Ultraviolet Excitation of M-O2 (M = Phenalenone, Fluorenone, Pyridine, & Acridine) Complexes Resulting in 1O2

In our experiment, a trace amount of an organic molecule (M = 1H-phenalen-1-one, 9-fluorenone, pyridine, or acridine) was seeded into a gas mix consisting of 3% O2 with a rare gas buffer (He or Ar) and then supersonically expanded. We excited the resulting molecular beam with ultraviolet light at either 355 nm (1H-phenalen-1-one, 9-fluorenone, or acridine) or 266 nm (pyridine) and used resonance enhanced multiphoton ionization (REMPI) spectroscopy to probe for the formation of O2 in the a-1Δg state, 1O2. For all systems, the REMPI spectra demonstrate that ultraviolet excitation results in the formation of 1O2 and the oxygen product is confirmed to be in the ground vibrational state and with an effective rotational temperature below 80 K. We then recorded the velocity map ion image of the 1O2 product. From the ion images, we determined the center-of-mass translational energy distribution, P(ET), assuming photodissociation of a bimolecular M-O2 complex. We also report results from electronic structure calculations that allow for a determination of the M-O2 ground state binding energy. We use the complex binding energy, the energy to form 1O2, and the adiabatic triplet energy for each organic molecule to determine the available energy following photodissociation. For dissociation of a bimolecular complex, this available energy may be partitioned into either center-of-mass recoil or internal degrees of freedom of the organic moiety. We use the available energy to generate a Prior distribution, which predicts statistical energy partitioning during dissociation. For low available energies, less than 0.2 eV, we find that the statistical prediction is in reasonable agreement with the experimental observations. However, at higher available energies, the experimental distribution is biased to lower center-of-mass kinetic energies compared with the statistical prediction, which suggests the complex undergoes vibrational predissociation.

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Practical Aspects in the Study of Biological Photosensitization Including Reaction Mechanisms and Product Analyses: A Do's and Don'ts Guide†

The interaction of light with natural matter leads to a plethora of photosensitized reactions. These reactions cause the degradation of biomolecules, such as DNA, lipids, proteins, being therefore detrimental to the living organisms, or they can also be beneficial by allowing the treatment of several diseases by photomedicine. Based on the molecular mechanistic understanding of the photosensitization reactions, we propose to classify them in four processes: oxygen-dependent (type I and type II processes) and oxygen-independent [triplet-triplet energy transfer (TTET) and photoadduct formation]. In here, these processes are discussed by considering a wide variety of approaches including time-resolved and steady-state techniques, together with solvent, quencher, and scavenger effects. The main aim of this survey is to provide a description of general techniques and approaches that can be used to investigate photosensitization reactions of biomolecules together with basic recommendations on good practices. Illustration of the suitability of these approaches is provided by the measurement of key biomarkers of singlet oxygen and one-electron oxidation reactions in both isolated and cellular DNA. Our work is an educational review that is mostly addressed to students and beginners.

Singlet O2 Produced by Ultraviolet Dissociation of the β-ionone-O2 Complex

We formed the gas-phase β-ionone-O₂ complex in a supersonic expansion and then photodissociated the complex with light near 312 nm. Photodissociation resulted in the production of O₂ in the a ¹Δ_g state, which was ionized at 312 nm using (2 + 1) resonance-enhanced multiphoton ionization (REMPI). We recorded the ¹O₂ REMPI action spectrum and O₂⁺ velocity map ion image following photodissociation of the complex. From the velocity map image, we determined the total recoil kinetic energy distribution from dissociation of the complex. Fitting the REMPI spectrum showed that the ¹O₂ product has an effective rotational temperature of about 50 K, while the recoil kinetic energy distribution was well fit with a statistical Boltzmann distribution having an effective translational temperature of 289 K. Using the average translational energy from the Boltzmann fit along with the complex dissociation energy from ab initio calculations, we determined that β-ionone was formed with an average of 2.87 eV of internal energy, which was 0.49 eV higher than previous measurements for the β-ionone triplet-state energy. Our own CCSD/cc-pVDZ//(U)MP2/cc-pVDZ calculations gave a minimum triplet-state energy of 2.04 eV. However, a large structural change occurs between the minimum singlet-ground-state geometry and the minimum triplet-excited-state geometry, and as a result, the calculated vertical energy for the triplet-state β-ionone was determined to be 3.30 eV. Comparing the ab initio and experimental results indicated that following excitation, β-ionone was formed in the triplet state but with significant internal vibrational energy. As such, complex dissociation likely proceeds following internal vibrational energy redistribution, which explains the statistical recoil kinetic energy distribution.

Investigation of O2-X (X = Pyrrole or Pyridine) Cluster Photodissociation Near 226 nm

We have recorded the O-atom velocity map photofragment ion images resulting from the photodissociation of O₂-pyrrole and O₂-pyridine clusters near 226 nm. To record the images, the O-atom photoproduct was state-selectively ionized and projected onto a two-dimensional (2D) position-sensitive detector. The resulting ion images from the clusters show evidence for three dissociation pathways. In the images from either cluster, we observe an isotropic process with kinetic energy near the one-photon limit, which is due to dissociation of the cluster into molecular subunits followed by secondary dissociation of molecular oxygen. Both O₂-pyrrole and O₂-pyridine also show a low kinetic energy process that appears to be isotropic in nature. This low kinetic energy process likely results from dissociation of the cluster resulting in significant internal energy in the organic fragment followed by secondary dissociation of O₂. Finally, our ion images for O₂-pyrrole show O atoms resulting from a two-photon dissociation channel, which has been previously attributed to the formation and subsequent photodissociation of excited O₂ (a ¹Δ_g). At similar laser intensities, O₂-pyridine does not show significant dissociation through the channel producing O₂ (a ¹Δ_g); however, this channel shows a laser intensity dependence for O₂-pyridine.

Singlet Oxygen Generation via UV-A, -B, and -C Photoexcitation of Isoprene-Oxygen (C5H8-O2) Encounter Complexes in the Gas Phase

The formation of singlet oxygen ¹O₂ provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C₅H₈-O₂) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 μm. The photogeneration of ¹O₂ is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of ¹O₂ is measured. This yield of ¹O₂ is governed mainly by photoexcitation of O₂ molecules to the Herzberg III (³Δ_u) state via enhanced absorption by C₅H₈-O₂ collision complexes. So excited triplet O₂ gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O₂ molecules. In the UV-B (308 nm) region the appearance of ¹O₂ is attributed to the excitation of a double spin-flip (DSF) transition in complex C₅H₈-O₂. In the UV-A region (355 nm) besides DSF the O₂-assisted T₁ ← S₀ excitation of isoprene to the triplet state takes place, which is a sensitizer of ¹O₂ formation. The contribution of the encounter complexes C₅H₈-O₂ to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.

Tungsten Isotope-Specific UV-Photodecomposition of W(CO)6 at 266 nm

UV photodissociation of tungsten hexacarbonyl W(CO)₆ has been studied in the molecular beam conditions using time-of-flight mass spectrometry and velocity map imaging. Irradiation of W(CO)₆ by pulsed laser radiation at 266 nm results in the appearance of singly and doubly charged tungsten ions. The isotope composition of these ions deviates essentially from natural abundance with deviation being pulse energy-dependent. The velocity map images of the tungsten ions indicate proceeding of several, more than two, parallel channels (sequences of the one-photon processes) of photodissociation, giving rise to tungsten atoms. Isotope effect is assigned to appear in a one-photon bound-bound transition in W(CO) intermediate followed by its predissociation. In the model suggested, the final state of this transition is a vibronic state with excited vibrational mode of W-C stretching vibration. This vibrational excitation is responsible for isotopic shift in the location of the final state. The suggested model fits the observed isotopic composition quantitatively.

Photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) via charge-transfer state: A velocity map imaging investigation

The photodissociation of van der Waals complexes of iodine X-I₂ (X = I₂, C₂H₄) excited via Charge-Transfer (CT) band has been studied with the velocity map imaging technique. Photodissociation of both complexes gives rise to translationally "hot" molecular iodine I₂ via channels differing by kinetic energy and angular distribution of the recoil directions. These measured characteristics together with the analysis of the model potential energy surface for these complexes allow us to infer the back-electron-transfer (BET) in the CT state to be a source of observed photodissociation channels and to make conclusions on the location of conical intersections where the BET process takes place. The BET process is concluded to provide an I₂ molecule in the electronic ground state with moderate vibrational excitation as well as X molecule in the electronic excited state. In the case of X = I₂, the BET process converts anion I₂^- of the CT state into the neutral I₂ in the repulsive excited electronic state which then dissociates promptly giving rise to a pair of I atoms in the fine states ²P_1/2. In the case of C₂H₄-I₂, the C₂H₄ molecules appear in the triplet T₁ electronic state. Conical intersection for corresponding BET process becomes energetically accessible after partial twisting of C₂H₄⁺ frame in the excited CT state of complex. The C₂H₄(T)-I₂ complex gives rise to triplet ethylene as well as singlet ethylene via the T-S conversion.

O2-·[Polar VOC] Complexes: H-Bonding versus Charge-Dipole Interactions, and the Noninnocence of Formaldehyde

Anion photoelectron imaging was used to measure the photodetachment spectra of molecular complexes formed between O₂^- and a range of atmospherically relevant polar molecules, including species with a carbonyl group (acetone, formaldehyde) and alcohols (ethanol, propenol, butenol). Experimental spectra are analyzed using a combination of Franck-Condon simulations and electronic structure calculations. Strong charge-dipole interactions and H-bonding stabilize the complex anions relative to the neutrals, resulting in a ca. 1 eV increase in electron binding energy relative to bare O₂^-, an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O₂-local π_g orbitals in the complexes results in the stabilization of the low-lying excited O₂ (a ¹Δ_g)·[polar VOC] state relative to the ground O₂ (X ³Σ_g^-)·[polar VOC] state when compared to bare O₂. The spectra of the O₂^-·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O₂^-·formaldehyde is unique in terms of both spectral profile and PAD. On the basis of these experimental results in addition to computational results, the complex anion cannot be described as a distinct O₂^- anion partnered with an innocent solvent molecule; the molecules are more strongly coupled through charge delocalization. Overall, the results underscore how the symmetry of the O₂ π_g orbitals is broken by different polar partners, which may have implications for atmospheric photochemistry and models of solar radiation absorption that include collision-induced absorption.

Photoelectron Imaging Spectra of O2-·VOC and O4-·VOC Complexes

The anion photoelectron imaging spectra of O₂^-·VOC and O₄^-·VOC (VOC = hexane, isoprene, benzene, and benzene-d₆) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O₂^-·VOC spectral features compared to the bare O₂^- indicate that photodetachment of the anion accesses a modestly repulsive region of the O₂-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O₂ (X ³Σ_g^-)·VOC ground state and O₂ (a ¹Δ_g)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O₂ (a ¹Δ_g)·VOC excited state relative to the O₂ (X ³Σ_g^-)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O₄^-·benzene and O₄^-·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.