Electron injection dynamics of RuII(dcbpy)2(SCN)2 on zirconia

The electron injection dynamics of Ru-II(4,4'-dicarboxy-2,2'-bipyridine)(2)(SCN)(2) (the so-called N3-dye) adsorbed on colloidal ZrO2 particles in room-temperature solution have been studied using time-resolved absorption polarization spectroscopy. The solvents used are acetonitrile and methanol. Electron injection is evidenced by the transient absorption anisotropy and/or the wavelength-dependent unpolarized absorption intensity. The results indicate that 630 nm excitation results in no N3* to ZrO2 electron transfer in both acetonitrile and methanol solvents, 515 nm excitation results in electron injection in acetonitrile, and 495 nm excitation results in electron injection in both acetonitrile and methanol. These results may be understood in terms of simple energetic considerations: 630 nm excitation produces N3* with insufficient energy for electron injection while 495 nm excitation produces N3* with an energy just sufficient for electron injection. In all cases where electron injection is energetically favorable, it occurs very rapidly, <200 fs. There is no indication of rapid (<5 ps) electron injection into deep trap states on the ZrO2 surface; however, the possibility of rapid injection into shallow trap states cannot be excluded. The observed kinetics also indicate that, following photoexcitation of the adsorbed N3, the ZrO2 surface relaxes on the 5-10 ps time scale.