π-Bridge-Engineered Near-Infrared Schaap’s Chemiluminescent Probes for Precise Tumor Diagnosis and Imaging-Guided Surgery

Chemiluminescence imaging exhibits markedly superior signal-to-noise ratios and tissue penetration compared with fluorescence imaging, as it requires no external excitation light. However, chemiluminescent molecules that simultaneously offer near-infrared (NIR) emission and long-term stability remain rare. In this paper, we present a design strategy for relatively stable NIR-emissive Schaap's chemiluminophores, in which 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile was used as the electron acceptor, and thiophene and its derivatives were used as π-conjugation bridges. Four chemiluminophores (ACHCL, ACHTCL, ACHTTCL, and ACHBTCL) were synthesized with the phenolic hydroxyl group caged by an H₂S-cleavable protecting group. Among them, ACHTTCL, with the π-bridge of thieno[3,2-b]thiophene, showed NIR-II emission and the highest brightness due to its extended and rigidified backbone. Encouragingly, after the expansion of the conjugated structure, the stability was not significantly affected, and over 70% of the molecules of the four chemiluminophores remained in their original structure even after three months of storage at room temperature. Such excellent storage stability is highly advantageous for transportation and end-use applications. Using the chemiluminophores as contrast agents, we achieved H₂S-activated chemiluminescence imaging and sensitive tumor detection. These NIR Schaap's chemiluminophores exhibited strong emission with high imaging signal-to-noise ratios, and enabled precise tumor diagnosis and chemiluminescence imaging-guided surgical navigation for the resection of deep-seated tumors.