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Published July 8, 2020 | Published
Journal Article Open

Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane


We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state co-conformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF²⁺ dication the CBPQT⁴⁺ ring is forced toward the molecular rotor leading to an increased energy barrier for the excited-state to rotate the rotor into the state with the high non-radiative rate constant, resulting in an overall 3.4-fold fluorescent enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge transfer state between the BODIPY rotor and the CBPQT⁴⁺ ring, the movement of the ring towards the BODIPY rotor produces an unexpectedly strong fluorescent signal decrease as the result of the photoinduced electron transfer from the BODIPY rotor to the CBPQT⁴⁺ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to the modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories and molecular logic gates.

Additional Information

© 2020 American Chemical Society. ACS AuthorChoice -- Made available through a Creative Commons CC-BY License. Publication Date: May 29, 2020. The authors would like to thank NU for their continued support of this research. The synthesis was supported by the National Science Foundation under CHE-1308107 (J.F.S.). This project was also supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-FG02-99ER14999 (M.R.W.). W.-G.L. and W.A.G. were supported by NSF (CBET). Y.W. thanks the Fulbright Scholar Program for a Fellowship and the NU International Institute of Nanotechnology for a Ryan Fellowship. We thank the personnel in the Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University (NU) for their assistance in the collection of the analytical data.

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August 19, 2023
October 20, 2023