Effect of Proton Substitution by Alkali Ions on the Fluorescence Emission of Rhodamine B Cations in the Gas Phase

The photophysics of chromophores is strongly influenced by their environment. Solvation, charge state, and adduct formation significantly affect ground and excited state energetics and dynamics. The present study reports on fluorescence emission of rhodamine B cations (RhBH⁺) and derivatives in the gas phase. Substitution of the acidic proton of RhBH⁺ by alkali metal cations, M⁺, ranging from lithium to cesium leads to significant and systematic blue shifts of the emission. The gas-phase structures and singlet transition energies of RhBH⁺ and RhBM⁺, M = Li, Na, K, Rb, and Cs, were investigated using Hartree–Fock theory, density functional methods, second-order Møller–Plesset perturbation theory, and the second-order approximate coupled-cluster model CC2. Comparison of experimental and theoretical results highlights the need for improved quantum chemical methods, while the hypsochromic shift observed upon substitution appears best explained by the Stark effect due to the inhomogeneous electric field generated by the alkali ions

From Planar to Cage in 15 Easy Steps: Resolving the C₆₀H₂₁F₉^– → C₆₀^– Transformation by Ion Mobility Mass Spectrometry

A combination of mass spectrometry, collision-induced dissociation, ion mobility mass spectrometry (IM-MS), and density functional theory (DFT) has been used to study the evolution of anionic species generated by laser-desorption of the near-planar, fluorinated polycyclic aromatic hydrocarbon (PAH), C₆₀H₂₁F₉ (s). The dominant decay process for isolated, thermally activated C₆₀H₂₁F₉^– species comprises a sequence of multiple regioselective cyclodehydrofluorination and cyclodehydrogenation reactions (eliminating HF and H₂, respectively, while forming additional pentagons and/or hexagons). The DFT calculations allow us to set narrow bounds on the structures of the resulting fragment ions by fitting structural models to experimentally determined collision cross sections. These show that the transformation of the precursor anion proceeds via a series of intermediate structures characterized by increasing curvature, ultimately leading to the closed-shell fullerene cage C₆₀^– as preprogrammed by the precursor structure

Gas-Phase Photoluminescence Characterization of Stoichiometrically Pure Nonanuclear Lanthanoid Hydroxo Complexes Comprising Europium or Gadolinium

Gas-phase photoluminescence measurements involving mass-spectrometric techniques enable determination of the properties of selected molecular systems with knowledge of their exact composition and unaffected by matrix effects such as solvent interactions or crystal packing. The resulting reduced complexity facilitates a comparison with theory. Herein, we provide a detailed report of the intrinsic luminescence properties of nonanuclear europium­(III) and gadolinium­(III) 9-hydroxy­phenalen-1-one (HPLN) hydroxo complexes. Luminescence spectra of [Eu₉(PLN)₁₆(OH)₁₀]⁺ ions reveal an europium-centered emission dominated by a 4-fold split Eu^III hypersensitive transition, while photoluminescence lifetime measurements for both complexes support an efficient europium sensitization via a PLN-centered triplet-state manifold. The combination of gas-phase measurements with density functional theory computations and ligand-field theory is used to discuss the antiprismatic core structure of the complexes and to shed light on the energy-transfer mechanism. This methodology is also employed to fit a new set of parameters, which improves the accuracy of ligand-field computations of Eu^III electronic transitions for gas-phase species

Vibronic Coupling Analysis of the Ligand-Centered Phosphorescence of Gas-Phase Gd(III) and Lu(III) 9‑Oxophenalen-1-one Complexes

The gas-phase laser-induced photoluminescence of cationic mononuclear gadolinium and lutetium complexes involving two 9-oxophenalen-1-one ligands is reported. Performing measurements at a temperature of 83 K enables us to resolve vibronic transitions. Via comparison to Franck–Condon computations, the main vibrational contributions to the ligand-centered phosphorescence are determined to involve rocking, wagging, and stretching of the 9-oxophenalen-1-one–lanthanoid coordination in the low-energy range, intraligand bending, and stretching in the medium- to high-energy range, rocking of the carbonyl and methine groups, and C–H stretching beyond. Whereas Franck–Condon calculations based on density-functional harmonic frequency computations reproduce the main features of the vibrationally resolved emission spectra, the absolute transition energies as determined by density functional theory are off by several thousand wavenumbers. This discrepancy is found to remain at higher computational levels. The relative energy of the Gd­(III) and Lu­(III) emission bands is only reproduced at the coupled-cluster singles and doubles level and beyond

Characterization of Nonanuclear Europium and Gadolinium Complexes by Gas-Phase Luminescence Spectroscopy

Gas-phase measurements using mass-spectrometric techniques allow determination of the luminescence properties of selected molecular systems with knowledge of their exact composition. Furthermore, isolated luminophores are unaffected by matrix effects like solvent interactions or crystal packing. As a result, the system complexity is reduced relative to the condensed phase and a direct comparison with theory is facilitated. Herein, we report the intrinsic luminescence properties of nonanuclear europium­(III) and gadolinium­(III) 9-hydroxyphenalen-1-one (HPLN)–hydroxo complexes. Luminescence spectra of [Eu₉(PLN)₁₆(OH)₁₀]⁺ ions reveal an europium-centered emission dominated by a 4-fold split Eu­(III) hypersensitive transition. The corresponding Gd­(III) complex, [Gd₉(PLN)₁₆(OH)₁₀]⁺, shows a broad emission from a ligand based triplet state with an onset of about 1000 wavenumbers in excess of the europium emission. As supported by photoluminescence lifetime measurements for both complexes, we deduce an efficient europium sensitization via PLN-based triplet states. The luminescence spectra of the complexes are discussed in terms of a square antiprismatic europium/gadolinium core structure as suggested by density functional computations

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)­(AuDppy)₆Ag₂·(BF₄)_<i>x</i>]^{(4–<i>x</i>)+} with <i>x</i> = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal–organic frameworks. The monocationic species (<i>x</i> = 3) is found to be less stable than the dicationic one (<i>x</i> = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation

Photoluminescence Spectroscopy of Mass-Selected Electrosprayed Ions Embedded in Cryogenic Rare-Gas Matrixes

An apparatus is presented which combines nanoelectrospray ionization for isolation of large molecular ions from solution, mass-to-charge ratio selection in gas-phase, low-energy-ion-beam deposition into a (co-condensed) inert gas matrix and UV laser-induced visible-region photoluminescence (PL) of the matrix isolated ions. Performance is tested by depositing three different types of lanthanoid diketonate cations including also a dissociation product species not directly accessible by chemical synthesis. For these strongly photoluminescent ions, accumulation of some femto- to picomoles in a neon matrix (over a time scale of tens of minutes to several hours) is sufficient to obtain well-resolved dispersed emission spectra. We have ruled out contributions to these spectra due to charge neutralization or fragmentation during deposition by also acquiring photoluminescence spectra of the same ionic species in the gas phase

Ion Mobility Spectrometry, Infrared Dissociation Spectroscopy, and ab Initio Computations toward Structural Characterization of the Deprotonated Leucine-Enkephalin Peptide Anion in the Gas Phase

Although the sequencing of protonated proteins and peptides with tandem mass spectrometry has blossomed into a powerful means of characterizing the proteome, much less effort has been directed at their deprotonated analogues, which can offer complementary sequence information. We present a unified approach to characterize the structure and intermolecular interactions present in the gas-phase pentapeptide leucine-enkephalin anion by several vibrational spectroscopy schemes as well as by ion-mobility spectrometry, all of which are analyzed with the help of quantum-chemical computations. The picture emerging from this study is that deprotonation takes place at the C terminus. In this configuration, the excess charge is stabilized by strong intramolecular hydrogen bonds to two backbone amide groups and thus provides a detailed picture of a potentially common charge accommodation motif in peptide anions