METAL-INDUCED SELF-ASSEMBLY OF A PYRENE-TETHERED HYDROXAMATE LIGAND FOR THE GENERATION OF MULTICHROMOPHORIC SUPRAMOLECULAR SYSTEMS - THE PYRENE EXCIMER AS SWITCH FOR IRON(III)-DRIVEN INTRAMOLECULAR FLUORESCENCE QUENCHING

We report the first example of intramolecular excimer formation in sup ramolecular trichromophoric systems resulting from the metal-directed self-assembly of chromophore-labeled ligand molecules. The trichromoph oric gallium(III) chelate, (1)(3)Ga, shows an intense pyrene excimer f luorescence emission in acetonitrile, while the iron(III) analogue, (1 )(3)Fe, exhibits a fluorescence of very low intensity compared to that of the free hydroxamate ligand 1. The trivalent gallium metal cation is demonstrated to play the role of an inert and transparent linker, w hereas the ferric center acts additionally as an extra chromophore tha t quenches the pyrene singlet state. On the basis of low-temperature f luorescence measurements, the mechanism of quenching within (l)(3)Fe i s attributed to a pyrene-to-iron electronic energy transfer process. H -1 NMR and electronic absorption spectroscopies show that intramolecul ar interactions between pyrene chromophores prevail in the ground stat e, which leads to the formation of preassociated excimers for both che lates. From the dependence of the solvent on the photophysical propert ies of (1)3Ga and (1)3Fe, it is inferred that the Fe(III)-induced quen ching process is triggered by the ground-state and excited-state intra molecular dimerization of the pyrene moieties, with the pyrene-to-iron energy transfer taking place more readily from the excimer singlet st ate as donor than from the locally excited state.