Organic electroluminescent materials and devices

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

• wherein:
• each of X1 to X6 is independently C or N;
• X is selected from the group consisting of CR1R2, SiR1R2, GeR1R2, O, S, Se, NR1, PR1, and BR1;
• Y is selected from the group consisting of CR3R4, SiR3R4, GeR3R4, O, S, Se, NR3, PR3, and BR3;
• 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 and RB independently represents mono to the maximum allowable substitutions, or no substitutions;
• each Rα, Rβ, R1, R2, R3, R4, RA, and RB is independently hydrogen or a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, germyl, boryl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, selenyl, and combinations thereof;
• LA is coordinated to a metal M, having an atomic mass of at least 40;
• M can be coordinated to other ligands;
• LA can be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• any two substituents can be joined or fused to form a ring;
• and
• at least one of the following is true:
• (1) at least one of X1 to X3 is N, with the proviso that if X is CR1R2 and Y is CR3R4, then at least one of the following is true (i) R1 is not joined with either of R3 or R4 to form a ring, or (ii) R2 is not joined with either of R3 or R4 to form a ring,
• (2) X is CR1R2 and R1 and R2 join to form a ring,
• (3) Y is CR3R4 and R3 and R4 join to form a ring,
• (4) at least one RB is a carbocyclic or heterocyclic ring, or
• (5) at least one of X or Y is substituted with a group comprising at least one fluorine atom,
• with the proviso that if M is Ir, K is a direct bond, X2 is N, X is NR1, and Y is CR3R4, then at least one of the following is true:
• (a) no RA substituent is joined or fused with X to form a ring, or (b) no RB joins with Y to form a ring.