%0 Generic
%A Yang, Yi-Fan
%C Heidelberg
%D 2020
%F heidok:28556
%R 10.11588/heidok.00028556
%T Electronic States of Fullerene Anions and Endohedral Fullerenes
%U https://archiv.ub.uni-heidelberg.de/volltextserver/28556/
%X In recent years, fullerenes, as rising stars in carbon clusters, have been widely applied in various fields of science and technology. The high electron affinity of fullerenes, due to unique geometric and electronic structures, leads to wide applications in many fields, e.g., organic solar cells, supercapacitors, catalyzers, and superconductive materials. Due to the difficulty to synthesize of carbon clusters and to determine their structures experimentally, researchers have paid much attention to the theoretical studies of their geometric and electronic structures. It is only recently that it became possible to apply state-of-the-art theoretical methods, e.g., equation of motion coupled cluster singles and doubles method for electron affinities (EA-EOM-CCSD) to these large molecular systems. With such high cost methods, the full picture of electronic states of the first known fullerene C60 has finally been revealed. Study of electronic structures of large molecular systems, such as fullerenes, has become a great challenge for modern theoretical and computational chemistry.    This thesis is devoted to the theoretical study of the electronic states of fullerene anions (e.g., C20–) and fullerene derivatives, utilizing accurate approaches. The latter includes endohedral fullerenes (e.g., Li@C20 and Li@C60) and carbon rings (e.g., C20).     To the best of our knowledge, our work is the first study on bound states of the C20– fullerene anion, employing accurate theoretical approaches. We find that the smallest fullerene anion C20–, can form one superatomic and a manifold of valence bound states. It indicates that possessing superatomic bound states is one of the common properties of fullerenes. We hope that this finding sheds light on the study of fullerenes applications in the future. Our theoretically estimated adiabatic electron affinity of the C20– fullerene, is consistent with the electron affinity obtained in the photoelectron experiment. It verifies the validity of the application of high accurate EA-EOM-CCSD method in studying electronic structures of fullerenes.    The endohedral fullerenes, e.g., Li@C20 and Li@C60, have attracted great attention due to their enhanced properties compared to the parent fullerenes. Our research on Li@C20 shows that the smallest fullerene, i.e., C20, can steal valence electron from the guest Li atom and form a charge separated donor-acceptor system. The Coulomb effect of Li+ is to stabilize the bound states, both valence and superatomic. Noteworthy, due to their different nature, the stabilizing effect on valence states is stronger than on superatomic states. The extra electron density distribution of superatomic states of the charge separated endohedral system is more compact compared to that of the parent fullerene, while the distribution of valence states does not exhibit this behavior.    Based on our calculations on Li@C60, we have found several excited states. Most of the electronic states are charge separated states, the appearance of Li+ stabilized the excited states of Li@C60 compared to those of the parent isolated anion without changing their characters, similarly to our finding for Li@C20. Importantly, for Li@C60 we reported a hitherto unknown non-charge-separated state, which we referred to as the caged-electron state. This state is neither a valence nor a superatomic state, since its extra charge density is mostly distributed at the center of the cage. We demonstrate that the caged-electron state is formed due to the large radius of the C60 cage, which reduces the Coulomb attraction effect between Li+ and the negative carbon cage of the endohedral fullerene. In much larger fullerenes, e.g., Li@C180, this state even becomes the ground state, due to the much weaker Coulomb attraction effect. It is a great example of the impact of the fullerene’s size on its electronic structures. Additionally, we have mentioned several possible applications of this new kind of state.    Last but not least, we turn to the carbon ring as the isomer of fullerenes. Carbon rings are intriguing and elegant species, but determining their geometry is an ongoing challenge. We have performed geometry optimization, vibrational frequency calculations and potential energy surface scans, based on EA-EOM-CCSD. Our work reveals that, similar to its fullerene isomer, the C20– ring can possess several bound states, including one superatomic state. Moreover, our calculation shows a symmetry breaking of the C20– ring anion structure occurring upon attaching an electron to the neutral ring. The discussion of the possible symmetry breaking mechanisms indicates that the shrinking and distortion of the ring upon electron attachment leading to the symmetry breaking, is a result of the interplay between the symmetry breaking and the totally symmetric modes. The discussion enriches the palette of possible symmetry breaking phenomena in carbon clusters.