Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [ n]cycloparaphenylene dications ([ n]CPP²⁺, n = 5-9), which have unique in-plane aromaticity. The fluorescence peak positions of the [ n]CPP²⁺s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP²⁺ was the highest among the [ n]CPP²⁺s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [ n]CPP²⁺s were smaller than those of neutral [ n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [ n]CPP²⁺s undergo smaller structural changes upon S₀-S₁ transition than [ n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S₀-S₁ transition of smaller [ n]CPP²⁺s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP²⁺ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap.