7,7,8,8-tetracyanoquinodimethane-based molecular dopants for p-type doping of OLEDs: a theoretical investigation

The array of organic conductivity dopants used for organic light-emitting devices (OLEDs) to reduce the operating voltage and improve power efficiency is extremely limited. Here we report a comparative theoretical study between newly proposed analogues and the standard state-of-the-art conductivity dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We used density functional theory to determine the bond lengths, bond angles, and electronic properties, such as the energy of the highest occupied molecular orbital (E(HOMO)) and the lowest unoccupied molecular orbital (E(LUMO)) states. The ground state structures of the proposed molecules were optimized at the B3LYP/6-31G* level. The results show that substitution of one or two fluorine groups in the F4-TCNQ core with a substituted phenyl ring or other electron-withdrawing moieties, will not substantially affect the geometry of the molecule or its electronic ability to accept electrons. The most significant finding was that the phenyl substitutions onto the TCNQ core are nearly perpendicular to the TCNQ plane, and thus there is no electronic communication between the two rings. This is extremely important, as such extension of the π conjugated system would negatively affect the E(LUMO) and thus the electron affinity of the molecule.