Dinuclear excited-state complexes from UV laser excitation of aqueous ammonia, methylamine, and ethylamine copper(Ⅰ) complexes

摘要

The temporal and spectral behavior of ammonia, methylamine, and ethylamine copper(Ⅰ) complexes in aqueous solutions at 1 M ionic strength were probed using 266-nm laser flash photolysis on the nanosecond time-scale. Since stability constants and UV spectra for the ground state systems are needed to understand this behavior, these had to be determined for the methylamine and ethylamine complexes, and are reported here for the first time. The stepwise stability constants are 0.067 +- 0.007 and 0.040 +- 0.004 for the tris(methylamine) and tris(ethylamine) complexes, respectively. Moreover, in order to know the activities of the two amines in the unit ionic strength solutions, it was also necessary to measure their Henry's law constants, and these are 5.002 and 7.987 Torr M~(-1), respectively. In each of the three systems an intermediate forms monophotonically, which absorbs in the 370-390-nm region, with a shoulder or weaker band at 420-430 nm, and a weaker, broad band at 550 nm. These species have been characterized as dinuclear triplet state complexes because their first-order decay lifetimes increase with copper(Ⅰ) concentration, consistent with the quasi-equilibria, ~3CuA_2~+ + CuA_2~+ = ~3Cu_2A_4~+ (an excimer) or ~3CuA_3~+ + CuA_2~+ = ~3Cu_2A_5~+ (an exciplex) in concert with the decay of the monomeric triplet species, ~3CuA_2~+ and ~3CuA_3~+, via delayed electron ejection. Thermodynamic and kinetic stability of the dinuclear excited state complexes increase for the ligand series: ammonia < methylamine < ethylamine. One-electron copper-copper bonding is proposed for these dinuclear species. Parallel to the formation of the dinuclear species, the hydrated electron is ejected monophotonically from laser photolysis of all three ligand systems on a subnanosecond time-scale with quantum yields that range from 0.34 to 0.47. A complete mechanistic scheme is described which shows that the prompt electron ejection is from the excited singlet state of a photolyzed monomer, CuA_2~+ or CuA_3~+, in parallel with intersystem crossing to a triplet monomer, ~3CuA_2~+ or ~3CuA_3~+, whose longer lifetime allows the reaction with ground-state CuA_2~+ and equilibration with the dinuclear excited state.