1 research outputs found
Conformer Ring Flip Enhances Mechanochromic Performance of <i>ansa</i>-Donor–Acceptor–Donor Mechanochromic Torsional Springs
Mechanochromophores
based on conformational changes of
donor–acceptor–donor
(DAD) springs allow sensing of forces acting on polymer chains by
monotonic changes of absorbance or photoluminescence (PL) wavelength.
Here, we identify a series of thiophene (D)-flanked quinoxalines (A)
as molecular torsional springs for force sensing in bulk polymers
at room temperature. The mode of DAD linkage to the polymer matrix
and linker rigidity are key parameters that influence the efficacy
of force transduction to the DAD spring and thus mechanochromic response,
as probed by in situ PL spectroscopy of bulk films during stress–strain
experiments. The largest shift of the PL maximum, and thus the highest
sensitivity, is obtained from an ansa-DAD spring
exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy
and density functional theory calculations, we reveal conformer redistribution
in the form of a thiophene ring flip as the major part of the overall
mechanochromic response. At forces as low as 27 pN at early stages
of deformation, the ring flip precedes mechanically induced planarization
of the ansa-DAD spring, the latter process producing
a PL shift of 21 nm nN–1. Within the stress–strain
diagram, the thiophene ring flip and DAD planarization are thus two
separated processes that also cause irreversible and reversible mechanochromic
responses, respectively, upon sample failure. As the thiophene ring
flip requires much smaller forces than planarization of the DAD spring,
such micromechanical motion gives access to sensing of tiny forces
and expands both sensitivity and the force range of conformational
mechanochromophores