A mixture of (a) at least one charge neutral metal-organic coordination compound, comprising a divalent Europium coordinated by a polycyclic organic ligand having the formula with • Y independently in each occurrence being selected from Si-R, C-R, B, N, P, preferably N, with R being a monovalent organic group formed by removing a hydrogen atom from an organic molecule containing at least one hydrogen atom, and • L 0, L 1, L 2 being independently each a divalent organic group formed by removing two hydrogen atoms from an organic molecule containing at least two hydrogen atoms, with • the shortest sequence of atoms linking the two Y atoms as part of L 1, and L 2 each being 4 to 7, preferably 5 atoms, with one of those 4 to 7, preferably 5 atoms, preferably one non-terminal carbon atom thereof, each independently being selected from B, N, P, S, O, preferably N, or O, and with • the shortest sequence of atoms linking the two Y atoms as part of L 0 being 8 or more atoms, and (b) at least one second organic compound, which is a charge-neutral host material, wherein the second charge-neutral organic compound has a higher electron binding energy than the charge-neutral metal-organic coordination compound, preferably wherein the second charge-neutral organic compound (b) has an electron binding energy higher than 4.5 eV, preferably higher than 5.3 eV and most preferably higher than 6.0 eV, and wherein compounds (a) and (b) are physically mixed or wherein the second charge-neutral compound (b) has a molecular weight M n above 1000 g/mol and the coordination compound (a) and the second charge-neutral compound (b) are chemically linked with each other, for example by a covalent or coordinative chemical bond. (a) at least one charge neutral metal-organic coordination compound, comprising a divalent Europium coordinated by a polycyclic organic ligand having the formula with • Y independently in each occurrence being selected from Si-R, C-R, B, N, P, preferably N, with R being a monovalent organic group formed by removing a hydrogen atom from an organic molecule containing at least one hydrogen atom, and • L 0, L 1, L 2 being independently each a divalent organic group formed by removing two hydrogen atoms from an organic molecule containing at least two hydrogen atoms, with • the shortest sequence of atoms linking the two Y atoms as part of L 1, and L 2 each being 4 to 7, preferably 5 atoms, with one of those 4 to 7, preferably 5 atoms, preferably one non-terminal carbon atom thereof, each independently being selected from B, N, P, S, O, preferably N, or O, and with • the shortest sequence of atoms linking the two Y atoms as part of L 0 being 8 or more atoms, and • Y independently in each occurrence being selected from Si-R, C-R, B, N, P, preferably N, with R being a monovalent organic group formed by removing a hydrogen atom from an organic molecule containing at least one hydrogen atom, and • L 0, L 1, L 2 being independently each a divalent organic group formed by removing two hydrogen atoms from an organic molecule containing at least two hydrogen atoms, with • the shortest sequence of atoms linking the two Y atoms as part of L 1, and L 2 each being 4 to 7, preferably 5 atoms, with one of those 4 to 7, preferably 5 atoms, preferably one non-terminal carbon atom thereof, each independently being selected from B, N, P, S, O, preferably N, or O, and with • the shortest sequence of atoms linking the two Y atoms as part of L 0 being 8 or more atoms, and (b) at least one second organic compound, which is a charge-neutral host material,