Double Aromaticity of Franck-Condon Excited States of Cyclo[16]carbon

Abstract

Excited-state aromaticity of molecular rings is a very interesting and meaningful topic. Inspired by the recent realization of cyclo[16]carbon (C16) with double anti-aromaticity characteristics of singlet ground state (S0 state), here we demonstrate that at the S0 minimum geometry, C16 not only exhibits evident triplet excited-state (T1 state) aromaticity, but also possesses more significant double aromaticity in quintet excited state (Q1 state), which are supported by robust aromaticity indicators in terms of induced current, magnetic shielding, and density distribution of globally delocalized π electrons. The comparative analysis also reveals the essential reason why the Q1 state of C16 has more aromaticity than the T1 state.

Theoretical insight into complexation between cyclocarbons and C60 fullerene

“…[8] Theoretical chemists have also conducted extensive research on the special properties and potential applications of cyclocarbon systems in recent years, among which we have achieved meaningful results in many aspects of C18 and related compounds, including geometric structure, electronic structure, ring strain, aromaticity, optical nonlinearity, intermolecular interaction, external field effect, dynamic behavior, excited state, and so on. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Multiple works have speculated or confirmed that the high reactivity of cyclocarbons and their precursors allows them to fuse through covalent coupling, opening up avenues for molecular polymerization into bigger carbon rings. [5,9] There are also indications that cyclocarbons are valuable precursors for building three-dimensional carbon networks [28] and forming fullerenes in graphite vapor.…”