Influence of solvent viscosity and permittivity on the harpooning mechanism in semirigidly bridged electron donor-acceptor systems

After the photoinduced electron transfer, the donor-bridge-acceptor system “WS4” executes a large scale motion which leads from an extended to a compact geometry (harpooning mechanism). The rate of this folding proceess, Kfold, was measured in three nonpolar solvents as a function of temperature (283 < T < 343 K) and hydroastatic pressure (0.1 < P < 350 MPa) and successfully fitted by the expression kfold = B · (A/η0.7exp[−(Ester−E′/ϵr+E″/ϵr2)/RT]. This implies that the effect of friction on the folding kinetics of WS4 is modeled best by the same stretched exponential, in contrast to the previously studied structurally related systems WS2 and WS3 where no friction dependence was observed.