Different back electron transfer from titanium dioxide nanoparticles to tetra (4-sulfonatophenyl) porphyrin monomer and its J-aggregate

The synthesized titanium dioxide (TiO2) nanoparticle was found to enhance the formation of J-aggregate of water-soluble porphyrin, tetra (4-sulfonatophenyl) porphyrin dye, 5,10,15,20-tetraphenyl-21H, 23 H-porphine tetrasulfonic acid (TPPS). The forward and back electron transfers (ETs) between adsorbed TPPS and TiO2 nanoparticles were examined by picosecond single-photon timing and femtosecond transient absorption spectroscopy. The ultrafast back ET of ∼0.8 ps was observed both for the protonated monomer and the J-aggregate. The back ET almost completes within a few tens of picoseconds for the protonated monomer, while >1 ns is required for the J-aggregate. The difference has been interpreted in terms the hole delocalization and electronic coupling of the protonated monomer and J-aggregate adsorbed on TiO2 nanoparticles.