Spectroscopy and Dynamics of Phosphonate-Derivatized Ruthenium Complexes on TiO₂

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.

Ultrafast kinetics of supramolecules with a Ru(II)- or Os(II)-polypyridyl light absorber, cis-Rh(III)Cl2-polypyridyl electron collector, and 2,3-bis(2-pyridyl)pyrazine bridge

“…An estimated  bET of ~1.3 eV suggests that for (OsT) 2 Rh and OsTRh the 1 GS surface crosses near the bottom of the 3 MMCT (Fig. 7, right), consistent with observation of 3 MMCT→ 1 GS on the timescale of vibrational relaxation ( vc ≈ 15 ps according to [5,40,47]). Back electron transfer for RuBRh and (RuB) 2 Rh is in the Marcus-inverted region, leading to slower return to the 1 GS.…”

Proton-Induced Trap States, Injection and Recombination Dynamics in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

“…Injection from the sensitizer into the TiO 2 occurs on the fs to ns time scale, 51,54,55 with the electrons appearing in the conduction band on the ps to ns time scale. 42 The TRTS scans shown in Figure 4 agree with this picture, with mobile electrons Recombination from the semiconductor occurs in the μs to ms range.…”

Light Harvesting and Charge Separation in a π-Conjugated Antenna Polymer Bound to TiO2

“…20 The presence of multiple kinetic components has been observed for other related sensitizers and arises from a number of factors, including the dye-binding motifs, electronic coupling and overlap of the dye donor levels with the TiO 2 acceptor states. [22][23][24] Furthermore, based on the analysis of the transient spectra 25 there is negligible ultrafast (τ<200 fs) electron injection in this complex, and by 1.4 ns the overall injection yield is 45%. 26 Photoexcitation of the PF-Ru-A//TiO 2 film at 450 nm gives rise to similar spectral features as seen for the Model-Ru-A//TiO 2 film at early times (Fig.…”