Conformational dynamics of semirigidly bridged electron donor-acceptor systems as revealed by stationary and time-resolved fluorescence spectroscopies at higher pressures

In low polarity solvents, several electron donor-acceptor systems D-B-A with a (semi-)flexible bridge B and sufficiently strong driving force exhibit the so-called harpooning process. That is, the primarily formed charge transfer state with extended geometry (ECT) relaxes via a change of the bridge conformation to a more compact exciplex (CCT). The dependence of the rate of transformation, k(fold), On solvent parameters epsilon and eta was studied by pressure tuning of these quantities (1 < p < 350 MPa). For the investigated compounds WS2 and WS3, epsilon-dependent activation energies between 16 and 30 kJ/mol were derived. In contrast, viscosity seems to have no effect on the rate of folding.