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| Top: The kinetics of charge transfer from a rhenium-bypyridine complex to a TiO2 , measured for several molecular linkers (purple, black, red, and blue traces). Insets show the structure of the molecular systems, and infrared spectroscopy demonstrating the attachment of Re1C to TiO2.
Bottom: Calculated transition densities associated with the 1MLCT excited state (left, shown for the ReEC complex) and biothiophene ππ* transition (right, Re2TC complex). The green shows areas of electron density depletion, and the yellow areas of excess density. |
Charge transfer across interfaces has an important fundamental role in materials with applications ranging from solar energy to photocatalysis. In dye sensitized solar cells, electrons are excited in a dye molecule by the absorption of light and subsequently injected into an inorganic semiconductor. IRG2 has resolved the electron injection kinetics of the dye sensitizer by optically probing the electrons transferred to TiO2 nanocrystal from a rhenium dye by using a mid-infrared femtosecond laser pulse to monitor free-carrier absorption. We have measured the electron injection kinetics of four rhenium−bipyridine complexes (Re1C, ReEC, Re1TC, and Re2TC) on TiO2 nanocrystalline films using transient infrared spectroscopy (Figure 1). We find that the insulating bridge leads to a slower injection rate and poorer injection yield compared with the conjugated spacers. Ground and electronically excited states of the dye complexes were characterized using ground-state and time-dependent density functional theory, providing fundamental insight into the processes involved (Figure 2). Theoretical results reveal that the anomalous kinetics of the Re2TC complex arise from charge transfer from the linker in addition to the rhenium complex.
Our work provides the proof-of-concept for a method where metal-based complexes absorbed on semiconductor surface can be used to read out the electron injection kinetics through molecular bridges. The fast, high-yielding injection of Re2TC shows that the use of sensitizers with multi injecting units could offer enhanced electron injection into inorganic semiconductors as long as the molecules self assemble in a highly dense-packed layer.
References
P. Paoprasert, J. E. Laaser, W. Xiong, R. A. Franking, R. J. Hamers, M. T. Zanni, J. R. Schmidt and P. Gopalan, “Bridge-Dependent Interfacial Electron Transfer from Rhenium-Bipyridine Complexes to TiO2 Nanocrystalline Thin Films,” J. Phys. Chem. C 114, 9898 (2010).