Quantum Mechanical and Experimental Validation that Cyclobis(paraquat-p-phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of π-electron-rich guests by the π-electron-deficient host, cyclobis(paraquat-p-phenylene) (CBPQT^(4+)), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge-transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT^(4+), is an emerald-green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas-phase calculations that reinterpreted this CT band in terms of an intermolecular side-on interaction of TTF with one of the bipyridinium (BIPY^(2+)) units of CBPQT^(4+), rather than the encapsulation of TTF inside the cavity of CBPQT^(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT^(4+) arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY^(2+) units of a CBPQT^(4+) ring residing on a separate [2]rotaxane in a side-on fashion. This [2]rotaxane has similar UV/Vis and ^1H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT^(4+). The [2]rotaxane exists as an equimolar mixture of cis- and trans-isomers associated with the disubstituted TTF unit in its dumbbell component. Solid-state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT^(4+)

Employment trends survey 1998

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Allosteric Modulation of Substrate Binding within a Tetracationic Molecular Receptor

The synthesis and recognition phenomena of a tetra­cationic molecular receptor that possesses a nanometer-sized molecular cavity are described. The host–guest properties of the molecular receptor can be tuned and modulated allo­sterically, where the association of a hetero­tropic effector at the periphery of the molecule serves to modulate its affinity for the globular, electron-rich guest that resides within its molecular cavity. This stimuli-responsive host–guest behavior was observed in both the solution phase and the crystalline solid state, and can be reversed with high fidelity by sequestration of the effector molecule

Quantum Mechanical and Experimental Validation that Cyclobis(paraquat-p-phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of -electron-rich guests by the -electron-deficient host, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge-transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT(4+), is an emerald-green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas-phase calculations that reinterpreted this CT band in terms of an intermolecular side-on interaction of TTF with one of the bipyridinium (BIPY2+) units of CBPQT(4+), rather than the encapsulation of TTF inside the cavity of CBPQT(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT(4+) arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY2+ units of a CBPQT(4+) ring residing on a separate [2]rotaxane in a side-on fashion. This [2]rotaxane has similar UV/Vis and (HNMR)-H-1 spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT(4+). The [2]rotaxane exists as an equimolar mixture of cis- and trans-isomers associated with the disubstituted TTF unit in its dumbbell component. Solid-state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT(4+)

Catenation through a Combination of Radical Templation and Ring-Closing Metathesis

Synthesis of an electro­chemically addressable [2]­catenane has been achieved following formation by templation of a [2]­pseudo­rotaxane employing radically enhanced molecular recognition between the bis­radical dication obtained on reduction of the tetra­cationic cyclo­phane, cyclo­bis­(paraquat-<i>p</i>-phenylene), and the radical cation generated on reduction of a viologen disubstituted with <i>p</i>-xylylene units, both carrying tetra­ethylene glycol chains terminated by allyl groups. This inclusion complex was subjected to olefin ring-closing metathesis, which was observed to proceed under reduced conditions, to mechanically inter­lock the two components. Upon oxidation, Coulombic repulsion between the positively charged and mechanically inter­locked components results in the adoption of a co-conformation where the newly formed alkene resides inside the cavity of the tetra­cationic cyclo­phane. ¹H NMR spectroscopic analysis of this hexa­cationic [2]­catenane shows a dramatic upfield shift of the resonances associated with the olefinic and allylic protons as a result of them residing inside the tetra­cationic component. Further analysis shows high dia­stereo­selectivity during catenation, as only a single (<i>Z</i>)-isomer is formed

Ferroelectric Polarization and Second Harmonic Generation in Supramolecular Cocrystals with Two Axes of Charge-Transfer

Ferroelectricity in organic materials remains a subject of great interest, given its potential impact as lightweight information storage media. Here we report supramolecular charge-transfer cocrystals formed by electron acceptor and donor molecules that exhibit ferroelectric behavior along two distinct crystallographic axes. The solid-state superstructure of the cocrystals reveals that a 2:1 ratio of acceptor to donor molecules assemble into nearly orthogonal mixed stacks in which the molecules are positioned for charge-transfer in face-to-face and edge-to-face orientations, held together by an extended hydrogen-bonding network. Polarization hysteresis was observed along the face-to-face and edge-to-face axes at room temperature. The noncentrosymmetric nature of the cocrystals, required to observe ferroelectric behavior, is demonstrated using second harmonic generation measurements. This finding suggests the possibility of designing supramolecular arrays in which organic molecules support multidimensional information storage

X‑Shaped Oligomeric Pyromellitimide Polyradicals

The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of <b>X</b>-shaped pyromellitimide (PI) oligomers (<b>X</b>_{<b><i>n</i></b>}<b>-R</b>, <b><i>n</i></b> = 2–4, <b>R</b> = <b>Hex</b> or <b>Ph</b>) linked together by single C–C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that <b>X</b>_<b>2</b><b>-Ph</b> crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers <b>X</b>_{<b><i>n</i></b>}<b>-Hex</b>, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, <b>X</b>_{<b><i>n</i></b>}<b>-Hex</b>^{<b>•–</b>} and <b>X</b>_{<b><i>n</i></b>}<b>-Hex</b>^{<b><i>n</i>(•−)</b>}, respectively, were probed extensively by UV–vis–NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions <b>X</b>_{<b><i>n</i></b>}<b>-Hex</b>^{<b>•–</b>} reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of <b>X</b>_{<b><i>n</i></b>}<b>-Hex</b>^{<b>•–</b>} yield EPR signals (in frozen solutions) that can be assigned to spin–spin interactions in <b>X</b>_<b>2</b><b>-Hex</b>^{<b>2(•−)</b>}, <b>X</b>_<b>3</b><b>-Hex</b>^{<b>3(•−)</b>}, and <b>X</b>_<b>4</b><b>-Hex</b>^{<b>4(•−)</b>}. Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C–C bond is a viable method for generating stabilized high-spin organic anionic polyradicals