Synthesis, electrochemical and optical properties of Ru(II)-diphenylphenanthroline-ethynyl-pyrene-phenanthroline systems

The synthesis, electrochemical and optical spectroscopic properties of a Ru(II)-diphenylphenanthroline complex decorated with ethynylpyrene (EP) appendages, 1, and those of its parent bromo-substituted complex, 2, are reported; complex 1 is [Ru(dpp)(2)(phen-EP(2))](PF(6))(2) and 2 is [Ru(dpp)(2)(phen-Br(2))](PF(6))(2), where dpp is 4,7-diphenyl-1,10-phenanthroline, phen-EP is 5,6-(1-ethynylpyrene)-1,10-phenanthroline and phen-Br2 is 5,6-dibromo-1,10-phenanthroline. In CH(3)CN solvent, both bromo- and EP-substituted complexes are redox active and metal-centred and ligand-centred processes are assigned with reference to the analogous steps for [Ru(phen)(3)](2+) and to steps for the EP appendages. The complexes exhibit strong absorption bands in the near-UV and visible regions due to ligand-centred (LC) and metal-to-ligand charge-transfer (MLCT) transition, respectively. For 1, the lowest-lying absorption band is slightly redshifted due to better delocalisation at the phen-EP ligand. At room temperature and in O(2)-free solvent, complex 1 appeared to be practically nonlummescent, whereas 2 exhibited intense (3)MLCT emission; quantum yields of f(em) = 9 x 10(-4) and 6.1 x 10(-2) were found for 1 and 2, respectively and lambda(exc) = 462 nm in both cases. The lowest-lying excited states of 1 were assigned as (3)LC states, which were localised on the EP moiety. These states are nonluminescent, which thus explains the weak room-temperature luminescence for this complex. At 77 K, 2 is emissive, as expected for (3)MLCT levels; however, a moderately intense (3)MLCT emission was also observed for I at this temperature. For the latter case, this outcome is tentatively explained in terms of local trapping of (3)MLCT states at the 4,7-diphenylphenanthroline ligands free of EP appendages.