Long-Lived Trimetallic Complexes of Fe(II), Ru(II), and Os(II) Based on a Heteroditopic Bipyridine-Terpyridine Bridge: Synthesis, Photophysics, and Electronic Energy Transfer.

Synthesis, characterization, and investigation of photophysical and redox behaviors of a new class of homo- and heterotrimetallic complexes of composition [(bpy/phen)Ru(dipy-Hbzim-tpy)M(tpy-Hbzim-dipy)Ru(bpy/phen)] (M = Fe, Ru, and Os) derived from a conjugated heteroditopic bipyridine-terpyridine bridge were carried out in this work. Trimetallic RuZnRu complexes of composition [(bpy/phen)Ru(dipy-Hbzim-tpy)Zn(tpy-Hbzim-dipy)Ru(bpy/phen)] were also synthesized in situ as their photophysical properties are of particular interest in demonstrating the absorption and emission spectra of the complexes in the presence of a metal (Zn) that has neither metal-to-ligand charge transfer (MLCT) nor metal-centered (MC) states. Complexes display intense absorption bands spanning almost the entire UV and visible region. The complexes also exhibit rich electrochemical behaviors with a number of metal-centered reversible oxidation and ligand-centered reduction waves. All complexes are luminescent at room temperature, and time-resolved emission spectral studies indicate that peripheral Ru-centered emissive MLCT states are quantitatively quenched, by intramolecular energy transfer to the low lying MLCT (for central Ru and Os) or MC states of the Fe center (nonluminescent). Interestingly, Fe(II) does not adversely deteriorate the photophysics of the RuFeRu assembly. Thus, multicomponent complexes in the present work can serve as well-organized light-harvesting antennas as the light absorbed by multiple chromophoric subunits is efficiently channeled to the distinct component having the lowest-energy excited state.