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.

C 60 solutions in benzonitrile have been found to show concentration dependent optical absorption behaviour. At lower concentrations (<100 µM) the UV-vis absorption characteristics of C 60 in benzonitrile are exactly similar to those in benzene and decalin. At higher concentrations (>100 µM), however, the C 60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also visually opaque. From picosecond laser flash photolysis experiments it is seen that the triplet quantum yield of C 60 in benzonitrile at higher concentrations (∼400 µM) is much less than unity and increases with the dilution while in decalin and benzene it is always close to unity and independent of the C 60 concentration. Dynamic light scattering experiments indicate the presence of particles of mean size of about 250 nm in C 60 solutions in benzonitrile with concentration >100 µM, while in <100 µM solutions no such particles have been observed. Such particles are also not observed for C 60 solutions in benzene and decalin, even up to ∼500 µM. Scanning electron microscopy also shows particles of size ∼250 nm. It is inferred that C 60 forms aggregates in benzonitrile at concentrations >100 µM and that the aggregated and the monomeric form of C 60 are in equilibrium.

The unusual solvatochromism of C70 is investigated in a variety of solvent mixtures by optical absorption and fluorescence techniques. Distinct reversible color change from pink to purple is seen in the solvent mixtures studied. Such changes are seen also for C60 solutions in some solvent mixtures. Formation of clusters is found to be responsible for the observed optical changes. Light scattering studies are carried out to confirm the presence of clusters which show that the particle size varies from ∼100 to ∼1000 nm depending on the concentration of the fullerene. It is found that the solubility of the fullerene in the solubilizing solvent and that in the solvent mixtures are the major factors governing the aggregation behavior of the fullerene. Polarity of the solvent plays a minor role in the formation of aggregates.

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