Jarugu Narasimha Moorthy

Mechanochemical catalytic oxidations in the solid state with in situ-generated modified IBX from 3,5-di-tert-butyl-2-iodobenzoic acid (DTB-IA)/Oxone

IBX is an indispensable reagent in contemporary organic oxidation chemistry, despite its explosive and insoluble attributes. The latter drawbacks can be overcome if a more reactive modified version of IBX can be generated in situ catalytically. We show that the sterically-crowded reactive I(V) species can be generated catalytically from the precursor 3,5-di-tert-butyl-2-iodobenzoic acid (DTB-IA) in the presence of a terminal oxidant, i.e., Oxone. The highly reactive modified IBX, i.e., I(V) species, generated in situ from DTB-IA in the presence of Oxone under mechanochemical ball milling conditions, permits unprecedented oxidation of alcohols to carbonyl compounds, vicinal diols to oxidative cleavage products, and non-vicinal diols to lactones in the solid state. Indeed, the results constitute first demonstration of catalytic oxidations in the solid state using a modified o-iodobenzoic acid, an I(I) species.

Helicity-Dependent Regiodifferentiation in the Excited-State Quenching and Chiroptical Properties of Inward/Outward Helical Coumarins

Influence of helicity on the excited-state as well as chiroptical properties of two sets of regiohelical coumarins that are differentiated by inward and outward disposition of the pyran-2-one ring has been investigated. A subtle difference in the helicities manifests in divergent excited-state properties and significant differences in the dipole moments. The latter permit heretofore unprecedented regiodifferentiation in the O-H⋅⋅⋅O hydrogen-bond assisted electron-transfer quenching by phenols. Furthermore, the enantiopure hexahelical coumarins exhibit strong Cotton effects and lend themselves to a very high differentiation in the specific rotations and anisotropic dissymmetry factors. The specific rotation observed for 6-in turns out being the highest of the values reported for all hexahelicenes reported so far.

New Insights into an Old Problem. Fluorescence Quenching of Sterically-Graded Pyrenes by Tertiary Aliphatic Amines

Although the quenching of singlet-excited states of aromatic molecules by amines has been studied for several decades, important aspects of the mechanism(s) remain nebulous. To address some of the unknowns, steric, and electronic factors associated with the quenching of the singlet-excited states of three electronically related aromatic molecules, pyrene, 1,3,6,8-tetraphenylpyrene (TPPy), and 1,3,6,8-tetrakis(4-methoxy-2,6-dimethylphenyl)pyrene (PyOMe), by a wide range of tertiary aliphatic amines have been assessed quantitatively. Correlations among the steric and electronic properties of the amines and the pyrenes (e.g., sizes, shapes, conformational labilities, excitation energies, and oxidation or reduction potentials) have been used in conjunction with the steady-state and dynamic fluorescence quenching data and DFT calculations on the ground and excited state complexes to make quantitative assessments of the steric and electronic factors controlling the quenching processes. PyOMe is a rather rigid bowl-like molecule that, in its electronic ground state, does not make stable complexes with amines in solution. TPPy has a shallower bowl-like shape that is much more flexible. Experiments conducted with a crystalline ground-state complex of an amine and PyOMe demonstrate (as assumed in many other studies but not shown conclusively heretofore) that the geometry needed for quenching the excited singlet state of PyOMe must place the lone-pair of electrons of the amines over the π-system of the pyrenyl group. Furthermore, there is a significant dependence on the shape and size of the amine on its ability to quench PyOMe, but not on the less conformationally constrained TPPy. The conclusions obtained from these studies are clearly applicable to a wide variety of other systems in which fluorescence from an aromatic moiety is being quenched, and they provide insights into how weak host-guest pairs interact.

Carbo[5]helicene versus planar phenanthrene as a scaffold for organic materials in OLEDs: the electroluminescence of anthracene-functionalized emissive materials

Aesthetically enticing helical structures are inextricable systems in biology, and have rapidly begun to pervade several aspects of materials science. The entry of helicenes in electroluminescent materials is very recent. In the present study, we have designed and synthesized two twisted carbo[5]helicenes functionalized with fluorescent phenylanthryl moieties, namely, CHMANT and CHDANT, for their applications as blue-emissive materials in nondoped OLED devices. Their lower analogs, i.e., PMANT and PDANT, based on a planar phenanthrene scaffold were also designed and synthesized to contrast the effect of helical versus planar cores in the development of emissive materials based on phenylanthracenes. It is shown that the helicenes functionalized with phenylanthryl group(s) not only display better fluorescence properties and thermal stabilities than analogous phenanthrenes, but also serve as superior emissive materials as well as host materials in electroluminescent devices. The comparative results bring out the fact that helicity (carbo[5]helicene) as a design element fares much better than a planar scaffold (phenanthrene) in developing anthracene-based emissive materials for OLEDs. The brightness (9820 cd m−2) and power and luminous efficiencies (3.48 lm W−1 and 4.22 cd A−1) obtained from a nondoped device fabricated with CHDANT are the highest by quite some margin among those of all helical materials reported so far.

Influence of Cations on the Fluorescence Quenching of an Ionic, Sterically Congested Pyrenyl Moiety by Iodide in Water

Quenching of the excited singlet states of a water-soluble, sterically congested tetraarylpyrene, 1,3,6,8-tetrakis(2,6-dimethyl-4-(α-carboxy)methoxyphenyl)pyrene (Py4C), by a series of iodide salts has been investigated by steady-state and time-resolved fluorescence measurements. Access to the pyrenyl group of Py4C is restricted sterically as a result of the four flanking (2,6-dimethylphenoxy)acetic acid groups and the energy costs associated with their rotation. Deprotonation of the carboxylic acid groups of Py4C permits examination of ion-ion electrostatic interactions on the rates of quenching by iodide salts in which different steric and electrostatic factors are introduced by varying the cationic portions. At the same concentrations and with the same cations, chloride anions are ineffective quenchers. The quenching rate constants of Py4C by iodide are found to correlate linearly with the ionic radii of the cations and their enthalpies of hydration. These correlations are discussed in terms of the Hofmeister series. Furthermore, the results indicate that the cations that flank Py4C decrease the quenching efficiency of iodide through polarization and shielding effects (i.e., lowering the effective charge), which isolate to varying degrees the π-system. The effects of the different cations on quenching the fluorescence of a simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), by iodide are much smaller. Overall, the results with Py4C indicate that the fluorescence quenching efficiency by iodide is influenced by direct interactions with the cations associated with the carboxylate groups of Py4C and not the solvation of water molecules. This observation is germane to a topic of current debate: Are the effects of the cations more closely related to bulk water properties or to direct ion-ion interactions? The conclusions obtained from these studies are applicable clearly to a wide variety of other systems in which ion pairing influences cooperative or inhibitory interactions.

Small molecular hole-transporting materials (HTMs) in organic light-emitting diodes (OLEDs): structural diversity and classification

Hole-transporting materials (HTMs) are integral to the construction of a wide variety of state-of-the-art semiconductor devices today. Insofar as display and lighting technologies based on organic light-emitting diodes (OLEDs) are concerned, HTMs are indispensable. Radical development of the area of OLEDs within the last two decades has led to creation of innumerable number of HTMs that are structurally diverse. A better understanding of the structural attributes and structure–property relationships is pivotal for improving the device performance results significantly. We have endeavored to collate, consolidate and discuss the structural milieu, classification, physical properties and electroluminescence data of diverse HTMs. Although our focus of the collection of HTMs is limited to those applied in OLEDs, the inferences drawn from an incisive scrutiny of the chosen structural landscape of HTMs and associated properties are applicable to research on solar cells, field effect transistors, photovoltaics, etc. Consolidated data such as HOMO–LUMOs, Tgs, Tds, hole mobilities, etc. of the HTMs should serve as an invaluable boon to the researchers working in interdisciplinary fields of chemistry, materials science, electrical engineering and physics. Indeed, the review is envisaged to serve as a concise resource of HTMs with important and necessary data.

Nitrogen-Free Bifunctional Bianthryl Leads to Stable White-Light Emission in Bilayer and Multilayer OLED Devices

White organic light-emitting diodes (WOLEDs) are at the center stage of OLED research today because of their advantages in replacing the high energy-consuming lighting technologies in vogue for a long time. New materials that emit white light in simple devices are much sought after. We have developed two novel electroluminescent materials, referred to as BABZF and BATOMe, based on a twisted bianthryl core, which are brilliantly fluorescent, thermally highly stable with high Td and Tg, and exhibit reversible redox property. Although inherently blue emissive, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15u2009600 cd/m2 in the fabricated multilayer nondoped OLED device. This device exhibited excellent color stability over a range of applied potential. Remarkably, similar white-light emission was captured even from a double-layer device, attesting to the innate hole-transporting ability of BABZF despite it being non-nitrogenous, that is, lacking any traditional hole-transporting di-/triarylamino group(s). Similar studies with BATOMe led to inferior device performance results, thereby underscoring the importance of dibenzofuryl groups in BABZF. Experimental as well as theoretical studies suggest the possibility of emission from multiple species involving BABZF and its exciplex and electroplex in the devices. The serendipitously observed white-light emission from a double-layer device fabricated with an unconventional hole-transporting material (HTM) opens up new avenues to create new non-nitrogenous HTMs that may lead to more efficient white-light emission in simple double-layer devices.