Electron Transfer from the Singlet and Triplet Excited States of Ru(dcbpy)₂(NCS)₂into Nanocrystalline TiO₂Thin Films

Abstract

Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpy) 2 (NCS) 2 (RuN3) into titanium dioxide (TiO 2 ) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of ∼30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO 2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs -1 for ET and 1/75 fs -1 for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from ∼1 to ∼60 ps.

Quantum Dynamics of Photoinduced Electron-Transfer Reactions in Dye−Semiconductor Systems: First-Principles Description and Application to Coumarin 343−TiO2

“…In particular, the process of electron injection from an electronically excited state of a dye molecule into a semiconductor substrate has been investigated in great detail experimentally in recent years. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] This process represents a key step for photonic energy conversion in nanocrystalline solar cells. 2,6,9,18,19 Employing femtosecond spectroscopy techniques, it has been demonstrated that electron-injection processes at dye-semiconductor interfaces often take place on an ultrafast (sub-picosecond) time scale.…”

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.…”

“…Negligible oxidized peak amplitude was observed at <200 fs, indicating a lack of instantaneous injection component that was observed for RuN3 and derivatives on TiO 2 . 19,[23][24][25][26][27]31,32 This has been attributed to the insertion of CH 2 spacer between the bipyridine and the anchoring group, which reduces its electronic coupling with TiO 2 , slowing down electron injection rate. 37 The injection rate can be better quantified by comparing injection kinetics in these systems.…”

“…24,31,32,35,39,48 In this model, shown in Figure 8, photoexcitation of the adsorbate prepares an unrelaxed excited state (S** or 1 MLCT * ). Electron injection from this state occurs with rate constant k 1 , which competes with intramolecular relaxation within the excited-state manifold (with rate constant k 2 ).…”

“…The value of 1/k 2 in RuN3 was measured to be ∼75 fs, and it is assumed to be the same for RuN3P. 31,32 The hot electron cross section decay was shown to be well characterized by a biexponential function with time constants and amplitudes (in parenthesis) of 90 ps (40%) and . 1 ns (60%) (or σ h (t)/σ c ) 0.67e -t/90ps + 1) for RuN3/TiO 2 .…”