Prompt and Delayed Electron Ejection from Photoexcited Aqueous Bromo- and Chlorocuprate(I) Complexes

Abstract

Two different routes for ejection of hydrated electrons by photoexcited CuCl 2 -and CuCl 3 2-have been observed: (1) a prompt (i.e. within the 5-ns laser pulse width) ejection directly from the charge-transfer excited states (characterized as CTTS or Rydberg states), and (2) a delayed ejection, with lifetimes of up to 105 ns, from the triplet states of the two complexes. These two processes have rather high quantum yields: 0.30-0.40 for the prompt ejection and 0.25-0.33 for the delayed ejection for the chlorocuprate(I) complexes. The delayed ejection process is the primary decay route for the two triplet species, 3 CuCl 2 -and 3 CuCl 3 2-, which are formed by intersystem crossing from the charge-transfer excited states formed by absorption of light by the bis-and triscoordinated complexes in equilibrium in the ground state. Both of the chlorocuprate triplet excited states are weakly luminescent species. The triplet tris species, 3 CuCl 3 2-, has a strong absorption band at 390 nm ( ≈ 27 000 M -1 cm -1 ), and the biscoordinated species, 3 CuCl 2 -, probably absorbs in the same region as well. Other intermediates such as Cu(0) complexes and Cl 2 •-radicals have also been observed as decay products. Prompt and delayed electron ejection has also been observed in CuBr 3 2-with quantum yields of 0.25 and 0.18, respectively. A mechanism that incorporates a triplet excited state equilibrium is invoked to account for these observations.

Formation and Photoinduced Electron Ejection of Amminedicyanocuprate(I) and Cyano-Bridged Trinuclear Copper(I) Complexes in Aqueous Ammonia Solution

“…Monomeric copper(I) complexes, CuL n , where n = 2 or 3 and L = Cl - , Br - , I - , NH 3 , CH 3 NH 2 , C 2 H 5 NH 2 , and CN - and mixtures thereof, have been shown − to undergo very efficient photoinduced electron ejection, when irradiated into their charge-transfer-to-solvent (CTTS) bands, with quantum yields on the order of 0.2−0.3. Interestingly, room-temperature luminescence is observed along with the electron ejection only when halo ligands are present in the complex, and this has been ascribed to the formation of exciplexes in an excited-state equilibrium. − Because of their very large stability constants, , the cyano complexes of copper(I), Cu(CN) 2 - , Cu(CN) 3 2- , and Cu(CN) 4 3- , are quite robust and air-stable in aqueous solution and can be formed, especially in the case of the first two, essentially by the quantitative reaction of copper(I) salts, such as CuCl or CuCN, with aqueous solutions of KCN or NaCN.…”

Luminescent Homoatomic Exciplexes in Dicyanoargentate(I) Ions Doped in Alkali Halide Crystals. 1. “Exciplex Tuning” by Site-Selective Excitation

“…On the other hand, inorganic exciplexes have been recognized only recently . Recent reported examples involve coordination compounds of Pt(II), , Cu(I), 9c,, Ru(II), and Ir(III) .…”

Prompt and Delayed Electron Photoejection from Triamminecopper(I) and Formation of a Dinuclear Excited State Complex in Aqueous Solution

“…We have previously reported the prompt and delayed UV photoejection of hydrated electrons from aqueous solutions of di- and trihalo anionic complexes of copper(I) with chloro , and bromo 2 ligands. The source of the prompt electron is attributed to direct ejection following excitation into the charge-transfer-to-solvent (CTTS) states of the complexes, whereas the delayed electron is ejected from two weakly luminescent, triplet, excited-state equilibrated species, a short-lived dihalo and a longer-lived trihalo complex, designated an exciplex. − Similarly to the halocuprate systems, copper(I) forms di- and triammine complexes, according to the equilibrium − ( K = 0.040) In an equilibrated mixture of these complexes, only the triammine species absorbs the laser UV output at 266 nm appreciably, also resulting in the ejection of hydrated electrons but without any accompanying luminescence. , Earlier laser flash studies at 266 nm revealed, along with the time-resolved spectrum of hydrated electrons, some other long-lived intermediate absorbing at about 400 nm .…”

“…The absorption peak at about 720 nm is clearly that of the hydrated electron, and by comparison of this peak to the electron signal produced by a ferrocyanide actinometer solution the quantum yield for this prompt electron photoejection is estimated to be about 0.3. Since earlier CW experiments 7,9 showed that the species oxidized by UV light in a solution of copper(I) in ammonia is the triamminecopper(I) cation, the primary photochemical step is absorption into the CTTS (or Rydberg d → s or d → p) state and prompt ejection of an electron, The electron is scavenged by the two copper(I) ammine complexes presumably forming copper(0) ammine species which appear to absorb around 400 nm, similarly to their halocuprate counterparts. , At very low or very high copper concentrations the electron decay is simple first order, but there is an intermediate range of copper concentrations where the signal becomes biexponential (see below). The initial decay lifetime of this electron signal decreases as the copper(I) concentration is increased, and from the slope of a plot of the observed electron decay constant vs [Cu(I)] (not shown) we obtain, at this ammonia concentration, the second-order scavenging constant of 3.0 × 10 9 M -1 s -1 .…”

“…The lack of sensitivity of decay lifetime to ammonia concentration implies that the ground-state species forming the exciplex is Cu(NH 3 ) 2 + rather than Cu(NH 3 ) 3 + since the relative concentration of the former is little affected by ammonia concentration whereas that of the latter is strongly affected (see eq 1). Although the kinetics of such coupled systems can be analyzed in detail, ,, the linearity and near zero intercept of lifetime-vs-copper(I) concentration plot of Figure can be interpreted in terms of the rapidly equilibrated system of eq 3 decaying primarily through the rate-controlling decay of species [I]. Thus the rate of decay of [II] should follow that of [I], which would be given by k [I] [*Cu(NH 3 ) 3 + ], where k [I] is the first-order decay constant of [I], which may decay by radiationless deactivation, and/or by the ejection of electron, Using the excited-state equilibrium constant, K ex , one can show that the lifetime of the decay is given by K ex [Cu(NH 3 ) 2 + ]/ k [I] .…”