| Abstract
| - The preparation and the physical and photophysical properties of three complexes containing the bridging ligand5-(2-(4-pyridyl)ethyl)-2-(2-pyridyl)pyrimidine (pypm-py) are described. Two of the complexes are monometallic,[Re(pypm-py)(py)(CO)3]+ and [Re(phen)(py-pypm)(CO)3]+, and one is bimetallic, [(CO)3(py)Re(pypm-py)Re(phen)(CO)3]2+, where phen is 1,10-phenanthroline and py is pyridine. [Re(pypm-py)(py)(CO)3](CF3SO3)crystallized in the space group P21/n with a = 13.98 (1) Å, b = 12.704 (6) Å, c = 15.66 (1) Å, and Z = 4. TheRe−N(pypm) bond distances for Re−N(py of pypm) = 2.167(9) Å and for Re−N(pm of pypm) = 2.202 (9) Å.The Re−N(py) bond distance is 2.203(9) Å. The complexes exhibit CO infrared-active bands in the 1900−2100cm-1 region of the infrared spectrum; three bands were present for [Re(pypm-py)(py)(CO)3]+, but only two forthe other complexes. Methylene protons for [Re(pypm-py)(py)(CO)3]+ are nonequivalent on the basis of NMRspectra analysis, but are equivalent for the other two complexes. Oxidation of the complexes occurs in the 1.83−1.87 V vs SSCE region and is attributed to the Re2+/+ redox couple; three to four reductions occur, the first ofwhich is assigned to reduction of the coordinated diimine ligand, either the (pypm0/--py) or the (phen0/-) coupleoccurring at ∼ −1.05 and ∼ −1.23 V, respectively. The three complexes undergo electronic excitation in the300−400 nm region assigned to dπ → π*(diimine) transitions and in the 200−300 nm region assigned to intraligandπ → π* transitions. Strong emission occurs from the complexes in solution at room temperature. The emissionbands are structureless, and the positions are solvent dependent, consistent with MLCT emitters. Quantumefficiencies and emission lifetimes in 4:1 ethanol/methanol at room temperature fall in the order [Re(phen)(py-pypm)(CO)3]+ (φem = 0.087, τ = 1.65 μs) > [(CO)3(py)Re(pypm-py)Re(phen)(CO)3]2+ (φem = 0.037, τ = 1.09μs) > [Re(pypm-py)(py)(CO)3]+ (φem = 0.014, τ = 0.11 μs), indicating that energy transfer occurs between theRe(phen) chromophore and the Re(pypm) center in the bimetallic complex.
- A comparison of the photophysical properties of the ReI monometallic complexes to those of the bimetallic complex containing an asymmetric bridging ligand reveals that energy transfer occurs from the “Re(phen)+” site to the “Re(pypm-py)+” unit enhancing its emission.
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