Quenching of Fluorescence in Bodipy-Derived Trifluoromethyl Enaminone Ligands upon Coordination to Copper(II)

The synthesis of two trifluoromethyl-derived enaminone ligands, L1H (bidentate) and L2H(2), (tridentate), which bear a fluorescent 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (Bodipy) moiety, leads to the preparation and characterization of two Cu-II complexes {[Cu(L1)(2)]2(CH2Cl2) (A) and [Cu-4(L2)(4)]4(C4H8O2) (B)} that exhibit different architectures. The magnetic properties of the cubane-like complex B evidence a dominant ferromagnetic behavior characterized by exchange coupling constants J(1) = +20.56(3) cm(-1) and J(2) = -10.29(2) cm(-1). Indeed, whereas the luminescence properties of the Bodipy-derived trifluoromethyl enaminones L1H and L2H(2) are typical of the Bodipy skeleton, the emission is totally quenched in both mononuclear (A) and cubane (B) Cu-II complexes. To elucidate this quenching phenomenon, the Rehm-Weller equation has been used based on experimental cyclic voltammetry redox potential measurements. The result remains ambiguous for complexes A and B, and the origin of the quenching phenomenon, photoinduced electron transfer (PET) versus energy transfer (Forster or Dexter), cannot be deduced.