School of Chemistry - Research Publications
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ItemA strong cis-effect in an imidazole/imidazolium substituted alkene(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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ItemIsomerisation of an intramolecular hydrogen-bonded photoswitch: protonated azobis(2-imidazole)(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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ItemA Strong cis‐Effect in an Imidazole‐Imidazolium‐Substituted Alkene(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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ItemHydrogen-adduction to open-shell graphene fragments: spectroscopy, thermochemistry and astrochemistry(Royal Society of Chemistry, 2017-02-01)We apply a combination of state-of-the-art experimental and quantum-chemical methods to elucidate the electronic and chemical energetics of hydrogen adduction to a model open-shell graphene fragment. The lowest-energy adduct, 1H-phenalene, is determined to have a bond dissociation energy of 258.1 kJ mol−1, while other isomers exhibit reduced or in some cases negative bond dissociation energies, the metastable species being bound by the emergence of a conical intersection along the high-symmetry dissociation coordinate. The gas-phase excitation spectrum of 1H-phenalene and its radical cation are recorded using laser spectroscopy coupled to mass-spectrometry. Several electronically excited states of both species are observed, allowing the determination of the excited-state bond dissociation energy. The ionization energy of 1H-phenalene is determined to be 7.449(17) eV, consistent with high-level W1X-2 calculations.
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ItemElectronic spectrum and photodissociation chemistry of the linear methyl propargyl cation H2C4H3+(AMER INST PHYSICS, 2017-01-28)The electronic spectrum of the methyl propargyl cation (2-butyn-1-yl cation, H2C4H3+) is measured over the 230-270 nm range by photodissociating the bare cation and its Ar and N2 tagged complexes in a tandem mass spectrometer. The observed A'1←A'1 band system has an origin at 37 753 cm-1 for H2C4H3+, 37738 cm-1 for H2C4H3+-Ar, and 37 658 cm-1 for H2C4H3+-N2. The methyl propargyl cation photodissociates to produce either C2H3++C2H2 (protonated acetylene + acetylene) or H2C4H++H2 (protonated diacetylene + dihydrogen). Photodissociation spectra of H2C4H3+, H2C4H3+-Ar, and H2C4H3+-N2 exhibit similar vibronic structure, with a strong progression of spacing 630 cm-1 corresponding to excitation of the C-C stretch mode. Interpretation of the spectra is aided by ground and excited state calculations using time dependent density functional theory at the ωB97X-D/aug-cc-pVDZ level of theory. Ab initio calculations and master equation simulations were used to interpret the dissociation of H2C4H3+ on the ground state manifold. These calculations support the experimentally observed product branching ratios in which acetylene elimination dominates and also suggests that channel switching occurs at higher energies to favor H2 elimination.
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ItemPhotophysics and aggregation effects of a triphenylamine-based dye sensitizer on metal-oxide nanoparticles suspended in an ion trap(ROYAL SOC CHEMISTRY, 2013)The photophysical behaviour of a triphenylamine-based organic dye sensitizer (Carbz-PAHTDTT) attached to alumina and titania nanoparticles (labelled Carbz-Al and Carbz-Ti, respectively) is examined in the absence and presence of the chenodeoxycholic acid (CDCA) coadsorber. The experiments are conducted in vacuo by suspending the target dye-sensitized nanoparticles within a quadrupole ion trap, where they are probed with laser radiation to obtain emission spectra and time-resolved excited state decay curves. For Carbz-Al, the dye's emission band is blue-shifted and the excited state lifetime is increased upon the coabsorption of CDCA, effects attributed to reduced dye aggregation. Compared to Carbz-Al, the Carbz-Ti excited state lifetimes are significantly shorter due to excited dye molecules injecting electrons into the titania conduction band. For Carbz-Ti, the electron injection quantum yields for the surfaces with CDCA (CDCA : dye = 25 : 1) and without CDCA are estimated to be 0.87 and 0.71, respectively. The gas-phase results demonstrate that Carbz-PAHTDTT dye aggregates are detrimental to the performance of a dye-sensitized solar cell.
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ItemBlue to near-IR energy transfer cascade within a dye-doped polymer matrix, mediated by a photochromic molecular switch(ROYAL SOC CHEMISTRY, 2016-02-21)The spectroscopic properties of a poly(methyl methacrylate) matrix doped with a coumarin dye, a cyanine dye, and a photochromic spiropyran dye have been investigated. Before UV irradiation of the matrix, excitation of the coumarin dye results in minimal energy transfer to the cyanine dye. The energy transfer is substantially enhanced following UV irradiation of the matrix, which converts the colourless spiropyran isomer to the coloured merocyanine isomer, which then acts as an intermediate bridge by accepting energy from the coumarin dye and then donating energy to the cyanine dye. This demonstration of a switchable energy transfer cascade should help initiate new research directions in molecular photonics.
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ItemPhoto and Collision Induced Isomerization of a Cyclic Retinal Derivative: An Ion Mobility Study(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.
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ItemPhotoisomerization of β-Ionone Protonated Schiff Base in the Gas Phase(AMER CHEMICAL SOC, 2016-08-25)The photoisomerization of β-ionone protonated Schiff base (BIPSB) is investigated in the gas phase by irradiating mobility-selected ions in a tandem ion mobility spectrometer with tunable radiation. Four distinguishable isomers are produced by electrospray ionization whose structures are deduced from their collision cross sections and photoisomerization behavior along with density functional theory calculations. They include two geometric isomers of BIPSB with trans or cis configurations about the polyene chain's terminal C═N double bond, a bicyclic structure formed through electrocyclization of the polyene chain, and a Z-retro-γ-ionone isomer. Although trans-BIPSB and 9-cis-BIPSB have similar photoisomerization action spectra, with a maximum response at 375 nm, they photoconvert to different isomers. The trans-BIPSB isomer transforms to the bicyclic form upon exposure to light over the 320-400 nm range, whereas the cis-BIPSB isomer is prevented by steric hindrance from forming the bicyclic BIPSB isomer following irradiation and is proposed instead to form the 7,9-di-cis isomer. Neither the bicyclic isomer nor the Z-retro-γ-ionone isomer respond strongly to near-UV light.
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ItemPhotoisomerization action spectroscopy: flicking the protonated merocyanine-spiropyran switch in the gas phase(ROYAL SOC CHEMISTRY, 2015)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.