Substituent-controlled energetics and barriers of mechanochromic spiropyran-functionalized Poly(ɛ-caprolactone)

In a joint theoretical and experimental study, it is shown that the onset of the mechanically-induced spiropyran (SP) to merocyanine (MC) isomerization can be controlled by both the regiochemistry and the substitution pattern of SP. Four SP-based bifunctional initiators with consistently varied polymer chain anchor point and substituent are used to synthesize poly-ε-caprolactone (PCL). Theoretical calculations (1S and 3S COGEF methods) and in-situ visible light absorption measurements of films during uniaxial stress-strain experiments consistently show varying activation barriers of the force-induced ring-opening reaction of SP to give MC. SPs with PCL chains attached in ortho-position to the pyran oxygen isomerize at lower stress than their para-analogs. NO2-substituted SP mechanophores exhibit a lower activation barrier compared with H-substituted ones, but only if the -NO2 substituent is located in para-position relative to the O at the pyran half. These results are consistent with theoretical loading rate-dependent rupture forces required to break the C-O bond of SP and may guide mechanophore design.