Tuning the Solid State Luminescence of Benzofuran-Cyanostilbenes by Functionalization with Electron Donors or...

Three series of linear extended benzofuran derivatives associating cyanovinyl unit and electron withdrawing systems such as paracyanophenyl (series 1) and pentafluorophenyl (series 2) units or the electron donor 3,4-dimethoxyphenyl (series 3) moiety have been prepared. The donor character of the benzofuran part is adjusted either without modifying the conjugation by adding electron-donating methoxy groups to positions 5 and 6 of the benzofuran (compounds BF2 and BF3) or by increasing the extension of the conjugation with the naphthofuran unit (BF4), and by the insertion of a furan ring between the benzofuran and the cyanovinylene bond (BF5). While most of the compounds show very low emission in solution, microcrystalline powders of almost all compounds show middle at strong emissions under excitation at 350-400 nm with emission colors ranging from blue - green to red. It is shown that this variation of the emission colors is essentially due to the type of stacking of the molecules in the solids for series 1 and 2. For series 3, it is above all the extension of the conjugation of the compounds which leads to the red shift.