FLUORESCENCE REDOX SWITCHING SYSTEMS OPERATING THROUGH METAL CENTERS - THE NI-III NI-II COUPLE/

The covalently linked two-component systems 3 and 4 display fluorescen ce redox switching activity: the Ni-III form quenches the fluorescence of the proximate aromatic fluorophore, whereas the Ni-II form does no t. Thus, fluorescence can be switched on and off at will through the r eversible Ni-II/Ni-III redox reaction, which is carried out both elect rochemically (in MeCN) and chemically (in EtOH). Quenching of the exci ted fluorophore F- is ascribed to a thermodynamically favoured F-*-to -Ni-III electron transfer mechanism. The more flexible system 5 does n ot work as a switch, since the fluorescence of the anthracene subunit is quenched in both Ni-II and Ni-III forms (an OFF/OFF situation), thr ough an energy transfer mechanism. The crystal and molecular structure of 4 in its protonated form is also described.