A compound of Formula I, wherein: M is Pd or Pt; each of X 1 to X 14 is independently C or N; one of Z 1 and Z 2 is C and the other is N; Y is selected from the group consisting of O, S, Se, N*R, CRR', SiRR', and GeRR'; K 1 is selected from the group consisting of a direct bond, O, and S; each of R A, R B, R C, and R D independently represents mono to the maximum allowable substitution, or no substitution; each R, R', R A, R B, R C, and R D 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, boryl, selenyl, and combinations thereof; at least one of R, R', R A, R B, R C, or R D is a substituent R *; wherein at least one R D is a substituent R *; one of the following two statements is true: (i) R * comprises a 5-membered or 6-membered heterocyclic ring; (ii) R * comprises Formula Ia, wherein each of R Y and R Z independently represents mono to the maximum allowable substitution, or no substitution; R 1, R 2, R 3, R 4, R Y and R Z 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, boryl, selenyl, and combinations thereof; wherein at least one of R 1, R 2, R 3, and R 4 is not hydrogen; and any two substituents may be joined or fused to form a ring. M is Pd or Pt; each of X 1 to X 14 is independently C or N; one of Z 1 and Z 2 is C and the other is N; Y is selected from the group consisting of O, S, Se, N*R, CRR', SiRR', and GeRR'; K 1 is selected from the group consisting of a direct bond, O, and S; each of R A, R B, R C, and R D independently represents mono to the maximum allowable substitution, or no substitution; each R, R', R A, R B, R C, and R D 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, boryl, selenyl, and combinations thereof; at least one of R, R', R A, R B, R C, or R D is a substituent R *; wherein at least one R D is a substituent R *; one of the following two statements is true: (i) R * comprises a 5-membered or 6-membered heterocyclic ring; (ii) R * comprises Formula Ia, wherein each of R Y and R Z independently represents mono to the maximum allowable substitution, or no substitution; R 1, R 2, R 3, R 4, R Y and R Z 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, boryl, selenyl, and combinations thereof; wherein at least one of R 1, R 2, R 3, and R 4 is not hydrogen; and (i) R * comprises a 5-membered or 6-membered heterocyclic ring; (ii) R * comprises Formula Ia, wherein each of R Y and R Z independently represents mono to the maximum allowable substitution, or no substitution; R 1, R 2, R 3, R 4, R Y and R Z 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, boryl, selenyl, and combinations thereof; wherein at least one of R 1, R 2, R 3, and R 4 is not hydrogen; and any two substituents may be joined or fused to form a ring.