Interfacial electron transfer between the photoexcited porphyrin molecule and TiO2 nanoparticles: effect of catecholate binding.

Interfacial electron transfer (ET) dynamics of 5,10,15-trisphenyl-20-(3,4-dihydroxybenzene) porphyrin (TPP-cat) adsorbed on TiO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region exciting at 400 and 800 nm. TPP-cat molecule forms a charge transfer (CT) complex with TiO2 nanoparticles through the catechol moiety with the formation of a five-membered ring. Optical absorption measurements have shown that the Q-band of TPP-cat interacts strongly with TiO2 due to chelation; however, the Soret band is affected very little. Optical absorption measurements indicate that the catechol moiety also interacts with TiO2 nanoparticles showing the characteristic band of pure catechol-TiO2 charge transfer (CT) in the visible region. Electron injection has been confirmed by monitoring the cation radical, instant bleach, and injected electron in the conduction band of TiO2 nanoparticles. Electron injection time has been measured to be < 100 fs and recombination kinetics has been best fitted with a multiexponential function, where the majority of the injected electrons come back to the parent cation radical with a time constant of approximately 800 fs for both excitation wavelengths. However, the reaction channel for the electron injection process has been found to be different for both wavelengths. Excitation at 800 nm, found to populate the CT state of the Q-band, and from the photoexcited CT state electron injection into the conduction band, takes place through diffusion. On the other hand, with excitation at 400 nm, a complicated reaction channel takes place. Excitation with 400 nm light excites both the CT band of Cat-TiO2 and also the Soret band of TPP-cat. We have discussed the reaction path in the TPP-cat/TiO2 system after exciting with both 400 and 800 nm laser light. We have also compared ET dynamics by exciting at both wavelengths.