The dynamics of interfacial charge transfer across (E)-3-(5-((4-(9H-carbazol-9-yl)phenyl)ethynyl)thiophen-2-yl)-2-cyanoacrylic acid (CT-CA) and TiO 2 nanocomposites was studied with femtosecond transient absorption, fluorescence upconversion, and molecular quantum dynamics simulations. The investigated dye, CT-CA is a push−pull chromophore that has an intramolecular charge-transfer (ICT) excited state and binds strongly with the surface of TiO 2 nanoparticles. Ultrafast transient absorption and fluorescence measurements, in both solution and thin film samples, were carried out to probe the dynamics of electron injection and charge recombination. Multiexponential electron injection with time constants of <150 fs, 850 fs, and 8.5 ps were observed from femtosecond fluorescence measurements in solution and on thin films. Femtosecond transient absorption measurements show similar multiexponential electron injection and confirm that the picosecond electron injection component arises from the excited ICT state of the CT-CA/TiO 2 complex. Quantum dynamics calculations also show the presence of a slow component (30%) in the electron injection dynamics although most of the electron injection (70%) takes place in less than 20 fs. The slow component of electron injection, from the local ICT state, is attributed to the energetic position of the excited state, which is close to, or slightly below, the conduction band edge. In addition, the transient bleach of CT-CA on the TiO 2 surface is shifted to longer wavelengths when compared to its absorption spectrum and the transient bleach is further shifted to longer wavelengths with charge recombination. These features are attributed to transient Stark shifts that arise from the local electric fields generated at the dye/TiO 2 interface due to charge-transfer interactions.

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 2 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 2 nanoparticles are prepared and the complexation is followed with optical absorption measurements. Although little visible luminescence is observed from these moleculeTiO 2 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 2 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 2 nanoparticles. However, the possibility of delocalized charge-transfer excitations is observed for salicylate and acetyl acetonate-TiO 2 nanoparticles. The decay of the charge transfer emission is ascribed to the relaxation of the localized states to delocalized states in the TiO 2 conduction band. Transient absorption measurements have shown long-lived charge separation in the case of surface-modified TiO 2 nanoparticles. Further measurements on the influence of charge-transfer excitations on the interfacial electron transfer in surface-modified TiO 2 nanoparticles are being carried out.