A compound of formula I: Y-X-Z, Formula I, wherein Y is selected from the group consisting of: A-B-C-D- A-B-C- A-B- wherein A, B, C, and D are independently selected from the compound of formula II: wherein X 1 is selected from the group C-R 1 and N; X 2 is selected from the group C-R 2 and N; X 3 is selected from the group C-R 3 and N; X 4 is selected from the group C-R 4 and N; X 5 is selected from the group C-R 5 and N; X 6 is selected from the group C-R 6 and N; X 7 is selected from the group C-R 7 and N; X 8 is selected from the group C-R 8 and N; wherein A, B, C, and D are connected to each other through C-N bonds, where N is a nitrogen in the 9-position; wherein at least one of A, B, C, and D is not carbazole; wherein X is an aryl or heteroaryl linker that is optionally further substituted; wherein Y and X are connected through a C-N bond, where N is the nitrogen in the 9-position in one of A, B, C, and D; wherein R 1, R 2, R 3, R 4, R 5, R 6, R 7, and R 8 are independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; and wherein Z is selected from the group consisting of dibenzothiophene, dibenzofuran, dibenzoselenophene, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene that is optionally further substituted. wherein Y is selected from the group consisting of: A-B-C-D- A-B-C- A-B- wherein A, B, C, and D are independently selected from the compound of formula II: wherein X 1 is selected from the group C-R 1 and N; X 2 is selected from the group C-R 2 and N; X 3 is selected from the group C-R 3 and N; X 4 is selected from the group C-R 4 and N; X 5 is selected from the group C-R 5 and N; X 6 is selected from the group C-R 6 and N; X 7 is selected from the group C-R 7 and N; X 8 is selected from the group C-R 8 and N; X 1 is selected from the group C-R 1 and N; X 2 is selected from the group C-R 2 and N; X 3 is selected from the group C-R 3 and N; X 4 is selected from the group C-R 4 and N; X 5 is selected from the group C-R 5 and N; X 6 is selected from the group C-R 6 and N; X 7 is selected from the group C-R 7 and N; X 8 is selected from the group C-R 8 and N; wherein A, B, C, and D are connected to each other through C-N bonds, where N is a nitrogen in the 9-position; wherein at least one of A, B, C, and D is not carbazole; wherein X is an aryl or heteroaryl linker that is optionally further substituted; wherein Y and X are connected through a C-N bond, where N is the nitrogen in the 9-position in one of A, B, C, and D; wherein R 1, R 2, R 3, R 4, R 5, R 6, R 7, and R 8 are independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; and wherein Z is selected from the group consisting of dibenzothiophene, dibenzofuran, dibenzoselenophene, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene that is optionally further substituted.