Photoisomerization of linear and stacked isomers of a charged styryl dye – a tandem ion mobility study

Eduardo Carrascosa, James N. Bull, Emilio Martínez-Núñez, Michael S. Scholz, Jack T. Buntine, Evan J. Bieske

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The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of EE (major) and ZE (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections. Two additional but minor isomers are generated by collisional excitation of the electrosprayed styryl 9M ions and are assigned to the EZ and ZZ geometric isomers, with the latter predicted to have a π-stacked configuration. The isomer assignments are supported through calculations of equilibrium structures, collision cross sections, and statistical isomerization rates. Photoexcitation of selected isomers using an IMS-photo-IMS strategy shows that each geometric isomer photoisomerizes following absorption of near-infrared and visible light, with the EE isomer possessing a S1 ← S0 electronic transition with a band maximum near 680 nm and shorter wavelength S2 ← S0 electronic transition with a band maximum near 430 nm. The study demonstrates the utility of the IMS-photo-IMS strategy for providing fundamental gas-phase photochemical information on molecular systems with multiple isomerizable bonds.
Original languageEnglish
Pages (from-to)2842-2851
Number of pages10
JournalJournal of the American Society for Mass Spectrometry
Issue number12
Early online date17 Nov 2021
Publication statusPublished - 1 Dec 2021


  • action spectroscopy
  • chemical master equation
  • dye
  • ion mobility mass spectrometry
  • isomerization

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