Ultrafast photon-mode recording based on photoinduced electron transfer in ion-pair charge-transfer complexes of 4,4'-bipyridinium salts

Ultrafast photon-mode recording based on photoinduced electron transfer reaction was proposed using ion-pair charge-transfer (IPCT) complexes of 4,4-bipyridinium salts. Results with two kinds of counter anions, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and iodide, were reported. These anions made electronic interactions with 4,4-bipyridinium ions in solutions and in solid films to give characteristic absorption in the visible region. The dynamics of color changes from pale yellow or orange to blue upon excitation of IPCT bands of these complexes in solutions were studied by femtosecond (fs) pulsed laser. Transient absorption at about 600 nm appeared in about 0.3 ps in both samples, which was controlled by the time-resolution of our fs laser system. Such extremely fast color changes were due to the fact that the IPCT absorption band is associated with the electronic transition from a partially charge-transferred ground state to an alomost completely charge-separated excited state. The decay behavior was totally different between two salts. Tetrakis[3,5- bis(trifluoromethyl)phenyl]borate salts showed a decay curve composed of a fast component with 80 ps lifetime and of an extremely slow one corresponding to steady and reversible color changes.