Electrochemically-induced TCNQ/Mn TCNQ 2 (H2O) 2 (TCNQ= 7, 7, 8, 8-Tetracyanoquinodimethane) solid-solid interconversion : two voltammetrically distinct processes that allow selective generation of nanofiber or nanorod network morphologies

Unlike the case with other divalent transition metal M\[TCNQ](2)(H(2)O)(2) (M = Fe, Co, Ni) analogues, the electrochemically induced solid-solid phase interconversion of TCNQ microcrystals (TCNQ = 7,7,8,8-tetracyanoquinodimethane) to Mn\[TCNQ](2)(H(2)O)(2) occurs via two voltammetrically distinct, time dependent processes that generate the coordination polymer in nanofiber or rod-like morphologies. Careful manipulation of the voltammetric scan rate, electrolysis time, Mn(2+)((aq)) concentration, and the method of electrode modification with solid TCNQ allows selective generation of either morphology. Detailed ex situ spectroscopic (IR, Raman), scanning electron microscopy (SEM), and X-ray powder diffraction (XRD) characterization clearly establish that differences in the electrochemically synthesized Mn-TCNQ material are confined to morphology. Generation of the nanofiber form is proposed to take place rapidly via formation and reduction of a Mn-stabilized anionic dimer intermediate, \[(Mn(2+))(TCNQ-TCNQ)(2)(*-)], formed as a result of radical-substrate coupling between TCNQ(*-) and neutral TCNQ, accompanied by ingress of Mn(2+) ions from the aqueous solution at the triple phase TCNQ/electrode/electrolyte boundary. In contrast, formation of the nanorod form is much slower and is postulated to arise from disproportionation of the \[(Mn(2+))(TCNQ-TCNQ)(*-)(2)] intermediate. Thus, identification of the time dependent pathways via the solid-solid state electrochemical approach allows the crystal size of the Mn\[TCNQ](2)(H(2)O)(2) material to be tuned and provides new mechanistic insights into the formation of different morphologies.

Scanning electrochemical microscopy study of the solid--solid interconversion of TCNQ to phase I and phase II CuTCNQ

The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals attached to a glassy carbon electrode in the presence of Cu2+(aq) to form CuTCNQ(s) has been investigated using scanning electrochemical microscopy in the substrate generation tip collection mode and shown to involve a generation of soluble TCNQ−(aq). The subsequent oxidation of CuTCNQ does not involve simple expulsion of Cu+ into solution but a soluble complex attributed to Cu2+TCNQ−(aq). Mechanistic insights relative to the electrochemical conversion of CuTCNQ phase I into phase II by repetitive cycling of potential and electrochemical formation of KTCNQ have also been established

A theoretical study of C4B isomers. The interconversion of CCBCC and CCCCB via cyclic C4B

Theory suggests that CCBCC (1) will rearrange to planar cyclo-C4B (19) if the excess energy of 1 is greater than or equal to16.1 kcal mol(-1) [calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-31G(d) level of theory]. Cyclo-C4B lies only 1.1 kcal mol(-1) above CCBCC. The planar nature of symmetrical cyclo-C4B is attributed to multicentered bonding involving boron. If cyclo-C4B (19) has an excess energy of greater than or equal to24.4 kcal mol-1, it may ring open to form CCCCB (3).

Morphology changes and mechanistic aspects of the electrochemically-induced reversible solid-solid transformation of microcrystalline TCNQ into Co TCNQ 2-based materials (TCNQ= 7, 7, 8, 8-Tetracyanoquinodimethane)

The chemically reversible solid−solid phase transformation of a TCNQ-modified glassy carbon, indium tin oxide, or metal electrode into Co\[TCNQ]2(H2O)2 material in the presence of Co2+(aq) containing electrolytes has been induced and monitored electrochemically. Voltammetric data reveal that the TCNQ/Co\[TCNQ]2(H2O)2 interconversion process is independent of electrode material and identity of cobalt electrolyte anion. However, a marked dependence on electrolyte concentration, scan rate, and method of electrode modification (drop casting or mechanical attachment) is found. Cyclic voltammetric and double potential step chronoamperometric measurements confirm that formation of Co\[TCNQ]2(H2O)2 occurs through a rate-determining nucleation and growth process that initially involves incorporation of Co2+(aq) ions into the reduced TCNQ crystal lattice at the TCNQ|electrode|electrolyte interface. Similarly, the reverse (oxidation) process, which involves transformation of solid Co\[TCNQ]2(H2O)2 back to parent TCNQ crystals, also is controlled by nucleation−growth kinetics. The overall chemically reversible process that represents this transformation is described by the reaction:  2TCNQ0(s) + 2e- + Co2+(aq) + 2H2O \[Co(TCNQ)2(H2O)2](s). Ex situ SEM images illustrated that this reversible TCNQ/Co\[TCNQ]2(H2O)2 conversion process is accompanied by drastic size and morphology changes in the parent solid TCNQ. In addition, different sizes of needle-shaped nanorod/nanowire crystals of Co\[TCNQ]2(H2O)2 are formed depending on the method of surface immobilization.

AFM study of morphological changes associated with electrochemical solid--solid transformation of three-dimensional crystals of TCNQ to metal derivatives (metal= Cu, Co, Ni; TCNQ= tetracyanoquinodimethane)

In situ atomic force microscopy (AFM) allows images from the upper face and sides of TCNQ crystals to be monitored during the course of the electrochemical solid–solid state conversion of 50 × 50 μm2 three-dimensional drop cast crystals of TCNQ to CuTCNQ or M[TCNQ]2(H2O)2 (M = Co, Ni). Ex situ images obtained by scanning electron microscopy (SEM) also allow the bottom face of the TCNQ crystals, in contact with the indium tin oxide or gold electrode surface and aqueous metal electrolyte solution, to be examined. Results show that by carefully controlling the reaction conditions, nearly mono-dispersed, rod-like phase I CuTCNQ or M[TCNQ]2(H2O)2 can be achieved on all faces. However, CuTCNQ has two different phases, and the transformation of rod-like phase 1 to rhombic-like phase 2 achieved under conditions of cyclic voltammetry was monitored in situ by AFM. The similarity of in situ AFM results with ex situ SEM studies accomplished previously implies that the morphology of the samples remains unchanged when the solvent environment is removed. In the process of crystal transformation, the triple phase solid∣electrode∣electrolyte junction is confirmed to be the initial nucleation site. Raman spectra and AFM images suggest that 100% interconversion is not always achieved, even after extended electrolysis of large 50 × 50 μm2 TCNQ crystals.

Formation of neutral C7H2 isomers from pour isomeric C7H2 radical anion precursors in the gas phase

Consideration of theoretical calculations \[E3LYP/aug-cc-pVDZ//B3LYP/6-31G(d)\] of the structures of ten C7H2 neutral isomers and the nine corresponding C7H2 radical anions have led us to synthesize four stable C7H2 radical anions in the ion source of our ZAB 2HF mass spectrometer, and to convert these to C7H2 neutrals. The four radical anion isomers prepared were (i) \[(HC equivalent to C)(2)C=C=C\](-.) \[from the reaction between (HC equivalent to C)(3)COCH3 and HO- \], (ii) \[HC=C=C=C=C=C=CH\](-.) \[from the reaction between HC equivalent to C-C equivalent to C-CD(OH)-C equivalent to CH and HO-\], (iii) \[C=C=C=C=C=C=CH2\](-.) \[from the reaction between DC equivalent to C-C equivalent to C-C equivalent to C-CH2OCH2CH3 and HO-\], and (iv) \[C equivalent to C-CH2-C equivalent to C-C equivalent to C\](-.) \[from the bis desilylation reaction of (CH3)(3)Si-C equivalent to C-CH2-C equivalent to C-C equivalent to C-Si (CH3)(3)With SF6-.\]. The four anions were further characterized by their collisional activation (negative ion) and charge reversal (CR, positive ion) mass spectra. The anions were converted into their corresponding neutrals by charge stripping, and the correspondence between the charge reversal (CR) and neutralization reionization (-NR+) mass spectra of each anion is taken as evidence that within the time frame of the -NR+ experiment (some 10(-6) s), each neutral is stable and undergoes no major rearrangement or interconversion to a more stable isomer. Theory and experiment are in accord for these systems.

Formation of neutral C7H2 isomers from four isomeric C7H2 radical anion precursors in the gas phase

Consideration of theoretical calculations [B3LYP/aug-cc-pVDZ//B3LYP/6-31G(d)] of the structures of ten C7H2 neutral isomers and the nine corresponding C7H2 radical anions have led us to synthesize four stable C7H2 radical anions in the ion source of our ZAB 2HF mass spectrometer, and to convert these to C7H2 neutrals. The four radical anion isomers prepared were (i) [(HC≡C)2C=C=C]-̇ [from the reaction between (HC≡C)3COCH3 and HO- ], (ii) [HC=C=C=C=C=C=CH]-̇ [from the reaction between HC≡C-C≡C- CD(OH)-C≡CH and HO-], (iii) [C=C=C=C=C=C=CH2]-̇ [from the reaction between DC≡C-C≡C- C≡C-CH2OCH2CH3 and HO-], and (iv) [C≡C-CH2-C≡C-C≡C]-̇ [from the bis desilylation reaction of (CH3)3Si-C≡C-CH2-C≡C-C≡C-Si (CH3)3 with SF6 -̇]. The four anions were further characterized by their collisional activation (negative ion) and charge reversal (CR, positive ion) mass spectra. The anions were converted into their corresponding neutrals by charge stripping, and the correspondence between the charge reversal (CR) and neutralization reionization (-NR+) mass spectra of each anion is taken as evidence that within the time frame of the -NR+ experiment (some 10-6 s), each neutral is stable and undergoes no major rearrangement or interconversion to a more stable isomer. Theory and experiment are in accord for these systems.

Reversible transdifferentiation of blood vascular endothelial cells to a lymphatic-like phenotype in vitro

Blood vascular cells and lymphatic endothelial cells (BECs and LECs, respectively) form two separate vascular systems and are functionally distinct cell types or lineages with characteristic gene expression profiles. Interconversion between these cell types has not been reported. Here, we show that in conventional in vitro angiogenesis assays, human BECs of fetal or adult origin show altered gene expression that is indicative of transition to a lymphatic-like phenotype. This change occurs in BECs undergoing tubulogenesis in fibrin, collagen or Matrigel assays, but is independent of tube formation per se, because it is not inhibited by a metalloproteinase inhibitor that blocks tubulogenesis. It is also reversible, since cells removed from 3D tubules revert to a BEC expression profile upon monolayer culture. Induction of the lymphatic-like phenotype is partially inhibited by co-culture of HUVECs with perivascular cells. These data reveal an unexpected plasticity in endothelial phenotype, which is regulated by contact with the ECM environment and/or cues from supporting cells.