We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO 2 para to the breaking spirocyclic C−O bond. The force required to achieve the rate constants of ∼10 s −1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the forcecoupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

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We have determined aqueous solubility (S w) for purified cis and trans isomers of azobenzene (AB) and AB n derivatives. S w for trans-AB n is 0.05-0.15 mM and the cis isomer is 6-40x more soluble. The effect of differential solubility is important for aqueous sample concentration > S w (trans) where cis/trans ratios >1 were observed via 1 H and 19 F NMR. This phenomenon is general for AB n (aq) that included semifluorinated AB n and peptide conjugates. The van't-Hoff-Dimroth relationship predicts the equilibrium cis/trans ratio is proportional to the ratio of S w values. Comparing NMR data to S w values confirms the trend observed in the cis/trans ratios, but the magnitude of the ratio is lower. This phenomenon is observed in μmol concentration and may be a factor in photopharmacology. We have also studied the thermal cis-to-trans isomerization as a function of temperature for eight AB n compounds. E a values AB n (aq) are ~ 25 kJ mol-1 lower compared to E a measured in nonpolar solvents and ΔS ‡ values in water are more negative so that aqueous rates are 2-5 times greater in water. This class of AB n compounds generally show little solvent effect and this change in E a is small compared to the behavior of donor-acceptor ABs. The inversion mechanism for this isomerization is supported by density functional theory.