Ultrafast multiexponential electron injection dynamics at a dye and ZnO QD interface: a combined spectroscopic and first principles study.

The photophysical properties of a push-pull dye and the dynamics of electron transfer across a dye and ZnO quantum dot (QD) interface have been studied using a combination of steady-state, time-resolved fluorescence and femtosecond transient absorption (TA) spectroscopies. Spectroscopic measurements, along with quantum chemical calculations, provide evidence for the photoinduced intramolecular charge transfer (ICT) in a donor-π-acceptor dye. Wavelength dependent fluorescence decay of the dye infers the solvent relaxation of the ICT state. TA spectral measurements suggest electron transfer from dye to ZnO QD through the ICT state by monitoring the dye radical cation. Multiexponential electron injection with time constants of 775 fs and 25 ps at the dye@QD interface is demonstrated using the TA kinetics results. The thermodynamics of fast and slow electron injections is discussed. Furthermore, density functional theory (DFT) and time-dependent (TD)-DFT simulations were performed to identify the dye cation radical and to get a deep insight into the experimental observations.