The impact of ligands on oligo(phenylene ethynylene) stability: a first-principles investigation

Abstract

This study utilizes density functional theory (DFT) calculations at the B3LYP/6-311++G(2df,2p)//B3LYP/6-31G(d,p) level to assess the stability and infrared (IR) spectral characteristics of oligo(phenylene ethynylene) (OPE) and its derivatives, which include substituents such as -CH₃, -NH₂, -Cl, -CN, and -NO₂. The results of structural optimization reveal that OPE and its derivatives are stable, as evidenced by the absence of imaginary frequencies. This confirms that the optimized molecular structures correspond to minima on the potential energy surface. Molecules containing -NH₂ and -NO₂ ligands displayed multiple stable symmetry point groups. Specifically, -NH₂ exhibits C2v symmetry, while -NO₂ showed C1 symmetry, both of which demonstrates the lowest energies and, consequently, the highest structural stability. This finding highlights that the careful selection and attachment of ligands not only optimize electronic properties but also significantly enhance structural stability, thereby increasing the practical applicability of OPE molecular dimers in molecular electronic devices. Additionally, the computed IR spectra reveal distinctive vibrational modes for each substituent, with notable increases in vibrational intensities occurring upon the incorporation of substituents into OPE. Overall, these results confirm the structural robustness of OPE molecules and aid in the identification of characteristic functional groups within the oligo structures.