Electronic and Photophysical Properties of [Re (L)(CO)3(phen)](+) and [Ru(L)2(bpy)2](2+) (L = imidazole), Building Units for Long-Range Electron Transfer in Modified Blue Copper Proteins.

The electronic, optical, and photophysical properties of Re(im)(CO)3(phen) and Ru(bpy)2(im)2 (im = imidazole; phen = 1,10-phenanthroline; bpy = 2,2'-bipyridine) in water, including spin-orbit coupling (SOC) effects, were studied by means of density functional theory (DFT) and time-dependent DFT. The main features of the visible experimental absorption spectra of both molecules are well-reproduced. Whereas the theoretical spectrum of the Re(I) complex is characterized by one metal-to-ligand charge transfer (MLCTphen) state of low intensity at 394 nm and a strongly absorbing MLCTphen state calculated at 370 nm, the spectrum of the Ru(II) complex presents a high density of singlet MLCTbpy excited states with significant oscillator strengths that contribute to the two broad bands centered at 490 and 340 nm. The absorption spectrum of Re(im) (CO)3(phen) is perturbed by SOC with non-negligible mixing between the low-lying triplet and singlet absorbing states, while SOC has no effect on the absorption spectrum of Ru(bpy)2(im)2. A detailed structural investigation of the two lowest singlet and four lowest triplet excited states of the Re(I) complex point to MLCTphen (S1, S2, T1, T2) and intra-ligand ILphen (T3) localized spin-densities characterized by small contractions from both Re-N and phen CC central bonds in the MLCT states and nearly no deformation in the IL state. A mechanism of luminescent decay of Re(im) (CO)3(phen) is proposed on the basis of the calculated energy minima and wavelengths of emission for the interpretation of the three frequency/time-scale signals put in evidence by ultrafast experiments. The long-lived emissive properties of Ru(bpy)2(im)2 are analyzed on the basis of the relative energies of the two lowest (3)MLCTbpy and metal-centered (3)MC excited states. The minimum corresponding to the (3)MC spin density shows a significant structural rearrangement with an increase of the Ru-N bond distance of 0.33 Å and a closure of the N-Ru-N bond angle of 20° inducing a large distortion of the octahedral motif. The spin-density associated with the lowest (3)MLCTbpy localized on one bpy ligand suggests the presence of a second degenerate (3)MLCTbpy minimum. The luminescence of the Ru(II) complex calculated at 669 nm is partially quenched by the presence of the low (3)MC nonradiative state at 1064 nm. When interacting with modified metal-based proteins the two complexes will behave differently because of these distinctive photophysical properties.