School of Chemistry - Research Publications
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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.
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ItemRetinal shows its true colours: photoisomerization action spectra of mobility-selected isomers of the retinal protonated Schiff base(ROYAL SOC CHEMISTRY, 2015)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.
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ItemElectronic spectrum of the propargyl cation (H2C3H+) tagged with Ne and N2(AMER INST PHYSICS, 2015-11-14)The Ã(1)A1 ← X̃(1)A1 band system of the propargyl cation (H2C3H(+)) is measured over the 230-270 nm range by photodissociation of mass-selected H2C3H(+)-Ne and H2C3H(+)-N2 complexes in a tandem mass spectrometer. The band origin occurs at 37 618 cm(-1) for H2C3H(+)-Ne and 37 703 cm(-1) for H2C3H(+)-N2. Ground and excited state ab initio calculations for H2C3H(+) using the MCSCF and coupled-cluster (CC) response methods show that the ion has C2v symmetry in the ground X̃(1)A1 and excited Ã(1)A1 states and that the strong vibronic progression with a spacing of 630 cm(-1) is due to the C-C stretch vibrational mode, ν 5.
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ItemUltraviolet photodissociation action spectroscopy of the N-pyridinium cation(AMER INST PHYSICS, 2015-01-07)The S1←S0 electronic transition of the N-pyridinium ion (C5H5NH(+)) is investigated using ultraviolet photodissociation (PD) spectroscopy of the bare ion and also the N2-tagged complex. Gas-phase N-pyridinium ions photodissociate by the loss of molecular hydrogen (H2) in the photon energy range 37,000-45,000 cm(-1) with structurally diagnostic ion-molecule reactions identifying the 2-pyridinylium ion as the exclusive co-product. The photodissociation action spectra reveal vibronic details that, with the aid of electronic structure calculations, support the proposal that dissociation occurs through an intramolecular rearrangement on the ground electronic state following internal conversion. Quantum chemical calculations are used to analyze the measured spectra. Most of the vibronic features are attributed to progressions of totally symmetric ring deformation modes and out-of-plane modes active in the isomerization of the planar excited state towards the non-planar excited state global minimum.
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ItemGas-phase electronic spectrum of the indole radical cation(TAYLOR & FRANCIS LTD, 2015-08-18)
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ItemDoes the triphenylamine-based D35 dye sensitizer form aggregates on metal-oxide surfaces?(ELSEVIER SCIENCE SA, 2015-04-01)