By optimizing the geometry of C 16 in S 0 state, the structure of alternating bond lengths and constant bond angles with D 8h point group was obtained (Figure (a) and Table ). The lengths of the two types of C-C bonds in C 16 are 1.363 and 1.213 Å, which are respectively larger and smaller than the corresponding bonds in C 18 ring. Therefore, the bond length alteration (BLA) of C 16 is more significant than that of C 18 , which implies weak aromaticity or anti-aromaticity of C 16 . The forces acting on the T 1 and Q 1 states of C 16 calculated based on the S 0 structure can be used to preliminarily determine the tendency of geometry distortion of the two excited states relative to the ground state. The atomic forces in the T 1 and Q 1 states tend to shorten and lengthen the long and short C-C bonds, respectively, meaning that the carbon ring inclines to equalize its bond lengths in the excited states (Figures ) and 1(c)). Comparatively, the bond equalization tendency is observed to be greater for the Q 1 state of C 16 than for the T 1 state. Considering the fact that aromaticity of molecules is usually related to bond length equalization, the above results imply that the two excited states of C 16 are more aromatic than the S 0 state, and Q 1 state shows evidently higher extent. 106.8/-47.8 ppm, respectively, indicating that the aromaticity of the ring in the two excited states is stronger than that in the ground state and the Q 1 state is more pronounced, as predicted by atomic forces (see Tables and for Cartesian coordinates of optimized T 1 and Q 1 states, respectively). The strong aromaticity of the Franck-Condon T 1 and Q 1 states of C 16 is even more impressive, which arouses our interest in conducting careful research on it. Therefore, all discussions on excited-state properties of C 16 below are based on its S 0 minimum structure, that is, aromaticity of Franck-Condon excited state, while the aromaticity change caused by geometry relaxation owing to electron excitation is not the main focus of this work (see Section Section S1 for Computational Details).