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

Abstract

We designed and synthesized a series 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-thyl-hexylamine chain, respectively is confirmed by different spectroscopic techniques such as FTIR, 1H-NMR, 13C-NMR spectroscopy and Mass spectrometry. Interestingly, incorporation of different numbers of 2-ethyl-hexylamine on electron deficient cNDI yields diverse photophysical and electrochemical properties. The change in number of alkyl chain on NDI core significantly influences the redox properties and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The crystal packing and changes in 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 were examined following the standard protocol of space-charge limiting current (SCLC) method. The NDI derivatives bearing various number of 2-ethyl-hexylamine chain at NDI core with thermal treatment at 170 degrees C exhibits very good electron mobility of the order of 10-6 to 10-4 cm2/Vs. The observed electron mobility trends are not only depends on number of 2-ethyl-hexylaminesubstituents but also on the changes in thin-film morphology.