The complexes CIRe(CO),L [L = N-(2-pyridinylmethylene)phenylamine , N-(2-pyridinylmethylene)cyclohexylamine (2-PC)] have been prepared and structurally characterized by X-ray diffraction. Their absorption spectra at 298 K, emission spectra and luminescence lifetimes at 77 K, and redox potentials are also reported. The absorption and emission are assigned to MLCT transitions, both of which blue shift upon cooling to 77 K. Increased polarity of the solvent also causes a blue shift in the MLCT absorption. The correlations between the electrochemical properties and both the absorption and emission energies are discussed.The photophysics, photochemistry, and redox properties of a,a'-diimine complexes of ruthenium] have been prolific areas of research over the past 15 years. The Ru(dr)diimine(?r*) metal-to-ligand charge-transfer (MLCT) excited states usually dominate the photophysics and photochemistry of these complexes. Although much less attention has been given to a,a'-diimine complexes of rhenium and osmium, early studies by Wrighton2 and more recent studies by Meyer3 show that they also typically exhibit MLCT absorptions and emission. To date, most studies of Re'(a,a'-diimine) complexes have been limited to either complexes of 2,2'-bipyridine or 1,lO-phenanthroline or their substituted deri~atives.~-~-l' The photophysics and photochemistry of these complexes can be "fine-tuned" to some extent by varying the substituents on the bipyridine or phenanthroline ligand. Since the ?r* level of t h e a,a'-diimine ligand is central to the MLCT absorption and emission properties, complexes containing other structural types of a&-diimine ligands should be explored. We report here the structural, electrochemical, and spectral properties of two new C1Re(CO)3(a,a'-diimine) complexes containing the following a,a'-diimine ligands: -2-PP 2-PC Both ClRe(C0),(2-PP) (1) and ClRe(C0)3(2-PC) (2) show unusually low MLCT transition energies and extremely short emission lifetimes at 77 K. Complex 1 (Figure 1) has the shortest emission lifetime and one of the lowest MLCT energies reported to date for such complexes. Experimental SectionThe ligands 2-PP and 2-PC were synthesized by refluxing 2pyridinecarboxaldehyde with a stoichiometric amount of the appropriate amine in toluene for about 12 h using a Stark trap. Details on the synthesis, purification, and characterization of the ligands 2-PP and 2-PC and the rhenium complexes 1 and 2 are available as supplementary material. All rhenium complexes ClRe(CO),L, where L = 2-PP. 2-PC, bpy (2,2'-bipyridine), phen (1,lO-phenanthroline), and dmphen (4,7-dimethyl-1,lO-phenanthroline), were synthesized for this study by the method of WrightonZb using CIRe(CO)5 and the appropriate ad-diimine ligand. Detailed X-ray diffraction data for 1 and 2 are also included in the supplementary material. Absorption and emission data were obtained using standard techniques as described in the l i t e r a t~r e .~~. ' * I~ Cyclic voltammograms were obtained on acetonitrile solutions containing 0.1 M tetrabutylammoniu...