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

• Published in: Angewandte Chemie International Edition Vol. 56; no. 10; pp. 2785 - 2789
• Main Authors: Sinha, Narayan, Stegemann, Linda, Tan, Tristan T. Y., Doltsinis, Nikos L., Strassert, Cristian A., Hahn, F. Ekkehardt
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.03.2017
• Edition: International ed. in English
• Subjects: Agglomeration; Complexation; Dilution; Emission; Emissions; Fluorescence; Metals; organometallic complexes; Phosphorescence; poly-NHC ligands; rigidification; Salts; tetraphenylethylene; organometallic complexes; fluorescence; tetraphenylethylene; rigidification; poly-NHC ligands
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201610971

Two tetraphenylethylene (TPE) bridged tetraimidazolium salts, [H4L‐Et](PF6)4 and [H4L‐Bu](PF6)4, were used as precursors for the synthesis of the dinuclear AgI and AuI tetracarbene complexes [Ag2(L‐Et)](PF6)2, [Ag2(L‐Bu)](PF6)2, [Au2(L‐Et)](PF6)2, and [Au2(L‐Bu)](PF6)2. The tetraimidazolium salts show almost no fluorescence (ΦF<1 %) in dilute solution while their NHC complexes display fluorescence “turn‐on” (ΦF 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). Stop rotating! Tetraphenylethylene (TPE) bridged tetrakis‐NHC precursors display fluorescence turn‐on in dilute solution upon formation of organometallic chelate complexes. The fluorescence enhancement can be attributed to the rigidification of the TPE moiety, caused by restriction of its intramolecular rotation. This ridigification method for the fluorescence turn‐on constitutes an alternative approach to aggregation‐induced emission.