Zinc-specific intramolecular excimer formation in TQEN derivatives: fluorescence and zinc binding properties of TPEN-based hexadentate ligandsElectronic supplementary information (ESI) available: The experimental procedure for synthesis of the compounds, Tables S1-S3, Scheme S1, Fig. S1-S31 and crystallographic data in CIF format. CCDC 998434-998437. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01847h

Zn 2+ -induced fluorescence enhancement of the TPEN ( N , N , N ′, N ′-tetrakis(2-pyridylmethyl)ethylenediamine)-based ligand, N , N -bis(1-isoquinolylmethyl)- N ′, N ′-bis(pyridylmethyl)ethylenediamine ( N , N -1-isoBQBPEN, 1b ), has been investigated. Upon Zn 2+ binding, 1b shows a fluorescence increase ( Zn = 0.028) at 353 and 475 nm. The fluorescence enhancement at longer wavelengths is due to intramolecular excimer formation of two isoquinolines and is specific for Zn 2+ ; Cd 2+ induces very small fluorescence at 475 nm ( I Cd / I Zn = 10%). The excimer formation of the [Zn( 1b )] 2+ complex in the excited state is supported by the time-dependent DFT calculation. Neither long-wavelength fluorescence nor excimer formation is observed in the Zn 2+ complex of N , N ′-1-isoBQBPEN ( 2b ). The quinoline analog N , N -BQBPEN ( 1a ) exhibits similar but significantly smaller excimer formation. Thermodynamic and kinetic comparisons of Zn 2+ binding properties of ethylenediamine-based hexadentate ligands with pyridines and (iso)quinolines are comprehensively discussed. The Zn 2+ -specific fluorescence enhancement of N , N -1-isoBQBPEN ( 1b ) at longer wavelengths is due to the intramolecular excimer formation, which is elucidated by time-dependent DFT calculation.