Thermal Back-Isomerization of Spirocyclic Naphtho-oxazine and Phenanthro-oxazine Derivatives in Alcohols, Nitriles, and Poly(alkyl methacrylates)

Abstract

Dedicated to Professor Andre M. Braun on the occasion of his 60th birthdayThe thermal back-isomerization of spiro[indole-naphtho-oxazine] 1 and spiro[indole-phenanthro-oxazine] 2 was studied in a series of primary alcohols, nitriles, and poly(methylmethacrylate), poly(ethylmethacrylate), and poly(isobutyl methacrylate) films by laser-flash photolysis in the temperature range of 0 ± 708. The decay is monoexponential in fluid solution, but deviates strongly from this behavior in polymeric environments even above the glass transition temperature of the polymers (Tg). In liquids, a very small solvent effect is observed on the isomerization rate constants (k iso ) for 1, which is attributed mostly to the solvent viscosity h. The values of k iso for 2 show influence of solvent viscosity and polarity, which were studied by application of a semiempirical relationship that accounts for non-Markovian processes. The decay kinetics in polymers was described by a Gaussian distribution of the activation energy and by a kinetic model that takes into account the simultaneous relaxation of the probe and the environment. For 1 and 2, the rate constant at the center of the Gaussian distribution is very similar to the first-order rate constant in nonpolar solvents. The Gaussian width of the distribution (s) decreases with temperature and is very similar in all polymers under Tg, and, above Tg, s decreases more abruptly. We make comparisons of the parameters derived from analysis of both 1 and 2 in polymers, as well as of their behaviors in solution and in polymers.

Phospholipid Bilayer Free Volume Analysis Employing the Thermal Ring-Closing Reaction of Merocyanine Molecular Switches

“…Comparison of eqs 1-3 shows that the Arrhenius activation energy is related to the molecular activation energy, E 0 , and the viscosity activation energy, E η 17 To estimate aE η , the E 0 should be determined independently. 36 In the temperature range used in our studies, the viscosity of toluene varies much less than the viscosity of the bilayer (viscosity activation energy for toluene E η,toluene ) 9.2 kJ mol -1 ).…”

“…Such analysis yields E 0 values in close agreement with the results for merocyanines derived from spirooxazines. 17 Results of the free volume analysis for MC1-MC3 reactions in DMPC and DPPC bilayers are summarized in Table 2.…”

“…Merocyanine reactions were studied in solvent mixtures 33 and in polymers. [15][16][17] In polymers, however, the kinetics are nonexponential, and more complex models are needed to analyze reaction rates. [15][16][17] In phospholipid bilayers, single-exponential thermal ring-closing kinetics allow solution-phase models to be used in the analysis.…”

“…The photochromic and solvatochromic properties of merocyanines have been applied to characterize complex systems, such as glasses. [15][16][17] The photochemical reactions of membrane-embedded merocyanines, similar to the ones used in this study, have been used to manipulate bilayer permeability to ions and small molecules. 18,19 Substitution of the indole moiety (R in Scheme 1) was used to control the localization and orientation of merocyanines in the bilayers, which allowed studies of well-defined membrane regions.…”

“…The products of such reactions, spiropyrans, consist of two orthogonal moieties, a substituted indole and benzopyran, connected by a spiro carbon atom. The photochromic and solvatochromic properties of merocyanines have been applied to characterize complex systems, such as glasses. − The photochemical reactions of membrane-embedded merocyanines, similar to the ones used in this study, have been used to manipulate bilayer permeability to ions and small molecules. , …”