Transient Absorption of N719 and its Electron Transfer Kinetics on ZnO Nanoparticles Surface

Abstract

The transient absorption and emission spectra of ruthenium complex sensitizer N719 under excitation in different solvents were studied. Isopropanol was found to stabilize the singlet excited state of N719. An emission band centered at 460 nm resulting from the singlet excited state of N719 was observed at 1 ns time delay, which is much longer than the reported observation time of the singlet excited state of N719. The triplet excited state of N719 undergoes photophysical decay in acetonitile and isopropanol with lifetimes of around 40 ns, while it may encounter photochemical reactions in water resulting in long living intermediate. The sensitizer was introduced to the surface of three types of ZnO nanoparticles with different morphology, which were used as the electron acceptors upon photoexcitation. The electron transfer dynamics between sensitized N719 and ZnO interface both in the presence and absence of electrolyte were studied by time-correlated single photon counting technique, nanosecond transient absorption and emission spectroscopies. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics at the N719-ZnO interface. In the absence of electrolyte, the electron transfer process at the N719-ZnO interface is dependent on the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, ZnO defects show no impacts on the electron transfer process at the N719-ZnO interface and effective electron injection happens similarly from the excited N719 to ZnO in spite of ZnO particle sizes.