Abstract
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.
Highlights
Molecules containing photoisomerizable subunits are commonly used as molecular triggers or sensors in technological and biomedical applications.[1]
The detector was connected to a multichannel scaler, which was used to generate a histogram of ion counts against arrival time, corresponding to an arrival time distribution (ATD)
To help ascertain contributions from secondary statistical isomerization, chemical master equation (CME) modeling was performed starting with a population of a single rotamer assuming a temperature T = 300 K plus the energy imparted through absorption of one or two photons at 430 or 680 nm
Summary
Molecules containing photoisomerizable subunits are commonly used as molecular triggers or sensors in technological and biomedical applications.[1]. We use tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy to investigate the charged styryl dye S9M in the gas phase, focusing on its electronic spectrum and its photoisomerization response over the visible spectral range. By irradiating the target S9M ions between two drift tube IMS stages, we can select a particular target isomer and distinguish photoproduct isomers This approach has been used previously to investigate photoinduced structural changes for a range of molecular ions in the gas phase.[39−42] The situation is relatively simple for charged molecules that have only two geometric isomers with significantly different collision cross sections, which can, for example, be interconverted through E−Z photoisomerization around a single double bond.[43] For molecules with several photolabile bonds such as S9M, the situation is more complicated as exposure to light could cause transformations between several different geometric isomers. The situation for S9M is more complicated than for Congo Red as the two photoisomerizable bonds are not equivalent and there are four rather than three geometric isomers
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