Effect of molecular structure on interfacial electron transfer dynamics of 7-N,N-dimethyl coumarin 4-acetic acid (DMACA) and 7-hydroxy coumarin 4-acetic acid (HCA) sensitized TiO₂ and ZrO₂ nanoparticles

Abstract

Ultrafast transient absorption spectroscopy has been employed to understand the effect of molecular structure on interfacial electron transfer (ET) dynamics of 7-N,N-dimethyl amino coumarin 4-acetic acid (DMACA) and 7-hydroxy coumarin 4-acetic acid (HCA) sensitized TiO₂ and ZrO₂ nanoparticles. Electron injection is confirmed by observing the cation radical of the dye molecules as well as the conduction band electron in the visible and near-IR regions. Electron injection efficiency has been found to be higher for the HCA/TiO₂ system as compared to the DMACA/TiO₂ system. Both dyes are structurally similar except that HCA has a hydroxyl group at the 7-position while DMACA has a dimethyl amino group at the 7-position. Steady-state and time-resolved fluorescence measurements confirmed that, in highly polar solvent, the excited state of DMACA dye exists both in twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT) states, whereas excited HCA exists only in the ICT state. Because the charge in the case of the TICT state of DMACA is localized away from the nanoparticle surface, electron injection from that state is not efficient. However, ICT states of both DMACA and HCA can inject electrons efficiently as the charge is delocalized. Hence, the quantum yield of electron injection is high in the case of HCA compared to DMACA. We have also observed that photoexcited DMACA whose energy level lies above the conduction band of ZrO₂ nanoparticle can inject electrons after exciting with 400 nm laser light. Back electron transfer (BET) rates have been determined by following the decay kinetics of the cation radical and conduction band electron in different systems. The BET rate is found to be slower for HCA/TiO₂ compared to DMACA/TiO₂, as they fall in the inverted region of Marcus electron transfer theory. The BET is faster in the DMACA/TiO₂ system as compared to that of the DMACA/ZrO₂ system. However, the residual absorption after 460 ps is less in the case of the DMACA/ZrO₂ system.