Comparison of Electron-Transfer Dynamics from Coumarin 343 to TiO₂, SnO₂, and ZnO Nanocrystalline Thin Films: Role of Interface-Bound Charge-Separated Pairs

Abstract

The role of the interface-bound charge-separated pair (IBCSP) in electron transfer at molecule/inorganic semiconductor interface remains poorly understood despite the importance of its counterpart in the solution-phase charge-separation process. To probe their role, we have compared the dynamics of electron transfer from C343 to TiO2, SnO2, and ZnO nanocrystalline thin films. The decay of the C343 excited state and the formation of oxidized C343 are measured by transient visible absorption to follow the rate of charge separation across the interface to form the IBCSPs. The dissociation of IBCSPs to form free electrons in semiconductor is probed by the free-carrier absorption in the mid-IR. For C343 on TiO2 and SnO2, the rate of the overall electron-transfer process is determined by the rate of the initial charge separation across the interface to form the IBCSP, which dissociates with negligible lifetime. The charge-separation rates are instrument-response-function limited (<150 fs) and ∼1 ps for C343 on TiO2 and SnO2, respectively. The faster electron-transfer rate to TiO2 than to SnO2 is attributed to the higher density of conduction-band states in the former. For C343 on ZnO, the initial charge-separation rate across the interface was shown to be instrument-response limited (<150 fs), but the IBCSP dissociates in ∼12 ps, which determines the overall electron transfer rate. We discuss possible reasons for the different lifetimes of IBCSP intermediates in interfacial electron transfer from C343 to TiO2, SnO2, and ZnO.

Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles

“…1. Further, this same assumption has been employed previously by Lian and coworkers in an investigation of electron transfer rate constants between organic dyes and metal oxide nanoparticles, which reported good agreement between the many-state Marcus formula and experimental findings (13).…”

“…Later, this model was extended to describe electron transfer from a single donating state to a continuum of accepting states, such as those present in the conduction band of a semiconductor (12). This model, which has been used to successfully describe the dependence of electron transfer rate on free energy driving force for systems of organic dyes coupled to various metal oxides (13)(14)(15)(16)(17)(18), has yet to be applied to a quantized semiconducting nanocrystal donor and nanoparticulate metal oxide acceptor (QD-MO) system. The functional form of this many-state Marcus model is as follows:…”

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye–Semiconductor Interface

“…13,29,30 Instead, the atomic-scale electronic coupling and transient interfacial electronic structure are believed to play key roles in determining the device function. [9][10][11][12][13]31 Ultrafast time-domain studies in the optical, infrared, and terahertz regimes have identified distinct differences between the dynamic response of dye−ZnO and dye−TiO 2 interfaces. 5,[8][9][10][11][12][13]17 In particular, transient signals from ZnO-based dye-sensitized systems are marked by prominent "slow" components, which evolve on time scales ranging from a few to hundreds of picoseconds.…”

Electronic Structure of the Perylene–Zinc Oxide Interface: Computational Study of Photoinduced Electron Transfer and Impact of Surface Defects

“…In such devices, the photoinduced electron-transfer (ET) reactions at the dye-semiconductor interface, in particular the processes of electron injection from an electronically excited state of a chemisorbed dye molecule into the semiconductor substrate, represent a key step for photonic energy conversion [33][34][35][36][37][38] . In recent years, photoinduced ET processes in dye-semiconductor systems have been studied in great detail experimentally with femtosecond spectroscopy techniques 34,35,37,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] . It has been shown that electron-injection processes at the dye-semiconductor interfaces often take place on an ultrafast sub-picosecond timescale 37,43,45,46,57,58 .…”