Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation

We compare the intersystem crossing rate, k_ISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]^2-), the singly deprotonated monoanion ([RB-H]^-), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]^- and [RB-2H + Cs]^-, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S₁) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopy─all with ca. 50 femtosecond temporal resolution. [RB-H]^- shows an S₁ lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]^2-, has a much longer lifetime. Superimposed on S₁ decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4-5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S₁ state, initially populated in the S₀ geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussed─also in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states.

Quantitation of Enantiomeric Excess in an Achiral Environment Using Trapped Ion Mobility Mass Spectrometry

We present a novel, straightforward method to determine the enantiomeric excess (ee) of tryptophan (Trp) and N-tert-butyloxycarbonyl-O-benzylserine (BBS) solutions without chiral additives. For this, lithium carbonate, sodium carbonate, or silver acetate was added to solutions of Trp or BBS. Singly negatively charged dimer and trimer clusters were then formed by electrospray ionization and analyzed using trapped ion mobility spectrometry (TIMS) and time-of-flight mass spectrometry. When a solution contains both enantiomers, homo- and heterochiral clusters are generated which can be separated in the TIMS-tunnel based on their different mobilities using a nitrogen buffer gas. The ratio of homochiral to heterochiral clusters shows a binomial distribution and can be calibrated with solutions of known ee to yield ee measurements of samples with better than 1% accuracy. Samples can be prepared rapidly, and measurements are completed in less than 5 min. Current instrumental limitations restrict this method to rigid molecules with large functional groups adjacent to the chiral centers. Nevertheless, we expect this method to be applicable to many pharmaceuticals and provide the example of 1-methyltryptophan to demonstrate this.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C₅H₈O₂) and methyl butyrate (MB, C₅H₁₀O₂) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO₂ is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E₀ = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO₂ loss as 66.6 ± 4.3 kJ mol^-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E₀ = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at Δ_fH^o_0K[CH₂C(OH)OCH₃⁺] = 502 ± 6 kJ mol^-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH₃ and CO₂ loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

High-resolution photoelectron imaging of MnB3 -: Probing the bonding between the aromatic B3 cluster and 3d transition metals

The B₃ triangular unit is a fundamental bonding motif in all boron compounds and nanostructures. The isolated B₃ ^- cluster has a D_3h structure with double σ and π aromaticity. Here, we report an investigation of the bonding between a B₃ cluster and a 3d transition metal using high-resolution photoelectron imaging and computational chemistry. Photoelectron spectra of MnB₃ ^- are obtained at six different photon energies, revealing rich vibrational information for the ground state detachment transition. The electron affinity of MnB₃ is determined to be 1.6756(8) eV, and the most Franck-Condon-active mode observed has a measured frequency of 415(6) cm^-1 due to the Mn-B₃ stretch. Theoretical calculations show that MnB₃ ^- has a C_2v planar structure, with Mn coordinated to one side of the triangular B₃ unit. The ground states of MnB₃ ^- (⁶B₂) and MnB₃ (⁵B₂) are found to have high spin multiplicity with a significant decrease in the Mn-B bond distances in the neutral due to the detachment of an Mn-B₃ anti-bonding electron. The Mn atom is shown to have weak interactions with the B₃ unit, which maintains its double aromaticity with relatively small structural changes from the bare B₃ cluster. The bonding in MnB₃ is compared with that in 5d MB₃ clusters, where the strong metal-B₃ interactions strongly change the structures and bonding in the B₃ moiety.

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.

Observation of Four-Fold Boron-Metal Bonds in RhB(BO-) and RhB

The maximum bond order between two main-group atoms was known to be three. However, it has been suggested recently that there is quadruple bonding in C₂ and analogous eight-valence electron species. While the quadruple bond in C₂ has aroused some debates, an interesting question is: are main-group elements capable of forming quadruple bonds? Here we use photoelectron spectroscopy and computational chemistry to probe the electronic structure and chemical bonding in RhB₂O^- and RhB^- and show that the boron atom engages in quadruple bonding with rhodium in RhB(BO)^- and neutral RhB. The quadruple bonds consist of two π-bonds formed between the Rh 4d_xz/4d_yz and B 2p_x/2p_y orbitals and two σ-bonds between the Rh 4d_z² and B 2s/2p_z orbitals. To confirm the quadruple bond in RhB, we also investigate the linear Rh≡B-H⁺ species and find a triple bond between Rh and B, which has a longer bond length, lower stretching frequency, and smaller bond dissociation energy in comparison with that of the Rh≣B quadruple bond in RhB.

ReB6-: A Metallaboron Analog of Metallabenzenes

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB₆^- and AlB₆^-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB₆^- cluster is found to be perfectly planar with a B-centered hexagonal structure (C_2v, ¹A₁), while AlB₆^- is known to have a similar structure, but with a slightly out-of-plane distortion (C_s, ¹A'). Chemical bonding analyses show that the closed-shell ReB₆^- is doubly σ- and π-aromatic, while AlB₆^- is known to be σ-aromatic and π-antiaromatic. The out-of-plane distortion in AlB₆^- is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The π-bonding in ReB₆^- is compared with that in both benzene and rhenabenzene [(CO)₄ReC₅H₅], and remarkable similarities are found. Hence, ReB₆^- can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.

High-Resolution Photoelectron Imaging of IrB3 - : Observation of a π-Aromatic B3 + Ring Coordinated to a Transition Metal

In a high-resolution photoelectron imaging and theoretical study of the IrB₃ ^- cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge-bonded Ir atom (C_s , ² A'), and the second isomer features a tetrahedral structure (C_3v , ² A₁ ). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir-B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir-(η³ -B₃ ⁺ ) complex. This study provides the first example of a boron-to-metal charge-transfer complex and evidence of a π-aromatic B₃ ⁺ ring coordinated to a transition metal.

Probing the coupling of a dipole-bound electron with a molecular core

A dipolar molecule can weakly bind an electron in a diffuse orbital. However, the spin-orbit coupling between this weakly bound electron and the electrons in the molecular core is not known. Here we probe this coupling using the linear C₂P^- anion with the ³Σ⁺ ground state, which possesses dipole-bound excited states because neutral C₂P (²Π) has a sufficiently large dipole moment. Photodetachment spectroscopy and resonant photoelectron spectroscopy are used to probe the nature of the dipole-bound states. Two dipole-bound excited states are observed with a binding energy of 37 cm^-1, corresponding to the two spin-orbit states of neutral C₂P (²Π_1/2 and ²Π_3/2). The current study demonstrates that the weakly bound electron in the dipole-bound excited states of C₂P^- is not spin-coupled to the electrons in the C₂P core and can be considered as a quasi-free electron.

Study of low temperature chlorine atom initiated oxidation of methyl and ethyl butyrate using synchrotron photoionization TOF-mass spectrometry

The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer.

Nb2©Au6: a molecular wheel with a short Nb 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Nb triple bond coordinated by an Au6 ring and reinforced by σ aromaticity

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb₂Au₆^- cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb₂Au₆^-/0 possess high-symmetry D_6h structures featuring a Nb-Nb axis coordinated equatorially by an Au₆ ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb₂ moiety in Nb₂©Au₆, as well as five totally delocalized σ bonds. The Nb 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Nb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb-Nb bond length comparable to the quintuple bond in gaseous Nb₂. The totally delocalized σ bonding in Nb₂©Au₆ is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb₂