Protomer-specific photochemistry investigated using ion mobility mass spectrometry

The utility of tandem ion mobility mass spectrometry coupled with electronic spectroscopy to investigate protomer-specific photochemistry is demonstrated by measuring the photoisomerization response for protomers of protonated 4-dicyanomethylene-2-methyl-6-para-dimethylaminostyryl-4H-pyran (DCM) molecules. The target DCMH⁺ species has three protomers that are distinguished by their different collision cross sections with He, N₂, and CO₂ buffer gases, trends in abundance with ion source conditions, and from their photoisomerization responses. The trans-DCMH⁺ protomers with the proton located either on the tertiary amine N atom or on a cyano group N atom exhibit distinct S₁← S₀ photoisomerization responses, with the maxima in their photoisomerization action spectra occurring at 420 and 625 nm, respectively, consistent with predictions from accompanying electronic structure calculations. The cis-DCMH⁺ protomers are not distinguishable from one another through ion mobility separation and give no discernible photoisomerization or photodissociation response, suggesting the dominance of other deactivation pathways such as fluorescence. The study demonstrates that isobaric protomers and isomers generated by an electrospray ion source can possess quite different photochemical behaviors and emphasizes the utility of isomer and protomer selective techniques for exploring the spectroscopic and photochemical properties of protonated molecules in the gas phase.