Organic electroluminescent materials and devices

A compound comprising a first ligand LA of Formula I:

• wherein:
• each of moiety A and moiety B is independently a monocyclic ring or a polycyclic fused ring system, wherein the monocyclic ring or each ring of the polycyclic fused ring system is independently a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• moiety C is a polycyclic fused ring system, wherein each ring of the polycyclic fused ring system is independently a 5-membered or 6-membered carbocyclic or heterocyclic rings;
• each of X1—X6, Z1, and Z2 is independently C or N;
• each of K1 and K2 is independently selected from the group consisting of a direct bond, O, S, N(Rα), P(Rα), B(Rα), C(Rα)(Rβ), and Si(Rα)(Rβ);
• if K1 is not a direct bond, then Z1 is C;
• if K2 is not a direct bond, then Z2 is C;
• each of RA, RB, and RC independently represents mono to the maximum allowable substitutions, or no substitutions;
• each Rα, Rβ, RA, RB, and RC is independently a hydrogen or a substituent selected from the group consisting of deuterium, halogen, fluorine, alkyl, fluoroalkyl, cycloalkyl, fluorocycloalkyl, heteroalkyl, heterocycloalkyl, boryl, arylalkyl, alkoxy, aryloxy, amino, silyl, germyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, fluoroaryl, fluoroheteroaryl, acyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, selenyl, and combinations thereof;
• any two substituents can be joined or fused into a ring;
• LA is coordinated to a metal M;
• M may be coordinated to other ligands;
• LA may join with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• and
• if M is Ir, then at least one of the following is true:
• (1) moiety C comprises four or more fused rings, with proviso that ligand LA does not comprise
• (2) at least one RC comprises an electron-withdrawing group, and if moiety A is pyridine, moiety B is benzene, moiety C is dibenzofuran, aza-dibenzofuran, dibenzothiophene, or aza-dibenzothiophene, then no RC is fluorine or an alkyl or cycloalkyl group containing fluorine;
• or
• (3) if moiety A is substituted or unsubstituted imidazole, then the compound is heteroleptic.