Strongly Coupled Ruthenium−Polypyridyl Complexes for Efficient Electron Injection in Dye-Sensitized Semiconductor Nanoparticles

Abstract

Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO(2) nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO(2) nanoparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (<100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where approximately 30% of the injected electrons are recombined with a time constant of <2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO(2) system as compared to other organic dye-sensitized TiO(2) nanoparticles with similar interactions.

The Effect of Heavy Atoms on Photoinduced Electron Injection from Nonthermalized and Thermalized Donor States of MII–Polypyridyl (M=Ru/Os) Complexes to Nanoparticulate TiO2 Surfaces: An Ultrafast Time‐Resolved Absorption Study

“…[13,14] The absorption bands at 400-550 nm are attributed to the MLCT singlet state for complexes 1 and 2. [5,13] After the addition of TiO 2 nanoparticles, the absorption spectrum (Figure 1c) of 1 becomes broad and redshifted with an appreciable increase in absorbance of the band at 460 nm. Once again, after the addition of TiO 2 nanoparticles, the absorption spectrum of complex 2 (Figure 1d) becomes broad and shifted to a longer wavelength with an appreciable increase in absorbance for both (dp Os !p* bpy and dp Os !p* L ) of the MLCT bands.…”

“…Simultaneous electron injection from the thermalized 3 MLCT states has also been observed and contributes to the multiexponential electron injection. In our earlier investigations [5,6] in which we used ruthenium-polypyridyl photosensitizers, namely, TiO 2 nanoparticles sensitized by [Ru [6] (bpy = 2,2'-bipyridine), we observed only a singleexponential electron injection in TiO 2 CB (CB = conduction band) from photoexcited MLCT states. We also observed that the strong electronic coupling between dye-TiO 2 systems facilitates electron injection from nonthermalized MLCT states.…”

“…The pK a value for the catecholate system is considerably higher (pK a > 9.6), which ensures more efficient binding with TiO 2 nanoparticles, even at higher pH values. [5,12] It would therefore be interesting to synthesize catecholate-based osmium complexes and study the electron-transfer dynamics onto a TiO 2 nanoparticle surface.…”

Injection Kinetics and Electronic Structure at the N719/TiO2 Interface Studied by Means of Ultrafast XUV Photoemission Spectroscopy

“…[7][8][9][10][11] This feature is associated with the population dynamics of different metalto-ligand charge-transfer (MLCT) states of the dye complex. Following excitation at the maximum of the absorption band (typically at a wavelength of 530 nm), the dye is initially prepared in the singlet 1 MLCT state.…”

Photo-Induced Electron Transfer from Dye or Quantum Dot to TiO2 Nanoparticles at Single Molecule Level

“…Second, slight difference of the fluorescence lifetimes implies that most IFET processes should be slow and inefficient. The oxazine 1 molecule lacks effective anchoring groups like carboxylate in metal-polypyridine complexes to covalently bind the dye to the semiconductor (Oregan & Gratzel, 1991;Ramakrishna et al, 2005). These dye molecules randomly selected are anticipated to be physisorbed to the TiO 2 NPs surface or loosely trapped at the NPs interstices.…”