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The development of novel metal-free solid-state near-infrared (NIR) emitters is a challenging task. Herein, the facile synthesis of two organic solid-state NIR emitters emitting at 705 and 673 nm are reported. The mechanism of NIR emission is shown to be an excited-state intramolecular proton transfer through a series of experiments as well as theoretical calculations. X-ray crystallography analyses revealed that the unique solid-state packing arrangement of the molecules with a slipped face-to-face arrangement helps to prevent the π−π stacking interaction and prevent aggregation-caused quenching in these molecules. A Hirshfeld analysis was performed to quantify the contribution of intra-and intermolecular interactions to the supramolecular assembly. Appropriate control compounds were synthesized, and their fluorescence and structural aspects have been studied to confirm the proposed NIR emission mechanism. These new solid-state NIR emitters have been tested as latent fingerprint (LFP) imaging probes on a variety of porous and nonporous surfaces including aluminum foil, glass, ceramic tile, wooden surface, and stainless steel using a powder dusting approach. High-quality LFP images with high selectivity, sensitivity, contrast, and low background interference were obtained using these probes with the clear identification of up to 16 characterizable level 2 and level 3 features.

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An organic solid-state near-infrared (NIR)-emitter (λ em = 738 nm) exhibiting large Stokes shift (Δλ = 293 nm) through the excited-state intramolecular proton transfer phenomenon has been synthesized and characterized. The present discovery points to the possibility of achieving a new family of solid-state NIR emitters starting from simple aldehyde and amine precursors.

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