Intramolecular dimer radical cations of α,ω-di(2-naphthyl)alkanes in solutions studied by near-IR transient absorption spectroscopy

Formation of intramolecular dimer radical cations in a series of α,ω-di(2-naphthyl)alkanes (DNpn, where n is the number of carbon atoms in the methylene chain) in solutions was investigated by near-IR transient absorption spectroscopy, which enables one to quantitatively evaluate the equilibrium constant (K) for the formation of the intramolecular dimer radical cations and stabilization energy (ECR) due to charge delocalization. The dependence of K on n corresponds to the probability of ring closure in a syn fashion where the chain attaches at the same side of the two naphthalene (Np) moieties. The dependence of ECR on n is similar to that of K except at n=3. For DNpn with a shorter chain, the mutual configuration of the two Np moieties cannot be optimal because the ring closure itself is difficult owing to the restriction by the short chain. For DNpn with a longer chain, the two Np moieties are in a more stable configuration because of the loose restriction of the long chain. The small value of ECR at n=3 indicates that the optimal configuration for the dimer radical cation of Np is not a sandwich-like one, but a distorted one.