Efficient electron injection from twisted intramolecular charge transfer (TICT) state of 7-diethyl amino coumarin 3-carboxylic acid (D-1421) dye to TiO2 nanoparticle

Electron injection and back-electron-transfer dynamics of 7-diethyl amino coumarin 3-carboxylic acid (D-1421) and coumarin 343 (C-343) dyes adsorbed on TiO2 nanoparticles have been studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. Electron injection has been observed by direct detection of electron in the conduction band of nanoparticle and cation radical of the dyes as detected by picosecond transient absorption spectroscopy. Electron injection efficiency has been found to be much higher for D-1421-sensitized TiO2 nanoparticle compared to C-343-sensitized TiO2 nanoparticle system. Steady state and time-resolved fluorescence measurements show that in high polar solvent, the excited state of D-1421 dye molecule predominantly exits as twisted intramolecular charge transfer (TICT) state, whereas excited C-343 exists as intramolecular charge transfer (ICT) state. On excitation with a laser pulse, D-1421 dye molecule in a highly polar solvent (water) goes to the ICT state and then relaxes very fast to the TICT state. Electron injection for D-1421 dye can take place both from ICT state and TICT state. As TICT is a highly charge-separated state, it can inject electrons very efficiently to the TiO2 nanoparticle. Quantum yield for electron injection for D-1421 dye has been found to be 0.90 compared to 0.60 for C-343.