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

Abstract

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.

Photosensitization of colloidal TiO2 nanoparticles with phycocyanin pigment

“…According to the equation E s à =sþ ¼ E s=sþ À E s , the oxidation potential of excited state phycocyanin is À1.41 vs SCE, where, E s/s+ is the oxidation potential of phycocyanin 0.53 V vs SCE, E s is the excited state energy 1.94 eV (excited state energy of the phycocyanin calculated [73] from the fluorescence maximum based on the reported method). The energy level of the conduction band of TiO 2 is À0.52 V vs SCE [74]. It suggests that the electron transfer from the excited state phycocyanin to the conduction band of TiO 2 is energetically favorable (Scheme 3).…”

Effect of strong coupling on interfacial electron transfer dynamics in dye-sensitized TiO2 semiconductor nanoparticles

“…So, in principle it cannot fulfill the condition, which has been demonstrated by Lian and co-workers in ReCO complexes on TiO 2 film. On the other hand, in our earlier studies, we have observed single exponential electron injection time constant in many organic dye-nanoparticle systems, 6,7 where electron injection would only take place from the singlet excited state since the singlettriplet ISC takes long time (>1 ns). In such systems strong electronic coupling of the dye molecules with the nanoparticles would govern the electron-injection dynamics.…”

“…The spectrum at each time delay consists of bleach in 470-500 nm wavelength region with a maximum at ≈480 nm, an absorption peak at ≈590 nm and another broad positive absorption band in the 700-1000 nm region which has been attributed to injected electrons in the conduction band. 6,7 The ab- sorption peak at 590 nm has been assigned to the Ru-cat cation radical. 11 Injection time in the present investigation has been determined by monitoring the appearance time of the signal of the injected electron.…”

A new porphyrin sensitizer with phenolic binding group for high efficiency dye-sensitized solar cells

“…3. It was reported that TiO 2 NPs can form a strong CT complex with dye molecules, such as alizarin [25], triphenylmethane dyes [26], and Ru-Cat through the catechol spacer. It has already been discussed by Moser et al [27] that TiO 2 NPs can form a strong complex with catechols by formation of a five-membered ring.…”