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

Abstract

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 versusSCE. 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.