Organic electroluminescent materials and devices

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

• wherein:
• each of moiety A and moiety B is independently a monocyclic ring or a polycyclic fused ring system, where the monocyclic ring or each ring of the polycyclic fused ring system is independently a 5-membered or 6-membered carbocyclic or heterocyclic ring;
• each of X1 to X4 is independently C or N;
• each of Z1 and Z2 is independently C 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)(Rβ), and Si(Rα)(Rβ);
• if K1 is not a direct bond, then Z1 is C;
• if K2 is not a direct bond, then Z2 is C;
• L1 is a direct bond or an organic linker;
• RA and RB each independently represent mono to the maximum allowable substitution, or no substitution;
• each 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;
• at least one RA or one RB comprises R*, where R* is an alkyl group, a silyl group, a germyl group, or a combination thereof;
• at least one of the following three statements is true:
• (1) R* is an alkyl group and comprises at least one tertiary carbon atom and at least two additional carbon atoms that are each independently secondary or tertiary carbon atoms;
• (2) R* is a substituted alkyl comprising at least two tertiary silicon atoms;
• or
• (3) moiety A is a fused polycyclic ring system, and R* has Formula II,
• wherein:
• Q is selected from the group consisting of C, Si, and Ge;
• each of R1, R2, and R3 is independently selected from the group consisting of hydrogen, deuterium, fluorine, alkyl, cycloalkyl, silyl, germyl, and combinations thereof;
• if Q is Si, then at least one of R1, R2, or R3 comprises at least two carbon atoms;
• if Q is C, then at least two of R1, R2, and R3 are joined to form a bridged polycyclic structure;
• 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 may be coordinated to other ligands;
• LA may join with other ligands to form a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• and
• any two substituents may be joined or fused to form a ring, with the proviso that R does not join with an RA substituent to form a ring, and with the proviso that LA does not include: