An organic amine dimmer and a method for synthesizing the same are disclosed. The method comprises the following steps: (A) mixing an iron-containing compound (e.g., Fe2O3, FeCl3), a nitrate-containing compound, oxalate acid, 4,4′-trimethylenedipiperidine (TMDP), H3PO4, and 2 to 8 ml of water to form a mixture; (B) heating the mixture and maintaining the mixture at 160° C. or above for a first predetermined time; (C) cooling the mixture; and (D) placing the mixture at room temperature for a second predetermined time, to obtain an organic amine dimmer, wherein Fe(NO3)3 can serve as the iron-containing compound as well as the nitrate-containing compound.