School of Chemistry - Research Publications

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    Electronic spectra of positively charged carbon clusters-C2n+ (n=6-14)
    Buntine, JT ; Cotter, M ; Jacovella, U ; Liu, C ; Watkins, P ; Carrascosa, E ; Bull, JN ; Weston, L ; Muller, G ; Scholz, MS ; Bieske, EJ (AIP Publishing, 2021-12-07)
    Electronic spectra are measured for mass-selected C2n +(n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 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 C2n +(n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C22 + and larger clusters. The C2n + electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C12 +, C16 +, C20 +, C24 +, and C28 + are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C14 +, C18 +, C22 +, and C26 + are narrower, consistent with slower non-radiative deactivation.
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    Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study
    Carrascosa, E ; Bull, JN ; Martinez-Nunez, E ; Scholz, MS ; Buntine, JT ; Bieske, EJ (AMER CHEMICAL SOC, 2021-12-01)
    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.
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    A strong cis-effect in an imidazole/imidazolium substituted alkene
    Georgiou, DC ; Haghighatbin, MA ; Hogan, CF ; Scholz, MS ; Bull, J ; Bieske, EJ ; Wilson, DJD ; Dutton, JL (John Wiley & Sons Ltd., 2017-07-10)
    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.
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    Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid
    Bull, JN ; da Silva, G ; Scholz, MS ; Carrascosa, E ; Bieske, EJ (American Chemical Society, 2019-05-23)
    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. For the E isomer of the carboxylate deprotomer, a one-way photoinitiated proton transfer generates the phenoxide deprotomer through a mechanism postulated to involve an excited-state enol–keto tautomerism followed by a series of ground-state rearrangements including a second proton transfer. This mechanism is supported by experiments in which the relevant intermediate keto isomer is prepared and spectroscopically probed and through master equation modeling of possible ground-state isomerization processes. The Z isomer of the carboxylate deprotomer shows a weak Z → E photoisomerization response that occurs in competition with photodestruction (presumably electron detachment), demonstrating that the E and Z isomers undergo different processes in their excited states. The study highlights the utility of isomer-selective spectroscopy for characterizing the photochemistry of isolated anions possessing multiple deprotonation sites.
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    Photodetachment and photoreactions of substituted naphthalene anions in a tandem ion mobility spectrometer
    Bull, JN ; Buntine, JT ; Scholz, MS ; Carrascosa, E ; Giacomozzi, L ; Stockett, MH ; Bieske, EJ (ROYAL SOC CHEMISTRY, 2019-07-01)
    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 458.5 nm, and a set of weaker peaks near the electron detachment threshold corresponding to transitions to dipole-bound states. The two deprotomers of 6-hydroxy-2-naphthoic acid are separated and spectroscopically probed independently. The molecular anion formed from deprotonation of the hydroxy group gives rise to a photodetachment action spectrum similar to that of the 2-naphtholate anion with an onset at 470 nm and a maximum at 420 nm. Near the threshold, the photoreaction with SF6 is observed with displacement of an OH group by an F atom. In contrast, the anion formed from deprotonation of the carboxylic acid group gives rise to a photodissociation action spectrum, recorded on the CO2 loss channel, lying at much shorter wavelengths with an onset at 360 nm and maximum photoresponse at 325 nm.
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    From E to Z and back again: reversible photoisomerisation of an isolated charge-tagged azobenzene
    Bull, JN ; Scholz, MS ; Carrascosa, E ; Bieske, EJ (ROYAL SOC CHEMISTRY, 2018-01-07)
    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 S1(nπ*) photoisomerisation response in the blue part of the spectrum peaking at 440 nm and a more intense S2(ππ*) photoisomerisation response in the near-UV with maxima at 370 and 310 nm for the E and Z isomers, respectively. The 60 nm separation between the S2(ππ*) photo-response maxima for the two isomers allows them to be separately addressed in the gas phase and to be reversibly photoisomerised using different colours of light. This is an essential characteristic of an ideal photoswitch. The study demonstrates that a sequence of light pulses at different stages in an ion mobility spectrometer can be deployed to generate and probe isomers that cannot be electrosprayed directly from solution or produced through collisions in the ion source.
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    Isomerisation of an intramolecular hydrogen-bonded photoswitch: protonated azobis(2-imidazole)
    Bull, JN ; Scholz, MS ; Coughlan, NJA ; Bieske, EJ (ROYAL SOC CHEMISTRY, 2017-05-28)
    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 N2 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. Loss of N2 is also the predominant collision-induced dissociation channel. Electronic structure calculations suggest that E-isomer photoisomerisation involves S1(ππ*) excitation, whereas the Z-isomer photoisomerisation involves S2(ππ*) excitation. Conversion between the E and Z isomers through collisional excitation, which is calculated to occur through both inversion and torsion pathways, is investigated experimentally by colliding the molecular ions with nitrogen buffer gas over a range of electric fields. This study demonstrates the versatility of tandem ion mobility mass spectrometry for exploring the isomerisation of molecular photoswitches initiated by either light or collisions.
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    A Strong cis‐Effect in an Imidazole‐Imidazolium‐Substituted Alkene
    Georgiou, DC ; Haghighatbin, MA ; Hogan, CF ; Scholz, MS ; Bull, JN ; Bieske, EJ ; Wilson, DJD ; Dutton, JL (Wiley, 2017-07-10)
    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.
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    Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light
    Carrascosa, E ; Petermayer, C ; Scholz, MS ; Bull, JN ; Dube, H ; Bieske, EJ (WILEY-V C H VERLAG GMBH, 2020-04-02)
    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. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40-80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.
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    Photo and Collision Induced Isomerization of a Cyclic Retinal Derivative: An Ion Mobility Study
    Coughlan, NJA ; Scholz, MS ; Hansen, CS ; Trevitt, AJ ; Adamson, BD ; Bieske, EJ (SPRINGER, 2016-09)
    A cationic degradation product, formed in solution from retinal Schiff base (RSB), is examined in the gas phase using ion mobility spectrometry, photoisomerization action spectroscopy, and collision induced dissociation (CID). The degradation product is found to be N-n-butyl-2-(β-ionylidene)-4-methylpyridinium (BIP) produced through 6π electrocyclization of RSB followed by protonation and loss of dihydrogen. Ion mobility measurements show that BIP exists as trans and cis isomers that can be interconverted through buffer gas collisions and by exposure to light, with a maximum response at λ = 420 nm.Graphical Abstract.