 Photochromism is the transformation of a molecule from one structural state to another where at least one of the states must be light-induced. Because of its unique properties, spiropyran is enjoying widespread use in many applications, particularly in light-responsive materials research, namely hybrid materials that utilize spiropyran to induce material changes or provide colormetric sensing of changes in the microenvironment. In our studies of spiropyran solution kinetics, we have observed deviations in expected behavior, arguing that spiropyran photoisomerization is not a simple first-order process involving two equilibrating structures. These results motivated us to examine details of this interconversion using ion mobility-mass spectrometry (IM-MS) which can separate the spiropyran isomers based on molecular collision cross-sectional area (CCS). Quantum chemical modeling based on Density Functional Theory was used to identify possible theoretical ground-state isomers of merocyanine. CCS calculations from the program MOBCAL were employed to aid conformer assignment to specific isomers. IM-MS experiments reveal the presence of three distinct conformers. CCS values and theoretical structures will be presented. The results indicate that the three conformers are comprised of one single structure and two groups of isomers; the ring-closed isomer, the cis-MC group; CCX, and the trans-MC group; XTX. The designations; cis (C) and trans (T), refer to configuration of the three methylene bonds linking the p-nitrophenolate and indoline moieties of MC. This work represents the first observation of a second isomer group on timescales greater than nanoseconds. A mechanism for the thermal reaction is postulated to occur via the formation of a stable cis intermediate, CCX, which has not been previously identified as a stable isomer in other work.

Ion-mobility mass spectrometry of a widely used photochromic system: Experimental support for the <i>cisoid</i> form of spiropyran

247th National Meeting (2014)

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Ion-mobility mass spectrometry of a widely used photochromic system: Experimental support for the cisoid form of spiropyran