Fluorescence of 5-Arylvinyl-5'-Methyl-2,2'-Bipyridyl Ligands and Their Zinc Complexes

摘要

The photophysical properties of 5-arylvinyl-5'-methyl-2,2'-bipyridyls (AVMBs, 1-9, 11) and theirzinc complexes were studied. Similar 2,2'-bipyridyl-based ligands have been applied as optical sensorsfor metal ions and sensitizers for solar energy conversion. The goal of this investigation is to reveal thefactors that determine the emission band shift and fluorescence quantum yield change of the titleligand system upon zinc binding. The outcome of this study will not only advance the fundamentalunderstanding of the coordination-driven photophysical processes embodied in the AVMB platformbut facilitate the rational design of fluorescent probes for metal ions, particularly zinc. The AVMBligands were synthesized using the Horner-Wadsworth-Emmons reaction. AVMBs containingelectron-donating aryl groups show absorption and emission in the visible region, which can beassigned to charge-transfer transitions as supported by solvent-dependency and computationalstudies. The binding between AVMB ligands and zinc ion in acetonitrile was studied using isothermaltitration calorimetry (ITC). A multicomponent equilibrium model is suggested that explains themultiple transitions evidenced in fluorescence titration isotherms. Coordination to zinc ion stabilizesthe charge-transfer excited state of an AVMB ligand with an electron-donating aryl substituent,consequently results in bathochromic shifts in both absorption and emission. However, unlike theemission band shift, the fluorescence quantum yield change upon zinc complex formation does nothave an intuitive correlation with the electronic nature of the aryl group. Lifetime measurementsusing the Time-Correlated Single Photon Counting method enabled the determination of nonradia-tive and radiative decay rate constants. Both rates of an AVMB ligand decrease upon zinc binding.The collective effect gives rise to the change in fluorescence quantum yield with the apparent lack ofcorrelation with the electronic property of the aryl group.