Interesting Aromaticity of Franck–Condon Excited States of Cyclo[16]Carbon

Inspired by the realization of cyclo[16]carbon (C₁₆) and driven by curiosity about its properties in excited states, a comprehensive theoretical study was conducted on the aromatic characteristics in the Franck-Condon states of C₁₆. This work demonstrates that at the ground-state (S₀ state) geometry, C₁₆ exhibits aromaticity not only in the triplet states (T_1(in→out), T_2(out→out), and T_3(in→in)), but also more prominently in the quintet state (Q₁ state), and different electronic transition modes result in different essential sources of aromaticity for each excited state. Quantitatively, C₁₆ is about three times more aromatic in the quintet state than in the triplet states. These conclusions are supported by multiple robust indicators such as atomic force, induced current, and magnetic shielding, from qualitative, quantitative, and graphical perspectives.