title: 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, [H4L-Et](PF6)4and [H4L-Bu](PF6)4, were used as precursors for the synthesis of the dinuclear AgIand AuItetracarbene 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" (ΦFup 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).
authors.affiliation: Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149, Münster, Germany.
authors.forename: Narayan
authors.initials: N
authors.lastname: Sinha
authors.affiliation: Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Heisenbergstrasse 11, 48149, Münster, Germany.
authors.forename: Linda
authors.initials: L
authors.lastname: Stegemann
authors.affiliation: Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149, Münster, Germany.
authors.forename: Tristan T Y
authors.initials: TT
authors.lastname: Tan
authors.affiliation: Institut für Festkörpertheorie and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149, Münster, Germany.
authors.forename: Nikos L
authors.initials: NL
authors.lastname: Doltsinis
authors.affiliation: Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Heisenbergstrasse 11, 48149, Münster, Germany.
authors.forename: Cristian A
authors.initials: CA
authors.lastname: Strassert
authors.affiliation: Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149, Münster, Germany.
authors.forename: F Ekkehardt
authors.initials: FE
authors.lastname: Hahn
keywords: fluorescence; organometallic complexes; poly-NHC ligands; rigidification; tetraphenylethylene