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 Tuning the self-assembly and photophysical properties of bi-1,3,4-thiadiazole derivatives through electron donor-acceptor interactions and their application in OLEDs(Royal Society of Chemistry, 2017) Yadav, A.K.; Pradhan, B.; Ulla, H.; Nath, S.; De, J.; Pal, S.K.; Satyanarayan, M.N.; Ammathnadu Sudhakar, A.S.We report several shape anisotropic molecules that contain two centrally placed 1,3,4-thiadiazole units, which vary from each other with respect to the number and length of the flexible chains at the termini. The number, position and length of the peripheral chains connected to the termini showed an impact on the thermal behavior of these compounds. The compounds with two terminal tails exhibited an enantiotropic smectic C phase, whereas the compounds with four terminal tails turned out to be crystalline. Surprisingly, among the compounds with six terminal tails, only the compound with a longer terminal chain exhibited a columnar phase with oblique symmetry. It is also to be noted that only compounds with six terminal chains exhibited gelation in long chain hydrocarbons. The xerogel of the hexacatenar with six n-decyloxy chains showed an entangled network of nanofibers of several micrometers in length. The aggregation behavior of the hexacatenar in the hydrocarbon solvent is mainly supported by the attractive ?-? interactions of the aromatic cores and the van der Waals interactions offered by the peripheral flexible tails. The emission behavior is dependent on the number of peripheral tails and not on the length. Furthermore, one of the hexacatenars exhibited solvatochromic emissive behavior. This molecular design helps in the development of long molecular nanowires with a central conducting core and insulating peripheral sheath, which will be helpful for the application in organic electronic devices. The application potential of the columnar liquid crystal material was tested by the fabrication of organic light emitting diodes (OLEDs) either as a single emissive material or as a guest material in a host polymer. Higher efficiency and brightness were noticed in the host guest OLED, which exhibited a technologically important bright blue emission. © 2017 The Royal Society of Chemistry.Item Star-Shaped Phenanthroimidazole-Triphenylamine-Based Yellow Organic Emitter for Organic Light Emitting Diodes(Wiley-Blackwell, 2017) Tagare, J.; Ulla, H.; Kajjam, A.B.; Satyanarayan, M.N.; Sivakumar, S.In this work, new star-shaped conjugated organic emitter, namely tris(4-(1-(9,9-diethyl-9H-fluoren-2-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIFTPA) with donor-?-acceptor (D-?-A) structure, was designed and synthesized by combining three fluoren-phenanthroimidazole arms into a triphenylamine core and characterized using 1H, 13C NMR and mass spectrometry. The detailed photophysical, thermal, electrochemical and electroluminescence properties were systematically studied. The organic emitter exhibits high glass transition temperature (230 °C) and high thermal stability with decomposition temperature. Electrochemical analysis reveals that the emitter has distinct oxidation and reduction behaviours. Furthermore, theoretical calculations (DFT) were performed to get a better understanding of the electronic structure. The OLEDs using this emitter as emissive material showed good device performance (6.45 cd/A, 4.39 lm/W, and 3.01%) with yellow emission and a low turn-on voltage (2.87 V). The results demonstrate that TPA integrated with phenthroimdazole plays an important role in the device performance. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimItem 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 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 Synthesis, photophysical and electroluminescence studies of new triphenylamine-phenanthroimidazole based materials for organic light emitting diodes(Elsevier B.V., 2018) Tagare, J.; Ulla, H.; Satyanarayan, M.N.; Sivakumar, S.In this work, two star-shaped small conjugated materials, namely tris(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl) amine (PIPTPA) and tris(4-(1-p-tolyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PITTPA) with donor-?-acceptor (D-?-A) structures, were designed and synthesized by combining three phenanthroimidazole arms into an triphenylamine core. A detailed photophysical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical calculations (DFT and TD-DFT) were performed to get a better understanding of the electronic structures. Both the materials were found to exhibit high glass transition temperatures (~ 238 °C) and high thermal stabilities with decomposition temperatures up to 298 °C. OLEDs using these materials as emissive materials showed excellent device performance (7.42 cd A?1, 5.77 lm W?1, 4.14% at 100 cd m?2) with green emission and low turn-on voltages. The results demonstrate that TPA integrated with phenthroimdazole plays an important role in the device performance. © 2017 Elsevier B.V.Item Photophysical and electrochemical properties of organic molecules: Solvatochromic effect and DFT studies(Elsevier B.V., 2018) Mohan, M.; Pangannaya, S.; Satyanarayan, M.N.; Trivedi, D.R.A series of five Schiff base molecules M1, M2, M3, M4 and M5 have been designed and synthesized by aldol condensation reaction. Synthesized molecules have been characterized by standard spectroscopic techniques in order to confirm their structural traits. The solvatochromic behavior of molecules M1 to M5 in solvents of varying polarity were investigated by UV–Vis, fluorescence spectroscopy and supported by TD-DFT calculations. DFT studies performed in the gas phase confirmed the energy stabilized structure of the molecules M1 to M5. Structural characteristics of molecule M2 favored higher fluorescence emission with a quantum yield of 0.35 and a solid-state emission of 512 nm. Fluorescence lifetime measurement of the molecules M1 to M5 exhibited a lifetime of order 2–5 ns. Overall, molecule M2 can find its application in organic light emitting diodes as a non-dopant emitter material. © 2018 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.