Key parameters of the host for long lifetimes in phosphorescent organic light-emitting diodes were investigated by synthesizing three isomeric hosts with a carbazolylcarbazole hole transport moiety and a benzonitrile electron transport moiety. The position of the CN unit was controlled as the ortho, meta and para position to the carbazolylcarbazole to study the relationship between material parameters and lifetime of the phosphorescent organic light-emitting diodes. It was revealed that the bond dissociation energy in the triplet excited state is well-correlated with the device lifetime of the phosphorescent organic light-emitting diodes.