Influência do sítio de protonação no processo de transferência de elétrons entre complexos polipiridínicos de Re(I) e diferentes doadores de elétrons.

Abstract

TIn this work, describes the spectroscopic, photochemical and photophysical properties of Re(I) complexes based on the polypyridyl ligand 2,2'-bipyridine (2,2'-bpy) andwith different ancillary ligands (L), namely pyridine (py, 1), 1,4-pyrazine (pz, 2), 4,4'-bipyridine (4,4'-bpy, 3) and 1,2-bis(4-pyridyl)ethane (bpa, 4). The complexes fac-[Re(CO)3(2,2'-bpy)(L)]PF6, were characterized by hydrogen nuclear magnetic resonance (1H NMR) and the experimental UV-Vis spectra were compared to theoretical simulations obtained by time-dependent density function theory (TD-DFT). The photophysical properties of the complexes were investigated in acetonitrile at 298 K and glassy medium (ethanol: methanol, 4: 1) at 77 K. For the complexes 1-4, large and unstructured bands were observed in both media and are assigned to the 3MLCT excited state. The emission quantum yields depend on the polypyridine ligandbinder, with 0.060 ( = 249 ns), 0.097 ( = 418 ns), 0.052 ( = 250 ns) and 0.057 ( = 210 ns), respectively for 1, 2, 3 and 4. The polypyridine complexes of Re (I) were investigated by quenching experiments in the presence of hydroquinone and 4-cyanophenol in acetonitrile and acetonitrile/water (1:1). The luminescence quenching rate constants (k_q) were determined by steady-state and lifetime measurements. For hydroquinone and 4-cyanophenol, in acetonitrile, the complexes 2 exhibits the highest k_q among the investigated species (3,49 ± 0,02) x 109 L mol-1 s-1 and (2,2 ± 0.1) × 107 L mol-1 s-1, respectively. Complex 2 is followed by 3 ((k_q = 2,50 ± 0,02) x 109 L mol-1 s-1 versus hydroquinone and (2,2 ± 0,1) x 107 L mol-1 s-1 versus 4-cyanophenol). In 1:1 CH3CN/H2O, the k_q values for all complexes are very similar, both in front of hydroquinone as toand in 4-cyanophenol, evidencing the role of water molecules as the proton acceptor acceptor of prostheses after the electron transferic transmission of the suppressors to the complexes. However, for complex 2, in the presence of 4-cyanophenol there is a decrease of k_q in the presence of water, which shows that the possible protonation of the 1,4-pyrazine base site (pKa = 0.37) by water decreases the efficiency of the electronic transfer process. These data reveal to us that the main mechanism of luminescence quenching of the complex 2 involves electron transfer, ET, accompanied by a proton transfer, PT. At high concentrations of 4-cyanophenol, the mechanism coupledaccopled electron- proton transfer, EPT, also occurs. In this case there is an equilibrium of association between the complex and the suppressor. These results are corroborated by time-dependent density functional theory (TD-DFT) calculations.