Dynamics of Anions: From Bound to Unbound States and Everything In Between.

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

Photoelectron spectroscopy of the deprotonated tryptophan anion: the contribution of deprotomers to its photodetachment channels.

Photoelectron spectroscopy and electronic structure calculations are used to investigate the electronic structure of the deprotonated anionic form of the aromatic amino acid tryptophan, and its chromophore, indole. The photoelectron spectra of tryptophan, recorded at different wavelengths across the UV, consist of two direct detachment channels and thermionic emission, whereas the hν = 4.66 eV spectrum of indole consists of two direct detachment features. Electronic structure calculations indicate that two deprotomers of tryptophan are present in the ion beam; deprotonation of the carboxylic acid group (Trp(I)-) or the N atom on the indole ring (Trp(II)-). Strong similarities are observed between the direct detachment channels in the photoelectron spectra of tryptophan and indole, which in conjunction with electronic structure calculations, indicate that electron loss from Trp(II)- dominates this portion of the spectra. However, there is some evidence that direct detachment of Trp(I)- is also observed. Thermionic emission is determined to predominantly arise from the decarboxylation of Trp(I)-, mediated by the ππ* excited state near λ = 300 nm, which results in an anionic fragment with a negative electron affinity that readily autodetaches.

The valence electron affinity of uracil determined by anion cluster photoelectron spectroscopy.

The unoccupied π* orbitals of the nucleobases are considered to play important roles in low-energy electron attachment to DNA, inducing damage. While the lowest anionic valence state is vertically unbound in all neutral nucleobases, it remains unclear even for the simplest nucleobase, uracil (U), whether its valence anion (U-) is adiabatically bound, which has important implications on the efficacy of damage processes. Using anion photoelectron spectroscopy, we demonstrate that the valence electron affinity (EAV) of U can be accurately measured within weakly solvating clusters, U-(Ar)n and U-(N2)n. Through extrapolation to the isolated U limit, we show that EAV = -2 ± 18 meV. We discuss these findings in the context of electron attachment to U and its reorganization energy, and more generally establish guidance for the determination of molecular electron affinities from the photoelectron spectroscopy of anion clusters.

Effect of N Atom Substitution on Electronic Resonances: A 2D Photoelectron Spectroscopic and Computational Study of Anthracene, Acridine, and Phenazine Anions.

The accommodation of an excess electron by polycyclic aromatic hydrocarbons (PAHs) has important chemical and technological implications ranging from molecular electronics to charge balance in interstellar molecular clouds. Here, we use two-dimensional photoelectron spectroscopy and equation-of-motion coupled-cluster calculations of the radical anions of acridine (C13H9N-) and phenazine (C12H8N2-) and compare our results for these species to those for the anthracene anion (C14H10-). The calculations predict the observed resonances and additionally find low-energy two-particle-one-hole states, which are not immediately apparent in the spectra, and offer a slightly revised interpretation of the resonances in anthracene. The study of acridine and phenazine allows us to understand how N atom substitution affects electron accommodation. While the electron affinity associated with the ground state anion undergoes a sizable increase with the successive substitution of N atoms, the two lowest energy excited anion states are not affected significantly by the substitution. The net result is that there is an increase in the energy gap between the two lowest energy resonances and the bound ground electronic state of the radical anion from anthracene to acridine to phenazine. Based on an energy gap law for the rate of internal conversion, this increased gap makes ground state formation progressively less likely, as evidenced by the photoelectron spectra.

Low-Energy Shape Resonances of a Nucleobase in Water.

When high-energy radiation passes through aqueous material, low-energy electrons are produced which cause DNA damage. Electronic states of anionic nucleobases have been suggested as an entrance channel to capture the electron. However, identifying these electronic resonances have been restricted to gas-phase electron-nucleobase studies and offer limited insight into the resonances available within the aqueous environment of DNA. Here, resonance and detachment energies of the micro-hydrated uracil pyrimidine nucleobase anion are determined by two-dimensional photoelectron spectroscopy and are shown to extrapolate linearly with cluster size. This extrapolation allows the corresponding resonance and detachment energies to be determined for uracil in aqueous solution as well as the reorganization energy associated with electron capture. Two shape resonances are clearly identified that can capture low-energy electrons and subsequently form the radical anion by solvent stabilization and internal conversion to the ground electronic state. The resonances and their dynamics probed here are the nucleobase-centered doorway states for low-energy electron capture and damage in DNA.

A Hückel Model for the Excited-State Dynamics of a Protein Chromophore Developed Using Photoelectron Imaging.

Chemistry can be described as the movement of nuclei within molecules and the concomitant instantaneous change in electronic structure. This idea underpins the central chemical concepts of potential energy surfaces and reaction coordinates. To experimentally capture such chemical change therefore requires methods that can probe both the nuclear and electronic structure simultaneously and on the time scale of atomic motion. In this Account, we show how time-resolved photoelectron imaging can do exactly this and how it can be used to build a detailed and intuitive understanding of the electronic structure and excited-state dynamics of chromophores. The chromophore of the photoactive yellow protein (PYP) is used as a case study. This chromophore contains a para-substituted phenolate anion, where the substituent, R, can be viewed as an acrolein derivative. It is shown that the measured photoelectron angular distribution can be directly related to the electronic structure of the para-substituted phenolate anion. By incrementally considering differing R groups, it is also shown that these photoelectron angular distributions are exquisitely sensitive to the conformational flexibility of R and that when R contains a π-system the excited states of the chromophore can be viewed as a linear combination of the π* molecular orbitals on the phenolate (πPh*) and the R substituent (πR*). Such Hückel treatment shows that the S1 state of the PYP chromophore has predominantly πR* character and that it is essentially the same as the chromophore of the green fluorescent protein (GFP). The S1 excited-state dynamics of the PYP chromophore probed by time-resolved photoelectron imaging clearly reveals both structural (nuclear) dynamics through the energy spectrum and electronic dynamics through the photoelectron angular distributions. Both motions can be accurately assigned using quantum chemical calculations, and these are consistent with the intuitive Hückel treatment presented. The photoactive protein chromophores considered here are examples of where a chemists' intuitive Hückel view for ground-state chemistry appears to be transferable to the prediction of photochemical excited-state reactivity. While elegant and insightful, such models have limitations, including nonadiabatic dynamics, which is present in a related PYP chromophore, where a fraction of the S1 state population forms a nonvalence (dipole-bound) state of the anion.

Photodissociation of permanganate (MnO4-) produces the manganese dioxide anion (MnO2-) in an excited triplet state.

Photoelectron imaging, electron action spectroscopy and electronic structure calculations are used to probe the structure and dynamics of MnO4-. Following excitation to the first bright absorption band of MnO4- (11T2), photodetachment, via ground state electron loss, and photodissociation, to produce MnO2-, are both observed to occur simultaneously. MnO2- is produced in an excited electronic state, identified as a triplet state, which indicates that the dissociation proceeds on singlet potential energy surfaces via spin conservation. Furthermore, electronic structure calculations indicate that both photodetachment and photodissociation are multiple photon processes that are mediated by the same 11T2 excited state. Taken together this data indicates that photodissociation of MnO4- occurs via a statistical dissociation on the MnO4- ground state at visible wavelengths.

Alkylated green fluorescent protein chromophores: dynamics in the gas phase and in aqueous solution.

Fluorescent labelling of macromolecular samples, including using the green fluorescent protein (GFP), has revolutionised the field of bioimaging. The ongoing development of fluorescent proteins require a detailed understanding of the photophysics of the biochromophore, and how chemical derivatisation influences the excited state dynamics. Here, we investigate the photophysical properties associated with the S1 state of three alkylated derivatives of the chromophore in GFP, in the gas phase using time-resolved photoelectron imaging, and in water using femtosecond fluorescence upconversion. The gas-phase lifetimes (1.6-10 ps), which are associated with the intrinsic (environment independent) dynamics, are substantially longer than the lifetimes in water (0.06-3 ps), attributed to stabilisation of both twisted intermediate structures and conical intersection seams in the condensed phase. In the gas phase, alkylation on the 3 and 5 positions of the phenyl ring slows the dynamics due to inertial effects, while a 'pre-twist' of the methine bridge through alkylation on the 2 and 6 positions significantly shortens the excited state lifetimes. Formation of a minor, long-lived (≫ 40 ps) excited state population in the gas phase is attributed to intersystem crossing to a triplet state, accessed because of a T1/S1 degeneracy in the so-called P-trap potential energy minimum associated with torsion of the single-bond in the bridging unit connecting to the phenoxide ring. A small amount of intersystem crossing is supported through TD-DFT molecular dynamics trajectories and MS-CASPT2 calculations. No such intersystem crossing occurs in water at T = 300 K or in ethanol at T ≈ 77 K, due to a significantly altered potential energy surface and P-trap geometry.

Unraveling the decarboxylation dynamics of the fluorescein dianion with fragment action spectroscopy.

The decarboxylation dynamics of the doubly deprotonated fluorescein dianion, Fl2-, are investigated by recording fragment action spectra for the anion, Fl-, and its decarboxylated analog, Fl-CO2-, using a new reflectron secondary mass spectrometer. The formation of the anion, Fl-, is directly investigated by photoelectron imaging. The Fl- and Fl-CO2- action spectra indicate that, for λ < 400 nm, one-photon dissociative photodetachment, i.e., simultaneous decarboxylation and electron loss, competes with photodetachment, whereas for λ > 400 nm, decarboxylation only proceeds following electron loss via a sequential two-photon process. The primary decarboxylation pathway is the ready loss of CO2 from the relatively short-lived intermediate excited state, Fl-[D1], which is formed by electron loss from the dianion via resonant tunneling through the repulsive Coloumb barrier associated with a high-lying excited dianion state, Fl2-[S2].

Photooxidation of the Phenolate Anion is Accelerated at the Water/Air Interface.

Molecular photodynamics can be dramatically affected at the water/air interface. Probing such dynamics is challenging, with product formation often probed indirectly through its interaction with interfacial water molecules using time-resolved and phase-sensitive vibrational sum-frequency generation (SFG). Here, the photoproduct formation of the phenolate anion at the water/air interface is probed directly using time-resolved electronic SFG and compared to transient absorption spectra in bulk water. The mechanisms are broadly similar, but 2 to 4 times faster at the surface. An additional decay is observed at the surface which can be assigned to either diffusion of hydrated electrons from the surface into the bulk or due to increased geminate recombination at the surface. These overall results are in stark contrast to phenol, where dynamics were observed to be 104 times faster and for which the hydrated electron was also a photoproduct. Our attempt to probe phenol showed no electron signal at the interface.

Photo-isomerization of the isolated photoactive yellow protein chromophore: what comes before the primary step?

Photoactive proteins typically rely on structural changes in a small chromophore to initiate a biological response. While these changes often involve isomerization as the "primary step", preceding this is an ultrafast relaxation of the molecular framework caused by the sudden change in electronic structure upon photoexcitation. Here, we capture this motion for an isolated model chromophore of the photoactive yellow protein using time-resolved photoelectron imaging. It occurs in <150 fs and is apparent from a spectral shift of ∼70 meV and a change in photoelectron anisotropy. Electronic structure calculations enable the quantitative assignment of the geometric and electronic structure changes to a planar intermediate from which the primary step can then proceed.

Photoelectron Imaging Study of the Diplatinum Iodide Dianions [Pt2I6]2- and [Pt2I8]2.

Photoelectron spectroscopy has been used to study the electronic structure, photodetachment, and photodissociation of the stable diplatinum iodide dianions [Pt2I6]2- and [Pt2I8]2-. Photoelectron spectra over a range of photon energies show the characteristic absence of low kinetic energy photoelectrons expected for dianions as a result of the repulsive Coulomb barrier (RCB). Vertical detachment energies of ∼1.6 and ∼1.9 eV and minimum RCBs of ∼1.2 and ∼1.3 eV are reported for [Pt2I6]2- and [Pt2I8]2-, respectively. Both of the diplatinum halides exhibit three direct detachment channels with distinct anisotropies, analogous to the previously reported spectra for PtI2- and PtI-, suggesting a platinum-centered molecular core that dominates the photodetachment. Additionally, evidence for two-photon photodissociation and subsequent photodetachment channels producing I- are observed for both dianions. Finally, an unexplained feature is observed at photon energies around 3 eV, whose origin is considered. Our work highlights the complex electronic structure of the heavy platinum-halide dianions that are characterized by a dense manifold of electronic states.

Photoelectron imaging of PtI2 - and its PtI- photodissociation product.

The photoelectron imaging of PtI2 - is presented over photon energies ranging from hν = 3.2 to 4.5 eV. The electron affinity of PtI2 is found to be 3.4 ± 0.1 eV, and the photoelectron spectrum contains three distinct peaks corresponding to three low-lying neutral states. Using a simple d-block model and the measured photoelectron angular distributions, the three states are tentatively assigned. Photodissociation of PtI2 - is also observed, leading to the formation of I- and of PtI-. The latter allows us to determine the electron affinity of PtI to be 2.35 ± 0.10 eV. The spectrum of PtI- is similarly structured with three peaks which, again, can be tentatively assigned using a similar model that agrees with the photoelectron angular distributions.

Resonances in nitrobenzene probed by the electron attachment to neutral and by the photodetachment from anion.

We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries. In both cases, the anions decay by electron emission and we record the electron spectra. Several types of emission are recognized, differing by the way in which the resulting molecule is vibrationally excited. In the excitation of specific vibrational modes, distinctly different modes are visible in electron collision and photodetachment experiments. The unspecific vibrational excitation, which leads to the emission of thermal electrons following the internal vibrational redistribution, shows similar features in both experiments. A model for the thermal emission based on a detailed balance principle agrees with the experimental findings very well. Finally, a similar behavior in the two experiments is also observed for a third type of electron emission, the vibrational autodetachment, which yields electrons with constant final energies over a broad range of excitation energies. The entrance channels for the vibrational autodetachment are examined in detail, and they point to a new mechanism involving a reverse valence to non-valence internal conversion.

Photoelectron spectroscopy of the protoporphyrin IX dianion.

Two-dimensional photoelectron spectroscopy using nanosecond and femtosecond lasers has been used to study the protopophyrin IX dianion at photon energies between 1.8-4.1 eV. The photoelectron spectra indicated the presence of two direct detachment channels, tunnelling through the repulsive Coulomb barrier (RCB) and thermionic emission from monoanions. A direct detachment feature suggested a near 0 eV electron affinity, which may be attributable to the repulsive through space interaction of the unshielded carboxylate groups. The minimum height of the repulsive Coulomb barrier (RCB) was found to be between 1.4-1.9 eV. Adiabatic tunnelling through the RCB was seen to occur on a timescale faster than rotational dephasing of the molecule. The observation of thermionic emission below the RCB in the nanosecond spectra originated from monoanions, which were produced via photon-cycling of the dianion.

Autodetachment dynamics of 2-naphthoxide and implications for astrophysical anion abundance.

Astrochemical modelling has proposed that 10% or more of interstellar carbon could be tied up as polycyclic aromatic hydrocarbon (PAH) molecules. Developing reliable models of the interstellar carbon lifecycle requires calibration data obtained through laboratory studies on relevant chemical and physical processes, including on the photo-induced and electron-induced dynamics of potential interstellar PAHs. Here, the excited state dynamics of the S1(ππ*) state of 2-naphthoxide are investigated using frequency-, angle-, and time-resolved photoelectron imaging. Frequency-resolved photoelectron spectra taken over the S1(ππ*) band reveal low electron kinetic energy structure consistent with an indirect, vibrational mode-specific electron detachment mechanism. Time-resolved photoelectron imaging using a pump photon energy tuned to the 0-0 transition of the S1(ππ*) band (hν = 2.70 eV) and a non-resonant probe photon provides the excited state autodetachment lifetime at τ = 130 ± 10 fs. There is no evidence for internal conversion to the ground electronic state or a dipole-bound state. These results imply that 2-naphthoxide has no resilience to photodestruction through the absorption of visible radiation resonant with the S1(ππ*) band, and that electron capture by the S1(ππ*) state, which is formally a shape resonance, is not a doorway state to a stable interstellar anion.

Modeling the Photoelectron Angular Distributions of Molecular Anions: Roles of the Basis Set, Orbital Choice, and Geometry.

A study investigating the effect of the basis set, orbital choice, and geometry on the modeling of photoelectron angular distributions (PADs) of molecular anions is presented. Experimental and modeled PADs for a number of molecular anions, including both closed- and open-shell systems, are considered. Guidelines are suggested for chemists who wish to design calculations to capture the correct chemical physics of the anisotropy of photodetachment, while balancing the computational cost associated with larger molecular anions.

Temporary Anion Resonances of Pyrene: A 2D Photoelectron Imaging and Computational Study.

The low-energy electron-scattering resonances of pyrene were characterized using experimental and computational methods. Experimentally, a two-dimensional photoelectron imaging of the pyrene anion was used to probe the dynamics of resonances over the first 4 eV of the continuum. Computationally, the energies and character of the anion states were determined using equation-of-motion coupled cluster calculations, while taking specific care to avoid the collapse onto discretized continuum levels, and an application of the pairing theorem. Our results are in good agreement with the predictions of electron-scattering calculations that include an offset and with the pyrene anion absorption spectrum in a glass matrix. Taken together, we offer an assignment of the first five electronic resonances of pyrene. Some of the population in the lowest-energy 2B1u resonance was observed to decay to the ground electronic state of the anion, while all other resonances decay by a direct autodetachment. The astronomical relevance of a ground-state electron capture proceeding via a low-energy resonance in pyrene is discussed.

Ultrafast Dynamics of the Isolated Adenosine-5'-triphosphate Dianion Probed by Time-Resolved Photoelectron Imaging.

The excited state dynamics of the doubly deprotonated dianion of adenosine-5'-triphosphate, [ATP-H2]2-, has been spectroscopically explored by time-resolved photoelectron spectroscopy following excitation at 4.66 eV. Time-resolved photoelectron spectra show that two competing processes occur for the initially populated 1ππ* state. The first is rapid electron emission by tunneling through a repulsive Coulomb barrier as the 1ππ* state is a resonance. The second is nuclear motion on the 1ππ* state surface leading to an intermediate that no longer tunnels and subsequently decays by internal conversion to the ground electronic state. The spectral signatures of the features are similar to those observed for other adenine-derivatives, suggesting that this nucleobase is quite insensitive to the nearby negative charges localized on the phosphates, except of course for the appearance of the additional electron tunneling channel, which is open in the dianion.

Time-resolved electronic sum-frequency generation spectroscopy with fluorescence suppression using optical Kerr gating.

Excited state dynamics of molecules at interfaces can be studied using second-order non-linear spectroscopic methods such as time-resolved electronic sum-frequency generation (SFG). However, as such measurements inherently generate very small signals, they are often overwhelmed by signals originating from fluorescence. Here, this limitation is overcome by optical Kerr gating of the SFG signal to discriminate against fluorescence. The new approach is demonstrated on the excited state dynamics of malachite green at the water/air interface, in the presence of a highly fluorescent coumarin dye, and on the photo-oxidation of the phenolate anion at the water/air interface. The generality of the use of optical Kerr gating to SFG measurements is discussed.