Lanthanide and Actinide Ion Complexes Containing Organic Ligands Investigated by Surface-Enhanced Infrared Absorption Spectroscopy.

A new technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, was used for the structural investigation of lanthanide (Ln) and actinide (An) complexes containing organic ligands. We synthesized thiol derivatives of organic ligands with coordination sites similar to those of 2-[N-methyl-N-hexanethiol-amino]-2-oxoethoxy-[N',N'-diethyl]-acetamide [diglycolamide (DGA)], Cyanex-272, and N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN), which have been used for separating Ln and An through solvent extraction. These ligands were attached on a gold surface deposited on an Si prism through S-Au covalent bonds; the gold surface enhanced the IR absorption intensity of the ligands. Aqueous solutions of Ln (Eu3+, Gd3+, and Tb3+) and An (Am3+) ions were loaded onto the gold surface to form ion complexes. The IR spectra of the ion complexes were obtained using Fourier transform infrared spectroscopy in the attenuated total reflection mode. In this study, we developed a new sample preparation method for SEIRA spectroscopy that enabled us to obtain the IR spectra of the complexes with a small amount of ion solution (5 µL). This is a significant advantage for the IR measurement of radiotoxic Am3+ complexes. In the IR spectra of DGA, the band attributed to C═O stretching vibrations at ∼1630 cm-1 shifted to a lower wavenumber by ∼20 cm-1 upon complexation with Ln and An ions. Moreover, the amount of the red shift was inversely proportional to the extraction equilibrium constant reported in previous studies on solvent extraction. The coordination ability of DGA toward Ln and An ions could be assessed using the band position of the C═O band. The Cyanex-272- and TPEN-like ligands synthesized in this report also showed noticeable SEIRA signals for Ln and An complexes. This study indicates that SEIRA spectroscopy can be used for the structural investigation of ion complexes and provides a microscopic understanding of selective extraction of Ln and An.

Electronic Interaction between Aromatic Chromophores in Dibenzo-Crown Ether Complexes with Alkali Metal Ions Investigated via Cold Gas-Phase Spectroscopy.

This study investigated the geometric and electronic structures of dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) complexes with alkali metal ions, identified as M+(DB21C7) and M+(DB24C8) (M = Na, K, Rb, and Cs), respectively. We observed the ultraviolet photodissociation (UVPD) spectra of these complexes under cold (∼10 K) gas-phase conditions. The conformations of the M+(DB21C7) and M+(DB24C8) complexes were determined by comparing the UVPD spectra with the calculated electronic transitions of the local-minimum forms. The interactions between the electronic excited states of the two benzene chromophores in the M+(DB21C7) and M+(DB24C8) complexes were examined and compared with those of previously studied complexes (dibenzo-15-crown-5 (DB15C5) and dibenzo-18-crown-6 (DB18C6)). The S1-S0 and S2-S0 electronic excitations of the M+(DB21C7) complexes were almost localized in one of the benzene rings. In contrast, the closed conformers of the M+(DB24C8) (M = K, Rb, and Cs) complexes were delocalized over the two chromophores for electronic excitations, exhibiting strong electronic interactions between the benzene rings. For the M+(DB24C8) complexes (M = K, Rb, and Cs), the short distance between the benzene rings (∼3.9 Å) led to a strong interaction between the benzene chromophores. We conclude that this strong interaction in the M+(DB24C8) complexes correlates strongly with the broad absorption in the UVPD spectra, suggesting the presence of an intramolecular excimer for the K+(DB24C8), Rb+(DB24C8), and Cs+(DB24C8) complexes.

Structures and Anharmonic Analyses of the O-H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H2O)n, and (BA)2(H2O)n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures.

The IR spectra of benzoic acid (BA), (BA)(H2O)n and (BA)2(H2O)n (n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm-1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H2O) and (BA)(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA)m(H2O)n and their ring-deuterated isotopologues (BA-d5)m(H2O)n for specific values of m and n. The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H2O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.

Structures of (Pyrazine)2 and (Pyrazine)(Benzene) Dimers Investigated with Infrared-Vacuum Ultraviolet Spectroscopy and Quantum-Chemical Calculations: Competition among π-π, CH···π, and CH···N Interactions.

The structures of a pyrazine dimer (pyrazine)2 and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C-H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine)2 and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π-π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm-1, with intervals of 8 cm-1 in (pyrazine)2 and 11 cm-1 in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene-d6), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine)2 and (pyrazine)(benzene) in a supersonic jet. For (pyrazine)2, the two isomers which were previously assigned to the H-bonded planar and the π-π stacked structures respectively were reassigned to the cross-displaced π-π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine)2, especially the split at ∼3065 cm-1. However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.

Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens.

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.

Structures of Pyridine-Water Clusters Studied with Infrared-Vacuum Ultraviolet Spectroscopy.

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd)m(H2O)n, with m, n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)m(H2O)n+ by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)m(H2O)n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)2+ cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)2(H2O)n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)2(H2O)n clusters had the characteristic structure in which H2O or (H2O)2 forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)2, and an additional H2O molecule(s) extends the H-bonded network. The π-stacked (Pyd)2(H2O)n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd)m(H2O)n was found to be less pronounced in the case of (Pyd)m(NH3)n studied previously.

IR-VUV spectroscopy of pyridine dimers, trimers and pyridine-ammonia complexes in a supersonic jet.

The infrared spectra of the C-H stretching vibrations of (pyridine)m, m = 1-3, and the N-H stretching vibrations of (pyridine)m-(NH3)n, m = 1, 2; n = 1-4, complexes were investigated by infrared (IR)-vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP0) of the pyridine monomer was determined to be 74 546 cm-1 (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C-H stretching region at 3000-3200 cm-1. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C-H stretching fundamentals. For (pyridine)2 and (pyridine)3, most C-H bands are blue-shifted by 3-5 cm-1 from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)2, while (pyridine)3 prefers the structures stabilized by dipole-dipole and C-Hπ interactions. For the (pyridine)m-(NH3)n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N-H stretching vibrations of the complexes showed four main bands in the 3200-3450 cm-1 region. These features are very similar to those of (NH3)n complexes, and the bands are assigned to the anti-symmetric N-H stretching band (ν3), the symmetric N-H stretching (ν1) band, and the first overtone bands of the N-H bending vibrations (2ν4). The anharmonic calculations including the Fermi-resonance between ν1 and 2ν4 well reproduced the observed spectra.

Electronic States and Nonradiative Decay of Cold Gas-Phase Cinnamic Acid Derivatives Studied by Laser Spectroscopy with a Laser-Ablation Technique.

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S1(1ππ*)-S0 absorption in the 31 500-33 500 cm-1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1ππ* state was measured by picosecond pump-probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1ππ* and its time profile were measured by nanosecond UV-deep UV pump-probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T1 state, and we concluded that the NRD process of 1ππ* is S1(1ππ*) → 1nπ* → T1(3ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T1 state was not detected in CafA, and its NRD route is suggested to be S1(1ππ*) → 1πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the 1ππ* energy and suppresses the NRD process for all the species.

Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition.

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans-cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1-S0 transition; UV-UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (-OCH3) groups orient in opposite directions to each other. Picosecond UV-UV pump-probe spectroscopy revealed the NRD process from the excited singlet (S1 (1ππ*)) state occurs at a rate constant of ∼1010-1011 s-1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV-deep UV (DUV) pump-probe spectroscopy identified a transient triplet (T1 (3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm-1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1 (trans, 1ππ*) → 1nπ* → T1 (3ππ*) → S0 (cis)".

Conformation of K+(Crown Ether) Complexes Revealed by Ion Mobility-Mass Spectrometry and Ultraviolet Spectroscopy.

The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K+ ion were examined by ion mobility-mass spectrometry (IM-MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in which the relative positions of the two benzene rings were different from each other were investigated. The IM-MS results at 86 K revealed a clear separation of two sets of conformers for the K+(SymDB24C8) and K+(Asym1DB24C8) complexes whereas the K+(Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in which the benzene-benzene distances in the complexes were long (>6 Å) and short (<6 Å), respectively. IM-MS at 300 K could not separate the two conformer sets of the K+(SymDB24C8) complex because the interconversion between the open and closed conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K+ ion in the complex, although the IM-MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300 K. The UVPD spectra of the K+(SymDB24C8) and K+(Asym1DB24C8) complexes at ∼10 K displayed broad features that were accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K+(SymDB24C8) among the three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K+ encapsulation. The encapsulation methods for K+ ion (the "wraparound" arrangement) are similar in the three structural isomers of DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study demonstrated that temperature-controlled IM-MS coupled with the introduction of appropriate bulky groups, such as aromatic rings to host molecules, could reveal the dynamic aspects of encapsulation in host-guest systems.

Laser Spectroscopy and Lifetime Measurements of the S1 State of Tetracyanoquinodimethane (TCNQ) in a Cold Gas-Phase Free-Jet.

The S1 electronic state of 7,7,8,8-Tetracyanoquinodimethane (TCNQ) has been investigated by laser induced fluorescence (LIF), dispersed fluorescence (DF) spectroscopy, and lifetime measurements under jet-cooled conditions in the gas-phase. The LIF spectrum showed a weak origin band at 412.13 nm (24262 cm-1 ) with prominent progression and combination bands involving vibrations of 327, 1098, and 2430 cm-1 . In addition, very strong bands appeared at ∼363.6 nm (3300 cm-1 above the origin). Both the LIF and DF spectra indicate considerable geometric change in the S1 state. The fluorescence lifetime of S1 at zero-point level was obtained to be 220 ns. This lifetime is 40 times longer than the radiative lifetime estimated from the S1 -S0 oscillator strength. Furthermore, the lifetimes of the vibronic bands exhibited drastic energy dependence, indicating a strong mixing with the triplet (T1 ) or intramolecular charge-transfer (CT) state. This study is thought to disclose intrinsic nature of TCNQ, which has been well known as a component of organic semiconductors and a versatile p-type dopant.

Conformation of alkali metal ion-calix[4]arene complexes investigated by IR spectroscopy in the gas phase.

We measure the IR spectra of calix[4]arene (C4A) complexes with K+, Rb+, and Cs+ ions in the 3200-3700 cm-1 region by IR-UV double-resonance spectroscopy performed under cold (∼10 K) gas-phase conditions. All the complexes show two bands that can be assigned to the stretching vibrations of hydrogen-bonded OH groups in the C4A part. Quantum chemical calculations predict several isomers having different IR spectra, but the IR spectrum of the "cone" conformer reproduces the IR-UV spectrum very well, indicating that all the complexes adopt the cone conformation including the metal ions in the cone. The frequency of the OH stretching vibrations decreases with increasing the ion size from K+ (3357 and 3513 cm-1) to Rb+ (3323 and 3463 cm-1) and Cs+ (3279 and 3379 cm-1), but it is substantially higher than that of hydrogen-bonded OH groups in bare C4A (3158 cm-1). These results suggest that C4A encapsulates the metal ions by distorting the cone cavity, and that the distortion of the cone conformation is reduced more and the hydrogen bond between the OH groups becomes stronger with increasing the ion size from K+ to Cs+. The Cs+ complex has the smallest distortion of the C4A cavity among the alkali metal ion complexes. This can be one origin for the predominant encapsulation of Cs+ ions by C4A over smaller alkali metal ions in solution.

The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens.

The electronic states and photochemistry including nonradiative decay (NRD) and trans(E) → cis(Z) isomerization of methylcinnamate (MC) and its hydrogen-bonded complex with methanol have been investigated under jet-cooled conditions. S1(1nπ*) and S2(1ππ*) are directly observed in MC. This is the first direct observation of S1(1nπ*) in cinnamate derivatives. Surprisingly, the order of the energies between the nπ* and ππ* states is opposite to substituted cinnamates. TD-DFT and SAC-CI calculations support the observed result and show that the substitution to the benzene ring largely lowers the 1ππ* energy while the effect on 1nπ* is rather small. The S2(ππ*) state lifetime of MC is determined to be equal to or shorter than 10 ps, and the production of the transient T1 state is observed. The T1(ππ*) state is calculated to have a structure in which propenyl C[double bond, length as m-dash]C is twisted by 90°, suggesting the trans → cis isomerization proceeds via T1. The production of the cis isomer is confirmed by low-temperature matrix-isolated FTIR spectroscopy. The effect of H-bonding is examined for the MC-methanol complex. The S2 lifetime of MC-methanol is determined to be 180 ps, indicating that the H-bonding to the C[double bond, length as m-dash]O group largely prohibits the 1ππ* → 1nπ* internal conversion. This lifetime elongation in the methanol complex also describes well a higher fluorescence quantum yield of MC in methanol solution than in cyclohexane, while such a solvent dependence is not observed in para-substituted MC. Determination of the photochemical reaction pathways of MC and MC-methanol will help us to design photofunctional cinnamate derivatives.

UV and IR Spectroscopy of Transition Metal-Crown Ether Complexes in the Gas Phase: Mn2+(benzo-15-crown-5)(H2O)0-2.

Ultraviolet photodissociation (UVPD) and IR-UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n (n = 0-2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n = 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n = 1 and 2 complexes exhibit sharp vibronic bands. The IR-UV and DFT results suggest that there is only one isomer each for the n = 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ ion through Mn2+···OH2 bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π-π* transition of the B15C5 part is highly mixed with the "ligand to metal charge transfer" transition in the n = 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π-π* transition for the n = 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.

Geometric and Electronic Structures of Ag+(benzo-18-crown-6), Ag+(dibenzo-18-crown-6), and Ag+(dibenzo-15-crown-5) Complexes Investigated by Cold Gas-Phase Spectroscopy.

The UV photodissociation (UVPD) spectra of Ag+ complexes with benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), and dibenzo-15-crown-5 (DB15C5) [Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5)] are observed under cold gas-phase conditions. Ag+(B18C6) and Ag+(DB18C6) show sharp vibronic bands in the 36000-37200 cm-1 region, while the UVPD spectrum of Ag+(DB15C5) is very broad. These UV bands are assigned to the π-π* transition, which is localized on the B18C6, DB18C6, and DB15C5 part of the complexes. Quantum chemical calculations suggest that the broad UV feature of Ag+(DB15C5) can be attributed to the short lifetimes of optically excited ππ* states due to internal conversion (IC) to low-lying excited states that are present only for this complex. The appearance of the π-π* transition in the same UV region as that of the neutral crown ethers and their complexes with alkali metal ions indicates that the positive charge is localized on the Ag atom in these complexes. However, the fragment ions produced after UV absorption are B18C6+, DB18C6+, and DB15C5+ radical ions, indicating that they are produced via charge transfer (CT) between the Ag+ ion and benzo-crown ethers. The CT during fragmentation is attributed to the higher ionization energy of Ag atom when compared to the benzo-crown ethers. In the complexes, the Ag+ ion is effectively encapsulated by the crown cavity of the benzo-crown ethers without transferring the positive charge from Ag+ to the crown. However, UV excitation of the Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5) complexes can reduce the Ag+ ion and produce a Ag atom with high efficiency in the gas phase.

Selective Probing of Potassium Ion in Solution by Intramolecular Excimer Fluorescence of Dibenzo-Crown Ethers.

Observation of an excimer fluorescence in solution is proposed for detecting the encapsulation of potassium ion as opposed to other alkali ions by dibenzo-crown ethers. The scheme has been validated by ultraviolet photodissociation (UVPD) spectroscopy of dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with potassium ion, K+ ⋅ DB21C7 and K+ ⋅ DB24C8, performed under cold (∼10 K) conditions in the gas phase and by quantum chemical calculations of the geometry and electronic structures of the complexes. Calculations suggest the formation of a closely spaced excimer structure of benzene rings only for the K+ ⋅ DB24C8. Interaction of the rings may lead to lifetime broadening in UV absorption, which is experimentally observed in the gas phase, indeed, only for this cold complex. Consistently, intramolecular excimer fluorescence of DB24C8 in solution is observed only for K+ ⋅ DB24C8. The excimer fluorescence is not observed with other alkali metal ions. The detection of such intramolecular excimer fluorescence can, therefore, potentially serve as a simple, background-free, selective probe of potassium ion in solution.

Pseudorotaxanes in the gas phase: structure and energetics of protonated dibenzylamine-crown ether complexes.

We observe UV spectra of protonated dibenzylamine (dBAMH+) and its complexes with 15-crown-5 (dBAMH+-15C5), 18-crown-6 (dBAMH+-18C6), and 24-crown-8 (dBAMH+-24C8) under cold (∼10 K) gas-phase conditions by UV photodissociation (UVPD) and UV-UV hole-burning (HB) spectroscopy. The UVPD spectrum of the dBAMH+-15C5 complex shows an extensive low-frequency progression, which originates from a unique conformation of the dBAMH+ part with benzene rings facing closely to each other, while UVPD and calculation results suggest open conformations of the dBAMH+ part for dBAMH+-18C6 and dBAMH+-24C8. UV-UV HB spectra of the dBAMH+-24C8 complex indicate that there exist at least two conformers; multiple conformations can contribute to high stability of dBAMH+-24C8 pseudorotaxane due to "conformational" entropic effects. The UVPD experiment indicates that the dissociation probability of dBAMH+-24C8 into dBAMH+ and 24C8 is substantially smaller than that of dBAMH+-15C5 and dBAMH+-18C6, which can be related to the barrier height in the dissociation process. The energetics of the dBAMH+-24C8 complex is investigated experimentally with NMR spectroscopy and theoretically with the global reaction route mapping (GRRM) method. An energy barrier of ∼60 kJ mol-1 is present in the pseudorotaxane formation in solution, whereas there is no barrier in the gas phase. In the course of the photodissociation, excited dBAMH+-24C8 complexes can be trapped at many local minima corresponding to multiple conformations. This can result in effective dissipation of internal energy into degrees of freedom not correlated to the dissociation and decrease the dissociation probability for the dBAMH+-24C8 complex in the gas phase. The energy barrier for the pseudorotaxane formation in solution originates not simply from the slippage process but rather from solvent effects on the dBAMH+-24C8 complex.

Different photoisomerization routes found in the structural isomers of hydroxy methylcinnamate.

An experimental and theoretical study has been carried out to elucidate the nonradiative decay (NRD) and trans(E) → cis(Z) isomerization from the S1 (1ππ*) state of structural isomers of hydroxy methylcinnamate (HMC); ortho-, meta- and para-HMC (o-, m- and p-HMC). A low temperature matrix-isolation Fourier Transform Infrared (FTIR) spectroscopic study revealed that all the HMCs are cis-isomerized upon UV irradiation. A variety of laser spectroscopic methods have been utilized for jet-cooled gas phase molecules to investigate the vibronic structure and lifetimes of the S1 state, and to detect the transient state appearing in the NRD process. In p-HMC, the zero-point level of the S1 state decays as quickly as 9 ps. A transient electronic state reported by Tan et al. (Faraday Discuss. 2013, 163, 321-340) was reinvestigated by nanosecond UV-tunable deep UV pump-probe spectroscopy and was assigned to the T1 state. For m- and o-HMC, the lifetime at the zero-point energy level of S1 is 10 ns and 6 ns, respectively, but it becomes substantially shorter at an excess energy higher than 1000 cm-1 and 600 cm-1, respectively, indicating the onset of NRD. Different from p-HMC, no transient state (T1) was observed in m- nor o-HMC. These experimental results are interpreted with the aid of TDDFT calculations by considering the excited-state reaction pathways and the radiative/nonradiative rate constants. It is concluded that in p-HMC, the trans → cis isomerization proceeds via a [trans-S1 → 1nπ* → T1 → cis-S0] scheme. On the other hand, in o- and m-HMC, the isomerization proceeds via a [trans-S1 → twisting along the C[double bond, length as m-dash]C double bond by 90° on S1 → cis-S0] scheme. The calculated barrier height along the twisting coordinate agrees well with the observed onset of the NRD channel for both o- and m-HMC.

Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase.

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K+, Rb+, and Cs+ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K+·DB18C6·(H2O) n, Rb+·DB18C6·(H2O) n, and Cs+·DB18C6·(H2O) n ( n = 1-8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb+·DB18C6·(H2O)1-5 and the Cs+·DB18C6·(H2O)3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K+·DB18C6, Rb+·DB18C6, and Cs+·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K+ to Cs+. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K+·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K+·DB18C6 conformer, while hydration occurs only on top of the Rb+·DB18C6 and Cs+·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb+·DB18C6 and Cs+·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K+·DB18C6 complex can accept H2O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K+·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K+ ion by DB18C6 in solution.

UV and IR Spectroscopy of Cryogenically Cooled, Lanthanide-Containing Ions in the Gas Phase.

We measure UV and IR spectra in the gas phase for EuOH+, EuCl+, and TbO+ ions, which are produced by an electrospray ionization source and cooled to ∼10 K in a cold, 22-pole ion trap. The UV photodissociation (UVPD) spectra of these ions show a number of sharp, well-resolved bands in the 30000-38000 cm-1 region, although a definite assignment of the spectra is difficult because of a high degree of congestion. We also measure an IR spectrum of the EuOH+ ion in the 3500-3800 cm-1 region by IR-UV double-resonance spectroscopy, which reveals an OH stretching band at 3732 cm-1. We perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these ions in order to examine the nature of the transitions. The DFT results indicate that the states of highest-spin multiplicity (octet for EuOH+ and EuCl+ and septet for TbO+) are substantially more stable than other states of lower-spin multiplicity. The TD-DFT calculations suggest that UV absorption of the EuOH+ and EuCl+ ions arises from Eu(4f) → Eu(5d,6p) transitions, whereas electronic transitions of the TbO+ ion are mainly due to the electron promotion of O(2p) → Tb(4f,6s). The UVPD results of the lanthanide-containing ions in this study suggest the possibility of using lanthanide ions as "conformation reporters" for gas-phase spectroscopy for large molecules.