Security feature, printing ink, document of value, and manufacturing method

A security feature for safeguarding a document of value, comprising an organometallic luminescent substance which, on excitation with radiation in a UV-A range, has emission of radiation in a visible wavelength range,

• wherein the luminescent substance is based on a polynuclear heteroleptic lanthanoid complex of the formula (I)
• [Mx(NN ligand)a(OO ligand)b] (I)
• where
• x, a and b are natural numbers, where x≥2, a≥2 and b≥6;
• M is a lanthanoid selected from the group consisting of Eu and Tb;
• the OO ligand is a ligand of the formula (II) that coordinates via oxygen atoms:
• where
• R1 to R5 are the same or different and are independently selected from the group consisting of hydrogen atom, OH, OCH3 and halogen atom;
• the NN ligand is a nitrogen ligand with a phenanthroline base skeleton of the general formula (III) or of the general formula (IV)
• where
• Y is NR9 or Y is an oxygen atom;
• R6 and R7 are the same or different and are independently selected from the group consisting of hydrogen atom and an electron-donating group;
• R8 and R9 are the same or different and are independently selected from the group consisting of hydrogen atom, substituted or unsubstituted alkyl radical having 1 to 8 carbon atoms, substituted or unsubstituted aryl radical, substituted or unsubstituted piperidine radical and substituted or unsubstituted benzyl radical.