Theoretical investigations of the electronic structures of carbazole-based triphenylphosphine oxide derivatives, potential bipolar host materials in blue-phosphorescent devices

Density functional theory calculations were performed to investigate the electronic structures of bipolar host materials comprising a backbone of linked acceptor moieties where each acceptor was also linked to a pendant donor moiety. The acceptor was triphenylphosphine oxide with two of its phenyls substituted with fluorine atoms or nitrile groups (CN). The donor was carbazole (CZ) substituted, or not, with t-butyl groups. The HOMO and LUMO energy levels of these host molecules were mainly influenced by their respective hole-and electron-transport units. The t-butyl substituents on the CZ moieties had an adverse effect on the triplet energies (E-T) of the host molecules, especially for molecules where the phenyls of the backbone chain were substituted with CN groups. While introducing CN substituents onto the backbone chain decreased the energy difference between the lowest singlet and triplet excited states (Delta E-ST), it also caused the energy gap between the HOMO and LUMO to narrow. Among the host molecules investigated, that in which one of the phenyls in the acceptor moiety was linked to the donor while the other two phenyls in the acceptor were substituted with CN substituents exhibited the highest E-T, balanced charge transport, a low charge-injection barrier, and a small Delta E-ST, and is therefore a promising candidate host material for use in blue-phosphorescent devices.