Fluorescence quenching in Zn2+-bis-terpyridine coordination polymers: a single molecule study

Abstract

A Zn²⁺-bis-terpyridine coordination polymer is investigated by single-molecule fluorescence spectroscopy (SMS). The bis-terpyridine ligands of the coordination polymers bear conjugated chromophores connecting the terpyridine spheres of the ligands, which resemble structural features of MEH-PPV. To the best of our knowledge this paper presents the first systematic SMS study on Zn²⁺-bis-terpyridine coordination polymers. Upon incorporation of free chromophores into the polymer, the fluorescence quantum yield of individual chromophores appears to be reduced. The reason for the reduced emission per chromophore in a polymer is investigated by brightness studies on single isolated polymer molecules. Furthermore, the experiments reveal that the Zn²⁺-bis-terpyridine coordination polymer contains a significantly larger fraction of effective chromophores, compared to one of the most common conjugated polymers, e.g., MEH-PPV. This finding is attributed to the particularly rigid geometry of the system at hand, which was corroborated by polarization-dependent experiments. Time-resolved experiments identified two different types of intensity fluctuations: small amplitude intensity fluctuations might correlate with switching of individual chromophores by structural fluctuations, while large amplitude jumps switch off the emission of the entire molecule. Finally, experiments under different atmospheric conditions offer further insights into the molecular mechanism and the nature of the quenchers involved in the blinking.