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]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 (MLCT_phen) state of low intensity at 394 nm and a strongly absorbing MLCT_phen state calculated at 370 nm, the spectrum of the Ru­(II) complex presents a high density of singlet MLCT_bpy 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]²⁺. A detailed structural investigation of the two lowest singlet and four lowest triplet excited states of the Re­(I) complex point to MLCT_phen (S₁, S₂, T₁, T₂) and intra-ligand IL_phen (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]²⁺ are analyzed on the basis of the relative energies of the two lowest ³MLCT_bpy and metal-centered ³MC excited states. The minimum corresponding to the ³MC 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 ³MLCT_bpy localized on one bpy ligand suggests the presence of a second degenerate ³MLCT_bpy minimum. The luminescence of the Ru­(II) complex calculated at 669 nm is partially quenched by the presence of the low ³MC nonradiative state at 1064 nm. When interacting with modified metal-based proteins the two complexes will behave differently because of these distinctive photophysical properties.