Dynamics of Back-Electron Transfer Processes of Strongly Coupled Triphenyl Methane Dyes Adsorbed on TiO₂ Nanoparticle Surface as Studied by Fast and Ultrafast Visible Spectroscopy

Electron injection and back-electron transfer (BET) dynamics of triphenylmethane (TPM) dyes adsorbed on TiO 2 nanoparticles have been studied by fast and ultrafast pump−probe spectroscopy in the subpico- to microsecond time domain. TPM dyes form charge-transfer complex with TiO 2 nanoparticles as they get adsorbed on the surface. Among the three TPM dyes, pyrogallol red (PGR) and bromo-pyrogallol red (Br−PGR) have higher electronic coupling to or interaction with TiO 2 nanoparticles compared to aurin tricarboxylic acid (ATC). Excitation of the dyes adsorbed on the TiO 2 nanoparticle surface leads to electron injection into the conduction band of TiO 2 . Electron injection has been confirmed by direct detection of an electron in the conduction band, a cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy in the visible and near-IR regions. The dynamics of BET from TiO 2 to the parent cation have been measured by monitoring the recovery kinetics of the bleach of the adsorbed dye. BET dynamics have been found to be multiexponential, and it is extremely fast for the strong coupling dyes (PGR and Br−PGR). The majority of the injected electrons are found to come back to the parent cation with a time constant of ∼2 ps. BET dynamics have been compared for the strong (PGR) and moderate (ATC) coupling dyes in the microsecond time domain. Intensity dependence experiments show BET reactions for PGR- and Br−PGR-sensitized TiO 2 nanoparticles follow a first-order ET rate, where as ATC-sensitized TiO 2 nanoparticles follow a second-order ET rate. It has been observed that for strongly coupled dyes, BET reaction occurs much faster than that for weakly coupled dyes. Coupling elements for the BET processes from TiO 2 nanoparticles to the parent cation have been determined for shallow and deep trap electrons in the above systems.