Interligand Electron Transfer Determines Triplet Excited State Electron Injection in RuN3−Sensitized TiO₂ Films

Abstract

Electron injection from the transition metal complex Ru(dcbpy) 2 (NCS) 2 (dcbpy ) 2,2′-bipyridine-4,4′dicarboxylate) into a titanium dioxide nanoparticle film occurs along two pathways. The dominating part of the electron injection proceeds from the initially excited singlet state of the sensitizer into the conduction band of the semiconductor on the sub-hundred-femtosecond time scale. The slower part of the injection occurs from the thermalized triplet excited state on the picosecond time scale in a nonexponential fashion, as was shown in a previous study et al. J. Am. Chem. Soc. 2002, 124, 489). Here we show that the slower channel of injection is the result of the excited state being localized on a ligand of the sensitizer that is not attached to the semiconductor; hence, the electron cannot be injected directly from such an excited state into the semiconductor. Before being injected, it has to be transferred from the non-surface-attached ligand to the attached one. The results show that the interligand electron-transfer time is on the picosecond time scale, depends on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer. The findings provide information relevant to the design of molecularbased assemblies and devices.

Comparison of Interfacial Electron Transfer through Carboxylate and Phosphonate Anchoring Groups†

“…[17][18][19] The injection kinetics has been shown to be biphasic, consisting of a primary <100 fs component and slower components on a few to tens of picosecond time scales. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The ultrafast component has been attributed to injection from the unrelaxed singlet metal-to-ligand charge transfer ( 1 MLCT) state and the slower components to injection from the 3 MLCT states near the band edge. 24,27,31,32,34,35 The ultrafast injection process from the unrelaxed excited-state competes with the ultrafast (∼75 fs) 31,32 intramolecular relaxation processes within the dense manifold of excited states.…”

Effect of the Presence of Iodide on the Electron Injection Dynamics of Dye-Sensitized TiO2-Based Solar Cells

“…Injection into the conduction band occurs on time scales ranging from 10 -13 to >10 -10 s and involves both thermalized and nonthermalized excited states of the chromophore. [6][7][8][9][10][11][12][13][14] Internal quantum yields approaching unity can be achieved provided that the lifetimes for injection and electron collection are significantly shorter than those for the injecting state and back-electron transfer. A detailed understanding of the interfacial electron-transfer step thus provides important information concerning a key performance characteristic of this class of photovoltaics.…”

“…7,8,[10][11][12] Specifically, the redox-active component of the electrolytic mixture, typically a combination of an I -salt and I 2 , is commonly excluded in time-resolved spectroscopic studies due to the optically opaque nature of the combination of these species. The absence of these redox components clearly prohibits regeneration of the cell in a manner similar to that under steadystate conditions; however, the effect of this change on other aspects of cell performance is not well elucidated.…”

Effect of Molecular and Electronic Structure on the Light-Harvesting Properties of Dye Sensitizers

“…These rare earth metal dyes prove to be successful in such applications because of their excellent light harvesting electronic spectra 14,15 , fast charge injection kinetics 16,17 and long-term molecular stability under various environmental conditions 18,19,20 . Their electronic structures have been studied extensively 16,17,21,22,23 . In addition to these experimental works there are several theoretical studies 24,25,26 , as well, which focus on the electronic spectra of these chromophores.…”