Ramesh Kumar Chitumalla

Blue and Blue-Green Light-Emitting Cationic Iridium Complexes: Synthesis, Characterization, and Optoelectronic Properties

Two new cationic iridium complexes, [Ir(ppy)2(phpzpy)]PF6 (complex 1) and [Ir(dfppy)2(phpzpy)]PF6 (complex 2), bearing a 2-(3-phenyl-1H-pyrazol-1-yl)pyridine (phpzpy) ancillary ligand and either 2-phenylpyridine (Hppy) or 2-(2,4-difluorophenyl)pyridine (Hdfppy) cyclometalating ligands, were synthesized and fully characterized. The photophysical and electrochemical properties of these complexes were investigated by means of UV-visible spectroscopy, emission spectroscopy, and cyclic voltammetry. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations were performed to simulate and study the photophysical and electrochemical properties of both complexes. Light-emitting electrochemical cells (LECs) were fabricated by incorporating complexes 1 and 2, which respectively exhibit blue-green (488 and 516 nm) and blue (463 and 491 nm) emission colors, achieved through the meticulous design of the ancillary ligand. The luminance and current efficiency measurements recorded for the LEC based on complex 1 were 1246 cd m(-2) and 0.46 cd A(-1), respectively, and were higher than those measured for complex 2 because of the superior balanced carrier injection and recombination properties of the former.

Density Functional Theory Study on the Cross-Linking of Mussel Adhesive Proteins

The water-resistant adhesion of mussel adhesive proteins (MAPs) to a wet surface requires a cross-linking step, where the catecholic ligands of MAPs coordinate to various transition-metal ions. Fe(III), among the range of metal ions, induces particularly strong cross-linking. The molecular details underlying this cross-linking mediated by transition-metal ions are largely unknown. Of particular interest is the metal-ligand binding energy, which is the molecular origin of the mechanical properties of cross-linked MAPs. Using density functional theory, this study examined the structures and binding energies of various trivalent metal ions (Ti-Ga) forming coordination complexes with a polymeric ligand similar to a MAP. These binding energies were 1 order of magnitude larger than the physisorption energy of a catechol molecule on a metallic surface. On the other hand, the coordination strength of Fe(III) with the ligand was not particularly strong compared to the other metal ions studied. Therefore, the strong cross-linking in the presence of Fe(III) is ascribed to its additional ability as an oxidant to induce covalent cross-linking of the catecholic groups of MAPs.

Impact of neutral and anion anchoring groups on the photovoltaic performance of triphenylamine sensitizers for dye-sensitized solar cells

Anchor groups play a vital role in dye-sensitized solar cells (DSSCs), acting as a bridge for electron injection from the sensitizers to the metal oxide semiconductor. Carboxylic acids (COOH) are widely used anchors for most sensitizers because of their strong adhesion to the semiconductor surface. Electron injection occurs through this adhesion, which is the main process that initiates the electrical circuit in a DSSC. Owing to the proton (H+) of the COOH anchoring group, the conduction band of the semiconductor is shifted positively after sensitizer adsorption, leading to open circuit photovoltage (VOC) loss. In this study, the triphenylamine-based sensitizer with a carboxylate group (COO−) was synthesized as an anchor. Although the power conversion efficiency of the anionic triphenylamine sensitized solar cells was lower than that of the neutral-base cell due to the decreased amount of dye on the photo-electrode surface, it exhibited an enhanced VOC compared to that of the neutral form. Density functional theory (DFT) and time dependent DFT studies were also carried out to theoretically characterize the two dyes and explore the difference between the carboxylic and carboxylate anchor groups.

Spectral, electrochemical and computational investigations on the host–guest interaction of Coumarin-460 with p-sulfonatocalix[4]arene

The supramolecular host–guest investigation of Coumarin 460 (C460), a salient coumarin family dye molecule is studied with a noteworthy host molecule, p-sulfonatocalix[4]arene (p-SC4). The investigation is carried out by both experimental and theoretical approach. The binding affinity of C460 with p-SC4 is experimentally studied using absorption, emission, excited state lifetime and Cyclic Voltammetry methods. The binding constant is around 103 M− 1, which shows potent binding. The binding stoichiometry is 1:1. The binding orientations and binding energies are studied using computational simulations. The mode of binding is also established using NMR spectral techniques.

Spectral, Electrochemical and Computational Investigations of Binding of n-(4-Hydroxyphenyl)-imidazole with p-Sulfonatocalix[4]arene

The interaction of n-(4-hydroxyphenyl)-imidazole with p-sulfonatocalix[4]arene is studied using fluorescence technique. The quenching of fluorescence intensity explains the efficiency of binding via binding constant and quenching constant. The excited state lifetime of n-(4-hydroxyphenyl)-imidazole is decreased upon interaction with p-sulfonatocalix[4]arene. The cyclic voltametric studies emphasized the interaction of n-(4-hydroxyphenyl)-imidazole with p-sulfonatocalix[4]arene. Quantum chemical calculations are carried out to study the interactions as well as charge transfer between the host and the guest upon complexation. The simulations revealed that the n-(4-hydroxyphenyl)-imidazole interacts with p-sulfonatocalix[4]arene with horizontal orientation with in the p-sulfonatocalix[4]arene cavity.

Synthesis of porphyrin sensitizers with a thiazole group as an efficient π-spacer: potential application in dye-sensitized solar cells

Herein, we report porphyrin sensitizers for DSSCs, coded CNU-OC8 and CNU-TBU, which were synthesized using a donor–π-bridge–acceptor approach. The porphyrin sensitizers were subjected to electrochemical experiments to study their electron distribution, intramolecular charge transfer and HOMO–LUMO levels. The optical and photovoltaic properties of these synthesized porphyrins were measured and compared with those of the YD2-OC8 benchmark dye. To further characterize, we simulated the electrochemical and optical properties of the dyes, which are perfectly in agreement with the experimental data. The new CNU-OC8 and CNU-TBU porphyrin sensitizers provided power conversion efficiencies of 6.49% and 3.19%, respectively, compared to a conversion efficiency of 6.10% for YD2-OC8 under similar conditions. These results indicate that CNU-OC8 exhibits better photovoltaic performance than the benchmark YD2-OC8 sensitizer in a liquid I−/I3− redox electrolyte.

Phenothiazine based blue emitter for light-emitting electrochemical cells

A butterfly shaped phenothiazine based electroluminescent material PPPSO2 was synthesized as an ionic small molecule emitter for light-emitting electrochemical cells (LECs). PPPSO2 shows deep blue emission in solution with high fluorescent quantum yield of 0.49. By employing the active molecule PPPSO2 as an emitter in LECs, the device shows electroluminescence centered at 498 nm with CIE coordinates of (0.21, 0.31) and maximum luminance of 509 cd m−2. The results show that the versatility of phenothiazine based luminescent materials such as high thermal stability and different emission color by tuning the oxidation state makes phenothiazine derivatives a prospective material for light-emitting devices.

Inkjet-Printable Hydrochromic Paper for Encrypting Information and Anticounterfeiting

Developing rewritable papers has gathered immense interest in recent times in view of developing sustainability in print media without exhausting environmental resources. We herein present a rapid and facile procedure for the fabrication of a communication medium by treating the surface of a paper with synthetic organic molecules, after which plain water could be used as an ink to print and reprint numerous times on the treated paper before disposal. Interestingly, as the paper comes in contact with water, the molecules are driven to reorganize in a slip-stacked arrangement. This alters their ground and excited state properties by hydrogen-bond-assisted nonradiative decay, in which the associated changes are visible to the naked eye. The changes evolved are sensitive to the solubility parameter of the solvent and thermally reversible, thus linking the hydrochromic property to the paper. Against a background of concerns over a rise in counterfeiting and leaks of confidential information, prospects for encrypted communications and anticounterfeiting is herein demonstrated.

Substituent effects on the croconate dyes in dye sensitized solar cell applications: a density functional theory study

AbstractUsing the density functional theory (DFT), we studied two model croconate dyes, one with an electron-donating substituent (CR1) and the other with an electron-withdrawing group (CR2). The geometric, electronic, and optical properties of these dyes were compared. Upon switching from CR1 to CR2, a considerable bathochromic shift was observed in the electronic absorption spectrum. We also investigated the adsorption behavior of the two dyes on a TiO2 (101) anatase surface by employing periodic DFT simulations. The periodic electronic-structure calculations revealed that the diketo group of CR1 bound more strongly to the TiO2 surface than that of CR2, with a binding strength comparable to that of a typical organic D–π–A dye. In this work we evaluate in particular the effect of the electron withdrawing/donating nature of the substituent on the electronic, optical, and adsorption properties of the croconate dyes. Finally, we hope that the present study will help in the design of highly efficient dyes for dye sensitized solar cells by considering substituent effects. Graphical abstractEffect of substituent on binding energy and charge transfer