Turn‐On Fluorescence in Tetra‐NHC Ligands by Rigidification through Metal Complexation: An Alternative to Aggregation‐Induced Emission

Abstract

Two tetraphenylethylene (TPE) bridged tetraimidazolium salts, [H L-Et](PF ) and [H L-Bu](PF ) , were used as precursors for the synthesis of the dinuclear Ag and Au tetracarbene complexes [Ag (L-Et)](PF ) , [Ag (L-Bu)](PF ) , [Au (L-Et)](PF ) , and [Au (L-Bu)](PF ) . The tetraimidazolium salts show almost no fluorescence (Φ <1 %) in dilute solution while their NHC complexes display fluorescence "turn-on" (Φ up to 47 %). This can be ascribed to rigidification mediated by the restriction of intramolecular rotation within the TPE moiety upon complexation. DFT calculations confirm that the metals are not involved in the lowest excited singlet and triplet states, thus explaining the lack of phosphorescence and fast intersystem crossing as a result of heavy atom effects. The rigidification upon complexation for fluorescence turn-on constitutes an alternative to the known aggregation-induced emission (AIE).

Alkali Blues: Blue‐Emissive Alkali Metal Pyrrolates

“…Similarly, emission enabled by confinement through metal‐ion chelation has been increasingly reported and is called the chelation‐enhanced fluorescence effect (CHEF effect) . Typically, the metal center itself is not directly involved in the excited states in these cases, as demonstrated, for example, by Hahn and Strassert for fluorescent tetra‐N‐heterocyclic carbene silver(I) complexes …”

Synthesis, Characterization, and Properties of Tetraphenylethylene‐Based Tetrakis‐NHC Ligands and Their Metal Complexes

“…Some organometallic complexes of monodentate, bidentate, and tetradentate NHC ligands with luminescent properties have been reported recently . The almost non‐emissive TPE‐bridged tetrakisimidazolium salts show large fluorescence enhancement upon formation of dinuclear silver(I) and gold(I) tetracarbene complexes in dilute solution . This behavior is ascribed to the rigidification of the TPE chromophore upon metal complex formation thereby preventing intramolecular rotation of the phenyl groups.…”

Boosting Luminescence of Planar‐Fluorophore‐Tagged Metal–Organic Cages Via Weak Supramolecular Interactions

“…The incorporation of organic fluorophores into MOCs does not only induce rigidification effect through metal complexation, but also offers an opportunity for designing add‐on sites on the vertices or edges of the targeted nanoscale polyhedra (i.e., spatial effect). Both effects may contribute to the enhancement of emission intensity and, moreover, the predefined spatial arrangement of chromophores facilitates the introduction of supramolecular interactions to modulate their luminescent properties .…”

“…The incorporation of organic fluorophores into MOCs does not only induce rigidification effect through metal complexation, but also offers an opportunity for designing add‐on sites on the vertices or edges of the targeted nanoscale polyhedra (i.e., spatial effect). Both effects may contribute to the enhancement of emission intensity and, moreover, the predefined spatial arrangement of chromophores facilitates the introduction of supramolecular interactions to modulate their luminescent properties . Inspired by newly developed strategies to achieve fluorescence enhancement of mononuclear or dinuclear metal complexes, we seek to introduce noncovalent interactions to regulate the luminescence behavior of our metal‐imidazolate cages, which were obtained through orthogonal subcomponent self‐assembly …”