Tuning Long‐Lived Mn(II) Upconversion Luminescence through Alkaline‐Earth Metal Doping and Energy‐Level Tailoring

Abstract Crystal‐site engineering of hexagonal‐phase NaGdF 4 :Mn through alkaline‐earth metal (A 2+ ) doping is used to tune the upconversion luminescence of the Mn 2+ dopants. Experimental and calculated results show the ability of A 2+ doping to alter the occupancy site of Mn 2+ ion from Na + to Gd 3+ , thus enabling an emission change from green (520 nm) to yellow (583 nm) upon excitation at 980 nm. The yellow emission of Mn 2+ shows a long emission lifetime of 65 ms, more than three times longer than the green emission of Mn 2+ (20 ms). The combination of tunable long‐lived Mn 2+ emission with lanthanide emission at a single‐particle level provides a convenient route to triple transient upconversion color codes upon dynamic excitation, which offers an attractive optical feature particularly suitable for efficient document encoding.