Fluorimetric redox switches, sensitive to external electrochemical stimuli, belong to a wide family of molecular sensors in which the switching process is achieved by shifting the molecules between two or more stable states. While the area of pH and ligand-based chemosensors has been extensively explored,^[1] the topic on luminescent molecular redox switches has remained rather underdeveloped^[2] even though redox modulation of fluorescence affords considerable opportunities in optoelectronic and biological applications^.[3]In this context, the development of new electrochemically switchable fluorescent probes is of high interest. Based on DFT calculation, an original nitrobenzoxadiazole (NBD) system containing a redox active ferrocene unit was designed (Figure 1) and its optical, electrochemical, and spectroelectrochemical properties were investigated. The system was nonfluorescent in its initial reduced state as a result of photoinduced electron transfer from ferrocene moiety to the photoexcited NBD part and considerable enhancement of emission intensity was observed during oxidation of NBD-ferrocene to NBD-ferrocenium, in which photoinduced electron transfer was disabled electrochemically.

References

[1]	(a) de Silva, A. P.; Gunaratne, H. Q. N. ; Gunnlaugsson, T. ; Huxley, A. J. M. ; McCoy, C. P. ; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515-1566; (b) Gale, P. A.; Caltagirone, C. Chem. Soc. Rev. 2015, 44, 4212-4227.
[2]	(a) Magri, D. C. Analyst 2015, 140, 7487-7495; (b) Audebert, P.; Miomandre, F. Chem. Sci. 2013, 4, 575-584.
[3]	Liu, Z.; Qi, W.; Xu, G. Chem. Soc. Rev. 2015, 44, 3117-3142.

Acknowledgement

The financial support from the EU Research and Innovation programme Horizon 2020 through the Marie Skłodowska-Curie action (H2020-MSCA-IF-2014, ID 656519) is gratefully acknowledged.

Figure 1 a) Electrochemical oxidation of the new NBD-ferrocene system. b) DFT calculations of orbital energy levels. c) Electrochemically mediated fluorescence enhancement.