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

Two tetraphenylethylene (TPE) bridged tetraimidazolium salts, [II4L-Et](PF6) 4 and [II4L-Bu](PF6)(4), were used as precursors for the synthesis of the dinuclear Ag-I and Au-I tetracarbene complexes [Ag-2(L-Et)](PF6)(2), [Ag-2(L-Bu)]( PF6)(2), [Au-2(L-Et)](PF6)(2), and [Au-2(L-Bu)](PF6)(2). The tetraimidazolium salts show almost no fluorescence (Phi(F)< 1%) in dilute solution while their NHC complexes display fluorescence turn-on (Phi(F) 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).