Significant structural relaxations of excited [n]cycloparaphenylene dications (n = 5–9)

Abstract

Hoop-shaped macrocycles such as cycloparaphenylenes ([n]CPPs, where n denotes the number of phenylene rings) have attracted considerable attention in recent years because of their interesting properties arising from the highly strained aromatic structure and radially oriented p-orbitals. While the radical cation and dication states of [n]CPPs have been characterized, there is no information available about their excited states, which are expected to exhibit enhanced redox properties. In this study, we investigated the S₁ state of [n]CPP²⁺ by transient absorption measurements in the visible and near-IR regions. The energy of the transient absorption peak exhibited a linear relationship with the reciprocal of the repeating unit, which indicated that the distribution of the excited state expanded with the size of the ring. In addition, smaller CPP²⁺s showed longer excited state lifetimes. Theoretical calculations suggested that there was a substantial structural relaxation of the smaller CPP²⁺s accompanying the changes in the charge distribution. Therefore, it was concluded that the smaller Franck–Condon factor resulting from the considerable structural change and larger S₁ energy were responsible for the longer S₁ state lifetime of smaller CPP²⁺s.