Experimental Fingerprints of Vibrational Wave-Packet Motion during Ultrafast Heterogeneous Electron Transfer

Abstract

By application of 20 fs laser pulses, vibrational wave packets of low-energy modes (mainly 357 and 421 cm-1) were generated in the perylene chromophore that gave rise to periodic beats that lasted longer than 1 ps in transient absorption signals. Electron transfer from the excited singlet state of the perylene chromophore, attached as molecule DTB−Pe via the −CH2−phosphonic acid group to anatase TiO2, was measured in ultrahigh vacuum with a time constant of 75 fs. The vibrational wave packet that was generated in the donor state continued its motion for several hundred femtoseconds in the product state of the reaction, i.e., in the ionized chromophore. This is direct proof for electron transfer occurring from a nonrelaxed vibrational population that was created by the short laser pulse in the donor molecule. The rise of the product state showed a staircase-like time dependence. The steps are attributed to electron transfer that occurs preferentially each time the vibrational wave packet (frequency 480 cm-1) reaches a crossing point for the potential curves of reactant and product state. Such wave-packet modulation of heterogeneous electron transfer can arise if the density of electronic acceptor states in the electrode is changing strongly over an energy range on the order of the reorganization energy below the excited molecular donor orbital.

Photoinduced electron transfer processes in dye-semiconductor systems with different spacer groups

“…2,7,10,[27][28][29] Employing femtosecond spectroscopy techniques, which allow the observation of ultrafast photoreactions in real time, it has been shown that electron injection processes at dyesemiconductor interfaces often take place on an ultrafast subpicosecond timescale. 3,10,11,[17][18][19]30 Other aspects of ultrafast interfacial ET reactions include the nonequilibrium character of the process, the importance of electronic-nuclear correlation, i.e., nonadiabatic dynamics, the influence of the anchor group on the electron injection dynamics, recombination processes, and the influence of intermediate states localized at the chromophore-substrate interface.…”

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.…”

“…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. 3,8,9,11,13,14,20 For example, electroninjection times as fast as 6 fs have been reported for alizarin adsorbed on TiO 2 nanoparticles 14 in time-resolved experiments, and even faster ET times have been found for biisonicotinic acid on a TiO 2 surface employing resonant photoemission spectroscopy. 11 Other interesting aspects of these ultrafast interfacial ET reactions are the nonequilibrium character and the influence of electronic-nuclear coupling.…”

Communication: Uncovering molecule-TiO2 interactions with nonlinear spectroscopy

“…For example, the present mechanism contrasts with the sequential process found in perylene-TiO 2 , where vibrational coherence is induced by a 75 fs electron transfer transition subsequent to photoexcitation. 26 Electron transfer induced vibrational coherences have also been observed in a rutile TiO 2 substrate sensitized with PbSe nanocrystals. 11 The key physical quantity governing the initiation of the vibrational coherences observed here is depicted in Fig.…”