Electron transfer from a new chalcone dye to TiO₂ nanoparticles: Synthesis, photophysics, and excited-state dynamics

Efficient electron injection from photosensitizers to semiconductor nanoparticles is essential in many applications such as solar energy harvesting and catalytic reactions. Herein, we present a promising photosensitizer, 4-dimethylamino-2′-hydroxy-4′-carboxychalcone (DHC-COOH), that upon adsorption on TiO₂ nanoparticles shows enhanced electron transfer from its excited state to the conduction band (CB) of TiO₂ via an intramolecular charge transfer (ICT). In order to fully characterize the current system, three more derivatives were synthesized and characterized, its analogue without the COOH group (DHC), and the molecules without the OH group (DC and DC-COOH). The presence of OH causes a red shift in the absorption and fluorescence spectra, whereas the COOH group induces more red shift due to efficient ICT which is also seen in the density functional theory calculations. Excited-state intramolecular proton transfer is evident in the crystalline/solid form of DHC and DHC-COOH as a large red shift in the fluorescence peak. The red shift was not observed when DHC-COOH was adsorbed on the TiO₂ surface due to the involvement of the OH group in binding. This was confirmed by XPS, in addition to a bidentate binding of the COOH group to TiO₂. The latter has a major contribution to the electron transfer mechanism which was observed as a much weaker fluorescence intensity and a 50% reduction in the lifetime component that involves the COOH dynamic (from 63 ps to 32 ps). On the other hand, electron-hole recombination dynamics were slowed down on the TiO₂ surface (from 311 ps to 440 ps and 1.4 ns to 2.4 ns). From cyclic voltammetry and steady state spectra, it was found that electron injection from the excited state of the dye to the CB of TiO₂ is energetically favorable, and regeneration of the oxidized dye by the I⁻/I₃⁻ redox pair (used in dye-sensitized solar cells) is also possible.