Ir(III) and Ir(III)/Re(I) complexes of a new bis(pyrazolyl-pyridine) bridging ligand containing a naphthalene-2,7-diyl spacer: Structural and photophysical properties

A new bridging ligand has been prepared in which two chelating bidentate pyrazolyl-pyridine termini are connected to a central naphthalene-2,7-diyl core via methylene spacer units. This ligand has been used to prepared mononuclear and dinuclear Ir(III) complexes in which {Ir(F2ppy)2} [F2ppy = cyclometallating anion of 2-(3,5-difluorophenyl)-pyridine] complex fragments are coordinated to one or both of the pyrazolyl-pyridine termini; in addition a heterodinuclear complex has been prepared containing one {Ir(F2ppy)2} unit and one {Re(CO)3Cl} unit in the two binding sites. X-ray crystallographic studies show that the bridging naphthyl group lies stacked with a coordinated F2ppy ligand from a terminal {Ir(F2ppy)2} unit in every case. Luminescence measurements show that the usual strong Ir(III)-centred blue luminescence is substantially quenched by the presence of a low-lying triplet state on the naphthyl group; in the Ir(III)/Re(I) dyad we observe both weak Ir(III)-based emission as well as lower-energy Re(I)-based emission which overlap; Ir(III) → Re(I) energy-transfer occurs on a timescale of <1 ns as no rise-time for sensitised Re(I)-based emission could be detected, in contrast to other Ir(III)/Re(I) dyads in which the Ir(III) → Re(I) energy-transfer is slower (10–100 ns timescale). We ascribe this to the spatial and energetic intermediacy of the naphthyl group whose triplet energy lies between that of the Ir(III) and Re(I) termini, providing an effective conduit for energy-transfer to occur.