Charge transfer transitions of the O2+-Ar and O2+-N2 complexes.

Electronic transitions are observed for the O2+-Ar and O2+-N2 complexes over the 225-350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O2+, O2, N2+, and N2) but rather are due to charge transfer transitions. Onsets of the O2+-Ar and O2+-N2 band systems occur at 3.

Wavelength dependent mechanism of phenolate photooxidation in aqueous solution.

Phenolate photooxidation is integral to a range of biological processes, yet the mechanism of electron ejection has been disputed. Here, we combine femtosecond transient absorption spectroscopy, liquid-microjet photoelectron spectroscopy and high-level quantum chemistry calculations to investigate the photooxidation dynamics of aqueous phenolate following excitation at a range of wavelengths, from the onset of the S-S absorption band to the peak of the S-S band. We find that for ≥ 266 nm, electron ejection occurs from the S state into the continuum associated with the contact pair in which the PhO˙ radical is in its ground electronic state.

UV Photoelectron Spectroscopy of Aqueous Solutions.

Knowledge of the electronic structure of an aqueous solution is a prerequisite to understanding its chemical and biological reactivity and its response to light. One of the most direct ways of determining electronic structure is to use photoelectron spectroscopy to measure electron binding energies. Initially, photoelectron spectroscopy was restricted to the gas or solid phases due to the requirement for high vacuum to minimize inelastic scattering of the emitted electrons.

Accurate Vertical Ionization Energy of Water and Retrieval of True Ultraviolet Photoelectron Spectra of Aqueous Solutions.

Ultraviolet (UV) photoelectron spectroscopy provides a direct way of measuring valence electronic structure; however, its application to aqueous solutions has been hampered by a lack of quantitative understanding of how inelastic scattering of low-energy (<5 eV) electrons in liquid water distorts the measured electron kinetic energy distributions. Here, we present an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. Our method combines Monte Carlo simulations of electron scattering and spectral inversion, with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution.

An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters.

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio.

Electronic spectra of positively charged carbon clusters-C (n = 6-14).

Electronic spectra are measured for mass-selected C (n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C (n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C and larger clusters.

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study.

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 (major) and (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections.

Electronic Spectrum of the Tropylium Cation in the Gas Phase.

The structure and properties of the tropylium cation (CH) have enthralled chemists since the prediction by Hückel in 1931 of the remarkable stability for cyclic, aromatic molecules containing six π-electrons. However, probing and understanding the excited electronic states of the isolated tropylium cation have proved challenging, as the accessible electronic transitions are weak, and there are difficulties in creating appreciable populations of the tropylium cation in the gas phase. Here, we present the first gas-phase S ←S electronic spectrum of the tropylium cation, recorded by resonance-enhanced photodissociation of weakly bound tropylium-Ar complexes.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light.

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers.

Electronic Spectra of Diacetylene Cations (HCH) Tagged with Ar and N.

Electronic spectra of mass-selected HCH-Ar ( = 1-3) and HCH-(N) ( = 1-2) complexes are measured over the 290-530 nm range using resonance-enhanced photodissociation spectroscopy in a tandem mass spectrometer. Vibronic transitions in the visible region are compared with previous experimental and theoretical results for the ÃΠ ← X̃Π band system of HCH. Hole burning experiments confirm that transitions over the 290-340 nm range involve the diacetylene cation (HCH).

Photodetachment and photoreactions of substituted naphthalene anions in a tandem ion mobility spectrometer.

Substituted naphthalene anions (deprotonated 2-naphthol and 6-hydroxy-2-naphthoic acid) are spectroscopically probed in a tandem drift tube ion mobility spectrometer (IMS). Target anions are selected according to their drift speed through nitrogen buffer gas in the first IMS stage before being exposed to a pulse of tunable light that induces either photodissociation or electron photodetachment, which is conveniently monitored by scavenging the detached electrons with trace SF6 in the buffer gas. The photodetachment action spectrum of the 2-naphtholate anion exhibits a band system spanning 380-460 nm with a prominent series of peaks spaced by 440 cm-1, commencing at 458.

Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid.

Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E → Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest ππ* transition.

Double Molecular Photoswitch Driven by Light and Collisions.

The shapes of many molecules can be transformed by light or heat. Here we investigate collision- and photon-induced interconversions of EE, EZ, and ZZ isomers of the isolated Congo red (CR) dianion, a double molecular switch containing two ─N═N─ azo groups, each of which can have the E or Z configuration. We find that collisional activation of CR dianions drives a one-way ZZ→EZ→EE cascade towards the lowest-energy isomer, whereas the absorption of a single photon over the 270-600 nm range can switch either azo group from E to Z or Z to E, driving the CR dianion to lower- or higher-energy forms.

Reversible Photoisomerization of the Isolated Green Fluorescent Protein Chromophore.

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.

Linkage Photoisomerization of an Isolated Ruthenium Sulfoxide Complex: Sequential versus Concerted Rearrangement.

Ruthenium sulfoxide complexes undergo thermally reversible linkage isomerization of sulfoxide ligands from S- to O-bound in response to light. Here, we report photoisomerization action spectra for a ruthenium bis-sulfoxide molecular photoswitch, [Ru(bpy)(bpSO)], providing the first direct evidence for photoisomerization of a transition metal complex in the gas phase. The linkage isomers are separated and isolated in a tandem drift tube ion mobility spectrometer and exposed to tunable laser radiation provoking photoisomerization.

Photoswitching an Isolated Donor-Acceptor Stenhouse Adduct.

Donor-acceptor Stenhouse adducts (DASAs) are a new class of photoswitching molecules with excellent fatigue resistance and synthetic tunability. Here, tandem ion mobility mass spectrometry coupled with laser excitation is used to characterize the photocyclization reaction of isolated, charge-tagged DASA molecules over the 450-580 nm range. The experimental maximum response at 530 nm agrees with multireference perturbation theory calculations for the S ← S transition maximum at 533 nm.

From E to Z and back again: reversible photoisomerisation of an isolated charge-tagged azobenzene.

Substituted azobenzenes serve as chromophores and actuators in a wide range of molecular photoswitches. Here, tandem ion mobility spectrometry coupled with laser excitation is used to investigate the photoisomerisation of selected E and Z isomers of the charge-tagged azobenzene, methyl orange. Both isomers display a weak S(nπ*) photoisomerisation response in the blue part of the spectrum peaking at 440 nm and a more intense S(ππ*) photoisomerisation response in the near-UV with maxima at 370 and 310 nm for the E and Z isomers, respectively.

Photoisomerization of Protonated Azobenzenes in the Gas Phase.

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 (NHABH) cations are investigated in the gas phase using a tandem ion mobility spectrometer. Both cations display a strong photoisomerization response across their S ← S 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 NHAB molecules.

Online measurement of photoisomerisation efficiency in solution using ion mobility mass spectrometry.

The photoisomerisation of charged molecules in solution is probed directly using ion mobility mass spectrometry with electrospray ionisation. The technique is demonstrated using a prototype azoheteroarene cation in methanol. By scanning the wavelength of the excitation light while monitoring the photoisomer intensity a photoisomerisation action spectrum is generated, which, when compared with a conventional absorption spectrum, allows the relative photoisomerisation quantum yield to be determined across a range of wavelengths.

Seleniranium Ions Undergo π-Ligand Exchange via an Associative Mechanism 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.

Isomerisation of an intramolecular hydrogen-bonded photoswitch: protonated azobis(2-imidazole).

Photoisomerisation of protonated azobis(2-imidazole), an intramolecular hydrogen-bonded azoheteroarene photoswitch molecule, is investigated in the gas phase using tandem ion mobility mass spectrometry. The E and Z isomers exhibit distinct spectral responses, with E-Z photoisomerisation occurring over the 360-520 nm range (peak at 460 nm), and Z-E photoisomerisation taking place over the 320-420 nm range (peak at 390 nm). A minor photodissociation channel involving loss of N is observed for the E-isomer with a maximum efficiency at 390 nm, blue-shifted by ≈70 nm relative to the wavelength for maximum photoisomerisation response.

A Strong cis-Effect in an Imidazole-Imidazolium-Substituted Alkene.

We report the first example of an alkene with two carbon-bound substituents (imidazole and imidazolium rings) where the Z-isomer has a greater thermodynamic stability than the E-isomer which persists in both the gas phase and in solution. Theoretical calculations, solution fluorescence spectroscopy and gas-phase ion mobility mass spectrometry studies confirm the preference for the Z-isomer, the stability of which is traced to a non-covalent interaction between the imidazole lone pair and the imidazolium ring.

Monitoring Isomerization of Molecules in Solution Using Ion Mobility Mass Spectrometry.

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.