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(center dot)-NA) was detected by ESR. The electron self-exchange rate constant (k(ex)) between QuPh(+)-NA and QuPh(center dot)-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(center dot)-NA(center dot+)) 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(center dot) and NA(center dot+) 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(center dot)-NA(center dot+) 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(center dot)-NA(center dot+), which obeyed second-order kinetics.