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

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.