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 Hydrogen bond-driven columnar self-assembly of electroluminescent D-A-D configured cyanopyridones(Royal Society of Chemistry, 2018) Vinayakumara, D.R.; Ulla, H.; Kumar, S.; Pandith, A.; Satyanarayan, M.N.; Shankar Rao, D.S.S.; Prasad, S.K.; Vasudeva Adhikari, A.Herein, we report the design and synthesis of a new series of flying bird-shaped liquid crystalline (LC) cyanopyridone derivatives with a D-A-D architecture, CPO-1 to CPO-4. Their mesomorphic, photophysical, electrochemical, and electroluminescence characteristics have been investigated in detail. Here, the H-bonding interactions through a central lactam core were shown to be the key driving force for their self-assembly into columnar mesophases. The key role of H-bonding has been confirmed by using newly synthesized similar shaped compounds, MCP-1 to MCP-3. New CPO mesogens were found to be intense greenish blue light emitters with narrow band-gap energies. Conclusions were drawn based on theoretical studies also. Finally, the application potential of the selected mesogen CPO-2 as an emissive material has been demonstrated for the fabrication of doped and non-doped OLED devices with different device architectures, which displayed encouraging results. In fact, this is the first report on the use of emissive H-bond-assisted columnar liquid crystals in devices. The present results provide a new guideline and a versatile approach to the design of new LC molecules for the fabrication of efficient OLEDs. © 2018 The Royal Society of Chemistry.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.Item Functionalized pyrene-based AIEgens: synthesis, photophysical characterization and density functional theory studies(John Wiley and Sons Ltd, 2019) Mohan, M.; James, J.; Satyanarayan, M.N.; Trivedi, D.Three new pyrene-based derivatives P1, P2 and P3 with a substituted pyrazole were designed, synthesized and characterized using standard spectroscopic techniques. Ultraviolet–visible (UV–vis) spectroscopic studies for P1–P3 uncovered a finite bathochromic shift of the molecules in solvents of varying polarity. Photoluminescence (PL) studies revealed the significant fluorescence emission of all molecules in higher polar solvents such as MeOH and dimethylformamide (DMF). Fluorescence quantum yield studies demonstrated the importance of P3 possessing cyanofunctionality for imparting higher emission with a quantum yield of 0.36%. Ratiometric studies performed in a tetrahydrofuran (THF)/H2O mixture indicated fluorescence enhancement with increasing overall percentage of water, confirming the aggregation-induced emission effect. Cyclic voltammetry study of molecules P1–P3 revealed an irreversible oxidation peak and the band gaps were calculated to be 2.26 eV for P1 and 2.31 eV for P2 and P3 respectively. Density functional theory (DFT) studies performed on molecules P1–P3 validate the structure correlation of the molecules. Theoretically estimated highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and bandgap correlated well with the experimental values. Furthermore, time-dependent (TD)DFT showed that the major contribution for the electronic transitions occurring in the system was governed by HOMO-1 and LUMO+1 orbitals. © 2019 John Wiley & Sons, Ltd.Item New cyanopyridine based conjugated polymers carrying auxiliary electron donors: From molecular design to blue emissive PLEDs(Elsevier Ltd, 2020) Pilicode, N.; Naik, P.; K M, K.M.; Acharya, M.; Satyanarayan, M.N.; Vasudeva Adhikari, A.V.Three new D-A (Donor-Acceptor) configured conjugated polymers, i.e. PPy1-3, centered on strong electron accepting cyanopyridine scaffold carrying varied auxiliary donors, viz. phenylene (PPy1), biphenyl (PPy2), and fluorene (PPy3) were designed and synthesized as blue emitters for PLEDs. The new polymers were subjected to spectral, thermal, photophysical and electrochemical characterization. Also, computational studies (DFT) were performed on the repeating units of polymer using Turbomole 7.2 V software package at the B3LYP/TZVP hybrid levels. Further, their weight average molecular masses were found to be 38.8 kDa, 38.9 kDa and 57.7 kDa, respectively as determined by GPC technique. Furthermore, the new polymers PPy1-3, were shown to be stable thermally up to 308–374 °C. Evidently, they exhibited good photophysical behavior with their optical energy band gaps of 2.53–2.64 eV. Finally, the polymers PPy1-3 were employed as an active emissive layer in standard ITO/PEDOT:PSS/Polymer/Al configured PLEDs. Interestingly, at 12 V all the newly fabricated devices exhibit a stable blue characteristic electroluminescence with low threshold voltages of 3.40–5.20 V, confirming an efficient injection of electrons in the diodes. From the results, it is clear that, the polymers PPy1-3, can be considered as prospective blue light emitters for PLED application. © 2019 Elsevier Ltd