Quenching of Triplet State Tetrakis(μ-pyrophosphito-PP‘)diplatinate(II) by Nickel(II) Macrocyclic and Tris-diimine Complexes

Abstract

The excited state of tetrakis(&mgr;-pyrophosphito-PP')diplatinate(II), Pt(2)(pop)(4)(4)(-), has been shown to be reductively quenched by a series of Ni(II) complexes. Steady-state photolysis of Pt(2)(pop)(4)(4)(-) in the presence of Ni(cyclam)(2+) and SO(4)(2)(-) ions was followed by absorption spectroscopy and showed the production of the Ni(III) complex with low efficiency. The Ni(III) yield was enhanced in the presence of oxygen presumably because of scavenging of Pt(2)(pop)(4)(5)(-) from the solvent-caged {Pt(2)(II,I).Ni(III)} primary product pair resulting from electron transfer. Cage escape yields of 0.028 in N(2) saturated solution and 0.054 in aerated solutions were estimated from the data. For a series of complexes with decreasing redox potential, the rate constants for their (3)Pt(2)(pop)(4)(4)(-) phosphorescence lifetime quenching decreased with driving force DeltaG, consistent with Rehm-Weller behavior and leading to a reorganization energy of approximately 60 kJ mol(-)(1). The diffusional rate constant calculated from the Debye-Smoluchowski equation was 1.6 x 10(10) M(-)(1) s(-)(1), in excellent agreement with the observed value of 1.6 x 10(10) M(-)(1) s(-)(1) based on the Rehm-Weller equation. The transmission factor for the electron transfer was estimated at 10(-)(4), in the weakly adiabatic region. Although the Rehm-Weller treatment has been used successfully in reactions involving mainly uncharged organic reactants, in this investigation it leads to an unrealistic ratio for the rate constants for the diffusive separation of the charged reactants and products. An alternative interpretation of the results based on an approach by Meyer and Nagle removes this problem and the observed linear dependence of ln k(q) on the square root of E degrees (Ni(II)/Ni(III)) in the endergonic region shows that the dominant back electron transfer produces ground-state species. The analysis also leads to an estimate of the (3)Pt(2)(pop)(4)(4)(-)/Pt(2)(pop)(4)(5)(-) potential of 1.29 +/- 0.05 V(vs NHE), in excellent agreement with the previous literature value of 1.34 V.

Dynamic quenching of porphyrin triplet states by two-photon absorbing dyes: Towards two-photon-enhanced oxygen nanosensors

“…k −esc/kreact >>1. Combining assumptions (i) and (ii) into k −d >>1/τ 0 +k d [Q] at low quencher concentrations (Q<<1), equation (3) can be rewritten as [47]: (4) In the last formula, k ET was expressed as [k e°× exp(−ΔG * ET /RT)] from the Arrhenius equation; and k −ET /k ET , which is the reciprocal of the electron transfer equilibrium constant (K ET ), was substituted for [exp(ΔG ET /RT)]. Here k e° is the frequency factor, ΔG * ET is the free energy of activation for the ET step, and ΔG ET is the driving force of the ET reaction.…”

“…In addition, the geometry of the encounter complexes involving bulky dyes and 2,6-substituted PtP's could be different, affecting the reorganization energy. In principle, fitting experimental points to the function given by equation (3) should produce kinetic parameters k d , k −d /k e°, ΔG * ET (0), k −d /k esc , as well as the reorganization energy λ [47]. However, such fitting in practice would be highly unstable because of the large number of parameters, their implicit dependence and noise in the data.…”

Intriguing multichannel photoinduced electron transfer in lanthanide(iii)–diphenylamine systems

“…As indicated in Table 3, the k q values for most of the studied DPA-Ln 3+ systems fall within the above estimate of the k diff value except for the cases with Eu 3+ and Yb 3+ ions where k q values significantly exceed the above k diff value. [81][82][83][84] For the present systems, even though the electrostatic interaction will be much weaker than in the cases of the oppositely charged donor-acceptor pairs, yet the effect can lead to a range of k q values as observed for some of the studied systems. It is likely that in these systems, along with the PET mechanism, which is evidently the major quenching channel as indicated from the LFP studies, there could be some contribution arising through a long range energy transfer mechanism leading to the enhancement in the quenching rate.…”

“…In the present context, we should recall the effect of stronger electrostatic interaction that participates in the cases of oppositely charged donor-acceptor pairs and in such cases the enhancement in k q values is so large that they become many times higher than the usual k diff value estimated using eqn (10). [81][82][83][84] For the present systems, even though the electrostatic interaction will be much weaker than in the cases of the oppositely charged donor-acceptor pairs, yet the effect can lead to a range of k q values as observed for some of the studied systems. Another important point to be mentioned here is that the electrostatic effect that we have discussed will be almost equally applicable for all the DPA-Ln 3+ systems studied.…”

Factors that Impact Photochemical Cage Escape Yields

“…The reductive quenching of the triplet state of tetrakis (μ-pyrophosphito-PP′)diplatinate(II), [Pt 2 (pop) 4 ] 4− , by [Ni(cyclam)] 2+ was studied in 0.01 M HClO 4 containing 0.1 M Na 2 SO 4 as electrolyte. 471 The cage escape yields were determined to be ϕ ce = 0.054 and 0.028 in air-saturated and in nitrogen-saturated solution, respectively. The larger cage escape yields for the formation of Ni(III) obtained under air point towards a scavenging of the reduced Pt complex {[Pt 2 (pop) 4 ] 5− } from the solvent cage by oxygen.…”