Photoinduced Charge Shift in Li+-Doped Giant Nested Fullerenes

Abstract

Over the last years, carbon nano-onions (CNOs) have been in focus in material science research. Their red-shifted absorption allows utilizing CNOs as promising photosensitizers. We report here a systematic study of excited state properties of six double-layered Li+-doped fullerenes of Ih symmetry: [Li@C60@C240]+, [Li@C60@C540]+, [Li@C60@C960]+, [Li@C240@C540]+, [Li@C240@C960]+, and [Li@C540@C960]+. On the basis of time-dependent density functional theory calculations, we show that the long-wave absorption by the Li+-doped species leads to charge transfer (CT) between the inner and the outer shells unlike their neutral double-layered precursors. The CT energy depends strongly on the size of the concentric fullerenes and it can easily be tuned by varying both the encapsulated metal ion and the size of the shells. Two types of low-lying excited states are identified: (1) capacitor-like structures, as Li+@C60–@C240+, with alternating positive and negative charges, and (2) states, where the positive charge is delocalized over the outer shell, as in Li@C240@C540+. We suggest a simple expression to estimate the energy difference of these excited states and to predict the type of the lowest CT state in nested fullerenes. The effect of nature of the encapsulated ion on the CT state energies is considered. The highest occupied molecular orbital–lowest unoccupied molecular orbital transition energy is found to vary significantly when going from [Li@C60@C240]+ to [Li@C540@C960]+.