Molecular shuttle kinetics

This study also demonstrated that the nature of the solvent can have an effea on the chain shuttling kinetics. Under constant conditions, a change in solvent from toluene to the more polar 1,2-difluorobenzene gave PPs with more narrow molecular weight distributions. In addition, Pjj was higher by a factor of 6-9 in samples made in 1,2-difluorobenzene. These data indicate that choice of solvent is an important consideration in design of new chain shuttling systems. 

Isotope-containing organic compounds as ion radical precursors have important significance for ion radical organic chemistry. The link between the isotopic substitution at the reaction center and the change in the kinetic properties is not so obvious for one-electron transfer. In that case, the highest occupied molecular orbit (MO) of a donor loses one electron. This electron is then shuttled to the lowest unfilled MO of an acceptor. 

We observed (Fig. 3) that in the absence of surface quinones, the relaxation of QD absorption bleach band (A,ex=528 nm) corresponding to 1 lSelSh> state reflects the trapping excited charge carriers at the surface. The picosecond kinetic analysis shows that in the presence of tCl-l,2-BQ the short time component of the transient bleach formation at A.reg=530 nm is additionally shortened from 93 ps down to 27 ps. It reflects the appearance of the additional non-radiative relaxation channel for electrons from QD conduction band to the lowest unoccupied molecular orbitals of quinone (LUMO). These results are in an agreement with calculations presented in [4]. We believe that long component (r> 3 ns) may reflect the electron shuttling from LUMO of the quinone to the QD valence band. 

This new kinetic evidence has stimulated further theoretical studies on the MBH mechanism, conducted initially by Xu and Sunoj. Recently, Aggarwal performed an extensive theoretical study, which supported their own kinetic observations and those of McQuade about the proton transfer step. They proposed that the proton-transfer step can proceed via two pathways (i) addition of a second molecular of aldehyde to form a hemiacetal alkoxide (hemil) followed by rate-limiting proton transfer as proposed by McQuade (non-alcohol-catalyzed pathway) and (ii) an alcohol that acts as a shuttle to transfer a proton from the a-position to the alkoxide of int2 (Scheme 1.4). 

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