Facile dione protection to benzo[1,2-:6,5-']dithiophene-4,5-dione (BDTD) in triggering ultraviolet emission - A new member of the emissive 3,3'-bridged dithiophenes.

To date, 3,3'-bridged dithiophenes with bridges developed from the first period elements (either pristine or oxidised) are non emissive. Benzo[1,2-:6,5-']dithiophene-4,5-dione (BDTD) is a typical 3,3' fused-dithiophene with a dione bridge. It is a critical building block for semiconducting materials, and it is non emissive. We serendipitously discovered that by protecting the diketone of BDTD with ethylene glycol, two isomers (BDTD-5 and 6) were obtained and both compounds effectively emit UV light in solution. Their maximum emission (382 and 375 nm for BDTD-5 and 6, respectively) are independent of the type of solvent. Both compounds exhibited fluorescence intensity enhancement in DMF-HO with the increase of water fraction from 0-90%. BDTD-6 can also effectively emit in its crystalline state with a quantum yield (QY) of 14% and an average fluorescence lifetime of 1.6 ns. X-ray crystallographic analysis indicates that BDTD-6 possesses a 3D C-HO interaction structure which produced its effective emission in the crystalline state. These two isomers not only have enlarged the emissive members of the 3,3'-fused dithiophene family, but also expand the emission boundary of emitters in this category to the UV area.