Substituent effects on the geometric and electronic properties of tetracyano-p-quinodimethane (TCNQ): a theoretical study

Electron acceptors are classes of molecules that are important in organic devices as they help to improve the conductivity of organic semiconducting molecules by forming p-type complexes or anion radical complexes. These molecules can be doped into hole transporting materials to provide good ohmic contact with the anode and to improve the carrier density of the hole transport layer. This results in organic light-emitting devices with low driving voltages and high power efficiencies. In this study, we investigate a series of tetracyano-p-quinodimethane derivatives with substituents expected to facilitate the electron-acceptor capabilities of the quinones using density functional theory (DFT) and time-dependent DFT (TDDFT). As expected, the cyano substitution stabilises both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and increases the adiabatic electron affinity of the dopants. The stabilisation effect on the LUMO levels is greater, and as a result the narrowing of the HOMOLUMO gap is seen. This fact was further confirmed by TDDFT studies, energy of the computed S1S0 transition red shifted upon CN substitution. However, perturbation to the ground-state geometry is negligible, and all anionic structures exhibit aromatisation independent of substitution. Our study suggests that the substituted derivatives reported herein show promise as conductivity dopants.