Formation of a long-lived electron-transfer state of a naphthalene- quinolinium ion dyad and the π-dimer radical cation

An electron donor-quinolinium ion dyad, 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh ⁺-NA), has been synthesized based on a rational design. The X-ray crystal structure of QuPh ⁺-NA indicates that the dihedral angle between the NA and QuPh ⁺ moieties of QuPh ⁺-NA is nearly perpendicular. The one-electron reduction potential (E _red) was observed as a well-defined reversible wave at -0.90 V versus SCE. The one-electron reduced species (QuPh-NA) was detected by ESR. The electron self-exchange rate constant (k _ex) between QuPh ⁺-NA and QuPh-NA has been determined from the ESR linewidth alternation. The reorganization energy (λ) of the electron self-exchange was determined to be 0.42 eV from the k _ex value. Femtosecond laser irradiation of QuPh ⁺-NA at 355 nm results in formation of the ET state (QuPh-NA ⁺) within 0.5 ps via photoinduced ET from NA to the singlet-excited state of QuPh ⁺. The transient absorption bands at 420 nm and 700 nm are assigned to the QuPh and NA ⁺ moieties, respectively. The nanosecond laser excitation of QuPh ⁺-NA affords the broad absorption band at 1000 nm and is due to the π-dimer radical cation formed between QuPh-NA ⁺ and QuPh ⁺-NA. The intramolecular back electron-transfer process was too slow to compete with the intermolecular back electron-transfer reaction judging from the decay time profile of QuPh-NA ⁺, which obeyed second-order kinetics.