Molecular Tweezers: Studies Directed Towards the Preparation of Effective Charge Transport Materials

ABSTRACT MOLECULAR TWEEZERS: STUDIES DIRECTED TOWARDS THE PREPARATION OF EFFECTIVE CHARGE TRANSPORT MATERIALS Khushabu Thakur, B.Sc, M.Sc. Marquette University, 2013 An important focus of the modern areas of photovoltaics and molecular electronics is to identify molecules or molecular assemblies that can promote effective charge/exciton transport over long distances. In order to be able to exert control and establish necessary structural parameters for reproducible production of self-assembled structures for long-distance charge transport, our laboratory has been actively engaged towards the preparation of a variety of well-defined, cofacially-arrayed polybenzenoid nanostructures in which aromatic donors are cofacially stacked. Herein, we undertake the design and synthesis of tweezer-like molecules for their usage for the preparation of long-range charge transport assemblies. We have successfully developed an efficient synthesis of molecular tweezers with different pincers using a common doubly-annulated m-terphenyl platform. The ready availability of these tweezers with different pincers allows us to demonstrate that they bind a variety of electron acceptors as guests via electron donor-acceptor or charge-transfer interactions. Moreover, the tweezers with electron-rich pincers [e.g. trimethylpyrene (TMP) pincers] undergo a ready self-assembly into cofacially-arrayed polybenzenoid nanostructures when prompted by 1-electron oxidation. The effectiveness of the functionality of various tweezers and the rigidity of the tweezer platforms was evaluated by comparison of the EDA complexation binding constants with the tweezers derived from completely rigid (CRP) and non-annulated platforms (NAP) and that of doubly-annulated platform (DAP). These studies established that rigidification of the tweezer platform does not contribute, in any significant way, to the functionality of the tweezers. Moreover, a comparative study of electron transfer prompted self-assembly of the NAP-TMP and DAP-TMP tweezers established that they both undergo ready self-association with comparable efficiency. We have also synthesized a triptycene scaffold and carefully evaluated its optoelectronic properties. This triptycene scaffold and its derivatives are expected to form two-dimensional self-assemblies which are potentially useful as long-range charge-transport materials in modern photovoltaic devices

Isolation of a Chiral Anthracene Cation Radical: X-Ray Crystallography and Computational Interrogation of its Racemization

Chiral cation-radical salts hold significant promise as charge-transfer materials, chiroptical switches, and electron-transfer catalysts for enantioselective synthesis. Herein we demonstrate that the readily-available chiral 9,10-diphenyleanthracene derivative (i.e.SANT) forms a robust cation radical, whose structure was elucidated by X-ray crystallography and DFT calculations. While SANT was observed to racemize on a timescale (t1/2) of 1.1 hours, a computational conformational search and kinetic analysis of the racemization pathway led us to identify a simple methyl substituted SANT derivative, which does not racemize (racemization t1/2 1013–1017 years)

A Search for Blues Brothers: X-ray Crystallographic/Spectroscopic Characterization of the Tetraarylbenzidine Cation Radical as a Product of Aging of Solid Magic Blue

Magic blue (MB+˙ SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+˙ SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’ (BC: Eox1 = 0.78 V vs. Fc/Fc+, λmax(BC+˙) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations

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

Serendipitous Discovery of Light-Induced \u3cem\u3e(In Situ)\u3c/em\u3e Formation of An Azo-Bridged Dimeric Sulfonated Naphthol as a Potent PTP1B Inhibito

Background Protein tyrosine phosphatases (PTPs) like dual specificity phosphatase 5 (DUSP5) and protein tyrosine phosphatase 1B (PTP1B) are drug targets for diseases that include cancer, diabetes, and vascular disorders such as hemangiomas. The PTPs are also known to be notoriously difficult targets for designing inihibitors that become viable drug leads. Therefore, the pipeline for approved drugs in this class is minimal. Furthermore, drug screening for targets like PTPs often produce false positive and false negative results. Results Studies presented herein provide important insights into: (a) how to detect such artifacts, (b) the importance of compound re-synthesis and verification, and (c) how in situ chemical reactivity of compounds, when diagnosed and characterized, can actually lead to serendipitous discovery of valuable new lead molecules. Initial docking of compounds from the National Cancer Institute (NCI), followed by experimental testing in enzyme inhibition assays, identified an inhibitor of DUSP5. Subsequent control experiments revealed that this compound demonstrated time-dependent inhibition, and also a time-dependent change in color of the inhibitor that correlated with potency of inhibition. In addition, the compound activity varied depending on vendor source. We hypothesized, and then confirmed by synthesis of the compound, that the actual inhibitor of DUSP5 was a dimeric form of the original inhibitor compound, formed upon exposure to light and oxygen. This compound has an IC50 of 36 μM for DUSP5, and is a competitive inhibitor. Testing against PTP1B, for selectivity, demonstrated the dimeric compound was actually a more potent inhibitor of PTP1B, with an IC50 of 2.1 μM. The compound, an azo-bridged dimer of sulfonated naphthol rings, resembles previously reported PTP inhibitors, but with 18-fold selectivity for PTP1B versus DUSP5. Conclusion We report the identification of a potent PTP1B inhibitor that was initially identified in a screen for DUSP5, implying common mechanism of inhibitory action for these scaffolds

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

Electrochemistry and electrogenerated chemiluminescence of π-stacked poly(fluorenemethylene) oligomers. multiple, interacting electron transfers

The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a series of π-stacked poly(fluorenemethylene) oligomers (Fn, n = 1-6) were investigated. The pendant cofacially oriented fluorene moieties are essentially in contact with each other by Van der Waals interaction promoting electronic delocalization in these species. All six compounds give successive cyclic voltammetric one-electron (1e) oxidations in 1:1 acetonitrile/benzene (MeCN/Bz), and the multiple 1e transfer properties of all these compounds were confirmed by chronoamperometric experiments with an ultramicroelectrode and digital simulations. The potentials for oxidation of the successive 1e transfers can be explained in terms of electrostatic interactions among the fluorenes. The monomer (F1) shows one irreversible wave, while F2 shows two reversible 1e waves. F3 shows only two reversible 1e oxidation waves, which is consistent with the large energy to remove a third electron because of the greater electrostatic repulsion, so the third wave is shifted toward more positive potentials. Both F4 and F5 show three reversible 1e oxidation waves, while F6 shows four reversible 1e waves. The removal of the first electron from an oligomer becomes easier as n increases. The stability of the radical cations also increases with n. The removal of consecutive electrons from Fn can be correlated with the distance between fluorene moieties. No reduction peaks were observed except for some broad ones at ∼-3.2 V vs SCE in THF, which is consitent with the wide highest occupied molecular orbital-lowest unoccupied molecular orbital gap in these compounds (absorbance at about 300 nm). No characteristic annihilation ECL signal was observed for these compounds in 1:1 MeCN/Bz mixed solvent. However, the ECL of F6 in the presence of the coreactant C 2O 42- showed a long-wavelength ECL emission that was proposed to be electrolyzed byproduct from the radical cation. © 2012 American Chemical Society

Nodal Arrangement of HOMO Controls the Turning On/Off the Electronic Coupling in Isomeric Polypyrene Wires

The charge transfer along π-conjugated wires is largely governed by the interchromophoric electronic coupling that depends on the geometry (e.g., interchromophoric dihedral angle) and electronic structure of the chromophores. Herein, we demonstrate that stabilization of the cationic charge (hole) in polypyrene cation radicals and the extent of hole delocalization can be easily controlled by modulating the nodal arrangement of the HOMO. For example, 2,2′-linked <i>para</i>-polypyrenes show nonexistent electronic coupling owing to a nodal arrangement of HOMO that is unfavorable for orbital overlap, despite a favorable interchromophoric dihedral angle. A repositioning of the linkage between two pyrenes from para to meta positions produces a far less favorable interchromophoric dihedral angle, yet the electronic coupling turns <b>on</b> due to a favorable nodal arrangement of HOMO, which allows interchromophoric orbital overlap. This surprising finding has been demonstrated through the synthesis and systematic examination of the redox and optical properties of <i>meta</i>-polypyrenes (<i>m</i>-<b>Py</b>_<i>n</i>), which reveals a sizable delocalization of the hole in <i>m</i>-<b>Py</b>_<i>n</i>^+• that extends to three pyrene units, only two benzenoid units less than in typical poly-<i>p</i>-phenylene wires. These findings of widespread interest, supported by density functional theory (DFT) and the Marcus-based multistate model, will impact the rational design of new charge-transfer materials for photovoltaic and molecular electronics applications

Nodal Arrangement of HOMO Controls the Turning On/Off the Electronic Coupling in Isomeric Polypyrene Wires

The charge transfer along π-conjugated wires is largely governed by the interchromophoric electronic coupling that depends on the geometry (e.g., interchromophoric dihedral angle) and electronic structure of the chromophores. Herein, we demonstrate that stabilization of the cationic charge (hole) in polypyrene cation radicals and the extent of hole delocalization can be easily controlled by modulating the nodal arrangement of the HOMO. For example, 2,2′-linked <i>para</i>-polypyrenes show nonexistent electronic coupling owing to a nodal arrangement of HOMO that is unfavorable for orbital overlap, despite a favorable interchromophoric dihedral angle. A repositioning of the linkage between two pyrenes from para to meta positions produces a far less favorable interchromophoric dihedral angle, yet the electronic coupling turns <b>on</b> due to a favorable nodal arrangement of HOMO, which allows interchromophoric orbital overlap. This surprising finding has been demonstrated through the synthesis and systematic examination of the redox and optical properties of <i>meta</i>-polypyrenes (<i>m</i>-<b>Py</b>_<i>n</i>), which reveals a sizable delocalization of the hole in <i>m</i>-<b>Py</b>_<i>n</i>^+• that extends to three pyrene units, only two benzenoid units less than in typical poly-<i>p</i>-phenylene wires. These findings of widespread interest, supported by density functional theory (DFT) and the Marcus-based multistate model, will impact the rational design of new charge-transfer materials for photovoltaic and molecular electronics applications