Organic electroluminescent materials and devices

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

• wherein:
• two RC substituents or two RD substituents are joined together to form a structure comprising Formula II,
• fused to ring C or ring D, respectively;
• each of moiety A and moiety B is independently a monocyclic ring or a polycyclic fused ring system, wherein the monocyclic ring or each ring of the polycyclic fused ring system is independently a 5-membered to 10-membered carbocyclic or heterocyclic ring;
• each of X1 to X13 is independently C or N;
• each of Z1 and Z2 is 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);
• 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═CRR′, S═O, SO2, CRR′, SiRR′, and GeRR′;
• L is selected from the group consisting of a direct bond, BR, BRR′, NR, PR, P(O)R, O, S, Se, C═O, C═S, C═Se, C═NR′, C═CRR′, S═O, SO2, CRR′, SiRR′, and GeRR′;
• each of RA, RB, RC, RD, and RE independently represents mono to the maximum allowable substitutions, no substitutions;
• each R, R′, Rα, Rβ, RA, RB, RC, RD, and RE 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;
• any two substituents can be joined or fused to form a ring;
• at least one of the following is true (i) one RA and one RD substituent are joined together to form a ring AD, or (ii) one RC and one RD substituent are joined together to form a ring CD;
• LA is coordinated to a metal M, having an atomic mass of at least 40;
• M can be coordinated to other ligands;
• and
• LA can be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand, with the proviso that if Y is NR, then R does not form a ring with an RE substituent.