Faculty Publications
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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 Efficient non-doped bluish-green organic light emitting devices based on N1 functionalized star-shaped phenanthroimidazole fluorophores(Elsevier B.V., 2018) Tagare, J.; Ulla, H.; Satyanarayan, M.N.; Sivakumar, S.In this paper, two star-shaped fluorescent phenanthroimidazole fluorophores, tris(4-(1-(3-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIMCFTPA) and tris(4-(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIPCFTPA) with a D–?–A structure were designed and synthesized by attaching a hole-transporting triphenylamine and an electron transporting phenanthroimidazole moiety. A detailed photophysical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical investigations (DFT) were performed to get a better understanding of the electronic structures. In particular, PIMCFTPA shows blue shifted emission due to the most twisted conformation and reduced intermolecular interaction as compared with PIPCFTPA. Both the fluorophores exhibit high glass transition temperatures and high thermal stabilities with decomposition temperatures up to 377 °C. The excellent stability renders them promising materials in electroluminescent devices. Non-doped organic light-emitting devices (OLEDs) using these fluorophores as emissive materials display bluish green emissions with high efficiency (6.58 cdA?1, 5.91 lmW?1, 3.62% at 100 cdm?2), low turn-on voltages (2.83 V) (PIMCFTPA) and excellent spectral stability. Our results suggest that the molecular design strategy of integrating TPA with phenthroimdazole play 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.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 Fluorescent MoS2 Quantum Dot-DNA Nanocomposite Hydrogels for Organic Light-Emitting Diodes(American Chemical Society service@acs.org, 2020) Pandey, P.K.; Ulla, H.; Satyanarayan, M.N.; Rawat, K.; Gaur, A.; Gawali, S.; Hassan, P.A.; Bohidar, H.B.In this study, we report the synthesis of water-soluble MoS2 quantum dots (MoS2, QD) by a hydrothermal one-step method. These QDs were mixed in an aqueous solution of 2 kbp DNA to form fluorescent nanocomposite hydrogels at a very low concentration of the nucleic acid (1.0% (w/v), normal gelation occurs at 2% (w/v)). The melting temperature Tmelt of these gels was 50 ± 2 °C while the hydrogels melt at 40 ± 2 °C, and the low-frequency storage modulus/gel strength G0 was 40 ± 2 Pa (9 ± 2 Pa for hydrogel). This clearly implied that MoS2 acted as a pseudo-cross-linker in the nanocomposite hydrogel formation. The remarkable synergy of interaction between DNA and QDs can be gauged from the fact that the gel strength and melting temperature increased with QD content regardless of the fact that both carried negative charge. Dynamic light scattering studies showed arrested dynamics at the onset of gelation, and the gel transition time or ergodicity breaking time ?EB decreased with the increase in QD concentration. Small-angle X-ray scattering data captured the internal structure of these gels. Thus, we have a unique nanocomposite DNA-based hydrogel that is fluorescent, and in 2-D, this soft matter remarkably exhibits the behavior of an organic light-emitting diode (OLED), which imparts sufficient novelty to this work. © © 2020 American Chemical Society.Item Design, Synthesis and Characterization of N-Substituted Heteroaromatics: DFT-Studies and Organic Light Emitting Device Application(Wiley-Blackwell info@wiley.com, 2020) Mohan, M.; John, R.; Satyanarayan, M.N.; Trivedi, D.R.Schiff's base condensation of 4,5-Dimethyl-1,2-phenylenediamine with various aldehydes namely fluorene-2-carboxaldehyde (MBF), N-ethyl-3-carboxaldehyde (MBE), 8-hydroxyjulolidine-9-carboxaldehyde (MBJ) and N-(4-formylphenyl) carbazole (MBB) functioning as light emitters have been synthesized. Solid-state emission reveals the exhibition of variant fluorescent color achieved by mere variation in the peripheral group attached to the core. Amongst the designed system, MBB showed a solid-state blue light emission with its emission peak centered at 453 nm. The fluorescence quantum yield of MBB displayed value of 44.82% in the solution state. Electrochemical studies on MBB estimated a HOMO energy level at ?5.7 eV and LUMO at ?3.2 eV. OLED realized with MBB as an active emitter material was successful in the generation of bluish-green emission with a maximum brightness of 280 cd/m2. Current efficiency of 2 cd/A and a power efficiency of 0.18 lm/W was observed for the fabricated device. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimItem Imidazole-Pyrene Hybrid Luminescent Materials for Organic Light-Emitting Diodes: Synthesis, Characterization & Electroluminescent Properties(Elsevier B.V., 2021) Umasankar, G.; Ulla, H.; Madhu, C.; Gontu, G.R.; Shanigaram, B.; Nanubolu, J.B.; Bhanuprakash, B.; Karunakar, G.V.; Satyanarayan, M.N.; Rao, V.J.A series of multichromophoric, pyrene-imidazole-phenyl based hybrid luminescent small molecules (PA, PI, PnB, PtB, PoM and PnDM) with different donating groups at the para position of the phenyl attached to N1 position of imidazole moiety were designed, synthesized, and characterized, for use in blue organic light-emitting diodes (OLEDs). The photophysical, thermal and electrochemical properties of the molecules were systematically investigated. All the molecules displayed delayed fluorescence at room temperature with a lifetime ranging from 7.1 to 8.5 µs. The synthesized fragment molecules (tetraphenyl-imidazoles: IA, IoM and InDM) revealed high triplet energies of ~ 2.90eV. Glass transition temperatures determined to range from 77°C to 123°C and decomposition temperatures are found to be above 280°C. The molecules possess appropriate HOMO and LUMO energy levels for effective charge injection. The crystal structure for PI is reported. OLED devices were fabricated based on the pyrene-imidazole-phenyl hybrids as emitters and as dopants with CBP as host. OLED devices with PI doped (5% wt.) with CBP exhibits excellent device performance with a current efficiency of 9.82 cd/A, the power efficiency of 8.32 lm/W and external quantum efficiency of 4.64%. © 2021Item Design and Synthesis of New Bithiophene Based Planar AIE Red Light Emitters: A Detailed Theoretical and Experimental Analysis**(John Wiley and Sons Inc, 2022) Mohan, M.; Satyanarayan, M.N.; Trivedi, D.R.Bithiophene core with −CN substituted pyrazole and imidazole side chains yielding B1, B2, and B3 have been designed and synthesized viz Schiff's base condensation reaction. Molecules, at their solid-state exhibited red-light emission supporting AIE mechanism. The series exhibited the highest relative quantum yield of value 18.66 % by B1 with the lowest recorded for B3 exhibiting 2.4 %. DFT results reveal the synthesized molecules at their optimized ground state adopt a planar structure. Theoretical calculation further validates the molecules to adopt a slip-stacking type of molecular packing arrangement at its condensed state, conducive for AIE. A large π-π stacking distance between the lattice plane of 3.4 Å, 3.6 Å and 3.5 Å possessed by B1, B2, and B3 respectively, favors AIE. Computational calculation on electron and hole reorganization energy reveals the effective role of two −CN groups in altering the hole mobility in comparison with the other molecules in the series. © 2022 Wiley-VCH GmbH.