Metal complexes, methods, and uses thereof

A metal complex comprising:

• palladium(II);
• and
• a tetradentate ligand bonded to the transition metal,
• wherein:
• the metal complex has a lowest triplet excited state and a lowest singlet excited state,
• the lowest triplet excited state has a lower energy level than the lowest singlet excited state,
• the lowest triplet excited state is associated with phosphorescence, and
• a transition from the lowest triplet excited state to the lowest singlet excited state yields delayed fluorescence from the lowest singlet excited state;
• wherein the metal complex is represented by one of the following formulas:
• wherein M is palladium(II),
• wherein A is
• which can optionally be substituted,
• wherein D is
• which can optionally be substituted,
• wherein C in structure (a) or (b) is
• which can optionally be substituted,
• wherein N in structure (a) or (b) is one of the following structures, which can optionally be substituted,
• wherein a2 is absent,
• wherein b1 is present or absent, and if present, comprises a linking group comprising one or more of the following
• wherein b2 is absent;
• wherein X is N and
• wherein each R independently is hydrogen, aryl, cycloalkyl, cycloalkenyl, heterocyclyl, heteroaryl, alkyl, alkenyl, alkynyl, deuterium, halogen, hydroxyl, thiol, nitro, cyano, amino, a mono- or di-alkylamino, a mono- or diaryl amino, alkoxy, aryloxy, haloalkyl, aralkyl, ester, nitrile, isonitrile, alkoxycarbonyl, acylamino, alkoxycarbonylamino, aryloxycarbonylamino, sulfonylamino, sulfamoyl, carbamoyl, alkylthio, sulfinyl, ureido, phosphoramide, mercapto, sulfo, carboxyl, hydrazino, substituted silyl, or polymerizable, or any conjugate or combination thereof.