Abstract
Invited for this months cover are the groups of Professors Marco Lucarini and Alberto Credi at the University of Bologna. The cover picture shows coupled and uncoupled states of a [2]rotaxane incorporating stable nitroxide radical units in both the ring and dumbbell components. Interaction between nitroxide radicals could be switched between noncoupled (three-line electron paramagnetic resonance (EPR) spectrum) and coupled (five-line EPR spectrum) upon deprotonation of the rotaxane NH2+ centers that effects a quantitative displacement of a dibenzocrown macroring to a 4,4’-bipyridinium recognition site. The complete base- and acid-induced switching cycle of the EPR pattern was repeated several times without an appreciable loss of signal, highlighting the reversibility of the process. Hence, this molecular machine is capable of switching on/off magnetic interactions by chemically driven reversible mechanical effects. For more details, see the Communication on p. 18 ff.
What was the inspiration for this cover design?
We decided to show the new rotaxane switch on a background image representing the old electron paramagnetic resonance (EPR) spectrometer used in Bologna in the 80s and 90s. Back then, we would have never imagined that one day we would be able to switch on/off EPR signals with a molecular machine!
Department of Chemistry “G. Ciamician” Alma Mater Studiorum-University of Bologna Via Selmi 2, 40126 Bologna (Italy) E-mail: alberto.credi@unibo.it E-mail: elisabetta.mezzina@unibo.it E-mail: marco.lucarini@unibo.it
What was the biggest challenge on the way to the results presented in this paper?
The use of movement to perform tasks is an open issue for artificial molecular machines. In this case, the challenge was to ensure that embedding the nitroxide radicals in the rotaxane components did not compromise the shuttling. Moreover, due to the presence of the paramagnetic centers, the complete characterization of the spin-labeled interlocked molecule was not a trivial task.
How did the collaboration on this project start?
All the groups involved in this work are interested in molecular machines. Together, we form an ideal team, as the Credi group has well-recognized experience in the field of molecular shuttles, while the Mezzina and Lucarini groups add their synthetic background and EPR spectroscopic expertise. Currently, we are extending our collaboration for the preparation and characterization of more sophisticated spin-labeled molecular devices.
What future opportunities do you see in light of the results presented in this paper?
A system of this kind represents a first step towards a new generation of nanoscale magnetic switches that may be of interest for information and communication technology (ICT) applications. Spin labeling in molecular machines is also interesting because correlations between radical pairs, monitored by pulsed-EPR methods, enable detailed conformational analyses, dynamics studies, and precise measurements of nanoscale distances. This is important to gain more understanding of the operation mechanism of such nanoscale devices.
Acknowledgments
This work was mainly supported by the University of Bologna as a part of the SLaMM Project (Finanziamenti di Ateneo alla Ricerca di Base).
