Here, we investigate the photochemistry of a catechol : o-quinone heterodimer as a model system for uncovering the photoprotective roots of eumelanin.

Changing the size of cerium oxide nanoparticles with UV light provides insights into the absorption properties of this redox-active photocatalyst.

Dye-sensitized electron injection into cerium oxide (CeO 2 ) nanoparticles is studied using femtosecond transient absorption spectroscopy. Following the 430 nm photoexcitation of coumarin 343 molecules adsorbed on CeO 2 nanoparticles, mid-infrared (mid-IR) transient absorption signals appear within 500 fs. Mid-IR signals are assigned to electrons injected into broadly distributed trap or defect states found roughly midway between the Ce 5d conduction band edge and the bottom of the empty band of Ce 4f states. These states are proposed to have Ce 5d character. In contrast, mid-IR signals are not observed when electrons in bare CeO 2 nanoparticles are promoted from the valence band to the band built from Ce 4f orbitals by UV excitation. Transient absorption signals from the oxidized dye rise within 300 fs and decay somewhat more slowly than the mid-IR signals, suggesting that electron relaxation occurs within CeO 2 in competition with back-electron transfer to the oxidized dye. These results reveal elementary events important for understanding photocatalysis using CeO 2 nanoparticles. Article pubs.acs.org/JPCC