Luminescent compounds with carbene ligands

An organic light emitting device, comprising:

• (a) an anode;
• (b) a cathode;
• (c) an organic layer disposed between the anode and the cathode, wherein the organic layer comprises a compound comprising one or more carbene ligands coordinated to a metal center, wherein the carbene ligand has a structure selected from:
• the dotted lines between A and R1 and between A or A′ and adjacent carbon atoms represent optional double bonds;
• each Z1, Z2, A, and A′ is C, N, O, P, or S;
• R1, R2, and R3 are independently selected from H, alkyl, aryl or heteroaryl, and additionally or alternatively, one or more of R1 and R2 and R2 and R3 together form independently a 5 or 6-member cyclic group, wherein the cyclic group is cycloalkyl, cycloheteroalkyl, aryl or heteroaryl, and wherein said cyclic group is optionally substituted by one or more substituents J;
• each substituent J is independently selected from the group consisting of R′, O—R′, N(R′)2, SR′, C(O)R′, C(O)OR′, C(O)NR′2, CN, NO2, SO2, SOR′, or SO3R′, and additionally, or alternatively, two J groups on adjacent ring atoms form a fused 5- or 6-membered aromatic group;
• each R′ is independently selected from H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl, aryl and heteroaryl;
• and
• a is 0, 1, or 2.