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.