Direct molecule-semiconductor interfacial charge transfer interactions have received considerable research attention for
their applications in various fields. In this study, the dynamics of molecule-TiO<sub>2</sub> interfacial charge transfer complexes is
monitored with femtosecond fluorescence upconversion and transient absorption. Small molecules (catechol, dopamine,
benzhydroxamic acid, acetyl acetonate and salicylate)-modified TiO<sub>2</sub> nanoparticles are prepared and the complexation is
followed with optical absorption measurements. Although little visible luminescence is observed from these molecule-
TiO<sub>2</sub> nanoparticles, ultrafast emission in broad range of wavelengths is detected with fluorescence upconversion which is
ascribed to the interfacial charge transfer emission. The charge transfer emission arose out of the radiative recombination
of the electrons in the conduction band of TiO<sub>2</sub> with holes in the molecule. Femtosecond fluorescence anisotropy
measurements have shown that the interfacial charge-transfer excitation is mostly a localized one for catechol, dopamine
and benzhydroxamate modified TiO<sub>2</sub> nanoparticles. However, the possibility of delocalized charge-transfer excitations is
observed for salicylate and acetyl acetonate-TiO<sub>2</sub> nanoparticles. The decay of the charge transfer emission is ascribed to the relaxation of the localized states to delocalized states in the TiO<sub>2</sub> conduction band. Transient absorption
measurements have shown long-lived charge separation in the case of surface-modified TiO<sub>2</sub> nanoparticles. Further
measurements on the influence of charge-transfer excitations on the interfacial electron transfer in surface-modified TiO<sub>2</sub>
nanoparticles are being carried out.
Two photon absorption cross-sections and fluorescence dynamics of Riboflavin, Fluorescein 548, Coumarin 519 and
Quinizarin adsorbed onto reactive (TiO<sub>2</sub>) and non-reactive (ZrO<sub>2</sub>) semiconductor nanoparticles have been investigated.
These dye molecules are chosen because of their inherently different anchoring groups with which they can bind to
semiconductor nanoparticles giving a handle to probe the influence of anchoring group as well as molecule-nanoparticle
electronic coupling on the two-photon absorption and nonlinear optical properties. Two-photon excited fluorescence
technique has been utilized to monitor the two photon absorption cross-sections and the dynamics of singlet states are
followed with femto second fluorescence upconversion. Interesting cross-section trends have been observed where the
TPA cross-section of chromophore on ZrO<sub>2</sub> surface is similar or lower to that of the free dye while the cross-sections
seem to be higher on the surface of reactive TiO<sub>2</sub> nanoparticle surface. Fluorescence upconversion investigations were
able to probe the electronic interactions of the chromophore with semiconductor nanoparticle and also the adsorption of
the chromophores on the surface of the nanoparticle.
Gold clusters with the sizes close to the Fermi wavelength of electron shows interesting quantum size effects. Linear and
nonlinear optical properties show dramatic trends when the sizes of clusters are in the range of quantum confinement.
We have investigated the size dependence of the non-linear optical property of two-photon absorption (TPA) cross-sections
of the gold clusters. Absolute TPA cross-sections are measured by a combination of one and two-photon excited
fluorescence upconversion measurements. Large cross-sections and abrupt changes in the trend of cross-sections are
observed in the size-dependence when size is reduced from nanoparticles to cluster. The results can be attributed to the
appearance of quantum confinement in these monolayer protected gold clusters.