Spectroscopy and dynamics of phosphonate-derivatized ruthenium complexes on TiO2

Giokas, Paul G., Miller, Stephen A., Hanson, Kenneth, Norris, Michael R., Glasson, Christopher R.K., Concepcion, Javier J., Bettis, Stephanie E., Meyer, Thomas J., and Moran, Andrew M. (2013) Spectroscopy and dynamics of phosphonate-derivatized ruthenium complexes on TiO2. Journal of Physical Chemistry Part C, 117 (2). pp. 812-824.

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Abstract

Knowledge of electronic structures and transport mechanisms in dye-sensitized semiconductors is motivated by their ubiquity in photoelectrochemical cells. In this work, optical spectroscopies are used to uncover the elementary dynamics initiated by light absorption at such molecule–semiconductor interfaces (e.g., electron transfer and nuclear relaxation). These processes are explored in a family of ruthenium bipyridyl complexes in aqueous solutions, wherein phosphonate groups are used to bind the molecules to TiO2 nanocrystalline films. The complexes differ in (i) the number of phosphonate groups and (ii) the presence (or absence) of a methylene bridge between the molecule and the TiO2 surface. A resonance Raman intensity analysis suggests that the electronic excitations possess very little charge transfer character for all complexes. That is, the electronic orbitals involved in light absorption are essentially localized to the molecules. Because the electronic resonances are molecular in character, the photophysics are most appropriately viewed as sequences in which light absorption precedes electron transfer. Transient absorption measurements conducted on the dye-sensitized films show that electron injection processes initiating directly from the photoexcited singlet states of the molecules occur in 100 fs or less. In contrast, the electron transfer rates slow down by at least a factor of 10 when intersystem crossing in the molecule precedes electron injection into TiO2. For ruthenium complexes linked to TiO2 with methylene bridges, intersystem crossing is more efficient than singlet electron injection because of attenuated molecule–TiO2 couplings; electron transfer primarily initiates in triplet states for these systems. Overall, the fundamental connections drawn in this work between molecular structure and photophysical behavior contribute to the general understanding of photoelectrochemical cells based on related molecule–semiconductor systems.

Item ID: 33464
Item Type: Article (Research - C1)
ISSN: 1932-7455
Funders: US Department of Energy (DOE)
Projects and Grants: DOE Award DE-SC0001011, DOE Award DE-FG02- 06ER1578, DOE Award DE-EE0003188
Date Deposited: 02 Jun 2014 05:28
FoR Codes: 03 CHEMICAL SCIENCES > 0302 Inorganic Chemistry > 030207 Transition Metal Chemistry @ 50%
03 CHEMICAL SCIENCES > 0305 Organic Chemistry > 030503 Organic Chemical Synthesis @ 50%
SEO Codes: 85 ENERGY > 8505 Renewable Energy > 850504 Solar-Photovoltaic Energy @ 50%
97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 50%
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