A pulse radiolysis study of the reactions of hydrogen atoms with dimethyl-, tetramethyl- and hexamethyl-substituted viologens

McAskill, NA

doi:10.1071/ch9841579

Cited by 14 publications

(6 citation statements)

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“…[52] As one can see, 8 shows the well-known, [53] very intense bands with l max 397 and 607 nm, whereas 7 shows a weaker and broader absorption band around 400 nm and an absorption band with l max 780 nm. [54] The observed transient spectrum ( Figure 12) is clearly very similar to that of 8 , indicating that, as expected, the photoinduced electron transfer process involves the A 1 station. Whether A 1 À is formed directly or via the intermediate formation of A 2 À followed by a very fast electron transfer from A 2 À to A 1 is a detail that will be discussed below.…”

Section: Designmentioning

confidence: 90%

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Ballardini²,

et al. 2000

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A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecu...

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Section: Designmentioning

confidence: 90%

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Ballardini²,

et al. 2000

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“…In what follows it is assumed (based on experimental evidence − ) that the initial propagation step, eq 2, is so fast that it is not rate-determining. The essence of the model is that there is no barrier to entry for a z -meric radical (i.e., I ), and thus the actual entry step is, or is close to being, diffusion-controlled.…”

Section: Background Considerationsmentioning

confidence: 99%

Entry in Emulsion Polymerization: Effects of Initiator and Particle Surface Charge

2003

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The rate coefficient for radical entry into latex particles in emulsion polymerizations is measured for styrene systems in which the entering species are anionic (from persulfate) and cationic (from 2,2′-azobis(2-methylpropionamidine) dihydrochloride, or V-50). These entry rate coefficients F are obtained by measuring rates in seeded emulsion polymerizations where the seeds have either cationic or anionic groups on the surface; "zero-one" conditions are employed, because these offer the advantage that particle size is sufficiently small (≈70 nm diameter) that intraparticle termination is not rate-determining. Data comprise steady-state rates with chemical initiator, combined with loss rates obtained using γ-radiolysis initiation and following the relaxation behavior after removal from the radiation source. Values for F as a function of initiator concentration can be meaningfully compared for different initiators through the dependence of initiator efficiency f entry on primary radical generation rate (radical flux). For the anionic latex, this dependence is seen to differ depending on the nature of the initiator used. This may be explained by the entry model [

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“…A useful technique for obtaining unambiguous kinetic parameters is the γ-relaxation experiment, which has been extensively used with various monomers including styrene, ,, methyl methacrylate, and vinyl esters, , as well as the RAFT-mediated polymerization of styrene using xanthates . The strength of the γ-relaxation experiment is that the external source of radicals (i.e., the γ-induced decomposition of water into various radical species , ) may be turned on or off at will, simply by introducing the sample into, or removing it from, the γ-source. Thus, γ-relaxation provides two steady-state values of n̄ (in-source and out-of-source) as well as the rate of approach to the new steady state; the latter non-steady-state behavior provides the data most sensitive to radical loss.…”

Section: Introductionmentioning

confidence: 99%

Radical Loss in RAFT-Mediated Emulsion Polymerizations

et al. 2005

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Radical loss processes in RAFT-controlled emulsion polymerization are investigated using γ-initiated seeded emulsion polymerization of styrene in “relaxation mode”, i.e., following the rate of polymerization after removal from the radical source. This provides direct measurement of radical loss processes. A water-insoluble RAFT agent, 2-phenylprop-2-yl phenyldithioacetate, was transported to preformed seed particles using acetone, the acetone then removed, the particles swollen with monomer, and RAFT-mediated polymerization initiated by γ-irradiation. The systems show good control over the molecular weight of the products and a retardation dependent on the concentration of RAFT agent. Kinetic parameters are obtained from the γ-relaxation experiments, using pseudo-bulk kinetics as an approximation to full compartmentalization/chain-length-dependent kinetics to describe the systems. The rate of radical loss was strongly affected by the presence of RAFT agent and was found to decrease with increasing length of the dormant chain. The interpretation of this observation is as follows. The termination rate coefficient depends on chain length; the dominant mode of termination is the reaction between chains of similar length in RAFT-mediated systems but between short and long chains in conventional systems. Radical entry into particles is assumed to be by chains of degree of polymerization z formed in the aqueous phase. Dormant z-meric chains in the particles are postulated to lead to an increase in the rate of radical exit from the particles, with the reactivation of these species generating a z-meric radical that is able to desorb from the particle surface in a RAFT-induced exit mechanism, leading to the rapid exchange of radicals between particles and hence radical loss. A simple model for the radical loss rate coefficient, expected to be valid for longer chain lengths, both reproduces much more computationally expensive Monte Carlo calculations and (when used with the scaling suggested by reptation theory) gives quantitative accord with the relaxation data.

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A pulse radiolysis study of the reactions of hydrogen atoms with dimethyl-, tetramethyl- and hexamethyl-substituted viologens

Cited by 14 publications

References 0 publications

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Entry in Emulsion Polymerization: Effects of Initiator and Particle Surface Charge

Radical Loss in RAFT-Mediated Emulsion Polymerizations

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