Ab Initio Nonadiabatic Molecular Dynamics of the Ultrafast Electron Injection across the Alizarin−TiO₂ Interface

Abstract

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Grätzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Grätzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.

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

“…However, significantly larger clusters had to be considered to minimize boundary effects. Well converged results could be obtained with the cluster (TiO 2 ) 24 (H 2 O) 30 , and only these results will be reported in the following.…”

“…To obtain the structure of the C343-(TiO 2 ) 24 (H 2 O) 30 complex shown in Figure 1 (right image) we have used the following protocol: The geometry of isolated C343 was taken from an earlier study. 42 The deprotonated form of C343 was aligned to the saturated (TiO 2 ) 8 (H 2 O) 14 cluster (Figure 1, middle image) by optimizing the ground-state energy in internal coordinates.…”

“…Thereby, the internal nuclear degrees of freedom of C343 and of the cluster were kept fixed, and only the six relative orientation coordinates (three translations and three rotations) were allowed to vary. Then, C343 was attached to the larger (TiO 2 ) 24 (H 2 O) 30 cluster (Figure 1, right image) using the six relative orientation coordinates from the alignment optimization at the smaller (TiO 2 ) 8 (H 2 O) 14 cluster. The proton detached from C343 upon adsorption was attached to a surface oxygen atom adjacent to the adsorption center.…”

“…Figure 2 shows the energies of the molecular orbitals of the overall system as well as those of the donor and acceptor orbitals obtained by the partitioning procedure outlined in section IIB. Also shown, in comparison, are the energy levels of the isolated (TiO 2 ) 24 (H 2 O) 30 cluster and the isolated chromophore C343. The isolated (TiO 2 ) 24 (H 2 O) 30 cluster exhibits a rather dense level structure with a valence and conduction band separated by a band gap.…”

Functional Assessment for Predicting Charge-Transfer Excitations of Dyes in Complexed State: A Study of Triphenylamine–Donor Dyes on Titania for Dye-Sensitized Solar Cells

“…To this end, numerous dyes with different donors, linkers and acceptors have been synthesized and their performance as photo-sensitizers evaluated experimentally (see, for example, review articles by Mishra et al 9 and Hagfeldt et al 4 ). On the theoretical side, numerous studies [11][12][13][14][15][16][17][18][19][20][21][22][23] have been carried out to understand the photo-physics of dyes by themselves and in complexes with titania thin-films or nanoparticles. Most of these studies utilize time-dependent density functional theory (TD-DFT) to calculate UV/Vis spectra of the systems of interest.…”