Organic electroluminescent materials and devices

A compound comprising a first ligand LA of Formula I,

• wherein:
• moiety A is a monocyclic 5-membered or 6-membered carbocyclic or heterocyclic ring;
• Q is selected from the group consisting of C, N, B, and Si;
• each of X1 to X5 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′;
• 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 RA, RB, and RC independently represent mono to the maximum allowable substitutions, or no substitutions;
• each R, R′, R″, Rα, Rβ, RA, RB, and RC 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;
• at least one RA, RB, or RC comprises a silyl group or a germyl group;
• LA is coordinated to a metal M, having an atomic mass of at least 40;
• metal M may be coordinated to other ligands;
• LA may be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• any two substituents may be joined or fused to form a ring except those explicitly excluded,
• two RA are not joined;
• and
• if M is Ir and K is a direct bond, then:
• (1) no two RB are joined or fused to form a ring;
• (2) if the RC substituents at X4 and X5 are fused to form a benzene ring, then at least one RA, RB, or RC comprises a germyl group;
• and
• (3) if Y comprises R, R′, or both, then neither the R nor the R′ are joined or fused with RA or RC to form a ring.