Photo-sensitized dyes

The molecular dyes are attached to a conducting medium, such TiO2, via de-protonated carboxylate groups, as shown in the schematic to the right. The dye electrons in the HOMO orbital absorb the photon. These electrons are then excited up into the LUMO, from which the electron can be deposited into the conduction band of the TiO2.

With over 7000 published papers (and counting) based on Ru^II(bpy)₃²⁺ and its derivatives, a considerable amount of time and energy has been directed toward understanding the HOMO-LUMO (d → π*) electronic transition, which corresponds to a metal-to-ligand charge transfer (MLCT). Based on their unique photophysical properties, they have been incorporated into solar conversion cells and utilized as chemical sensors. In terms of solar cell efficiency, scientists are seeking to identify molecular dyes (based on the parent Ru^II(bpy)₃²⁺ complex) that absorb into the near-IR in order to maximize the number of solar photons converted into electric current.

Even though iron and ruthenium exist in the same group in the periodic table, there is comparatively little known about the physical and photophysical properties of the iron-based analogs of these complexes. Consequently, we have recently initiated a theoretical investigation into the structural, energetic, and photophysical characteristics of Fe^II polypyridyl complexes utilizing density functional theory (DFT). These results are subsequently compared to the Ru^II-based analog.

A common Ru^II-based molecular dye is the (bipyridine)₂Ru(3,3’-dicarboxy-2,2’-bipyridine)²⁺, shown in Figure A. In terms of structural reliability, the DFT calculation accurately predicts the dihedral angle between the bipyridine rings with the carboxylate groups to be ~ 30^o. More importantly is the theoretical prediction of the location and energetics of the HOMO and LUMO orbitals, depicted in Figures B and C, respectively.

Figure A	Figure B	Figure C

Note that the HOMO is expressed as the d_z2 molecular orbital and that the LUMO is located out on the carboxylate bipyridine ring system. This is one of the important properties of these dyes, the HOMO and LUMO are located on distinctly different locations within the structure of the dye. Although is most cases, the absorbing wavelength of the Fe^II complex is comparable to, or shorter than, that in the Ru^II complex, we have identified one derivative of the MII(bpy)₃²⁺ complex in which the Fe^II has a significantly longer wavelength. Consequently, this Fe-based complex (1,10-phenanthroline)₂Fe(2,2’-bipyrimidine)²⁺ has the potential to be a more efficient solar conversion dye in this particular case. We continue to search for additional derivatives of the Fe- and Ru-based complexes that have the potential to absorb photons of longer and longer wavelengths.

1. Investigate the HOMO/LUMO relationships for additional metal-centered complexes.
2. Investigate the HOMO/LUMO relationships for derivatives of previously studied metal-complexes.

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