Artificial molecular pumps (AMPs), inspired by the active cellular transportation exhibited in biological systems, enable cargoes to undergo unidirectional motion following molecular ratchet mechanisms in presence of energy sources. Recently, significant progress has been achieved using alternatively radical interactions and Coulombic repulsive force to create AMPs. In an attempt to widen the horizon of applications of AMPs, we have explored the effect of molecular pumping on the photophysical properties of the collecting chain on the dumbbell. A dumbbell bearing a central pyrene fluorophore and two terminal AMPs has been synthesized. The two pumping cassettes are operated so as to sequester two tetracationic cyclophanes out of solution, generating mechanical bonds and forming a [3]rotaxane. The fluorescent dumbbell and the resulting [3]rotaxane have been fully characterized by high resolution mass spectrometry in addition to UV-Vis and NMR spectroscopies. Emission spectroscopy has been employed to determine the structure and photophysical properties of the [3]rotaxane. The formation of [3]rotaxane results in the quenching of the fluorescence of dumbbell. This work opens up a new opportunity of employing molecular pumping to generate mechanical bonds and tune precisely the photophysical properties of fluorophores, which cannot be achieved in a mixture of these components.

Fluorescence quenching by molecular pumping

ACS Spring 2022

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