An ion mobility mass spectrometry apparatus for investigating the photoisomerization and photodissociation of electrosprayed molecular ions in the gas phase is described. The device consists of a drift tube mobility spectrometer, with access for a laser beam that intercepts the drifting ion packet either coaxially or transversely, followed by a quadrupole mass filter. An ion gate halfway along the drift region allows the instrument to be used as a tandem ion mobility spectrometer, enabling mobility selection of ions prior to irradiation, with the photoisomer ions being separated over the second half of the drift tube. The utility of the device is illustrated with photoisomerization and photodissociation action spectra of carbocyanine molecular cations. The mobility resolution of the device for singly charged ions is typically 80 and it has a mass range of 100-440 Da, with the lower limit determined by the drive frequency for the ion funnels, and the upper limit by the quadrupole mass filter.

A new approach for studying the photoisomerization of molecular ions in the gas phase is described. Packets of molecular ions are injected into a drift tube filled with helium buffer gas, where they are irradiated with tunable laser light. Photoisomerization changes the ions' cross section for collisions with helium atoms so that they arrive at the ion detector slightly earlier or later than the parent ions. By monitoring the photo-isomer peak as a function of laser wavelength one can record an action spectrum that is related to the ions' absorption spectrum modulated by the photoisomerization probability. The approach is demonstrated using the polymethine dye HITC (1,3,3,1',3',3'-hexamethylindotricarbocyanine). The data show that both trans and cis forms of HITC(+) exist in the gas phase with trans→cis photoisomerization predominating over the 550-710 nm range and cis→trans photoisomerization occurring over the 735-770 nm range. The gas-phase photoisomerization action spectrum is comparable to the absorption spectra of trans HITC and cis HTIC in the condensed phase, but with the absorption peaks shifted to shorter wavelength. The gas-phase photoisomerization action spectrum of the (HITC)2(2+) dication dimer is also reported. (HITC)2(2+) cations photoisomerize over the 550-770 nm range to form more compact structures.

Laser spectroscopy and ion mobility spectrometry are combined to provide structural and photochemical information on photoisomerizing molecules in the gas phase. The strategy exploits the fact that an ion packet propelled through buffer gas by an electric field separates spatially and temporally into its constituent isomers because of small differences in their collision cross sections. Isomers selected by an electrostatic ion gate are exposed to wavelength tunable radiation, promoting formation of photoisomers that are separated in a second ion mobility stage. The approach is demonstrated for protonated merocyanine and spiropyran isomers formed through electrospray ionization. Four isomers are observed whose relative abundances depend on pretreatment of the electrosprayed solution with either ultraviolet or visible light, and on collisional excitation before the ions are launched into the drift tube. The observations are interpreted in the light of accurate double-hybrid density functional theory calculations for the protonated spiropyran and merocyanine isomers that are used to predict structures, relative energies, isomerization barriers, collision cross sections and electronic absorption spectra. The two most abundant isomers, are merocyanine forms, in which the proton resides on the quinone oxygen atom, with either a trans or cis central bond in the linking polymethine chain. These two mero forms can be interconverted through photoexcitation, with different wavelength dependences for the forward and reverse photoisomerization processes. Protonated spiropyran is formed from protonated merocyanine isomers through collisional activation, but in only minor amounts through their photo-excitation over the 300-700 nm range.

Fluorescent proteins have revolutionized the visualization of biological processes, prompting efforts to understand and control their intrinsic photophysics. Here we investigate the photoisomerization of deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI), the chromophore in green fluorescent protein and in Dronpa protein, where it plays a role in switching between fluorescent and nonfluorescent states. In the present work, isolated HBDI molecules are switched between the Z and E forms in the gas phase in a tandem ion mobility mass spectrometer outfitted for selecting the initial and final isomers. Excitation of the S ← S transition provokes both Z → E and E → Z photoisomerization, with a maximum response for both processes at 480 nm. Photodetachment is a minor channel at low light intensity. At higher light intensities, absorption of several photons in the drift region drives photofragmentation, through channels involving CH loss and concerted CO and CHCN loss, although isomerization remains the dominant process.

The photophysical behaviour of the isolated retinal protonated n-butylamine Schiff base (RPSB) is investigated in the gas phase using a combination of ion mobility spectrometry and laser spectroscopy. The RPSB cations are introduced by electrospray ionisation into an ion mobility mass spectrometer where they are exposed to tunable laser radiation in the region of the S1 ← S0 transition (420-680 nm range). Four peaks are observed in the arrival time distribution of the RPSB ions. On the basis of predicted collision cross sections with nitrogen gas, the dominant peak is assigned to the all-trans isomer, whereas the subsidiary peaks are assigned to various single, double and triple cis geometric isomers. RPSB ions that absorb laser radiation undergo photoisomerization, leading to a detectable change in their drift speed. By monitoring the photoisomer signal as a function of laser wavelength an action spectrum, extending from 480 to 660 nm with a clear peak at 615 ± 5 nm, is obtained. The photoisomerization action spectrum is related to the absorption spectrum of isolated retinal RPSB molecules and should help benchmark future electronic structure calculations.

Retinal is one of Nature's most important and widespread chromophores, exhibiting remarkable versatility in its function and spectral response, depending on its protein environment. Reliable spectroscopic and photochemical data for the isolated retinal molecule are essential for calibrating theoretical approaches that seek to model retinal's behaviour in complex protein environments. However, due to low densities and possible co-existence of multiple isomers, retinal is a challenging target for gas-phase investigations. Here, the photoisomerization behaviour of the trans isomer of the retinal protonated Schiff base (RPSB) is investigated in the gas phase by irradiating mobility-selected RPSB ions with tunable light in a tandem ion mobility spectrometer. trans RPSB ions are converted to single cis isomers and also more compact isomers through irradiation with visible light. The S1← S0 photoisomerization action spectrum of trans RPSB, obtained by monitoring production of cis isomers as a function of wavelength, exhibits a single well-defined peak with a maximum at 618 ± 5 nm. Corresponding action spectra of cis RPSB isomers exhibit broader peaks, conclusively demonstrating an isomeric dependence for the RPSB spectrum in the gas phase.

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, N, and CO 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 S← S 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.

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.