The preparation and study of photocatalysts: from core-shell palladium-titanium dioxide nanoparticles to iridium(III) complexes

Titanium dioxide nanoparticles (NPs) and iridium(III) complexes have been prepared and studied as photocatalysts towards enhanced efficiency and mechanistic understanding of photocatalysis. Core-shell palladium-titanium dioxide NPs, Pd@TiO₂, were prepared using monodisperse Pd@SiO₂ core-shell NPs as a template. The Pd cores and porous, high surface area TiO₂ shells are expected to prevent Pd loss and increase surface-substrate interactions, respectively, thereby improving photocatalytic efficiency. Carbon dioxide was photocatalytically reduced in water by the Pd@TiO₂ NPs with methane being the major product. Iridium(III) complexes were tailored to increase the excited state lifetime through minor ligand modification. [Ir(ppy)₂phen]PF₆, [Ir(ppy)₂dtbbpy]PF₆, [Ir(ppy)₂dmbpy]PF₆, and [Ir(ppy)₂bpy]PF₆ were prepared where ppy = 2-phenylpyridine, bpy = 2,2ʹ-bipyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, dtbbpy = 4,4’-di-tert-butyl-2,2ʹ-bipyridine and phen = 1,10-phenanthroline. Their excited state lifetimes range from 0.3 µs to 0.7 µs and correlate to the rate of single-electron transfer (SET) from excited state to substrate, CF₃SO₂Cl (1.9 × 10⁸ M‾¹ s‾¹ to 9.3 × 10⁸ M‾¹ s‾¹), but the rate of final product formation is unchanged. The photocatalyzed trifluoromethylation of quinoline was used as a prototypical reaction. The unchanged rate of product formation indicates that SET involving the excited state is not rate-limiting in this system. Further increase in the SET rate was attempted by attaching pyrene onto a ligand in the complex for Reversible Electron Energy Transfer (REET) to increase the photocatalyst excited state lifetime more significantly. [Ir(npy)₂bpyethylpyr]PF₆, [Ir(npy)₂bpypyr]PF₆, and [Ir(npy)₂dmbpy]PF₆ were prepared where npy = 2-(naphthalen-1-yl)pyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, bpyethylpyr = 4-methyl-4ʹ-[2-(pyren-1-yl)ethyl]-2,2ʹ-bipyridine and bpypyr = 4-(1ʹʹ-pyrenyl)-2,2ʹ-bipyridine. The excited state lifetimes are 13.8 µs, 4.8 µs and 3.2 µs, respectively. Excited state lifetime and transient absorption studies indicate that only the complex with pyrene separated from bpy by an alkyl bridge displays REET. The rates of SET and product formation in the trifluoromethylation of quinoline are slower upon incorporation of pyrene. This is attributed to new, less reactive excited states and increased photocatalyst size, which slows down diffusion. The findings presented herein reveal the importance of photophysical and structural properties, such as photocatalyst size and excited state lifetime, in SET and photocatalytic efficiency, thereby contributing to guided optimization of photocatalytic systems.