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.

Beyond Duality: Rationalizing Repulsive Coulomb Barriers in Host-Guest Cyclodextrin-Dodecaborate Complexes

The repulsive Coulomb barrier (RCB), an intrinsic potential energy barrier along electron detachment or charge-separation coordinates in multiply charged anions (MCAs), provides dynamic stability to MCAs whose electronic and thermodynamic stabilities are largely dictated by strong internal Coulomb repulsions. Spectroscopic and theoretical characterizations of the RCB have been focused on isolated MCAs. In this work, we extend the RCB investigation beyond the previous scope by including noncovalent host-guest cyclodextrin-closo-dodecaborate dianionic complexes χCD·B₁₂X₁₂^2- (χ = α, β, γ; X = H, F-I). Photodechment photoelectron spectroscopy reveals the existence of two distinctly different RCBs, derived from detaching electrons from the guest dianions (RCB₁) or ionizing the host neutrals (RCB₂), respectively, with the latter being substantially smaller than the former. Theoretical calculations support the duality of RCBs in these complexes and further exhibit highly anisotropic nature of the RCBs.

Kasha's Rule and Koopmans' Correlations for Electron Tunnelling through Repulsive Coulomb Barriers in a Polyanion

The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl2-, partially decays through electron tunnelling from the S2 excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans' correlations, thus disentangling different channels.

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.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B₂(CN)₆]^2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)₃]^•- is energetically favorable. Quantum chemical calculations demonstrate a D_3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Size and charge-state dependence of detachment energies of polyanionic silver clusters

The electronic properties of silver clusters (N up to 800) charged by attachment of up to z = 7 excess electrons are investigated. As an essential preparation step, the technique of in-trap electron attachment to size-selected monoanions within a linear Paul trap is applied. By taking advantage of tunable laser pulses, the photoelectron spectra allow us to evaluate details of the electronic structure of polyanionic metal clusters, giving a multidimensional dataset. The subsequent analysis based on the liquid drop model provides information about the atomic structure and the bulk work function at a hitherto unknown quality.

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.

Measuring Electronic Structure of Multiply Charged Anions to Understand Their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [B_nX_n]^2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N₂. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [B_nX_n]^2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are 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.

Site-Specific Photo-oxidation of the Isolated Adenosine-5'-triphosphate Dianion Determined by Photoelectron Imaging

Photoelectron imaging of the isolated adenosine-5'-triphosphate dianion excited to the ¹ππ* states reveals that electron emission is predominantly parallel to the polarization axis of the light and arises from subpicosecond electron tunneling through the repulsive Coulomb barrier (RCB). The computed RCB shows that the most probable electron emission site is on the amino group of adenine. This is consistent with the photoelectron imaging: excitation to the ¹ππ* states leads to an aligned ensemble distributed predominantly parallel to the long axis of adenine; the subsequent electron tunneling site is along this axis; and the negatively charged phosphate groups guide the outgoing electron mostly along this axis at long range. Imaging of electron tunneling from polyanions combined with computational chemistry may offer a general route for probing the intrinsic photo-oxidation site and dynamics as well as the overall structure of complex isolated species.

Electron Binding in a Superatom with a Repulsive Coulomb Barrier: The Case of [Ag44(SC6H3F2)30]4- in the Gas Phase

The electron binding mechanism in [Ag₄₄(SC₆H₃F₂)₃₀]^4- (SC₆H₃F₂ = 3,4-difluorobenzenethiolate) tetra-anion was studied by photoelectron spectroscopy (PES), collision-induced dissociation mass spectrometry (CID-MS), and density functional theory (DFT) computations. PES showed that [Ag₄₄(SC₆H₃F₂)₃₀]^4- is energetically metastable with respect to electron autodetachment {[Ag₄₄(SC₆H₃F₂)₃₀]^3- + e^-} and features a repulsive Coulomb barrier (RCB) with a height of 2.7 eV. However, CID-MS revealed that [Ag₄₄(SC₆H₃F₂)₃₀]^4- does not release an electron upon collisional excitation but undergoes dissociation. DFT computations performed on the known structure of [Ag₄₄(SC₆H₃F₂)₃₀]^4- confirmed the negative adiabatic electron affinity of [Ag₄₄(SC₆H₃F₂)₃₀]^3- and interpreted the experimental PE spectrum by taking into account tunneling electron photodetachment through the RCB.

Ultrafast Intersystem Crossing in Isolated Ag29(BDT)123- Probed by Time-Resolved Pump-Probe Photoelectron Spectroscopy

The photophysics of the isolated trianion Ag₂₉(BDT)₁₂^3- (BDT = benzenedithiolate), a ligand-protected cluster comprising BDT-based ligands, terminating a shell of silver thiolates and a core of silver atoms, was studied in the gas phase by femtosecond time-resolved, pump-probe photoelectron spectroscopy. UV excitation at 490 nm populates one or more singlet excited states with significant charge transfer (CT) character in which electron density is shifted from shell to core. These CT states relax on an average time scale of several hundred femtoseconds by charge recombination to yield either the vibrationally excited singlet ground state (internal conversion) or a long-lived triplet (intersystem crossing). Our study is the first ultrafast spectroscopic probe of a ligand-protected coinage metal cluster in isolation. In the future, it will be interesting to study how cluster size, overall charge state, or heteroatom doping can be used to tune the corresponding relaxation dynamics in the absence of solvent.

Properties of perhalogenated {closo-B10} and {closo-B11} multiply charged anions and a critical comparison with {closo-B12} in the gas and the condensed phase

Dependence of electronic properties and reactivity of closo-borates with size and halogen substituent was investigated.

Ion mobility action spectroscopy of flavin dianions reveals deprotomer-dependent photochemistry

Photo-induced proton transfer, deprotomer-dependent photochemistry, and intramolecular charge transfer in flavin anions are investigated using action spectroscopy.

The intrinsic optical properties and photochemistry of flavin adenine dinucleotide (FAD) dianions are investigated using a combination of tandem ion mobility spectrometry and action spectroscopy. Two principal isomers are observed, the more stable form being deprotonated on the isoalloxazine group and a phosphate (N-3,PO₄ deprotomer), and the other on the two phosphates (PO₄,PO₄ deprotomer). Ion mobility data and electronic action spectra suggest that photo-induced proton transfer occurs from the isoalloxazine group to a phosphate group, converting the PO₄,PO₄ deprotomer to the N-3,PO₄ deprotomer. Comparisons of the isomer selective action spectra of FAD dianions and flavin monoanions with solution spectra and gas-phase photodissociation action spectra suggests that solvation shifts the electronic absorption of the deprotonated isoalloxazine group to higher energy. This is interpreted as evidence for significant charge transfer in the lowest optical transition of deprotonated isoalloxazine. Overall, this work demonstrates that the site of deprotonation of flavin anions strongly affects their electronic absorptions and photochemistry.

Electronic structure of the para-dinitrobenzene radical anion: a combined 2D photoelectron imaging and computational study

2D photoelectron spectroscopy combined with high-level ab initio calculations provides insights into the dissociative electron attachment of para-dinitrobenzene.

Influence of long-range Coulomb interaction in velocity map imaging

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical

UV excitation of mass-selected hexachloroplatinate–nucleobase clusters provides detailed insight into the photophysics and photochemistry of a model DNA photopharmaceutical.

Time-Resolved Photodetachment Anisotropy: Gas-Phase Rotational and Vibrational Dynamics of the Fluorescein Anion

The photoelectron signal of the singly deprotonated fluorescein anion is found to be highly dependent on the relative polarization between pump and probe pulses, and time-resolved photodetachment anisotropy (TR-PA) is developed as a probe of the rotational dynamics of the chromophore. The total photoelectron signal shows both rotational and vibrational wavepacket dynamics, and we demonstrate how TR-PA can readily disentangle these dynamical processes. TR-PA in fluorescein presents specific opportunities for its development as a probe for rotational dynamics in large biomolecules as fluorescein derivatives are commonly incorporated in complex biomolecules and have been used extensively in time-resolved fluorescence anisotropy measurements, to which TR-PA is a gas-phase analogue.

Incorporating real time velocity map image reconstruction into closed-loop coherent control

We report techniques developed to utilize three-dimensional momentum information as feedback in adaptive femtosecond control of molecular dynamics. Velocity map imaging is used to obtain the three-dimensional momentum map of the dissociating ions following interaction with a shaped intense ultrafast laser pulse. In order to recover robust feedback information, however, the two-dimensional momentum projection from the detector must be inverted to reconstruct the full three-dimensional momentum of the photofragments. These methods are typically slow or require manual inputs and are therefore accomplished offline after the images have been obtained. Using an algorithm based upon an "onion-peeling" (also known as "back projection") method, we are able to invert 1040 × 1054 pixel images in under 1 s. This rapid inversion allows the full photofragment momentum to be used as feedback in a closed-loop adaptive control scheme, in which a genetic algorithm tailors an ultrafast laser pulse to optimize a specific outcome. Examples of three-dimensional velocity map image based control applied to strong-field dissociation of CO and O2 are presented.

Mapping the UV Photophysics of Platinum Metal Complexes Bound to Nucleobases: Laser Spectroscopy of Isolated Uracil·Pt(CN)4(2-) and Uracil·Pt(CN)6(2-) Complexes

We report the first UV laser spectroscopic study of isolated gas-phase complexes of platinum metal complex anions bound to a nucleobase as model systems for exploring at the molecular level the key photophysical processes involved in photodynamic therapy. Spectra of the Pt(IV)(CN)6(2-)·Ur and Pt(II)(CN)4(2-)·Ur complexes were acquired across the 220-320 nm range using mass-selective photodepletion and photofragment action spectroscopy. The spectra of both complexes reveal prominent UV absorption bands (λmax = 4.90 and 4.70 eV) that we assign primarily to excitation of the Ur π-π* localized chromophore. Distinctive UV photofragmentation products are observed for the complexes, with Pt(IV)(CN)6(2-)·Ur photoexcitation resulting in complex fission, while Pt(II)(CN)4(2-)·Ur photoexcitation initiates a nucleobase proton-transfer reaction across 4.4-5.2 eV and electron detachment above 5.2 eV. The observed photofragments are consistent with ultrafast decay of a Ur localized excited state back to the electronic ground state followed by intramolecular vibrational relaxation and ergodic complex fragmentation.