Extended, π‑Conjugated, Pyrene‑Based, Red‑Emissive Luminogens: Synthesis, Photophysical Properties, and Latent Fingerprint Imaging

Abstract

We report two pyrene-based D-π-A luminogens, Py-Th-CN and Py-2Th-CN, synthesized through π-conjugation engineering by incorporating mono- and bithiophene bridges between the pyrene donor and dicyano-isophorone acceptor. The crystal structures of Py-Th-CHO, a precursor to Py-Th-CN, and Py-2Th-CN have been determined. Py-Th-CN exhibits aggregation-enhanced emission in THF/H₂O aggregate system, while partial aggregation-caused quenching (ACQ) occurs with a red emission maximum at 656 nm in the solid state, whereas the more extended Py-2Th-CN shows ACQ behavior. Solvatochromic investigations confirm strong intramolecular charge transfer effects, which are further enhanced by the extended bithiophene bridge. Furthermore, Py-Th-CN was successfully applied as a fluorescent developer for latent fingerprint visualization on various substrates, producing high-contrast images with clearly distinguishable ridge details. This study provides valuable insights into the structure-property relationship of pyrene-based red emitters and offers an effective strategy for developing long-wavelength fluorescent materials for sensing and security applications.