Facile synthesis of naphthalene diimide (NDI) derivatives: aggregation-induced emission, photophysical and transport properties

Naphthalene diimide (NDI) derivatives have been synthesized via facile synthesis process and comprehensively studied their photophysical, thermal-activated conductivity and electron transport properties. Four different substituents at imide nitrogen, i.e. 2,6 diisopropyl phenyl (iPrP-NDI), diphenylmethylene (DPM-NDI), 2-nitrophenyl (NO₂P-NDI) and pentafluorophenyl (PFP-NDI) have been studied for their effect on self-assembling, photophysical and electronic properties. Electrochemical analysis has been performed to evaluate their redox properties and calculation of HOMO and LUMO energy levels. These NDI derivatives have been analysed for their aggregation behaviour and aggregation-induced emission (AIE) by absorption and emission spectroscopy in fresh and aged solutions in different polarity solvents without using any external additive. Among all, NO₂P-NDI showed strong AIE property in fresh as well as aged samples. Conductivity of NDI derivatives has been measured as a function of temperature where the highest conductivity ~ 10⁻⁴ S⁻¹ cm⁻¹ was obtained at 200 °C in iPrP-NDI. SEM images clearly showed different types of assembly formation in solid state for all the materials. All the materials showed excellent electron mobility of the order of 10⁻⁴ to 10⁻³ cm² V⁻¹ s⁻¹ measured following the standard protocol of SCLC model. Such NDI materials with excellent photophysical and electronic properties are potential candidates to be used as n-type semiconductor material in organic electronics. NO₂P-NDI which also shows aggregation-induced emission can be used on OLEDs or other bio-medical applications as luminescent material.