Quantifying the barrier for the movement of cyclobis(paraquat-p-phenylene) over the dication of monopyrrolotetrathiafulvalene

Abstract

A bistable [2]pseudorotaxane 1⊂CBPQT•4PF6 and a bistable [2]rotaxane 2•4PF6 have been synthesised to measure the height of an electrostatic barrier produced by double molecular oxidation (0 to +2). Both systems...

Evaluating the energy landscape of an out-of-equilibrium bistable [2]rotaxane containing monopyrrolotetrathiafulvalene

“…7 a , c ,13 Despite the importance of these redox-switchable [2]rotaxanes there are only a few studies aiming toward the use of TTF as a stimuli-induced electrostatic barrier in [2]rotaxanes. 7 c ,14…”

“…MPTTF 2+ ) in both the forward ( k 1 ) and the backward ( k −1 ) directions. The dumbbell component of the [2]rotaxane R 4+ is comprised of a diisopropyl-phenyl (DIPP) stopper (left), a redox-active MPTTF unit and an oxyphenylene (OP) moiety, 15 which is directly connected to a triarylmethyl stopper 14 (right). The MPTTF unit (green) acts as the primary binding site (station) for CBPQT 4+ (blue), while the OP moiety (red) only acts as a station when MPTTF is oxidised.…”

“…Upon oxidation of MPTTF firstly to the radical cation (MPTTF˙ + ) and secondly to the dication (MPTTF 2+ ), the tetracationic CBPQT 4+ ring is forced to leave the oxidised MPTTF station as a result of electrostatic repulsion. Since the planar pyrrole moiety (<15 kcal mol −1 ) 16 of MPTTF constitutes a much smaller steric barrier for CBPQT 4+ compared to the combination of the thiomethyl (SMe) group and the thiotriethyleneglycol substituent (>22 kcal mol −1 ), 14 the oxidation process initiates a directional movement of CBPQT 4+ toward the glycol chain (orange) attached to the DIPP stopper, producing R ·DIPP 6+ . This co-conformation represents a high energy (out-of-equilibrium) state of the oxidised [2]rotaxane, which slowly will interconvert into the thermodynamically more stable co-conformation R ·OP 6+ when CBPQT 4+ moves across MPTTF 2+ .…”

“…S2c, ESI†) clearly reveals that CBPQT 4+ is mechanically bonded to the dumbbell component in R 4+ , with CBPQT 4+ encircling the MPTTF unit. 14 Since R 4+ contains two different stations ( i.e. MPTTF and OP) for CBPQT 4+ , it can, in theory, exist as an equilibrium between two interconverting co-conformations.…”

“…S2b, ESI†) recorded of R 4+ , it is evident that R 4+ exclusively exists as R ·MPTTF 4+ in CD 3 CN at 298 K, which is consistent with the fact that MPTTF has a significantly higher affinity to CBPQT 4+ relative to the OP moiety. 7 a ,14…”