Relatore:  Marco Frasconi, Dipartimento Scienze Chimiche, Università di Padova.

Abstract:  The self-assembly of molecular building blocks into the desire structures plays a fundamental role for the development of functional materials. The precise control of noncovalent bonding interactions has allowed to develop a variety of template-directed protocols and to explore the possibility of wholly synthetic molecules to perform advanced tasks, such as muscle-like contraction and expansion. Here, we highlight the introduction of radical-pairing interactions into mechanically interlocked molecules (MIMs), from the development of protocols for the template-directed synthesis of MIMs to their electrochemically controlled actuation.1 The formation of reversible weak noncovalent π-homodimers by radical species, particularly by tetrathiafulvalene (TTF) and 1,1′-dialkyl-4,4′-bipyridinium (BIPY), has received much attention in the past half-century because of the ease of their preparation and well-organized assemblies. The tendency of bipyridinium units in their reduced radical-cation state to form inclusion complexes with the diradical dicationic cyclobis(paraquat-phenylene) ring has been investigated mechanistically and quantitatively. We have employed the remarkable stabilization associated with this inclusion complex for the development of radical template-directed strategies for synthesizing MIMs, such as catenanes and rotaxanes. In addition to providing supramolecular assistance in the syntheses of MIMs, radical-pairing interactions introduce switching behaviour into the resulting MIMs along with optical, conducting, and magnetic properties. We have used chemical, electrochemical, and photochemical approaches to achieve the reversible actuation of BIPY-based MIMs. Electrochemical investigations performed on the rotaxanes indicate that their mechanically interlocked structures lead to the enforced interactions between the dumbbell and ring components, allowing them to contract in their reduced states and expand in their fully oxidized states as a result of Coulombic repulsions.2 The ability to tailor the properties of MIMs by radical-pairing interactions and the electrochemical modulation of their actuation paves the way for the investigation of more complex processes at the molecular and supramolecular levels, such as the operation of artificial molecular machines that functions by transducing electrochemical energy into mechanical work at the nanoscale.
References:
1) Y. Wang, M. Frasconi, J. F. Stoddart “Introducing stable radicals into molecular machines” ACS Cent. Sci. 2017, 3, 927–935.
2) Y. Wang, T. Cheng, J. Sun, Z. Liu, M. Frasconi, W. A. Goddard III, J. F. Stoddart “The neighboring component effect in a tristable [2]rotaxane” J. Am. Chem. Soc. 2018, 140, 13827−13834.