Conformational dynamics of semiflexibly bridged electron donor–acceptor systems comprising long aliphatic tails

Abstract

In continuation of our previous work on the conformational dynamics (harpooning mechanism) of semiflexibly bridged electron donor–acceptor systems we have studied a derivative with two long aliphatic chains tethered to the donor and acceptor moieties, respectively. The fitting of the time- and wavelength-resolved fluorescence curves required three components whose lifetimes varied smoothly with pressure (solvent viscosity). It is concluded that these three components correspond to three distinct conformations of the charge transfer state. Consequently, the previously established reaction scheme must be expanded by introducing an additional conformer termed the intermediate charge transfer (ICT) state, which differs from the previously defined extended charge transfer (ECT) state by rotation around the single bond connecting the acceptor to the bridge. Due to the enlarged hydrodynamic volume, this rotation is slowed down sufficiently to be easily detectable (1.7 × 10⁸⩽k_rot/s⁻¹⩽16 × 10⁸). The rate of the folding process of the bridge leading from the ICT to the compact (CCT) conformational state is also reduced by a factor of 5 compared to the parent compound (5 × 10⁷⩽k_fold/s⁻¹⩽40 × 10⁷).