Synthesis and Photoinduced Electron Transfer Studies of Ligand Exchanged Mn-Doped ZnSe Nanocrystals in Water

We report the synthesis and ligand exchange of Mn-doped ZnSe (Mn:ZnSe) nanocrystals with high photoluminescence (PL) quantum yields (QYs) and long excited state lifetimes and examine the influence that ligand chemistry has on diffusion-controlled photoinduced electron transfer (PET). Comparisons of the PL QY and excited state properties of the Mn:ZnSe nanocrystal ligand exchanged with 3-mercapto proprionic acid (MPA) in water versus the as-synthesized nanocrystals in organic solvents reveal an increase in the average PL QY from 22 to 47% in aqueous solution, with a doubling of the excited state lifetime to greater than 500 μs. Time-resolved PL and transient absorption measurements demonstrate that these long excited state lifetimes enable diffusion of the nanocrystals over hundreds of nanometers in solution while in their excited states, allowing them to undergo PET to molecular species at nanomolar concentrations that are orders of magnitude lower than those used in traditional quenching studies. Interestingly, MPA-passivated Mn:ZnSe nanocrystals in water exhibit a factor of 2 increase in the length scale over which they can diffuse within their excited state lifetime in comparison to the as-synthesized nanocrystals passivated by stearic acid ligands in organic solution. The ability to achieve long excited state lifetimes and to support diffusion-controlled charge-transfer processes over hundreds of nanometer length scales suggests that these nanocrystals are promising model systems for photocatalytic applications.