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 supramolecular trichromophoric systems resulting from the metal-directed self-assembly of chromophore-labeled ligand molecules. The trichromophoric gallium(III) chelate, (1)3Ga, shows an intense pyrene excimer fluorescence emission in acetonitrile, while the iron(III) analogue, (1)3Fe, 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, whereas the ferric center acts additionally as an extra chromophore that quenches the pyrene singlet state. On the basis of low-temperature fluorescence measurements, the mechanism of quenching within (1)3Fe is attributed to a pyrene-to-iron electronic energy transfer process. 1H NMR and electronic absorption spectroscopies show that intramolecular interactions between pyrene chromophores prevail in the ground state, which leads to the formation of preassociated excimers for both chelates. From the dependence of the solvent on the photophysical properties of (1)3Ga and (1)3Fe, it is inferred that the Fe(III)-induced quenching process is triggered by the ground-state and excited-state intramolecular dimerization of the pyrene moieties, with the pyrene-to-iron energy transfer taking place more readily from the excimer singlet state as donor than from the locally excited state.