Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component

Abstract

Five pentiptycene-derived stilbene systems (1 R; R = H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1 H<1 OM<1 NO<1 Pr<1 Bu). At room temperature (298 K), rotation of the pentiptycene rotor is fast (k(rot)=10(8)-10(9) s(-1)) with little interaction with the brake component in the trans form ((E)-1 R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)-1 R), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)-1 NO, (Z)-1 Pr, and (Z)-1 Bu, the rotation is nearly blocked (k(rot)=2-6 s(-1)) at 298 K. It is also demonstrated that the rotation is slower in [D(6)]DMSO than in CD(2)Cl(2). A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)-1 H, (Z)-1 OM, and (Z)-1 NO, and it levels off on going from (Z)-1 NO to (Z)-1 Pr and (Z)-1 Bu. DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E-Z photoswitching is up to 46%, and both the E and Z isomers are stable under the irradiation conditions.

Pentiptycene-based polyimides with hierarchically controlled molecular cavity architecture for efficient membrane gas separation

“…[6,[15][16][17][18] As demonstrated in our previous paper on triptycene-polyimide membranes, [15] a unique and appealing feature of this simplest iptycene molecule is that the open space between the benzene rings, called "internal free volume", not only introduces significant amount of "intrinsic" free volume into the polymer for fast transport, but also provides a feasible means to finely tune the size of internal free volume or molecular cavity by varying the substituent groups neighboring the triptycene units. [22][23][24][25] However, incorporation of penti structure has been sparsely reported, possibly because the not readily available. As a natural extension of the triptycene-based structure, pentiptycene-containing polyimides are reported here to demonstrate their potential as high performance gas separation membranes.…”

Light‐Gated Molecular Brakes Based on Pentiptycene‐Incorporated Azobenzenes

“…Variable‐temperature NMR experiments were performed to investigate the rotational kinetics of the Z isomers of 2 R , 2 OR , and 2 NR in CDCl 3 from 268 K to 303 K. Our previous work used CD 2 Cl 2 as the solvent,20–22, 29, 31 but CDCl 3 was more suitable for measurements at elevated temperatures, owing to its higher boiling point. Nevertheless, the rotational kinetics in terms of solvent viscosity were expected to be similar in both solvents.…”

“…We have previously reported a series of light‐gated molecular brakes based on pentiptycene‐incorporated alkenes 20–22. 29–31 One particular example is stilbene derivative 1O R , in which the H‐shaped pentiptycene scaffold serves as a four‐blade rotor component and the dinitrophenyl group as a brake pad (Scheme ) 20. The brake‐off and brake‐on states correspond to the E and Z configurations, whilst the difference in rotation rate (braking power) is as large as ×10 8 at ambient temperature.…”

Crystal structure of 5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene-6,13-dione

“…Pentiptycene and its derivatives are members of the iptycene family (Hart et al, 1981). They possess a rigid, bulky, aromatic, three-dimensional scaffold which makes them suitable for specific applications in porous material construction (Yang & Swager, 1998a), fluorescent polymers, chemical sensing (Yang & Swager, 1998b) and molecular machines (Sun et al, 2010). The first iptycene derivative was reported 85 years ago (Clar, 1931).…”