Lubomír Pospíšil

Helquats: A Facile, Modular, Scalable Route to Novel Helical Dications

The synthesis and properties of helical extended diquat (helquat), and derivatives that bear resemblance to diquat and azoniahelicene, was reported. Triyne with elongated tethers connecting the heterocyclic moiety with the pendant alkyne functionalities undergoing cycloisomerization give helquat featuring two seven-membered rings. The seven helquats reported are accessed uniformly in three steps from commercially available starting materials, entailing a Sonogashira coupling, bisquaternization, cycloisomerization, and 2+2+2 cycloisomerization. The evidence for the reversible electrochemical Weiz-type manifold and regular columnar stacks in crystal structures suggest the potential of helquats as electroactive functional elements.

Intense Chiroptical Switching in a Dicationic Helicene-Like Derivative: Exploration of a Viologen-Type Redox Manifold of a Non-Racemic Helquat

Two-step redox switching in enantiopure helquat system [P-1](2+) ⇌ [P-1](•+) ⇌ [P-1](0) is demonstrated. The viologen-type electroactive unit embedded directly in the helical scaffold of 1 is responsible for the prominent chiroptical switching at 264 nm. This process is associated with a marked sign-reversal of Cotton effect ramping between Δε = +35 M(-1) cm(-1) for [P-1](2+) and Δε = -100 M(-1) cm(-1) for [P-1](0). This helically chiral system features the most intense chiroptical switch response documented in the field of helicenoids.

ELECTROCHEMISTRY OF S-TRIAZINE HERBICIDES - REDUCTION OF ATRAZINE AND TERBUTYLAZINE IN AQUEOUS-SOLUTIONS

Abstract The electrochemical behavior of both herbicides is qualitatively similar. The two-electron reduction of atrazine (2-chloro-4-ethylamino 6-isopropylamino- 1,3,5- triazine) proceeds only in acidic media where the preceding protonation takes place (pK = 1.44). In solutions of higher pH (2–4) the observed reduction current is kinetically controlled by protonation and the estimated value of the protonation rate constant is kr = 2400 ± 400 mol−1 1 s−. The products of reduction were identified by large-scale electrolysis followed by GC-MS analysis. The electron transfer reaction causes cleavage of the Cl atom and loss of the ethyl group yielding the respective products in the ratio approximately 4:1. The electrochemical properties of terbutylazine are characterized by the protonation constant pK = 2.37 and rate constant kr = 1900 ± 200 mol−1 1 s−1.

Single-Molecule Conductance in a Series of Extended Viologen Molecules.

Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold-molecule-gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm(-1) (0.006 ± 0.0004 Å(-1)) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires.

Voltammetry in benzene using lithium dodecamethylcarba-closo-dodecaborate, LiCB11Me12, as a supporting electrolyte: reduction of Ag+

Abstract The electrical conductivity of 0.5 M lithium dodecamethylcarba- closo -dodecaborate, Li + CB 11 Me 12 − (Li + - 1 ), in benzene is sufficient for performing voltammetric and other electrochemical measurements. The potential window is conveniently wide: it is limited by oxidation of 1 at +0.7 V and by reduction of Li + at −2.0 V. The reduction of Ag + - 1 follows a similar pattern as its redox reaction in more conventional electrochemical solvents. When the negative potential scan extends beyond −1.6 V, the formation of an intermetallic alloy of Ag and Li is observed.

Electrochemical conversion of dinitrogen to ammonia mediated by a complex of fullerene C60 and γ-cyclodextrin

We report on an electrochemical conversion of N2 to NH3 at ambient pressure and 60 degrees C, which is mediated by reduced C(60) inside the molecular cavity of gamma-cyclodextrin.

Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Bovine serum albumin film as a template for controlled nanopancake and nanobubble formation: in situ atomic force microscopy and nanolithography study.

Air nanobubbles and nanopancakes were investigated in situ by both tapping mode atomic force microscopy (TM AFM) and atomic force nanolithography techniques employing bovine serum albumin (BSA) film supported by highly oriented pyrolytic graphite (HOPG). The BSA denaturation induced by the water-to-ethanol exchange served for conservation of nanobubble and nanopancake sites appearing as imprints in BSA film left by gaseous cavities formerly present on the interface in the aqueous environment. Once the BSA film was gently removed by the nanoshaving technique applied in ethanol, a clean basal plane HOPG area with well-defined dimensions was regenerated. The subsequent reverse ethanol-to-water exchange led to the re-formation of nanopancakes specifically at the nanoshaved area. Our approach paves the way for the study of gaseous nanostructures with defined dimensions, formed at solid-liquid interface under controlled conditions.

Voltammetry of hypoxic cells radiosensitizer etanidazole radical anion in water

Cytotoxic properties of radiosensitizers are due to the fact that, in the metabolic pathway, these compounds undergo one-electron reduction to generate radical anions. In this study we focused our interest on the electrochemical transfer of the first electron on radiosensitizer Etanidazole (ETN) and, consequently, on the ETN radical-anion formation in the buffered aqueous media. ETN was electrochemically treated in the broad pH range at various scan rates. Three reduction peaks and one oxidation peak were found. At strong alkaline pH the four-electron reduction peak was separated into one-electron and three-electron reductions. Under these conditions the standard rate constant k(0) for the redox couple ETN-NO(2)+e(-) <--> ETN-NO(2)(*-) was calculated. Moreover, the value of a so called E(7)(1) potential that accounts for the energy necessary to transfer the first electron to an electroactive group at pH=7 in aqueous medium to form a radical anion was also determined. The obtained value of E(7)(1) indicates that lower energy compared to the other possible chemical radiosensitizers is necessary for the system to transfer the first electron to ETN. On the other hand, the necessity of the strong alkaline pH may decrease the ability of ETN to act as hypoxic radiosensitizer in the human body.

Novel redox label for proteins.: Electron transfer properties of (η5-cyclopentadienyl) tricarbonyl manganese bound to bovine serum albumin

Abstract (η 5 -Cyclopentadienyl) tricarbonyl manganese (cymantrenyl) derivatives of the protein bovine serum albumin (BSA) with coupling ratios ranging from 7 to 20 were prepared by reaction of cymantrenyl methyl imidate with some of the lysine residues of BSA. Electrochemical measurements based on the AC voltammetric detection of the reduction of cymantrene bound to BSA protein were successfully set up in aqueous solutions and the detection limit of 2×10 −7 M BSA was achieved. This limit is almost two orders of magnitude lower than that measured by IR spectroscopy. Electrochemical detection of cymantrene label will be used for immunoassay analysis of water-soluble analytes in our future work.