Dual Nature of Thiophene Harnessed in the Design of Near-Infrared Emitters via Antiaromaticity Relief

Near-infrared (NIR) emitters are of great interest for applications in bioimaging and modern technology. Yet the design of such materials with decent characteristics is challenging due to intrinsic limitations. In a recent article, Murai and Yamaguchi report the synthesis of NIR emitters with appreciable fluorescence quantum yields reaching 0.02 at 878 nm in CH2Cl2 solution. The low band gaps were achieved by a new design strategy exploiting antiaromaticity relief. This concept was realized for compounds consisting of an antiaromatic azepine central ring fused to thiophene moieties. In these systems, thiophene unfolds its dual nature. On the one hand, it contributes to the high antiaromaticity of azepine; on the other hand, it exerts a stabilizing effect on azepine through the formation of a quinoid structure, which reduces its antiaromaticity and shifts the absorption and emission maxima into the NIR region. Murai and Yamaguchi report on near-infrared emitters derived from the dithieno[2,3-b : 3 ',2 '-f]azepine core. Low band gaps were achieved by fusing antiaromatic azepine with thiophene rings equipped with electron-accepting moieties, which relieves the antiaromaticity of azepine through the formation of a quinoid structure. This novel approach provides dyes with fluorescence quantum yields of up to 0.02 at 878 nm.image