Faculty Publications
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Item Blue organic light emitting materials: Synthesis and characterization of novel 1,8-naphthalimide derivatives(Elsevier B.V., 2014) Ulla, H.; Garudachari, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.A series of naphthalimide derivatives were designed and synthesized by substituting electron-donating phenoxy groups at the 4th position of 1,8-naphthalimide. Photophysical, thermal, electrochemical properties of the synthesized derivatives were studied. The photophysical studies revealed that by varying the substituents at the 4th position of the 1,8-naphthalimide backbone, the photoluminescence spectra can be readily tuned in the range 410-423 nm (solution) and 457-468 nm (thin film). The derivatives have high Stokes' shifts and the Commission Internationale de l'Eclairage (CIE) coordinates are positioned in the deep blue region of the chromaticity diagram. Thermal analysis showed that the melting points are in the range 135-270 C with good thermal stability of 260-275 C. Electrochemical studies show the derivatives to have low-lying energy levels revealing that they possess good electron-transporting and hole-blocking properties. The ionization potentials and electron affinity are in the region of 6.30-6.36 eV and 3.31-3.43 eV, respectively, with energy band-gaps in the range 2.93-3.0 eV. The studies reveal that these energy values are relatively higher than the commonly used electron transporting materials. Hence these derivatives are potential candidates not only as electron transporting but also as hole blocking blue emitters for organic light-emitting diode applications. © 2013 Elsevier B.V. All rights reserved.Item Blue emitting halogen-phenoxy substituted 1,8-naphthalimides for potential organic light emitting diode applications(Elsevier B.V., 2014) Ulla, H.; Raveendra Kiran, M.R.; Garudachari, B.; Satyanarayan, M.N.; Umesh, G.; Isloor, A.M.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; photophysical, 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. © 2014 Elsevier B.V. All rights reserved.Item Pyrene-Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials(American Chemical Society service@acs.org, 2016) Chidirala, S.; Ulla, H.; Valaboju, A.; Raveendra Kiran, M.R.; Mohanty, M.E.; Satyanarayan, M.N.; Umesh, G.; Bhanuprakash, K.; Rao, V.J.Three pyrene-oxadiazole derivatives were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and electron transporters in three different device configurations, involving hole-injection/hole-blocking materials that showed good current and power efficiencies. To understand the underlying mechanisms involved in the application of these molecules as emitters and transporters, a detailed photophysical characterization of molecules 4-6 was carried out. The absorption, steady-state fluorescence, phosphorescence, fluorescence lifetime, and phosphorescence lifetime measurements were carried out. The high quantum yield and efficient reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a good emitter, and the charge delocalization properties leading to excimer formation make them efficient electron transporters. Isoenergetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in the absence of thermal activation. © 2015 American Chemical Society.Item Blue emitting 1,8-naphthalimides with electron transport properties for organic light emitting diode applications(Elsevier B.V., 2017) Ulla, H.; Raveendra Kiran, M.R.; Garudachari, B.; Ahipa, T.N.; Tarafder, K.; Vasudeva Adhikari, A.; Umesh, G.; Satyanarayan, M.N.In this article, the synthesis, characterization and use of two novel naphthalimides as electron-transporting emitter materials for organic light emitting diode (OLED) applications are reported. The molecules were obtained by substituting electron donating chloro-phenoxy group at the C-4 position. A detailed optical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical calculations (DFT) were performed to get a better understanding of the electronic structures. The synthesized molecules were used as electron transporters and emitters in OLEDs with three different device configurations. The devices with the molecules showed blue emission with efficiencies of 1.89 cdA-1, 0.98 lmW?1, 0.71% at 100 cdm-2. The phosphorescent devices with naphthalimides as electron transport materials displayed better performance in comparison to the device without any electron transporting material and were analogous with the device using standard electron transporting material, Alq3. The results demonstrate that the naphthalimides could play a significant part in the progress of OLEDs. © 2017 Elsevier B.V.Item Enhancement in fluorescence quantum yield of MEH-PPV:BT blends for polymer light emitting diode applications(Elsevier B.V., 2018) K M, K.M.; Satyanarayan, M.N.; Umesh, G.We have investigated the effect of blending electron deficient heterocycle Benzothiadiazole (BT) on the photo-physical properties of conjugated polymer Poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Quantum yield (QY) value has been found to increase from 37% for pure MEH-PPV to 45% for an optimum MEH-PPV:BT blend ratio of 1:3. This can be attributed to the efficient energy transfer from the wide bandgap BT (host) to the small bandgap MEH-PPV (guest). The FTIR spectrum of MEH-PPV:BT blended thin film indicates suppression of aromatic C-H out-of-plane and in-plane bending, suggesting planarization of the conjugated polymer chains and, hence, leading to increase in the conjugation length. The increase in conjugation length is also evident from the red-shifted PL spectra of MEH-PPV:BT blended films. Single layer MEH-PPV:BT device shows lower turn-on voltage than single layer MEH-PPV alone device. Further, the effect of electrical conductivity of PEDOT:PSS on the current-voltage characteristics is investigated in the PLED devices with MEH-PPV:BT blend as the active layer. PEDOT:PSS with higher conductivity as HIL reduces the turn on voltage from 4.5 V to 3.9 V and enhances the current density and optical output in the device. © 2018 Elsevier B.V.Item Capacitance and impedance spectroscopy studies of polymer light emitting diodes based on MEH-PPV:BT blends(Elsevier Ltd, 2019) K M, N.K.; Sterin, N.S.; Das, P.P.; Umesh, G.; Satyanarayan, M.N.Light emitting polymer poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is blended with a wide bandgap electron transport material benzothiadiazole (BT) and its effect on the electronic properties has been studied by capacitance and impedance spectroscopy (IS) in PLEDs. The impedance data is fitted using equivalent circuit models and the minimum parallel resistance (Rp) at zero bias have been obtained for 1:3 ratio of MEH-PPV:BT blended devices. The negative capacitance (NC) shows the occurrence of the trap-assisted non-radiative recombination mechanism at low frequencies in the unblended MEH-PPV PLEDs. Further, this behavior is seen to be reduced in PLEDs with MEH-PPV:BT blends. This clearly suggests that the blending of MEH-PPV and BT at different weight ratios results in the suppression of trap-assisted recombination. This can be attributed to the elimination of trap states due to the dilution of semiconductor material on account of the addition of wide bandgap host material. Moreover, the blended devices have shown a significant improvement in the conductivity at small bias voltages. © 2019 Elsevier B.V.