A compound having the structure of Formula I is disclosed, where Ga has the structure and Gb has the structure In the structure of Formula I, Z is selected from Si and Ge; Xa and Xb are independently selected from the group consisting of O, S, and Se; the Y and Z atoms forming the six-membered rings in Formula I, Ga, and Gb are each independently selected from C or N; each R1, R2, Ra1, Ra2, Rb1, and Rb2 is independently selected from a variety of substituents; at least one Ra1 is La-Aa; at least one Rb1 is Lb-Ab; Aa and Ab are each independently selected from carbazole, dibenzofuran, dibenzothiophene, dibenzoselenophene, triphenylene, and nitrogen-substituted variants thereof, which are optionally further substituted; and La and Lb are each independently an organic linker. Formulations and devices, such as an OLEDs, that include the compound of Formula I are also described.