Mechanistic Studies of the Thermal Spiropyran-Merocyanine Interconversion Using Kinetic Analysis, Ion Mobility-Mass Spectrometry and Quantum Chemical Computation Methods
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The spiropyran-merocyanine system of four derivatives of the parent spiropyran system were studied on the thermal merocyanine to spiropyran interconversion pathway using kinetic analysis, ion mobility-mass spectrometry and several supplemental computational approaches. The effect of tethering a spiropyran moiety to the surface of a silica nanoparticle suspended in ethanol was kinetically characterized. Ion mobility mass-spectrometry chromatographs of three derivatives were analyzed for conformers of spiropyran detected in the gas phase. Three singly charged monomeric conformer groups were identified for all derivatives used in ion mobility-mass spectrometry studies and were assigned to the spiropyran, cisoid and transoid isomers based on theoretical collision cross-sections of potential isomers. Collision cross-sections were predicted through optimization of seven protonated spiropyran isomers at the DFT-B3LYP/6-31G++(d,p) level of theory followed by analysis using the program MOBCAL. Six spiropyran isomers for three of the most photoactive spiropyrans studied were identified to be the dominant isomers on the thermal MC to SP interconversion pathway and their geometries were optimized at the DFT-B3LYP/6-31G++(d’,p’) level of theory with the inclusion of the conductor-like polarizable continuum model for methanol solvent (CPCM). Following geometric optimization, ZINDO/CPCM and TDDFT-B3LYP/CPCM were used to predict the absorption maxima of each geometric isomer and isomer-specific rate constants were determined through conventional UV-visible spectroscopic analysis. Based on the collective results of each portion of this work, phase-dependent mechanisms for the thermal MC to SP interconversion are postulated.