Effect of Halogen Ions on the Photocycle of Fluorescent Carbon Nanodots

Carbon dots (C-dots) are well-known for their strong sensitivity to the environment, which reflects on intensity and shape changes of their fluorescence, induced by various interacting ions and molecules in solution. Although these interactions have been extensively studied in the last few years, especially in view of their possible sensing applications, the existing works have mostly focused on the quenching of C-dot fluorescence induced by metal cations. In fact, these latter easily bind to C-dots surfaces, which are negatively charged in most cases, promoting an electron transfer from the surface to them. Much less is known from the literature on the effect induced on C-dots by prototypical negative species in solutions, motivating more systematic studies on this different class of interactions. Here, we analyzed the effect of halogen ions on the fluorescence of C-dots, by combining steady-state optical absorption and photoluminescence, time-resolved fluorescence and femtosecond pump/probe spectroscopy. We demonstrate a quenching effect of C-dots fluorescence in the presence of halogen ions, which becomes more and more pronounced with increasing atomic number of the halogens, being negligible for chloride, appreciable for bromide and stronger for iodide. We find that quenching is mostly static, due to the binding of halogen ions on suitable surface sites at C-dots surfaces, while collisional quenching becomes obvious only at very high iodide concentrations. Finally, nanosecond and femtosecond time-resolved spectroscopies provide information on the quenching mechanism and time scales. Based on these data, we propose that the fluorescent state is deactivated by intersystem crossing to a dark triplet state, induced by close-range interactions with the heaviest halogen ions.