Efficient Charge Separation in TiO₂ Films Sensitized with Ruthenium(II)–Polypyridyl Complexes: Hole Stabilization by Ligand‐Localized Charge‐Transfer States

Abstract

We have studied the interfacial electron-transfer dynamics on TiO(2) film sensitized with synthesized ruthenium(II)-polypyridyl complexes--[Ru(II)(bpy)(2)(L(1))] (1) and [Ru(II)(bpy)(L(1))(L(2))] (2), in which bpy=2,2'-bipyridyl, L(1)=4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L(2)=4-(N,N-dimethylaminophenyl)-2,2'-bipyridine-by using femtosecond transient absorption spectroscopy. The presence of electron-donor L(2) and electron-acceptor L(1) ligands in complex 2 introduces lower energetic ligand-to-ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO(2) surface. The hole is transferred directly or stepwise to the electron-donor ligand (L(2)) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L(2)) ligand and the electron injected in TiO(2) nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO(2) system relative to the 1/TiO(2) system. Our results suggest that Ru(II) -polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru(II)-polypyridyl complexes comprising MLCT states only.

Catechol‐Based Biomimetic Functional Materials

“…With regard to the original Ru II ‐catechol complex, the dimethylaminophenyl‐substituted complex showed a 20% higher efficiency in the electron injection yield, without impairment of the electronic coupling of the dye to the TiO 2 nanoparticles. The beneficial effect of the ligand disappeared at very low pHs, which was explained by the fact that the amino group responsible for the charge delocalization becomes protonated in these conditions, giving further proof of its role in the increase of charge separation 597. Similar results have been recently observed for Ru II ‐dye‐catecholate complexes with other conjugated electron‐donating moieties 598…”

“…Yoon and co‐workers reported a notable increase in net injection efficiency through the design of ligands with conjugated electron‐donating groups for Ru II ‐dye‐catecholate complexes,387 to which similar studies by other groups followed 597, 598. Another approach involved coating of the semiconductor NPs with a nanometer‐thick SrTiO 3 shell barrier,599 which led to a 70% improvement in charge collection at the interface.…”

Ruthenium Sensitizers and Their Applications in Dye-Sensitized Solar Cells

“…It is important to understand the interfacial electron transfer pathways within such systems in which charge separation/recombination can be controlled by changing the electron-donor ability of ancillary bipyridyl ligands. Verma et al [57] have studied the interfacial electron-transfer dynamics on TiO 2 film sensitized with synthesized ruthenium(II)-polypyridyl complexes 17 and 18 (Figure 33) by using femtosecond transient absorption spectroscopy. They have observed significantly redshifted absorption spectra for complex 18 relative to complex 17 that were attributed to ligand-to-ligand charge-transfer states of complex 18.…”

ZnO Nanoparticles Photosensitization Using Ruthenium(II)‐polypyridyl Isomeric Complexes

“…The relatively high efficiencies of the ruthenium(II)‐polypyridyl DSSCs can be attributed to their wide absorption range from the visible to the near‐infrared (NIR) regime. In this regard, several attempts have been made to molecularly engineer the structure of these complexes in order to increase their molar absorption coefficient, and thus their long‐term stability . This research focuses on the synthesis and characterization of ruthenium(II) complexes modified ZnO nanoparticles to use them in the design of photoanodes for DSSCs.…”