SUBSTITUENT CONSTANT CORRELATIONS AS PREDICTORS OF SPECTROSCOPIC, ELECTROCHEMICAL, AND PHOTOPHYSICAL PROPERTIES IN RING-SUBSTITUTED 2,2'-BIPYRIDINE COMPLEXES OF RHENIUM(I)

The synthesis and the spectroscopic, electrochemical and photophysical properties of the homologous series of photosensitizers, fac-[4,4'-X2-5,5'-Y2-2,2'-bipyridine]Re(CO)3Etpy+ (X = NEt2, Me, OMe, H, Ph, Cl, CO2Me, NO2, Y = H; X = Y = Me; Etpy = 4-ethylpyridine) are described. Both the quasi-reversible or irreversible oxidation of the Re(I) center and the reversible or quasi-reversible, one-electron reduction of the coordinated bipyridyl ligand are observed to vary with the electron donor/acceptor abilities of X, Y as measured by the sum of the Hammett substituent constants sigma(m) + delta(p). Hammett (sigma(T) = sigma(p) + sigma(m)) values for the X, Y groups are observed to correlate linearly with the metal-to-ligand charge-transfer (MLCT) absorption and emission energies and provide a convenient tool for the estimation of excited-state properties of the complexes. The complexes are moderately strong excited-state oxidants (E1/2(+*/0)) = 0.73-1.12 V vs SSCE) and exhibit emission maxima in the range 528 nm (X = NEt2, Y = H) to 755 nm (X = NO2, Y = H). MLCT excited-state decay is dominated by nonradiative decay from the 3MLCT state to the ground state and is governed by an "energy gap law". It is shown that excited-state properties such as absorption energy, emission energy, rate of nonradiative decay (k(nr)), and the rate of radiative decay (k(r)) correlate with sigma(T). The correlations can be derived from more fundamental considerations. The dependence of 1n k(nr) on emission energy is similar to values obtained in earlier studies with bipyridyl Ru(II) or Os(II) complexes which do not contain coordinated CO and somwhat less than that observed in a previous study for fac-(bpy)Re(CO)3(L')+ complexes (L' = monodentate, neutral ligands). This behavior is interpreted as an indication that nonradiative decay involves acceptor modes that are predominantly ring-based vibrations of the substituted bipyridine ligand, but that CO modes are also involved.