Self-Resetting Bistable Redox Molecular Machines for Fullerene Recognition

Abstract

Addressing control over molecular machines resulting in variable output modulation by mimicking nature mechanisms is a current hot topic. The exploitation of reversibility in thiol/disulfide motifs in chemical systems flanked by nonplanar corannulene moieties capable to recognize fullerenes is presented herein. Two redox-based machines have been conceived for this purpose: an ON/OFF switch that activates its binding properties upon dimerization and a self-resetting (i.e., with an automated backward process) host that substantially modulates its affinity.

From Induced-Fit Assemblies to Ternary Inclusion Complexes with Fullerenes in Corannulene-Based Molecular Tweezers

“…There is a trend of a lower QY as the bridgehead group becomes more acceptor. Measured decay lifetimes (τ) range from 5.08 to 10.54 ns, fitted with a mono-exponential function, for most of the hosts, which are typical values for this kind of compounds, confirming the existence of a fluorescent singlet excited state (Table ). Interestingly, hosts 11-CO and 11-C­(CN) 2 display a different behavior with respect to the rest of the family.…”

“…This arrangement is typically set to avoid internal hydrogen repulsion (H 3 in this case). This is very common in tethers with a short spacer group and a lack of conformational mobility (see the Supporting Information for the full set of optimized structures). ,,, In a closer inspection to computed geometries, key structural factors such as penetration depths (distance between the centroid of the corannulene bowl and the centroid of the fullerene moiety) and clamping depth (distance between the midpoint of the bond connecting both phenyl units in the tether and the centroid of the fullerene moiety) reveal that, while the location of C 60 between two corannulenes is kept throughout the whole series (6.86 Å on average) it is farther within the S-derived subfamily than in the other hosts (7.40 vs 7.18 Å on average) (Figure S172 and Table S8). This seems to be a consequence of the longer S–C 1 bond distance (1.80 vs 1.48 Å on average) (Table S8) pointing toward a better adaptation ability of the tether within hosts 4 .…”

“…This is a non-negligible effect worth discussing. Experimental association constants are in the expected range of other similar corannulene-based molecular pincers. ,,, But the difference in this case is not immediate to grasp a priori as all of the hosts are not expected to experience great variations in the structure of their host–guest complexes as the tweezer scaffold is generally maintained throughout the whole series. Correlation between experimental Gibbs free energy of association (Δ G a in Table ) and Hammett constants − is nonexistent either (Figure S136), suggesting that the electrostatic interactions do not play a major role despite the great influence of the bridgehead group on the electrostatic surface of corannulene moieties (Figures c,d and S171).…”

“…Reagents were purchased from regular suppliers and used without further purification. 1-Bromocorannulene (Br-cora) was acquired from Synoi Chemicals (http:// synoichemicals.uva.es/), whereas 2,2′-bis(methylthio)-5,5′-bis-(corannulyl)-1,1′-biphenyl, 20 3,6-dibromophenanthrene-9,10-dione (6), 77 3,6-dibromo-fluoren-9-one (7), 77 3,6-dibromo-9H-fluorene (8), 77 and 3,6-dibromo-9,9-dimethyl-fluorene (9) 77 were prepared according to the reported procedure. Solvents were of analytical grade or spectrophotometric grade.…”

“…We recently reported a self-resetting bistable molecular machine that bears a redox effector whose capability to modify its affinity toward fullerenes can be efficiently modulated through an external stimulus . We however noted that, under certain conditions (see the Experimental Section), the resulting compound was dibenzothiophene 4-S (Figure ), which shows outstanding performance on fullerene recognition (see below).…”