Influences of the number of 2-ethylhexylamine chain substituents on electron transport characteristics of core-substituted naphthalene diimide analogues

Abstract

We designed and synthesized a series of naphthalenediimide (NDI) derivatives through core-substitution (coded as cNDI) with various number of 2-ethyl-hexylamine (EHA) chains at different positions. The molecular structure of cNDI derivatives such as cNDI-1EHA, cNDI-2EHA, cNDI-3EHA and cNDI-4EHA bearing one, two, three and four 2-ethyl-hexylamine chains, respectively, was confirmed by different spectroscopic techniques such as FTIR, ¹H-NMR, ¹³C-NMR spectroscopy and mass spectrometry. Interestingly, the incorporation of different numbers of 2-ethyl-hexylamine on electron-deficient cNDI yields diverse photophysical and electrochemical properties. The change in the number of alkyl chains on the NDI core significantly influences the redox properties and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The changes in the morphology of the spin-coated films before and after annealing are reorganized differently depending on the number of 2-ethyl-hexylamine topology proved by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The electron mobility of cNDIs was examined by following the standard protocol of the space-charge limiting current (SCLC) method. The NDI derivatives bearing various number of 2-ethyl-hexylamine chains at the NDI core after thermal treatment at 170 °C exhibited very good electron mobility of the order of 10⁻⁶ to 10⁻⁴ cm² V⁻¹ s⁻¹. The observed electron mobility trends depend not only on the number of 2-ethyl-hexylamine substituents but also on the changes in thin-film morphology.