Correlation equation for the activation free energy of the inversion of configuration in the closed form of indolinospiropyrans

Soluble and stable alternating main-chain merocyanine copolymers through quantitative spiropyran–merocyanine conversion

“…Secondly, the varying energy levels of SP and MCH + for the two substituents R ¼ NO 2 and PhOMe indicate that an alteration of the equilibria involved is possible using substituent effects. This result itself is not surprising, [28][29][30] but nevertheless shows that the synthetic approach of Suzuki polycondensation used here has electronic consequences that alter the relative stabilities of all forms involved, as a phenyl unit is installed in para-position to spiropyran oxygen. Therefore, using other comonomers with altered electronic effects should provide a means to tune the SP/ MC equilibrium in such main chain architectures.…”

“…It is known that low molecular weight spiropyrans differently substituted in 6-position exhibit altered isomerization behavior and stabilities. [28][29][30] We were interested in elucidating the electronic effect of the phenyl ring on the stabilities of SP, MC and MCH + . This is especially interesting as most spiropyran derivatives carry a nitro group at the same position.…”

“…Firstly, spiropyran substituent effects can be introduced by the neighboring comonomer, which is expected to inuence the SP/MC equilibrium by stabilizing or destabilizing the merocyanine unit. [28][29][30] As a result, the dynamic properties of such stimuli-responsive polymeric materials might efficiently be tuned. Secondly, the chemical variation of the comonomer can alter the miscibility behavior of the copolymer with another polymer matrix to yield mechanochromically altered materials via a simple and versatile blending approach.…”

Thermal Racemization of Substituted Indolinobenzospiropyrans: Evidence of Competing Polar and Nonpolar Mechanisms

“…Few studies on the configurational stability of indolinobenzospiropyrans have been carried out so far. Zaichenko and co-workers measured free energies of activation for the inversion of configuration (Δ G ‡ inv ) of a series of spiropyrans 3 at 25 °C by dynamic NMR spectroscopy in deuterated DMSO and correlated those values to Hammett σ p constants for substituents R 1 and R 3 , and to Taft σ* constants for substituent R 2 . The results are consistent with a rate-determining heterolytic cleavage of the C(sp 3 )−O bond to give a zwitterionic cisoid intermediate that can rapidly invert and close back to the spiropyran of opposite configuration, as shown in Scheme .…”

Substituent Correlations Characterized by Hammett Constants in the Spiropyran–Merocyanine Transition

“…Substituents on both the indoline and benzopyran parts of SP have been shown to critically affect both Gibbs free energy and rate − constants of SP/MC isomerization. While the classical NO 2 substituent at the C 6 -carbon stabilizes the MC form and increases the rate of MC formation in polar solvents, other, less used derivatives allow one to shift SP/MC equilibrium drastically. ,, There are several experimental studies that correlate reaction (decolorization) rates ,− and racemization rates as well as spectral shifts of SP and related compounds, in which various variants of Hammett constants are used. However, a deeper analysis of substituent effects and their correlation with Hammett constants in SP in particular within theory has yet to be established.…”