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Regiospecific Quenching of a Photoexcited Platinum(II) Complex at Acidic and Basic Sites

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journal contribution
posted on 16.05.2011, 00:00 by Daniel P. Lazzaro, Robert McGuire, David R. McMillin
The carbometalated complex Pt(dppzϕ*)Cl, where dppzϕ* denotes the 6-(4-tert-butylphenyl)-dipyrido[3,2-a:2′,3′-c]phenazine ligand, exhibits emission in a dichloromethane solution at room temperature with a concentration-dependent excited-state lifetime. Extrapolation to zero Pt(dppzϕ*)Cl concentration yields a limiting lifetime of 11.0 μs in the absence of dioxygen along with an impressive emission quantum yield of 0.17. The visible absorption of Pt(dppzϕ*)Cl has intraligand charge-transfer as well as metal-to-ligand charge-transfer character, but the oscillator strength may derive, in part, from π−π* excitation within the phenazine moiety. An intriguing aspect of the Pt(dppzϕ*)Cl system is that its reactive excited state is subject to regiospecific quenching by Lewis bases and hydrogen-bonding Lewis acids. Base-induced quenching involves an attack at the platinum center. The rate constant increases with the donor strength of the quencher and reaches the order of 108 M−1 s−1 with a relatively strong base like dimethyl sulfoxide. The orbital parentage of the excited state probably influences the quenching rates by affecting the charge density at platinum, as well as at the phenazine nitrogen atoms, where attack by Lewis acids occurs. With mildly acidic alcohols like 1,1,1,3,3,3-hexafluoropropan-2-ol and 2,2,2-trifluoroethanol, high concentrations of the quencher are necessary to suppress the emission. Carboxylic acids are stronger quenchers, and the quenching constant increases with the acid strength according to tabulated pKa values. Cyanoacetic acid exhibits the highest measured quenching rate constant (2.6 × 109 M−1 s−1), which only decreases 30% when the acid is in the (NC)CH2CO2D form. A weaker acid, CH3CO2H, exhibits an even smaller kinetic isotope effect. Literature comparisons suggest that acid-induced quenching probably involves hydrogen-bond formation as opposed to net proton transfer.