Quenching Mechanism of Zn(Salicylaldimine) by Nitroaromatics

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Abstract
Nitroaromatics and nitroalkanes quench the fluorescence of Zn(Salophen) (H2Salophen = N,N′-phenylene-bis-(3,5-di-tert-butylsalicylideneimine); ZnLR) complexes. A structurally related family of ZnLR 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 ZnLR were generally high (Φ ∼ 0.3) with sub-nanosecond fluorescence lifetimes. The fluorescence of ZnLR was quenched by nitroaromatic compounds by a mixture of static and dynamic pathways, reflecting the ZnLR ligand bulk and reduction potential. Steady-state Stern-Volmer plots were curved for ZnLR 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 ZnLR 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 ZnLR as sensors, as well as for potential photoinduced charge transfer chemistry.