Solvent-Switching Gelation and Orange-Red Emission of Ultrasmall Copper Nanoclusters.

By tuning the Cu⋅⋅⋅Cu and hydrogen-bonding interactions, the small cluster Cu3 L can be selectively synthesized to develop a stable and highly fluorescent material, as confirmed by matrix-assisted laser desorption ionization-time of flight mass spectroscopy. Further characterizations, including absorbance spectroscopy, XPS, and XRD demonstrate the formation of tiny Cu nanoclusters (NCs). In water, the as-prepared Cu NCs can exhibit high orange fluorescence via solution evaporation to eliminate hydrogen-bonding, and in dimethylformamide, a strong orange fluorescent gel is obtained by solvent induction to enhance the Cu⋅⋅⋅Cu and hydrogen-bonding interactions. More importantly, the Cu NCs in their substantial form exhibit nonlinear optical properties upon two-photon excitation. These results will shed light on Cu and related cluster applications in two-photon biological imaging, optical power limiting, and solar energy conversion.