5 years ago

On low-lying excited states of extended nanographenes

On low-lying excited states of extended nanographenes
Ayako Nakata, Takao Tsuneda, Raman K. Singh
Low-lying excited states of planarly extended nanographenes are investigated using the long-range corrected (LC) density functional theory (DFT) and the spin-flip (SF) time-dependent density functional theory (TDDFT) by exploring the long-range exchange and double-excitation correlation effects on the excitation energies, band gaps, and exciton binding energies. Optimizing the geometries of the nanographenes indicates that the long-range exchange interaction significantly improves the CC bond lengths and amplify their bond length alternations with overall shortening the bond lengths. The calculated TDDFT excitation energies show that long-range exchange interaction is crucial to provide accurate excitation energies of small nanographenes and dominate the exciton binding energies in the excited states of nanographenes. It is, however, also found that the present long-range correction may cause the overestimation of the excitation energy for the infinitely wide graphene due to the discrepancy between the calculated band gaps and vertical ionization potential (IP) minus electron affinity (EA) values. Contrasting to the long-range exchange effects, the SF-TDDFT calculations show that the double-excitation correlation effects are negligible in the low-lying excitations of nanographenes, although this effect is large in the lowest excitation of benzene molecule. It is, therefore, concluded that long-range exchange interactions should be incorporated in TDDFT calculations to quantitatively investigate the excited states of graphenes, although TDDFT using a present LC functional may provide a considerable excitation energy for the infinitely wide graphene mainly due to the discrepancy between the calculated band gaps and IP–EA values. © 2017 Wiley Periodicals, Inc. Excited states play a central role in electron conductance. Nanographenes and graphenes have very different excited-state natures, despite the latter are the extension of the former. In this study, we explored this difference focusing on long-range exchange and double-excitation correlation effects on the excited states. As a result, we revealed that long-range exchange effects dominate the exciton binding energies, that is, the difference between excitation energies and band gaps, for nanographenes, and consequently play a crucial role in the excited states, while double-excitation correlation effects hardly affect them. Conversely, we also found that the long-range exchange effects gradually decrease to near zero as nanographenes being extended to graphenes.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/jcc.24846

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