High‐Temperature Solid‐State Post‐Synthetic Modification of Highly Luminescent Cu(I) Metallacycles toward New Luminescent Thermic Tracers

A new luminescent Cu(I) tetrametallic metallacycle B is reported that features very rare semi‐bridging aqua ligands. When heated markedly above room temperature, this compound undergoes a post‐synthetic transformation in the solid‐state, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single‐crystal X‐ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, but is caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye‐perceived green (B) to blue (C) room‐temperature luminescence for these molecular solids. Photophysical measurements and time‐dependent density‐functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, and suggest that thermally activated delayed fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering access to stimuli‐sensitive materials that can witness, a posteriori, the exceeding of critical temperatures in their environment. The TADF luminescent Cu(I) metallacycle B carrying unusual semi‐bridging aqua ligands has been prepared in a one‐step high‐yield reaction. When B is heated markedly above room temperature, a solid‐state reaction affords a new metallacycle C displaying TADF luminescence different from that of B. The removal of the aqua ligands and the formation of cuprophilic interactions makes this transition an innovative example of a solid‐state photoactive tracer.