Mono- and di-nuclear Re(i) complexes and the role of protonable nitrogen atoms in quenching emission by hydroquinone.

Abstract

Mono-nuclear fac-[Re(CO)3(ph2phen)(4,4'-bpy)]+ and di-nuclear [(ph2phen)(CO)3Re(4,4'-bpy)Re(ph2phen)(CO)3]2+ complexes, ph2phen = 4,7-diphenyl-1,10-phenanthroline and 4,4'-bpy = 4,4'-bipyridine, were synthesized and characterized by 1H NMR, UV-visible and FT-IR spectroscopy. Also, their photophysical properties were investigated using steady-state and time-resolved emission spectroscopy. Both complexes showed UV absorption assigned to intraligand, 1ILph2phen, and metal-to-ligand charge transfer, 1MLCTRe→ph2phen, transitions, and typical 3MLCTRe→ph2phen emission (ϕ = 0.360 and τ = 3.81 μs; ϕ = 0.177 and τ = 1.90 μs for mono- and di-nuclear, respectively). Additionally, the luminescence of these complexes is quenched by hydroquinone with approximately 4 × 109 L mol-1 s-1 rate constant for the bimolecular excited state, kq. The Stern-Volmer constants, KSV, determined by the emission intensity and lifetime showed excellent correlation, which is indicative of the dynamic quenching. The similarity of the bimolecular rate constants between the two complexes implies that the photoinduced electron transfer is the main pathway with a very small (or no) influence of the proton transfer step. The results provide additional insight into the role of the protonable nitrogen atom in the photophysical properties of rhenium(i) complexes, using a dyad architecture.