Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetra‐stable Donor–Acceptor [2]Rotaxane

Abstract

At etra-stable donor-acceptor [2]rotaxane 1·4PF 6 has been synthesized. The dumbbell component is comprised of an oxyphenylene (OP), at etrathiafulvalene (TTF), a monopyrrolo-TTF (MPTTF), and ah ydroquinone (HQ) unit, which can act as recognition sites (stations) for the tetra-cationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT 4 + ). The TTF and the MPTTF stations are located in the middle of the dumbbell component and are connected by at riethylene glycol (TEG) chain in such aw ay that the pyrrole moiety of the MPTTFs tation points towardt he TTF station, while the TTF and MPTTFs tations are flankedb yt he OP and HQ stations on their left hand side and right hand side, respectively.T he [2]rotaxane was characterized in solution by 1 HNMR spectroscopy and cyclic voltammetry.T he spectroscopic data revealed that the majority (77 %) of the tetrastable [2]rotaxane 1 4 + exist as the translational isomer 1·MPTTF 4 + in which the CBPQT 4 + ring encircles the MPTTF station.T he electrochemical studiess howedt hat CBPQT 4 + in 1·MPTTF 4 + undergoes ring translation as result of electrostatic repulsion from the oxidized MPTTFu nit. Following tetra-oxidation of 1·MPTTF 4 + ,ahigh-energy state of 1 8 + was obtained (i.e., 1·TEG 8 + )i nwhich the CBPQT 4 + ring waslocated on the TEGl inker connecting the di-oxidized TTF 2 + and MPTTF 2 + units. 1 HNMR spectroscopy carriedo ut in CD 3 CN at 298 Ko nac hemically oxidized sample of 1·MPTTF 4 + revealed that the metastable state 1·TEG 8 + is only short-lived with al ifetime of af ew minutes and it was found that 70 % of the positivelyc harged CBPQT 4 + ring movedf rom 1·TEG 8 + to the HQ station,w hile 30 %m oved to the much weaker OP station. These resultsc learly demonstrate that the CBPQT 4 + ring can cross both an MPTTF 2 + and aT TF 2 + electrostatic barriera nd that the free energy of activation required to cross MPTTF 2 + is ca. 0.5 kcal mol À1 smaller as compared to TTF 2 + .[a] Dr.Scheme2.Synthesis of the dumbbell 8 and the tetra-stable [2]rotaxane 1·4PF 6 together with cartoon representations of the three different translational isomers 1·4PF 6 ·TTF, 1·4PF 6 ·MPTTF, and 1·4PF 6 ·HQ present in solution.N otethat compounds 1·4PF 6 , 2, 4,and 8 all exist as amixture of E and Z isomers.

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

“…S6†). Previous studies 25 have shown that such behaviour is fully consistent with the formation of the MPTTF 2+ dication and, hence, the formation of the oxidised [2]rotaxane 2 6+ and [2]pseudorotaxane 1 ⊂CBPQT 6+ , respectively, upon addition of TBPASbCl 6 .…”

“…These interlocked compounds have been used extensively and successfully to create artificial molecular machines (AMMs) 10 in an effort to mimic both biological processes (biomimetic 11 ), such as the controlled molecular motion taking place in ATP-synthase 12,13 or kinesin's ability to walk along the microtubule filament, 14 and macroscopic processes (technomimetic 11 ), such as pumps, 15–19 cars 20 and elevators. 21 These designs often rely on controlling 15,22–24 the mechanisms of stimuli-driven motion, and thus motivate studies of kinetics 25–29 and the underlying energy landscapes. 30,31 We do this here by measuring the impact of double oxidation on the barrier to the motion of a mobile ring (Scheme 1).…”

“…By combining spectroscopic and computational data, it was concluded that the electrostatic barrier energy for the CBPQT 4+ ring to cross the TTF 2+ unit is 65 kcal mol −1 . 47 However, in a recent study by some of us, it was reported that in a tetra-stable [2]rotaxane 25 incorporating CBPQT 4+ as the ring component and TTF, monopyrroloTTF (MPTTF), HQ and oxyphenylene (OP) as stations, the CBPQT 4+ ring can cross both an MPTTF 2+ and a TTF 2+ unit under stress-free conditions ( i.e. no pulling of CBPQT 4+ is applied) at room temperature and in a time scale of less than 20 min.…”

“…In the [2]rotaxane 2 ·4PF 6 , the second stopper is a smaller and more unconventional one that is composed of a thioethyl (SEt) group and a thiodiethyleneglycol (TDEG) substituent. 40 By replacing the SEt group with a less bulky thiomethyl (SMe) group, the CBPQT 4+ ring (blue) possesses enough thermal energy at room temperature to permit its slow passage over the combination of the SMe group and the TDEG substituent 25,27,52 destroying the interlocked nature of the system and therefore convert it into a [2]pseudorotaxane ( i.e. 1 ⊂CBPQT·4PF 6 ).…”