Lasala, F.; Rojo, J.; Delgado, R.; Martín, N. J. Am. Chem. Soc. 2019, 141, 15403–15412. 2 Rodríguez-Pérez, L.; Ramos-Soriano, J.; Pérez-Sánchez, A.; Illescas, B. M.; Muñoz, A.; Luczkowiak, J.; Lasala, F.; Rojo, J.; Delgado, R.; Martín, N. J. Am. Chem. Soc. 2018, 140, 9891–9898. 3 Nierengarten, J. F. Chem. Commun. 2017, 53, 11855 –11868. 4 Muñoz, A.; Sigwalt, D.; Illescas, B. M.; Luczkowiak, J.; Rodríguez-Pérez, L. Nierengarten, I.; Holler, M.; Remy, J. S.; Buffet, K.; Vincent, S. P.; Rojo, J.; Delgado, R.; Nierengarten, J. F.; Martín, N. Nat. Chem. 2016, 8, 50 –57. 5 Ramos-Soriano, J.; Pérez-Sánchez, A.; Ramírez-Barroso, S.; Illescas, B. M.; Azmani, K.; Rodríguez-Fortea, A.; Poblet, J.M.; Hally, C.; Nonell, S.; García-Fresnadillo, D.; Rojo, J.; Martín. N. Angew. Chem. Int. Ed. 2021, 60, 16109–16118. Figure 1