Quenching Mechanism of Zn(Salicylaldimine) by Nitroaromatics

详细信息    查看全文

• 作者：Meaghan E. Germain ; Thomas R. Vargo ; Beth Anne McClure ; Jeffrey J. Rack ; P. Gregory Van Patten ; Michael Odoi ; Michael J. Knapp
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：July 21, 2008
• 年：2008
• 卷：47
• 期：14
• 页码：6203 - 6211
• 全文大小：1043K
• 年卷期：v.47,no.14(July 21, 2008)
• ISSN：1520-510X

文摘

Nitroaromatics and nitroalkanes quench the fluorescence of Zn(Salophen) (H₂Salophen = N,N′-phenylene-bis-(3,5-di-tert-butylsalicylideneimine); ZnL^R) complexes. A structurally related family of ZnL^R complexes (R = OMe, di-tBu, tBu, Cl, NO₂) were prepared, and the mechanisms of fluorescence quenching by nitroaromatics were studied by a combined kinetics and spectroscopic approach. The fluorescent quantum yields for ZnL^R were generally high (Φ ~ 0.3) with sub-nanosecond fluorescence lifetimes. The fluorescence of ZnL^R was quenched by nitroaromatic compounds by a mixture of static and dynamic pathways, reflecting the ZnL^R ligand bulk and reduction potential. Steady-state Stern-Volmer plots were curved for ZnL^R with less-bulky substituents (R = OMe, NO₂), suggesting that both static and dynamic pathways were important for quenching. Transient Stern-Volmer data indicated that the dynamic pathway dominated quenching for ZnL^R with bulky substituents (R = tBu, DtBu). The quenching rate constants with varied nitroaromatics (ArNO₂) followed the driving force dependence predicted for bimolecular electron transfer: ZnL* + ArNO₂ → ZnL⁺ + ArNO₂⁻. A treatment of the diffusion-corrected quenching rates with Marcus theory yielded a modest reorganization energy (λ = 25 kcal/mol), and a small self-exchange reorganization energy for ZnL*/ZnL⁺ (ca. 20 kcal/mol) was estimated from the Marcus cross-relation, suggesting that metal phenoxyls may be robust biological redox cofactors. Electronic structure calculations indicated very small changes in bond distances for the ZnL → ZnL⁺ oxidation, suggesting that solvation was the dominant contributor to the observed reorganization energy. These mechanistic insights provide information that will be helpful to further develop ZnL^R as sensors, as well as for potential photoinduced charge transfer chemistry.