Synthesis, Fluorescence, and Two-Photon Absorption Properties of Push-Pull 5-Arylthieno[3,2-b]thiophene Derivatives

Three series of novel push–pull 5‐arylthieno[3,2‐b]thiophene derivatives functionalized with potent electron‐withdrawing terminal moieties have been synthesized in moderate to excellent yields by Suzuki coupling followed by Knoevenagel condensation. These novel chromophores show intense absorption in the near‐UV region through to the orange visible region, related to a strong intramolecular charge‐transfer transition. By combining a strong donor and acceptor, large fluorescence quantum yields were achieved as well as large two‐photon absorption responses. Interestingly, due to the improved rigidity and electronic delocalization provided by the thienothiophene moiety (as compared with the bithiophene unit), higher one‐ and two‐photon brightness values have been achieved. As a result, several new fluorescent dyes showing enhanced brightness and tunable fluorescence (ranging from the blue to the NIR region) have been obtained that have potential for bioimaging applications. Fusion makes it better! Push–pull compounds built from a thienothiophene π‐connector have been synthesized. Due to improved rigidity and conjugation, derivatives with strong donor end groups have been shown to combine strong and tunable fluorescence with large two‐photon absorption in the NIR region. These dyes provide a route towards bright fluorophores spanning the visible region to the NIR.