Organic electroluminescent materials and devices

A compound comprising a first ligand LA of Formula I,

• wherein:
• moiety A is a fused polycyclic ring system, where each of at least two rings is independently a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• moiety B is a monocyclic ring or a fused polycyclic system, wherein the monocyclic ring and each ring of the fused polycyclic system is independently a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• ring C is a 5-membered ring;
• ring D is a 6-membered ring;
• Y is selected from the group consisting of N, B, and P;
• each of Z1 to Z4 is independently C or N;
• X1 is CR1 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)(RR), and Si(Rα)(RR);
• each of RA and RB independently represents mono to the maximum allowable substitutions, or no substitutions;
• each R1, R2, Rα, Rβ, RA, and RB is independently a hydrogen or a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, boryl, arylalkyl, alkoxy, aryloxy, amino, silyl, germyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, selenyl, and combinations thereof;
• LA is joined to a metal M by the dashed lines;
• metal M has an atomic mass of at least 40 and can be coordinated to other ligands;
• LA can be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• and
• any two substituents may be joined or fused to form a ring.