Blue emitting halogen–phenoxy substituted 1,8-naphthalimides for potential organic light emitting diode applications

• A series of 1,8-naphthalimides are synthesized and characterized.
• They show PL in blue region with good chromaticity and high Stokes’ shifts.
• They have high melting points and decomposition temperatures.
• They have low-lying HOMO–LUMO energy levels.
• Electron-transporting behavior was revealed by unoptimized OLEDs.

In this paper, we report the synthesis and characterization of six 1,8-naphthalimides [4a–4c and 5a–5c] obtained by the substitution of electron donating halogen–phenoxy groups at the C-4 position. The derivatives were characterized using 1H NMR, 13C NMR, mass spectra, FT-IR, single crystal XRD; photo-physical, thermal, surface morphological and electrochemical properties were also investigated. The derivatives exhibit deep blue photoluminescence in the range 414–423 nm (in CHCl3) and 457–466 nm (in thin film state) on UV excitation with high Stokes’ shifts and good chromaticity. The TGA and DSC analysis showed that the derivatives possess good thermal stability (271–284 °C) and melting points (138–201 °C). The HOMO and LUMO energy levels estimated by cyclic voltammetry are in the range 6.21–6.34 eV and 3.31–3.41 eV respectively corresponding to energy band gaps of 2.98–3.15 eV. These energy values are relatively higher than the commonly used electron transporting materials. The optical and electronic properties of the derivatives were tuned by the introduction of different electron donating halogen–phenoxy groups through C-4 position of the naphthalimide moiety. The emissive and electron-transporting properties of the naphthalimide derivative 4a were studied by fabricating a bi-layer and tri-layer devices. Further a phosphorescent device with 4a as electron transport layer (ETL) exhibited superior performance than the device without any ETL and was comparable with the device using standard Alq3 as ETL. These results indicate that the synthesized naphthalimide derivatives could play an important role in the development of OLEDs.

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