Solid-State Emission Enhancement via Molecular Engineering of Benzofuran Derivatives

A series of linear benzo­furan derivatives consisting of either a vinylene or a cyano­vinylene were prepared in order to investigate their emission properties. The X-ray crystallography of structurally similar derivatives was also evaluated. The crystalline structures of the vinylene derivatives showed only lateral contacts that involved the benzo­furans and no π-stacking. In contrast, π-stacking was observed for the bisbenzo­furan and benzo­furan-phenyl cyano­vinylene derivatives. No inter­molecular π–π stacking was observed for the extended cyano­vinylene structures. Inter­molecular bonding between the nitrile and a furan atom was found. The fluorescence quantum yields (Φfl) of the vinylene derivatives were consistently high (>50%) in both solution and the crystal state. The exception was the benzo­furan-furan-vinylene-phenyl, the Φfl of which was <10% when in the solid state. The cyano­vinylene counterparts emitted weakly in solution (Φfl < 2%). Their luminogenic property was demonstrated with a ca. 15-fold increase in emission in the solid state. A 6-fold emission enhancement was also found when they were aggregated in a 90 vol% methanol/water mixture. The solid-state emission enhancement of the cyano­vinylene benzo­furans was in part attributable to inter­molecular contacts that suppressed excited-state deactivation by molecular motion.