Organic electroluminescent materials and devices

A compound comprising a first ligand LA of Formula I,

• wherein:
• moiety B is a fused polycyclic ring system, wherein each ring of the fused polycyclic ring system is a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• X1 to X4 are each independently C or N;
• 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);
• LA is coordinated to a metal M;
• M may be coordinated to other ligands;
• LA may be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• one of the following three conditions is true:
• (1) at least one of RA or RB comprises a silyl group, and moiety B comprises four or more fused rings,
• (2) at least one RA has a structure of Formula II,
• and at least one of RA or RB comprises a silyl group, wherein if the at least one of RA that is Formula II is joined to X3 and K is a direct bond, then at least one RC is not H, and
• (3) at least one RB comprises a silyl group and at least one RA comprises a cyclic group or a silyl group;
• wherein each of RA, RB, and RC independently represents mono to the maximum allowable substitutions, or no substitutions;
• each RA, RB, RC, Rα, and Rβ is independently 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;
• and
• any two RA, RB, RC, Rα, and Rβ may be joined or fused to form a ring.