Mono- and Rare Trinuclear Zn(II) Complexes with Near-Infrared Emissive Ligands: Anion-Responsive Nuclearity Control, Interconversion, Solid-State NIR Emission, and Latent Fingerprint Imaging

Abstract

The ability to predictably control the nuclearity of metal complexes is vital in developing functional coordination compounds and metal–organic frameworks. However, consistent nuclearity control achieving complexes of different nuclearities using a series of ligands is difficult, especially when the metal ion involved does not show any crystal field stabilization energy for a particular geometry. Herein, a series of three mononuclear (M1–M3) and the corresponding trinuclear (T1–T3) Zn(II) complexes have been synthesized from a series of near-infrared (NIR)-emitting ligands (HL1–HL3) through counterion control (ZnCl2 vs Zn(OAc)2). The structural analyses revealed a regular distorted tetrahedral geometry for the mononuclear complexes, M1–M3. However, the complexes T1–T3 showed a rare trinuclear structure, in which two deprotonated ligand molecules coordinate with three Zn(II) centers such that a central octahedral Zn ion is connected to two terminal tetrahedral Zn ions through phenolate and acetate bridging groups. Intramolecular π–π stacking interactions between the constituting ligands play significant roles in forming such a rare geometry. The plausible reasons for the counterion-controlled nuclearity have been proposed along with successful interconversion of trinuclear complexes to mononuclear complexes at 65 °C. The complexes M1–M3 and T1–T3 exhibited substituent and nuclearity-dependent photophysical properties. In the solid state, two of the mononuclear complexes, M1 and M2, displayed NIR emissions (661–670 nm), while the trinuclear complexes T1–T3 emitted yellow/orange fluorescence (572–607 nm). The complexes M2 and T1 were successfully tested as latent fingerprint (LFP) imaging agents on several non-infiltrating and semi-infiltrating substrates, including everyday household objects, using the powder dusting method. In all these cases, highly fluorescent LFP images with high sensitivity, selectivity, and contrast and low background interference have been obtained.