Abstract
Terpyridine 4H-imidazole-ruthenium(II) complexes are considered promising candidates for use as sensitizers in dye sensitized solar cells (DSSCs) by displaying broad absorption in the visible range, where the dominant absorption features are due to metal-to-ligand charge transfer (MLCT) transitions. The ruthenium(III) intermediates resulting from photoinduced MLCT transitions are essential intermediates in the photoredox-cycle of the DSSC. However, their photophysics is much less studied compared to the ruthenium(II) parent systems. To this end, the structural alterations accompanying one-electron oxidation of the RuIm dye series (including a non-carboxylic RuIm precursor, and, carboxylic RuImCOO in solution and anchored to a nanocrystalline TiO2 film) are investigated via in situ experimental and theoretical UV-Vis absorption and resonance Raman (RR) spectroelectrochemistry. The excellent agreement between the experimental and the TDDFT spectra derived in this work allows for an in-depth assignment of UV-Vis and RR spectral features of the dyes. A concordant pronounced wavelength dependence with respect to the charge transfer character has been observed for the model system RuIm, and both RuImCOO in solution and attached on the TiO2 surface. Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety. Therefore, these 4H-imidazole ruthenium complexes investigated here are potential multi-photoelectron donors. One electron is donated from MLCT states, and additionally, the 4H-imidazole ligand reveals electron-donating character with a significant contribution to the excited states of the ruthenium(III) complexes upon blue-light irradiation.
Highlights
Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety
Recently it was reported that injection for closely related complexes occurs partially from hot vibrational states, since the thermalized long-lived relaxed 3MLCT states fall within the bandgap of TiO2.32. In this contribution we focus on the spectroscopic properties of the photo-oxidized ruthenium(III) species of RuImCOO, which appear as essential mechanistic intermediates in the photoelectrochemical cycle underlying the function of the dye sensitized solar cells (DSSCs)
A pronounced wavelength dependency with respect to the charge transfer (CT) character of the electronic transitions is observed for the absorption spectrum of the oxidized ruthenium(II) metal center (RuIm): low-energy excitation leads to the population of ligand-to-metal charge transfer (LMCT) states and, to the formation of ruthenium(II), while high-energy excitation leads to an intra-ligand charge transfer (ILCT) of the 4H-imidazole ligand
Summary
Ruthenium polypyridyl dyes have attracted considerable interest in various applications due to their chemical stability, strong absorption of visible light, and unique redox and catalytic properties.[1,2,3,4,5,6,7,8] For example, ruthenium polypyridyl complexes are used as photosensitizers in dye-sensitized solar cells (DSSCs).Besides the nanocrystalline semiconductor, the redox electrolyte and the platinum counter electrode the ruthenium polypyridyl complexes are one of the main components of these devices.[9,10] In DSSCs, the direct conversion of light into electrical energy occurs via visible light absorption by the dye molecules, followed by a generally very fast electron transfer into the wide-bandgap semiconductor.[11,12] Subsequently, the oxidized form of the adsorbed dye is re-reduced by a redox couple in the electrolyte – a process which is dominated by molecular collisions of the redox couple with the adsorbed dye.[13,14,15] The optimization of the performance of DSSCs is based on a detailed understanding of the system and its light-induced dynamics at a molecular level, which allows deeper insight into the structure-dynamics-function interplay in complex systems for the conversion of sunlight into electricity[16] and other storable forms of energy.[17,18,19] A significant amount of work focused on improving the light-absorbing properties of the sensitizer,[6,20,21,22,23,24,25,26,27] which should cover the visible range of the solar spectrum and stretch far into the IR. A pronounced wavelength dependency with respect to the charge transfer (CT) character of the electronic transitions is observed for the absorption spectrum of the oxidized RuIm: low-energy excitation leads to the population of LMCT states and, to the formation of ruthenium(II), while high-energy excitation leads to an ILCT of the 4H-imidazole ligand.
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