The rate of electron injection at the organic/semiconductor interface is an important parameter in the design of light driven voltaic and catalytic devices. As a result, different ways of controlling the injection time have been proposed. One approach is to tune the energy level alignment at the interface of the reaction to modify the efficiency of the transfer process. In this work we present ultrafast time-resolved measurements performed on porphyrinoids (phlorin and zinc-porphyrin derivatives) in solution and when attached to TiO2. Porphyrinoids are molecules that have attracted attention as sensitizers for different photo activated devices and constitute a good model to study the dependence of energy alignment on the electron transfer process. Our study involves two model systems with well-defined variations in excess energy and dipole moment. In the first model we excited the S1 and S2 states of a novel phlorin. Excited state dynamics in solution was first studied and found to involve several subsequent relaxation steps, including a flattening of the phlorin ring. On the electrode, heterogeneous electron transfer (HET) times from both states were similar and around 50 fs. Surprisingly, the large difference in the density of acceptor states that are resonant with the respective donor level of the molecules does not significantly influence HET dynamics.a For the second model, dipolar groups were introduced in the porphyrin-bridge-anchor architecture to manipulate the energy alignment with respect to the band edges of the semiconductor.b The influence of the dipole moment on the energy level alignment can be observed in the injection dynamics of the porphyrins bound to TiO2. Preliminary results indicate that the three different molecules tested show the expected dipole dependent electron transfer dynamics. In addition, measurements in solution suggest intermolecular energy transfer that precedes electron transfer. a Nieto-Pescador J., Abraham B., Pistner A. J., Rosenthal J., Gundlach L., Phys. Chem. Chem. Phys., 2015, 17, 7914-7923 b Rangan S., Batarse A., Chitre K. P., Kopecky A., Galoppini E., Bartynski R. A., J. Phys. Chem. C 118 (2014) 12923

Effect of energy level alignment on heterogeneous electron transfer: Injection from porphyrinoids into TiO2

250th National Meeting (2015)

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