Paramagnetic Molecular Grippers: The Elements of Six-State Redox Switches

Abstract

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices.

The Quest for Molecular Grippers: Photo‐Electric Control of Molecular Gripping Machinery

“…Such unique potential of quinones in controlling the gripping machinery was primarily the result of their propensity to alter noncovalent interactions, such as hydrogen bonding, as a function of the redox state . This is complemented by their versatility, the high reversibility of the redox interconversion, accompanied by long lifetimes of the different redox states, and accessibility through various processes …”

“…The capability of quinones to adopt multiple redox states provided the basis for such development (Figure c), as the quinone‐based cavitands could potentially perform the desired conformational switch in three different redox states ( Q , SQ , and HQ ) . To achieve the objective of effectively altering the properties electrochemically with a fast response time, we set out to establish systems that operate in aprotic environments by using the SQ radical anion …”

“…Each of the reduction steps was shown to follow a two‐electron (2 e − ) process by rotating disc voltammetry (RDV). Moreover, the first and fully reversible step is related to the formation of the SQ dianion species, whereas the two successive quasi‐reversible reductions could be ascribed to the quinone dianion Q 2− and the reduction of the quinoxaline walls, respectively …”

“…One of the most powerful features of these nanosystems is their switchability, which can be defined as the capacity to change properties in response to external stimuli; this feature lies at the core of their predisposition for smart nanotechnologies . In this context, although a variety of switches that mimic different macroscopic objects have already been established, a system that has remained underdeveloped is that of molecular grippers . These systems are defined by the ability to switch between an open (expanded) and a closed (contracted) state, which as a result enables reversible encapsulation of smaller molecular objects.…”

“…Scaffolds that have emerged as particularly promising for the design of molecular grippers are the resorcin[4]arene cavitands . This class of molecules was introduced by Cram and co‐workers in 1982 .…”