Organic electroluminescent materials and devices

A compound comprising a first ligand LA having a structure of Formula I,

• wherein:
• moiety A is a monocyclic ring or a polycyclic fused ring system, where the monocyclic ring and each ring of the polycyclic fused ring system is independently a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• K is selected from the group consisting of a direct bond, O, S, N(Rα), P(Rα), B(Rα), C(Rα)(Rβ), and Si(Rα)(Rβ);
• each of Z1 and Z2 is independently C or N;
• each of X1 to X8 is independently C or N;
• Y is selected from the group consisting of BR, BRR′, NR, PR, P(O)R, O, S, Se, C═O, C═S, C═Se, C═NR′, C═CR′R″, S═O, SO2, CR, CRR′, SiRR′, and GeRR′;
• each of R1, R2, and R3 independently represents mono to the maximum allowable number of substitutions, or no substitution;
• at least one R2 or R3 is a substituted or unsubstituted 5-membered or 6-membered carbocyclic or heterocyclic ring, a silyl group, a germyl group, or an electron-withdrawing group;
• each Rα, Rβ, R, R′, R″, R1, R2, and R3 is independently hydrogen or a substituent selected from the group consisting of deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, boryl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, selenyl, germyl, and combinations thereof;
• LA is coordinated to a metal M selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Pd, Ag, Au, and Cu;
• metal M can be coordinated to other ligands;
• LA can be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• wherein any two substituents can be joined or fused to form a ring;
• with the proviso that if an R3 substituent is a heterocyclic ring, then no other R3 substituent is F or CN;
• with the proviso that no R2 substituent is carbazole if moiety A is imidazole or pyridine;
• with the proviso that LA does not comprise
• wherein X is O or S;
• and
• with the proviso that the compound is not