Fluorescent Metallacycles via Coordination-Driven Self-Assembly: Preparation, Regulation, and Applications

Fluorescence by small molecular dyes is renowned for its real-time, dynamic, and noninvasive nature. It has become indispensable across scientific domains, including information storage, optoelectronic materials, biosensing, and both diagnosing and treating diseases. Despite their widespread use, these molecular dyes suffer from several limitations due to the sensitivity of their photophysical properties to environmental factors, such as concentration, solvent composition, and polarity. These challenges become particularly prominent when assembling or aggregating fluorescent molecules; their optical characteristics often become unpredictable or uncontrollable. Alternative strategies to stabilize and tune fluorescence during preparation are therefore crucial.Metal coordination, a classical approach in supramolecular chemistry, offers a promising solution. Coordinating fluorescent dyes to metals precisely directs self-assembly, ensuring defined stoichiometries, geometries, and reversibility. The resulting multifunctional metallacycles combine the advantages inherent to molecular design and fluorescence, pushing the boundaries of fluorescence-based assemblies. We present a modular, directional, and controllable strategy for the self-assembly of supramolecular metallacycles with well-defined geometries, providing a new avenue to address the limitations of traditional small molecular dyes.A key innovation in this research lies in the incorporation of photochromic units into the metallacycles, tuning their photophysical properties reversibly under external illumination. Their emission wavelengths, chiralities, and circularly polarized luminescence (CPL) signals can all be modulated dynamically. These characteristics offer the potential for holographic imaging, where fine control of fluorescence behavior is crucial. We introduce a novel multistep Forster resonance energy transfer (FRET) strategy that enables real-time monitoring of the metallacycle assembly dynamics. Our FRET approach has been employed to develop photosensitized oxygenation reactions and highly efficient light-harvesting systems, highlighting its versatility. The unique photophysical properties of our fluorescent metallacycles have been applied successfully in several fields. They detect heparin quantitatively, showcasing their potential in biosensing. They have been integrated into nanoagents for photothermal, photodynamic, and chemotherapeutic therapies guided by imaging, offering a multimodal approach to therapeutic intervention. Such precise control over fluorescence, energy transfer, and assembly dynamics not only opens new avenues in materials design but also underscores supramolecular metallacycles' potential for advancing fluorescence technologies. Integrating metal coordination into fluorescence represents a significant step in the design and application of functional fluorescent metallacycles. This design strategy both advances fundamental supramolecular chemistry and provides new insights into photophysical systems for sensing, imaging, and therapeutics.