Advancing the Mechanosensitivity of Atropisomeric Diarylethene Mechanophores through a Lever-Arm Effect

Abstract

Understanding structure-mechanical activity relationships (SMARs) in polymer mechanochemistry is essential for the rational design of mechanophores with desired properties, yet SMARs in noncovalent mechanical transformations remain relatively underexplored. In this study, we designed a subset of diarylethene mechanophores based on a lever-arm hypothesis and systematically investigated their mechanical activity toward a noncovalent-yet-chemical conversion of atropisomer stereochemistry. Results from Density functional theory (DFT) calculations, single-molecule force spectroscopy (SMFS) measurements, and ultrasonication experiments collectively support the lever-arm hypothesis and confirm the exceptional sensitivity of chemo-mechanical coupling in these atropisomers. Notably, the transition force for the diarylethene M3 featuring extended 5-phenylbenzo[b]thiophene aryl groups is determined to be 131 pN ± 4 pN by SMFS. This value is lower than those typically recorded for other mechanically induced chemical processes, highlighting its exceptional sensitivity to low-magnitude forces. This work contributes a fundamental understanding of chemo-mechanical coupling in atropisomeric configurational mechanophores and paves the way for designing highly sensitive mechanochemical processes that could facilitate the study of nanoscale mechanical behaviors across scientific disciplines.

Figure 1

(a) Representative mechanisms for mechanically induced molecular transformations. (b) This study introduces a lever-arm effect that enables fine-tuning mechanical reactivity in force-triggered atropisomerization.

Figure 2

(a) DFT-calculated structures of model mechanophores in equilibrium geometry. (b) CoGEF calculations (B3LYP/6-31G*) predict the mechanical stereochemical conversion of parallel diarylethenes to their antiparallel forms. Elongating the constrained distance results in distortion of the dihedral angle between the benzothiadiazole bridge and the side-arm aryl groups and eventually leads to a sudden rotation of one side-arm aryl plane around the BBT-aryl σ bond, inducing a flip at that chirality axis. M3 structures corresponding to the data point indicated by the arrow are shown (see the SI for details).

Figure 3

(a) Synthetic scheme for multimechanophore copolymers P1-P3. (b) Overlay of representative force–extension curves obtained for P1-P3. Curves are normalized to the corresponding extension at 0.8 nN force. (c) Multicycle SMFS experiment of P2 shows a characteristic plateau in the first withdraw, corresponding to the stereochemical conversion from parallel diarylethenes to the antiparallel. No plateau is observed in subsequent cycles.

Figure 4

(a) Synthesis of PMA3 containing a chain-centered mechanophore, and its stimuli-responsive properties. Force-triggered atropisomerization of the parallel diarylethene generates a racemate of antiparallel isomers, but only one antiparallel isomer is shown for simplicity. (b) Ultrasonication-dependent photochromism of PMA3.

Figure 5

(a) UV-induced absorbance changes (photostationary-state spectrum minus non-photoirradiated spectrum, see Figure S11 for data processing) for sonicated PMA3 solutions. (b) Time-course sonomechanical activation of PMA1-PMA3 fitted into pseudo-first-order rate expressions (Figure S13).

Figure 6

Partial ¹H NMR spectra of PMA3 (acetone-d₆) subjected to different ultrasonication conditions: no sonication (top trace), 5 and 60 min of ultrasonication (2nd and 3rd traces, respectively). The bottom trace corresponds to a separately synthesized control polymer PMA3ap incorporating an antiparallel diarylethene. Orange shade: parallel diarylethene; blue shade: antiparallel diarylethene; blended: overlapped peaks.