Formation of a long-lived electron-transfer state of a naphthalene-quinolinium ion dyad and the pi-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 (lambda) 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 pi-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.