Negative electron binding energies observed in a triply charged anion: Photoelectron spectroscopy of 1-hydroxy-3,6,8-pyrene-trisulfonate

Effect of total charge on the electronic structure of thiolate-protected X@Ag12 superatoms (X = Ag, Au)

Electronic structures of chemically synthesized silver-based clusters [XAg₁₆(TBBT)₁₂]^3- (X = Ag or Au; TBBT = 4-tert-butylbenzenethiolate) having an icosahedral X@Ag₁₂ superatomic core were studied by gas-phase photoelectron spectroscopy and density functional theory calculations. The electron binding energy of the highest occupied molecular orbital (HOMO) with a 1P superatomic nature was determined to be 0.23 and 0.29 eV for X = Ag or Au, respectively. Resonant tunnelling electron emission through the repulsive Coulomb barrier (RCB) was observed. From the kinetic energy of the tunnelling electrons, it was estimated that the lowest unoccupied molecular orbital (LUMO) was supported at 1.51 and 1.62 eV above the vacuum level by the RCB for X = Ag or Au, respectively. The HOMO of [XAg₁₆(TBBT)₁₂]^3- (X = Ag or Au) was destabilized by 3.74 and 3.71 eV, respectively, compared with those of [XAg₂₄(DMBT)₁₈]^- (DMBT = 2,4-dimethylbenzenethiolate) having the icosahedral X@Ag₁₂ core due to the larger negative charge imparted by the ligand layers.

Highly photostable "super"-photoacids for ultrasensitive fluorescence spectroscopy

The photoacid 8-hydroxypyren-1,3,6-trisulfonic acid (HPTS, pyranine) is a widely used model compound for the examination of excited state proton transfer (ESPT). We synthesized five "super"-photoacids with varying hydrophilicity and acidity on the basis of HPTS. By chemical modification of the three sulfonic acid substituents, the photoacidity is enhanced by up to more than five logarithmic units from pK*≈ 1.4 to ∼-3.9 for the most acidic compound. As a result, nearly quantitative ESPT in DMSO can be observed. The novel photoacids were characterized by steady-state and time-resolved fluorescence techniques showing distinctively red shifted spectra compared to HPTS while maintaining a high quantum yield near 90%. Photostability of the compounds was checked by fluorescence correlation spectroscopy (FCS) and was found to be adequately high for ultrasensitive fluorescence spectroscopy. The described photoacids present a valuable palette for a wide range of applications, especially when the properties of HPTS, i.e. highly charged, low photostability and only moderate excited state acidity, are limiting.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Guiding electron emissions by excess negative charges in multiply charged molecular anions

Using photoelectron imaging, we show the effects of excess negative charges on the directions of outgoing electrons in multiply charged anions. Photoemissions are observed to occur either in a perpendicular or parallel direction, depending on the molecular configurations and origins of the detached electrons. Detachment of the π electrons from biphenyl-disulfonate dianions is shown to occur in a perpendicular direction due to the Coulomb repulsion from the two terminal charges, whereas detachment from the sulfonate groups in linear aliphatic disulfonates occurs in parallel directions.

Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap

The ability to control ion temperatures is critical for gas phase spectroscopy and has been a challenge in chemical physics. A low-temperature photoelectron spectroscopy instrument has been developed for the investigation of complex anions in the gas phase, including multiply charged anions, solvated species, and biological molecules. The new apparatus consists of an electrospray ionization source, a three dimensional (3D) Paul trap for ion accumulation and cooling, a time-of-flight mass spectrometer, and a magnetic-bottle photoelectron analyzer. A key feature of the new instrument is the capability to cool and tune ion temperatures from 10 to 350 K in the 3D Paul trap, which is attached to the cold head of a closed cycle helium refrigerator. Ion cooling is accomplished in the Paul trap via collisions with a background gas and has been demonstrated by observation of complete elimination of vibrational hot bands in photoelectron spectra of various anions ranging from small molecules to complex species. Further evidence of ion cooling is shown by the observation of H2-physisorbed anions at low temperatures. Cold anions result in better resolved photoelectron spectra due to the elimination of vibrational hot bands and yield more accurate energetic and spectroscopic information. Temperature-dependent studies are made possible for weakly bonded molecular and solvated clusters, allowing thermodynamic information to be obtained.