Photoreductive dissolution of cerium oxide nanoparticles and their size-dependent absorption properties

Cerium oxide has attracted attention recently for its photocatalytic properties, but there are gaps in understanding its performance, especially at low and high pH. UV irradiation of ceria nanoparticles causes electrons from photogenerated electron-hole pairs to localize as small polarons, yielding Ce3+ ions. In pH 10 solution, ceria nanoparticles capped with polyacrylic acid ligands can accumulate large numbers of Ce3+ defects as revealed by strong bleaching of the absorption onset. In contrast, we show that UV irradiation of several-nanometer diameter ceria nanoparticles in acidic (pH < 3) aqueous solution releases Ce3+ ions into solution with a quantum yield that approaches 70% and that varies with excitation wavelength, particle size, and the presence of a hole scavenger (glycerol) on the nanoparticle surface. The instability of Ce3+ at the nanoparticle surface and the ability of electron small polarons to migrate to the surface by hopping strongly suggest that nanoceria is fully oxidized and essentially free of Ce3+ centers at pH < 3. Efficient photoreduction and the excellent stability of unirradiated nanoparticles make it easy to shrink the nanoparticles using only light, while maintaining them in a fully oxidized state. This enables study of the size-dependent absorption properties of ceria nanoparticles that are free of Ce3+ defects. No evidence of quantum confinement is observed, consistent with highly localized excited states. The observed quantum yields of photoreduction are higher than reported for other metal oxides, revealing that a significant fraction of electron-hole pairs are available for driving surface redox reactions, even in fully oxidized particles.

Triplet Harvesting with a Simple Aromatic Carbonyl

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive ¹ ππ* excitation may rapidly equilibrate with an upper triplet state (³ nπ*). This equilibrium may persist for nanoseconds. Population of the ³ nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light.

Photoinduced Electron Transfer between Psoralens and DNA: Influence of DNA Sequence and Substitution

Psoralens are heterocyclic compounds which are, among other uses, used to treat skin deseases in the framework of PUVA therapy. In the dark, they intercalate into DNA and can form photoadducts with thymines upon UV-A excitation, which harms the affected cells. We have recently discovered that after excitation of intercalated psoralens, an efficient photoinduced electron transfer (PET) from DNA occurs. Here, the PET is studied in detail by means of femtosecond transient absorption spectroscopy. Using DNA samples that contain either only GC or AT base pairs, we show that only guanine donates the electrons. Additionally, the substituent effects on PET are studied relying on three different psoralen derivatives. The substitution alters spectroscopic and electrochemical properties of the psoralens, which are determined by cyclic voltammetry and steady state spectroscopy. These experiments allow us to estimate the PET energetics, which are in line with the measured kinetics. Implications for the applications of psoralens are discussed.

DNA Intercalated Psoralen Undergoes Efficient Photoinduced Electron Transfer

The interaction of psoralens with DNA has been used for therapeutic and research purposes for decades. Still the photoinduced behavior of psoralens in DNA has never been observed directly. Femtosecond transient absorption spectroscopy is used here to gain direct insight into the photophysics of a DNA-intercalated psoralen (4'-aminomethyl-4,5',8-trimethyl-psoralen (AMT)). Intercalation reduces the excited singlet lifetime of AMT to 4 ps compared with 1400 ps for AMT in water. This singlet quenching prohibits the population of the triplet state that is accessed in free AMT. Instead, a DNA to AMT electron transfer takes place. The resulting radical pair decays primarily via charge recombination with a time constant of 30 ps. The efficient electron transfer observed here reveals a completely new aspect of the psoralen-DNA interaction.

The photoformation of a phthalide: a ketene intermediate traced by FSRS

Femtosecond stimulated Raman spectroscopy, transient absorption and quantum chemistry are combined to unravel the complex path of phthalide photoformation.