A compound including a first ligand LX of Formula II is disclosed, where F is a 5-membered or 6-membered carbocyclic or heterocyclic ring; each RF and RG independently represents mono to the maximum possible number of substitutions, or no substitution; Z3 and Z4 are each independently C or N and coordinated to a metal M to form a 5-membered chelate ring; G is a fused ring structure comprising five or more fused heterocyclic or carbocyclic rings, of which at least one ring is of Formula III the fused heterocyclic or carbocyclic rings in the fused ring structure G are 5-membered or 6-membered; of which if two or more 5-membered rings are present, at least two of the 5-membered rings are fused to one another; Y can be one of BR′, NR′, PR′, O, S, Se, C═O, S═O, SO2, CR′R″, SiR′R″, and GeR′R″; the metal M can be coordinated to other ligands; and the ligand LX can be linked with other ligands to comprise a tridentate, tetradentate, pentadentate, or hexadentate ligand.