From Wires to Cables: Attempted Synthesis of 1,3,5-Trifluorenylcyclohexane as a Platform for Molecular Cables

Multiple molecular wires braided together in a single assembly, termed as molecular cable, are promising next-generation materials for effective long-range charge transport. As an example of the platform for constructing molecular cables, 1,3,5-trifluorenylcyclohexane (TFC) and its difluorenyl analogues (DFCs) were systematically investigated both experimentally (X-ray crystallography) and theoretically (DFT calculations). Although the syntheses of DFCs were successfully achieved, the synthesis of TFC, which involved a similar intramolecular Friedel–Crafts cyclization as the last step, was unsuccessful. An exhaustive study of the conformational landscape of cyclohexane ring of TFC and DFCs revealed that TFC is a moderately strained molecule (∼17 kcal/mol), and computational studies of the reaction profile show that this steric strain, present in the transition state, is responsible for the unusually high (∼5 years) reaction half-life. A successful synthesis of TFC will require that the steric strain is introduced in multiple steps, and such alternative strategies are being currently explored

Charge-Transfer or Excimeric State? Exploring the Nature of The Excited State in Cofacially Arrayed Polyfluorene Derivatives

It is well known that upon electronic excitation various π-stacked dimers readily exhibit excimer formation, facilitated by a perfect sandwich-like arrangement between the chromophores. However, it is unclear whether such a dimer is also capable of electron transfer upon excitation, if a strong electron-donating group is covalently attached. In this work, we probe the nature of the excited state in a series of cofacially arrayed polyfluorene derivatives with electron-rich aromatic donor attached via a methylene linker. Our studies show that in all cases excimer formation is energetically favorable, and promotion of a charge-transfer state in such systems is possible but requires a free energy for electron transfer far exceeding 1 V. These findings shed light on important design principles for molecular scaffolds capable of stabilizing both excimeric and charge-transfer states upon their excitation

Preliminary studies directed towards the design and synthesis of electro-active fluorene and calixarene derivatives

We have designed and synthesized a modified calixarene derivative that allows, for the first time, the isolation of a stable cation radical salt that binds a single molecule of nitric oxide deep within its cavity with remarkable efficiency (KNO \u3e108 M-1 ), as demonstrated by isolation of a crystalline complex [Calixarene derivatives, NO]+ and its characterization by X-ray crystallography as well as by optical spectroscopy. Furthermore, the ready accessibility of the calixarene cation radical will allow the exploration of its use for developing efficient sensing devices for nitric oxide based on the accompanied color changes. Interactions between aromatic rings via π-stacking are at the origin of many phenomena of organic material science and biological chemistry including the electron transport in DNA through stacked π-bases. Moreover, owing to the synthetic difficulties, the examples of such π-stacked molecules with multiple layers are scarce. Recently discovered multi-layered π-stacked polyfluorenes in which the van der Waals contact between the cofacially-oriented fluorene moieties allows effective electronic coupling amongst them (F1-F6). These stacked p-systems provide the unique opportunity to investigate the electron and energy transport phenomenon through stacked aromatic systems as spacers for the development of (next-generation) conducting wire-like materials for potential applications in the emerging field of molecular electronics and nanotechnology

Preliminary Studies Directed towards the Design and Synthesis of Electro-Active Fluorene and Calixarene Derivatives

Nitric Oxide as a Biological Messenger. The Nobel Prize in Medicine in year 1998 was awarded to Murad, Furchgott, and Ignarro for the discovery of the signaling properties of nitric oxide - a small diatomic radical. The flurry of activities made in last two decades showed that NO could be synthesized by mammalian cells and could act as a physiological messenger as well as a cytotoxic agent and thus has increased the importance of its detection. For example, (i) The endothelium (inner lining) of blood vessels use nitric oxide (synthesized by endothelial NO synthase eNOS ) to signal the surrounding smooth muscles to relax and thus dilate the arteries which allow increase in blood flow. The nitroglycerin, amyl nitrate and other nitrate derivatives that are used in the treatment of heart diseases, such as angina, are now believed to function by release of NO from these nitrated material that reduce myocardial wall tension and are also believed to reduce arteriolar resistance that in turn decrease myocardial oxygen demand. (ii) Macrophages are types of cells in the immune system that produce nitric oxide by the aid of inducible nitric oxide synthase (iNOS) to kill invading bacteria.5 Under certain conditions, the uncontrolled release of nitric oxide in these cells can prove to be highly undesirable..

Soluble cycloannulated tetroxa[8]circulane derivatives: Synthesis, optical and electrochemical properties, and generation of their robust cation-radical salts

Syntheses of soluble bicycloalkane-annulated tetraoxa[8]circulane derivatives via Lewis acid-catalyzed tetramerization of readily available cycloannulated benzoquinons is described. The ready availability of the soluble circulane derivatives allows the evaluation of their optical and electrochemical properties. These circulanes form stable (isolable) cation-radical salts upon 1-electron oxidation using antimony pentachloride and various aromatic oxidants. © 2004 Elsevier Ltd. All rights reserved

Design, Synthesis, Electronic Properties, and X-ray Structural Characterization of Various Modified Electron-Rich Calixarene Derivatives and Their Conversion to Stable Cation Radical Salts

We have designed and synthesized electron-rich calixarene derivatives, which undergo reversible electrochemical oxidation in a well-accessible potential range that allows the ready preparation and isolation of the corresponding cation radicals. Preparation of mono- or tetra-radical cation can be achieved by using stable aromatic cation-radical salts such as MA+•, MB+•, and NAP+• as selective organic oxidants. The cation radicals of calixarenes are stable indefinitely at ambient temperatures and can be readily characterized by UV-vis-NIR spectroscopy. These cation radicals bind a single molecule of nitric oxide within its cavity with remarkable efficiency

Synthesis, structure, and evaluation of the effect of multiple stacking on the electron-donor properties of π-stacked polyfluorenes

A versatile synthesis of π-stacked polyfluorenes is described. These polyfluorenes retain their cofacial conformations both in solution and in the solid state as was judged by NMR spectroscopy and X-ray crystallography. The experimental electron-detachment energies of F1-F4 showed linear correlations with the quantity 1/n, where n is the number of fluorene moieties. These correlations allowed the estimation of the vertical ionization potential (IP) of 7.10 eV and the oxidation potential (Eox) of 0.97 V versus SCE for the multiply stacked polyfluorene donor with an infinite number of fluorene moieties. These observations with π-stacked polyfluorenes may prove to be highly relevant to the electron-transport phenomenon observed in DNA through π-stacked bases. Copyright © 2003 American Chemical Society

Synthesis of a calix[4]arene derivative for isolation of a stable cation radical salt for use as a colorimetric sensor of nitric oxide

We have designed and synthesized a modified calixarene derivative (1) that allows, for the first time, the isolation of a stable cation radical salt that binds a single molecule of nitric oxide deep within its cavity with remarkable efficiency (KNO \u3e108 M-1), as demonstrated by isolation of a crystalline complex [1, NO]+ and its characterization by X-ray crystallography as well as by optical spectroscopy. Furthermore, the ready accessibility of the calixarene cation radical will allow the exploration of its use for developing efficient sensing devices for nitric oxide based on the accompanied color changes. Copyright © 2004 American Chemical Society

Molybdopterin biosynthesis - Mechanistic studies on a novel MoaA catalyzed insertion of a purine carbon into the ribose of GTP

Abstract The first step in the biosynthesis of the molybdopterin cofactor involves an unprecedented insertion of the purine C8 carbon between the C2′ and C3′ carbons of the ribose moiety of GTP. Here we review mechanistic studies on this remarkable transformation. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications