Evidence of Multiple Electron Injection and Slow Back Electron Transfer in Alizarin-Sensitized Ultrasmall TiO2 Particles

We have investigated the interfacial electron-transfer dynamics in ultrasmall TiO2 nanoparticles sensitized by alizarin using a femtosecond transient absorption technique. Electron injection has been confirmed by direct detection of the electron in the conduction band and cation radical of the adsorbed dye as monitored by transient absorption spectroscopy in the visible and near-IR region. The electron injection event was followed by monitoring the formation of the dye cation at 550 nm and also the conduction band electron at 900 nm, which revealed a multiexponential dynamics with time constants of 100 fs and 17 and 50 ps. This observation was explained on the basis of discreteness of the conduction band levels due to the finite size effect of the nanoparticle. The multiexponential injection indicated the event to be nonadiabatic contrary to debated literature reports. The back electron-transfer (BET) dynamics followed at the same wavelength revealed a fast component of value 0.2 ps and very slow BET time of > 1 ns. The comparison with previous reports on bulk TiO2 revealed a very slow BET in the present case which has been explained within the framework of Marcus theory. The result gives us direct proof of a nonadiabatic electron-transfer reaction in a strong binding dye like alizarin.