Synthesis and characterization of luminescent metal-silicon 8-quinolyl chelate complexes of rhodium and iridium

An investigation was conducted in the photophysics of a new class of luminescent transition metal complexes containing the metal-silicon sigma-bond chromophore. A series of 8-(diorganosilyl)quinoline and bis(8-quinolyl)tetramethyldisilane compounds having methyl and/or phenyl substituents on the silyl group were prepared to serve as N^Si-chelate precursors for a variety of rhodium andiridium metal-silyl complexes. The N^Si-chelate precursors were used to preparea number of unique bis, and tris(N^Si-chelate) derivatives of Rh(III) andmono, bis, and tris(N^Si-chelate) derivatives of Ir(III) through oxidativeaddition reactions with respective Rh(I) and Ir(I) starting materials. The metal complexes were characterized by elemental analysis, NMR and mass spectroscopy, and for several compounds, x-ray crystallography. In some instances, reaction intermediates observed by 1H NMR spectroscopy in reactions using the 8-(diorganosilyl)quinoline precursors were used to propose reaction mechanisms for the formation mono, bis and tris(N^Si-chelate) products. The redox properties of the metal complexeswere examined using cyclic voltammetry while photophysical characterization was conducted using a combination of absorption, emission and time resolved spectroscopy. The values of the oxidation potentials are consistent with an interpretation of the highest occupied molecular orbital having a strong metal-silicon bonding character. The complexes displayed photophysical properties that strongly varied depending on the identity of the N^Si-chelate or ancillary ligandsor the identity of metal center (Rh or Ir). Luminescence spectra recorded in glassy solvents at 77K ranged between vibronically structured, high energy emission bands with decay lifetimes in the millisecond range to broad, featureless low energy emission with microsecond decay lifetimes. The highly structured luminescence is ascribed to emission from a triplet pi*-state on the quinolyl moiety that is strongly perturbed by coordination to the heavy metal center, whereas the low energy emission originates from a triplet state that can best be described as having a metal-silicon, sigma-bond to ligand charge-transfer (SBLCT) character. Several of the tris(N^Si-chelate) chelated Ir complexesare also luminescent in fluid solution at room temperature. The presence of visible emission under ambient conditions suggests that these complexes with SBLCT excited states could be employed in applications that proceed by photo-initiated electron transfer reactions.