Interfacial electron transfer molecular excitations

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

The addition of electron donors or acceptors to the external electrolyte has allowed sustained photocurrents to be measured at sensitized metal interfaces, but the mechanism(s) often remain speculative. A photocurrent can be generated by excited state interfacial electron transfer like that shown in Figure 5, or by inter-molecular excited state electron transfer followed by dark redox reactions at the electrode. It can be experimentally difficult to distinguish between these distinct mechanisms and strong evidence exists only for the latter pathway which forms the basis of the photogalvanic cell. 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

These observations of an excitation wavelength dependence of the charge injection process show that photoinduced interfacial electron transfer from a molecular excited state to a continuum of acceptor levels can take place in competition with the relaxation from upper excited levels. The rather slow growth of the injection... 

Molecular-based sensitization of sol-gel processed Ti02 to visible light is a rapidly growing area of research with potential practical applications in photovoltaics, sensing, and the remediation of environmental pollutants (Arakawa, 2003 Gratzel, 2001, 2001 Kalyanasundaram, 1998 Kamat, 1998). Three different interfacial electron-transfer mechanisms have been identified. The scheme in Figure 21 -1 (a) shows the typical excited-state(s) interfacial electron injection. Upon light excitation, a sensitizer S forms an excited state. 

Interfacial Charge Separation Mechanisms There are three interfacial charge separation mechanisms for electron transfer from a molecular donor to a semiconductor such as Ti02 (1) excited-state transfer, that is, Rum(dcb )(bpy)22 + 7... 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

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