Organic electroluminescent materials and devices

A compound comprising a first ligand of Formula I:

• wherein:
• moiety A is 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 or 6-membered carbocyclic or heterocyclic ring;
• ring C is independently a 5-membered or 6-membered aryl or heteroaryl ring;
• each of ring D and ring E is independently a 5-membered or 6-membered aryl or heteroaryl ring, or a 5- or 6-membered alicyclic ring or non-aromatic heterocyclic ring, with the following conditions:
• (1) if ring D is a 5-membered or 6-membered aryl or heteroaryl ring, then ring E is a 5- or 6-membered alicyclic ring or non-aromatic heterocyclic ring, and ring E does not have any ring carbons bonded to two hydrogens (CH2), and ring E is not annulated by an additional ring;
• or
• (2) if ring E is a 5-membered or 6-membered aryl or heteroaryl ring, then ring D is a 5- or 6-membered alicyclic ring or non-aromatic heterocyclic ring, and ring D does not have any ring carbons bonded to two hydrogens (CH2), and ring D is not annulated by an additional ring;
• each of X1, X2, Z1 to Z5, and W1 to W4 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═CRR′, S═O, SO2, CR, CRR′, SiRR′, and GeRR′;
• 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 the one of Z2, Z3, Z4, or Z5 to which it is bonded is C;
• L is selected from the group consisting of a direct bond, BR, NR, PR, O, S, Se, C═O, C═S, C═Se, C═NR, C═CRR′, S═O, SO2, CRR′, P(O)R, SiRR′, and GeRR′;
• each of RA, RB, RC, RD, and RE independently represents mono to the maximum allowable substitutions, or no substitutions;
• each R, R′, Rα, Rβ, RA, RB, RC, RD, and RE is independently a 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, with the proviso that an RA and an RB are not joined or fused to form a ring;
• LA is coordinated to a metal M, having an atomic mass of at least 40;
• none of ring C, ring D, and ring E form a direct bond to metal M;
• metal M can be coordinated to other ligands;
• and
• LA can be joined with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand.