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-TiO2 interfacial charge transfer complexes is
monitored with femtosecond fluorescence upconversion and transient absorption. Small molecules (catechol, dopamine,
benzhydroxamic acid, acetyl acetonate and salicylate)-modified TiO2 nanoparticles are prepared and the complexation is
followed with optical absorption measurements. Although little visible luminescence is observed from these molecule-
TiO2 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 TiO2 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 TiO2 nanoparticles. However, the possibility of delocalized charge-transfer excitations is
observed for salicylate and acetyl acetonate-TiO2 nanoparticles. The decay of the charge transfer emission is ascribed to the relaxation of the localized states to delocalized states in the TiO2 conduction band. Transient absorption
measurements have shown long-lived charge separation in the case of surface-modified TiO2 nanoparticles. Further
measurements on the influence of charge-transfer excitations on the interfacial electron transfer in surface-modified TiO2
nanoparticles are being carried out.
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