Impact of cyclacene size on their electronically excited singlet states

Cyclacenes are cyclic analogs of acenes consisting of linearly fused benzene rings rolled into a cylindrical shape. Despite extensive theoretical investigations, their experimental synthesis remains challenging. This study employs the high-level NEVPT2-CASSCF computational method to explore the electronic excitation spectra of [n]-cyclacenes (n = 5-12). Our results reveal an oscillation in electronic properties, with even-n cyclacenes exhibiting strong electronic transitions in the visible range, while odd-n cyclacenes generally lack such features until n = 11. Additionally, the HOMO-LUMO transition in odd-n cyclacenes occurs at higher energies with greater intensity than in even-n counterparts. These findings provide crucial insights into the optical characteristics of cyclacenes and will guide future experimental identification efforts.