Donor/Acceptor Adsorbates on the Surface of Metal Oxide Nanoporous Films: A Spectroscopic Probe for Different Electron Transfer Pathways

Abstract

A composite model system which consists of a molecular donor/acceptor pair coupled to the surface of metal oxide nanoporous films is proposed to study the contribution of the surface trap states and their influence on the interfacial ET as well as charge trapping and spatial diffusion processes in films. The photophysics of this donor/acceptor system is investigated by time-resolved transient absorbance spectroscopy in the UV/Vis (347-675 nm) spectral region. Variation of the band gap allows one to disentangle the ET pathways. Adsorption of the donor/acceptor pair on the nonreactive Al(2)O(3) surface shows that coupling to the surface assists electron transfer between adsorbed donor and acceptor molecules, resulting in ultrafast intermolecular electron transfer of approximately 100 fs. On the other hand, a competition between interfacial and intermolecular electron transfer is observed for the donor/acceptor pair coupled to a reactive TiO(2) surface. The subsequent transfer of the conduction band electron to the electron acceptor is examined by monitoring the free charge carrier absorption in the mid-IR (approximately 5 microm) spectral region.

Charge Transfer-Induced State Filling in CdSe Quantum Dot–Alizarin Complexes

“…Although the signal is weak, the spectral position coincides well with the oxidized alizarin observed in earlier studies on the TiO 2 –alizarin system. In these studies an efficient ET from photoexcited alizarin to the conduction band of TiO 2 was found. − …”

“…This assignment is corroborated by the quantitative decay of the alizarin signal at >625 nm where the alizarin SE dominates. Such a quenching of SE is a typical feature of ET reactions. − At the same time the alizarin GSB at 550–625 nm persists throughout the complete investigated time scale, demonstrating that the alizarin ground state is not completely repopulated during the investigated process. Additionally, positive absorption changes appear at >625 nm and long delay times.…”

“…The spectral position of the absorption band of surface-bound alizarin is quite different from that of pure alizarin (approximately 430 nm). A spectral shift of the alizarin absorption has been already observed in TiO 2 – and Al 2 O 3 –alizarin complexes and explained by the interaction between the inorganic surface and the alizarin molecules. − It is therefore rational to assume the formation of stable QD–alizarin complexes. A bidentate binding mode of the alizarin on metal oxide surfaces has been discussed previously as the energetically most favorable − which may also apply for alizarin on the CdSe surface.…”

“…The alizarin dye is frequently used as sensitizer for the large band gap semiconductor TiO 2 . In that system the ET from the photoexcited alizarin to the conduction band of TiO 2 occurs on the femtosecond time scale. − Other studies used different anthraquinone dyes, including alizarin, as electron acceptors in combination with CdTe, as well as type I and type II , core–shell QD. Therefore, the alizarin dye is an interesting candidate to study the fundamentals of charge transfer processes at the QD–organic molecule hybrid interface.…”