An ion mobility spectrometer (IMS) with an electrospray ion source is used to investigate photo and thermal isomerization of photoactive molecules in the electrospray syringe. A light emitting diode ...adjacent to the syringe establishes a photostationary state that relaxes thermally toward the more stable isomer once illumination ceases. The arrangement is demonstrated by measuring Z–E thermal isomerization rates for several azoheteroarene compounds. The IMS technique has a distinct advantage over UV–vis spectrophotometry for measuring isomer populations in situations where there are multiple isomers with overlapping absorption profiles. In another development, an LED array adjacent to the silica capillary connecting the syringe to the electrospray ion source, is used to activate photochromic molecules, and investigate sequential photoswitching events.
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.
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 S1 ← S0 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 CH3 loss and concerted CO and CH3CN loss, although isomerization remains the dominant process.
Photoisomerization of Protonated Azobenzenes in the Gas Phase Scholz, Michael S; Bull, James N; Coughlan, Neville J. A ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
08/2017, Letnik:
121, Številka:
34
Journal Article
Recenzirano
Because of their high photoisomerization efficiencies, azobenzenes and their functionalized derivatives are used in a broad range of molecular photoswitches. Here, the photochemical properties of the ...trans isomers of protonated azobenzene (ABH+) and protonated 4-aminoazobenzene (NH2ABH+) cations are investigated in the gas phase using a tandem ion mobility spectrometer. Both cations display a strong photoisomerization response across their S1 ← S0 bands, with peaks in their photoisomerization yields at 435 and 525 nm, respectively, red-shifted with respect to the electronic absorption bands of the unprotonated AB and NH2AB molecules. The experimental results are interpreted with the aid of supporting electronic structure calculations considering the relative stabilities and geometries of the possible isomers and protomers and vertical electronic excitation energies.
The preferential solvation behavior for eight different derivatives of protonated quinoline was measured in a tandem differential mobility spectrometer mass spectrometer (DMS-MS). Ion-solvent cluster ...formation was induced in the DMS by the addition of chemical modifiers (i.e., solvent vapors) to the N
2
buffer gas. To determine the effect of more than one modifier in the DMS environment, we performed DMS experiments with varying mixtures of water, acetonitrile, and isopropyl alcohol solvent vapors. The results show that doping the buffer gas with a binary mixture of modifiers leads to the ions binding preferentially to one modifier over another. We used density functional theory to calculate the ion-solvent binding energies, and in all cases, calculations show that the quinolinium ions bind most strongly with acetonitrile, then isopropyl alcohol, and most weakly with water. Computational results support the hypothesis that the quinolinium ions bind exclusively to whichever solvent they have the strongest interaction with, regardless of the presence of other modifier gases.
We describe the modification of a commercially available tandem differential mobility mass spectrometer (DMS) that has been retrofitted to facilitate photodissociation (PD) of differential ...mobility-separated, mass-selected molecular ions. We first show that a mixture of protonated quinoline/isoquinoline (QH+/iQH+) can be separated using differential mobility spectrometry. Efficient separation is facilitated by addition of methanol to the DMS environment and increased residence time within the DMS. In action spectroscopy experiments, we gate each isomer using appropriate DMS settings, trap the ions in the third quadrupole of a triple quadrupole mass spectrometer, and irradiate them with tunable light from an optical parametric oscillator (OPO). The resulting mass spectra are recorded as the OPO wavelength is scanned, giving PD action spectra. We compare our PD spectra with previously recorded spectra for the same species and show that our instrument reproduces previous works faithfully.
Two ion populations of protonated Rivaroxaban, C19H18ClN3O5S + H+, are separated under pure N2 conditions using differential mobility spectrometry prior to characterization in a hybrid triple ...quadrupole linear ion trap mass spectrometer. These populations are attributed to bare protonated Rivaroxaban and to a proton-bound Rivaroxaban–ammonia complex, which dissociates prior to mass-selecting the parent ion. Ultraviolet photodissociation (UVPD) and collision-induced dissociation (CID) studies indicate that both protonated Rivaroxaban ion populations are comprised of the computed global minimum prototropic isomer. Two ion populations are also observed when the collision environment is modified with 1.5% (v/v) acetonitrile. In this case, the protonated Rivaroxaban ion populations are produced by the dissociation of the ammonium complex and by the dissociation of a proton-bound Rivaroxaban–acetonitrile complex prior to mass selection. Again, both populations exhibit a similar CID behavior; however, UVPD spectra indicate that the two ion populations are associated with different prototropic isomers. The experimentally acquired spectra are compared with computed spectra and are assigned to two prototropic isomers that exhibit proton sharing between distal oxygen centers.
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.
Incorporation of fluorescent proteins into biochemical systems has revolutionized the field of bioimaging. In a bottom-up approach, understanding the photophysics of fluorescent proteins requires ...detailed investigations of the light-absorbing chromophore, which can be achieved by studying the chromophore in isolation. This paper reports a photodissociation action spectroscopy study on the deprotonated anion of the red Kaede fluorescent protein chromophore, demonstrating that at least three isomers-assigned to deprotomers-are generated in the gas phase. Deprotomer-selected action spectra are recorded over the S
← S
band using an instrument with differential mobility spectrometry coupled with photodissociation spectroscopy. The spectrum for the principal phenoxide deprotomer spans the 480-660 nm range with a maximum response at ≈610 nm. The imidazolate deprotomer has a blue-shifted action spectrum with a maximum response at ≈545 nm. The action spectra are consistent with excited state coupled-cluster calculations of excitation wavelengths for the deprotomers. A third gas-phase species with a distinct action spectrum is tentatively assigned to an imidazole tautomer of the principal phenoxide deprotomer. This study highlights the need for isomer-selective methods when studying the photophysics of biochromophores possessing several deprotonation sites.
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