Photoinduced charge separation in cyclohexylidene-based donor - (σ-bridge) - acceptor compounds - Building blocks for materials

We describe the syntheses and properties of five semi-rigid donor - (σ-bridge) - acceptor (D-σ-A) compounds. These compounds, which are candidates for incorporation in materials, have aniline-type donors and (mixed) cyano- and alkoxycarbonyl-1,1-difunctionalized vinyl acceptors that are separated by a four-σ-bond bridge. Their electronic properties were studied by means of photoelectron spectroscopy in combination with ab initio MO calculations, cyclic voltammetry, UV-Vis absorption spectroscopy, (time-resolved) fluorescence spectroscopy, and time-resolved microwave conductivity (TRMC). We found that these D-σ-A compounds exhibit a weak intramolecular charge-transfer absorption band that borrows its intensity from local transitions. Upon photoexcitation, a charge-separated state is generated in virtually quantitative yield. Conformational contraction or "folding" of this state takes place for the two D-σ-A compounds with a dicyanovinyl acceptor in apolar solvents. The absence of folding in compounds with an alkoxycarbonyl-functionalized acceptor is attributed to a reduced lifetime of the charge-separated state.