Dual-state emission versus no emission by manipulating the molecular structures of cyanovinyl–benzofuran derivatives†

A very small change in the molecular structure of cyanostilbene derivatives leads to high luminescence both in solution and in the solid state. For benzofuran–cyanovinyl–pentafluorophenyl derivatives, the simple transfer of the cyano group from the β position of the benzofuran moiety to the α position induces strong changes in the emission properties: from a non-emissive compound for the β-isomer to dual state emission for the α-isomer with quantum yields reaching 80% in solution and 42% in the solid state. In solution, this behaviour difference illustrates the concept of fluorescence regulation by the more or less lowering of Franck–Condon energy through positional isomers. In the solid state, the stacking modes, mainly due to π–π_F intermolecular interactions between the benzofuran and perfluorophenyl units, induce either a positioning between two stacked molecules allowing a photoinduced cycloaddition under excitation and no luminescence properties, or in contrast, high luminescence due to the rigidification of the structure that suppresses the non-radiative deactivation modes.