Achieving Adaptive Aromaticity in Cyclo[10]carbon by Screening Cyclo[n]carbon (n=8−24)

Abstract

Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8−24), which is supported by nucleus‐independent chemical shift (NICS), anisotropy of the current‐induced density (ACID), π contribution of electron localization function (ELFπ) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in‐plane ππ* excitation. Thus, the major contribution to the aromaticity from out‐of‐plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out‐of‐plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon.

Adaptive σ‐Aromaticity in an Unsaturated Three‐Membered Ring

“…In addition, benzene can also display adaptive aromaticity by strong π‐accepting substituent groups (Figure 1d) [7] . Very recently, cyclo[10]carbon was demonstrated to possess adaptive aromaticity whose excitation mode is formed from in‐plane π to π* molecular orbital (Figure 1e) [8] . And a highly stable singlet fission material based on a polycyclic dipyrrolonaphthyridinedione skeleton was found to have adaptive aromaticity in the pyrrole ring (Figure 1f) [9] …”

“…[7] Very recently, cyclo [10]carbon was demonstrated to possess adaptive aromaticity whose excitation mode is formed from in-plane π to π* molecular orbital (Figure 1e). [8] And a highly stable singlet fission material based on a polycyclic dipyrrolonaphthyridinedione skeleton was found to have adaptive aromaticity in the pyrrole ring ( Figure 1f). [9] Note that the adaptive aromatic species mentioned above are all π-aromaticity due to the delocalization of πelectrons in unsaturated rings.…”

Vibrational Spectra and Molecular Vibrational Behaviors of All‐Carboatomic Rings, cyclo[18]carbon and Its Analogues

“…Furthermore, in view of other cyclic oligomers such as C 12 Br 4 and C 24 Br 8 were also detected in the debromination reaction mixture, the properties of other kinds of cyclocarbons are not only interesting in theory, but also very important in practical research. Chenshu Dai et al., for instance, have screened the aromaticity in cyclo[2 n ]carbon ( n =4 to 12) in their S 0 and T 1 states and confirmed that cyclo[10]carbon is adaptive aromatic, that is, it is being aromatic in both states [37] . Therefore, size‐dependent spectral features of a broad range of cyclo[2 n ]carbon systems for n =3 to 15 will also be explored and compared here.…”

Strong Interaction between Cyclo[18]Carbon and Graphene

“…The aromaticity is an interesting property for carbon rings, and C 18 has aromaticity in the singlet state and antiaromaticity in the triplet state. [ 9 ] The carbon ring strongly tends to adsorb some small molecules to its ring center, and its relatively stable dimer is stacked parallelly with a D 9h symmetry by density functional theory (DFT) calculations. [ 10 ] Not only the interactions with small molecules, but also were the typical electron donor and acceptor molecules studied by theoretical calculations.…”