Anisotropic circular dichroism of jet-cooled chiral molecules.

Anisotropic circular dichroism (CD) refers to the CD of oriented molecules, which varies with the direction of light propagation toward the molecules. Thus, anisotropic CD spectroscopy has been used to investigate the orientations of molecules in anisotropic media such as liquid crystals and thin films. However, it is unclear whether anisotropic CD results from isolated chromophores or their intermolecular interactions with other atoms or molecules that form anisotropically aligned structures. Herein, anisotropic CD of isolated chiral molecules was observed for the first time. The resonant two-photon ionization CD spectra of jet-cooled pseudoephedrine and styrene oxide indicated a difference between the CD values of the P/R and Q branches of the origin bands of the S0-S1 transition. This difference may have resulted from the anisotropic CD phenomena of these molecules, which are oriented via photoselection. Although jet-cooled molecules may have nearly random orientations, those excited to the P/R or Q branch become oriented because the transition probability to these branches depends on the molecular orientation relative to the direction of light propagation. These results demonstrate that the CD spectra of cold, isolated molecules, such as those in an interstellar medium, may exhibit anisotropic CD values.

Cryogenic Ion Spectroscopy of Protonated and Sodiated Methyladenine Derivatives.

Ultraviolet photodissociation (UVPD) spectra of protonated 9-methyladenine (H+9MA), protonated 7-methyl adenine (H+7MA), protonated 3-methyladenine (H+3MA), and sodiated 7-methyladenine (Na+7MA) near the origin bands of the S0-S1 transition were obtained using cryogenic ion spectroscopy. The UV-UV hole burning, infrared (IR) ion-dip, and IR-UV double resonance spectra showed that all the ions were present as single isomers in a cryogenic ion trap. The UVPD spectrum of H+9MA exhibited only a broad absorption band, whereas the spectra of H+7MA, H+3MA, and Na+7MA displayed moderately or well-resolved vibronic bands. Potential energy profiles were computed to understand the reason for the different bandwidths of the vibronic bands in the spectra. The broadening of the bands was correlated with the slopes between the Franck-Condon point and the conical intersection between the S1 and S0 states in the potential energy profiles, thus reflecting the deactivation rates in the S1 state.

Dual-Beam Circular Dichroism Spectroscopy of Jet-Cooled Chiral Molecules.

Circular dichroism (CD) spectroscopy of jet-cooled molecules provides conformation-specific CD spectra. However, its widespread utilization has been limited by the weak CD effects and the low density of gas-phase molecules. Here, we developed a dual-beam method to improve the sensitivity and accuracy of gas-phase CD measurements. Circularly and linearly polarized pulses were generated from a single laser pulse and used to irradiate a single molecular-beam pulse to produce two ion peaks. The ion peaks induced by linearly polarized pulses were subtracted from those induced by circularly polarized pulses to correct the CD values for the pulse-to-pulse fluctuations in laser power and gas density. The resonant two-photon ionization CD spectrum of (1R,2R)-(-)-pseudoephedrine revealed that the standard deviations of CD values measured using the dual-beam method were three times lower than those measured using a single-beam method. The dual-beam method provides an effective, accurate, and easy-to-use tool to obtain gas-phase CD spectra.

Ultraviolet photodissociation circular dichroism spectroscopy of protonated L-phenylalanyl-L-alanine in a cryogenic ion trap.

We obtained ultraviolet photodissociation (UVPD) circular dichroism (CD) spectra of protonated L-phenylalanyl-L-alanine (L-H+PheAla) near the origin band of the S0-S1 transition using cryogenic ion spectroscopy. Infrared (IR) ion-dip, IR-UV hole burning (HB) and UV-UV HB spectra showed that L-H+PheAla existed as two different conformers in a cryogenic ion trap, and they had nearly identical peptide backbones but different conformations in the Phe side chain. The UVPD CD spectra revealed that the two conformers had opposite CD signs and significantly different CD magnitudes from each other. These results demonstrate that the CD value of L-H+PheAla near the origin band is strongly influenced by the conformation of the Phe side chain.

Cryogenic ion spectroscopy of adenine complexes containing alkali metal cations.

Cryogenic ion spectroscopy was used to characterize adenine complexes containing alkali metal cations (M+A, M = Cs, Rb, K, Na, and Li) produced by electrospray ionization. The ultraviolet (UV) photodissociation spectra of the complexes stored in a cryogenic ion trap exhibited well-resolved vibronic bands near their origin bands of the S0-S1 transition. The UV-UV hole-burning and infrared ion-dip spectra showed that all the M+A ions in the ion trap were single isomers of M+A7a, where the M+ ion was not bound to canonical 9H-adenine (A9) but bound to a rare tautomer, 7H-adenine (A7). Density functional theory calculations showed lower tautomerization barriers for M+A9 than for bare A9 in aqueous solution. We suggest that M+ ions not only play a catalytic role in the tautomerization of A9 to A7 but also increase the tautomerization yield by forming stable M+A7a isomers.

Induced Circular Dichroism of Jet-Cooled Phenol Complexes with (R)-(-)-2-Butanol.

The induced circular dichroism (ICD) of phenol complexed with (R)-(-)-2-butanol [PhOH-(-)BOH] in a supersonic jet is investigated using resonant two-photon ionization circular dichroism (R2PICD) spectroscopy. The R2PICD spectrum of PhOH-(-)BOH exhibits nonzero ICD bands near the absorption region of bare PhOH, where (-)BOH is transparent. Two different conformers containing a single hydrogen bond between PhOH and (-)BOH are identified using ultraviolet-ultraviolet hole-burning and infrared ion-dip spectroscopy combined with quantum theoretical calculations. The ICD values of the two conformers are similar to each other. To understand these similar ICD effects of the conformers, the geometrical asymmetry of the PhOH moiety bound to (-)BOH and the coupling strength of the electric transition dipole moments between PhOH and (-)BOH are estimated. Comparing the ICD values of PhOH-(-)BOH with those of PhOH-(-)-l-methyl lactate in the previous report [ Hong , A. ; J. Phys. Chem. Lett. 2018 , 9 , 476 -480 ], we investigate the physical properties that may govern the differences of the ICD values between the two complexes.

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)-Platinum(II) and Iridium(III)-Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine.

We investigated the electrochemical and excited-state properties of 2,3-bis(2-pyridyl)pyrazine (dpp)-bridged bimetallic complexes, (L)2Ir-dpp-PtCl [1, L = 2-(4',6'-difluorophenyl)pyridinato-N,C2 (dfppy); 2, L = 2-phenylpyridinato-N,C2 (ppy)] and [(L)2Ir]2(dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes, (L)2Ir-dpp (5, L = dfppy; 6, L = ppy) and dpp-PtCl (dpp-PtIICl2; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed that 1 and 3 have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in 5 and 6 is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir-Pt and Ir-Ir bimetallic complexes (1-4) could be explained by assuming the involvement of crossing to platinum- and iridium-based d-d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d-d states from the emissive triplet metal-to-ligand charge-transfer (3MLCT) state, followed by cleavage of the dpp-Pt and (L)2Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based 3MLCT to the Ir d-d or Pt d-d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d-d or Pt d-d) with a significant dependency on the relative 3MLCT levels of the (L)2Ir-dpp component.

Site-dependent effects of methylation on the electronic spectra of jet-cooled methylated xanthine compounds.

We obtained the electronic spectra of various methylated xanthine compounds including caffeine in a supersonic jet by resonant two-photon ionization spectroscopy. The methyl group in the tested methylated xanthine compounds has a distinct, site-dependent effect on the electronic spectrum. Methylation at the N3 position causes a significant red shift of the ππ* state, whereas methylation at the N1 position has only minimal effects on the electronic spectrum. The notably broad spectra of theobromine and caffeine result from methyl substitution at the N7 position, which causes a large displacement between the potential energy surfaces of the S0 and S1 states, and a strong vibronic coupling. We also investigated the internal rotation of the methyl group and its effect on the electronic spectrum of the methylated xanthine compounds. We found that the barrier height for the torsional motion in the ground state is significantly affected by a carbonyl or methyl group that lies close to the methyl group of interest. In contrast, the torsional barrier in the excited state is governed by the hyperconjugation interaction in the lowest unoccupied molecular orbital. The agreement between the experimental and simulated spectra of torsional vibronic bands suggested that the low frequency torsional vibrations arising from the tunneling splitting and the coupling between the torsional and molecular motions give theobromine and theophylline the multiplet nature of their origin bands. This study provides a new level of understanding for the methyl substitution effects on the electronically excited states of xanthine compounds, which may very well be applicable to many other methyl substituted biomolecules including DNAs and proteins.

Fluorescence-detected circular dichroism spectroscopy of jet-cooled ephedrine.

The resonant two-photon ionization circular dichroism (R2PICD) spectrum represents the cumulative circular dichroism (CD) of one-photon excitation and the subsequent one-photon ionization, whereas the fluorescence-detected circular dichroism (FDCD) spectra exhibit only the CD of one-photon excitation, similar to conventional CD spectra. We obtained the FDCD spectra of jet-cooled ephedrine (EPD) near the origin band of the S0-S1 transition to measure the CD of one-photon absorption and thus the CD of the ionization process in R2PI in comparison with the R2PICD spectra. The CD effects of the ionization following excitation of the A (0-0) and C (930 cm(-1)) bands in the spectrum are small, whereas those of the B band (530 cm(-1)) are anomalously large, leading to opposite CD signs for the FDCD and R2PICD spectra. Based on the intermediate state-selective fragmentation patterns in the R2PI spectra, this large CD effect is attributed to the state-selective isomerization that occurs after excitation of the B band. By comparing the experimental and theoretical spectra, we determined that the B band corresponds to an asymmetric ring distortion mode that involves torsional motions of the side chain, which may facilitate the isomerization process. This study demonstrates that FDCD spectroscopy combined with R2PICD spectroscopy provides a powerful tool to measure the CD effects of the excitation and ionization processes separately in R2PI and thus probe the structural changes that occur during the ionization process following excitation to an intermediate state.

Conformation-specific circular dichroism spectroscopy of cold, isolated chiral molecules.

The CD spectroscopy of a chiral compound in solution yields an average CD value derived from all of the conformations of a chiral molecule. By contrast, CD spectroscopy of cold chiral molecules in the gas phase distinguishes specific conformers of a chiral molecule, but the weak CD effect has limited the practical application of this technique. Reported herein is the first resonant two-photon ionization CD spectra of ephedrines in a supersonic jet using circularly polarized laser pulses, which were generated by synchronizing the oscillation of the photoelastic modulator with the laser firing. The spectra exhibited well-resolved CD bands which were specific for the conformations and vibrational modes of each enantiomer. The CD signs and magnitudes of the jet-cooled chiral molecules were very sensitive to their conformations and thus offered crucial information for determining the three-dimensional structures of chiral species, as conducted in combination with quantum chemical calculations.

Conformation-Selective Ultraviolet-Ultraviolet Hole Burning Spectra of Ubiquitin Ions in a Cryogenic Ion Trap.

Understanding the structural variations of conformational isomers in proteins is crucial for elucidating protein folding mechanisms. Here, we present a novel method for obtaining conformation-selective ultraviolet (UV)-UV hole burning (HB) spectra of ubiquitin ions ((Ubi+zH)+z, z = 7-10) produced via electrospray ionization. Our approach involves binding multiple N2 molecules to ubiquitin ions ((Ubi+zH)+z(N2)m, m = 1-55) within a cryogenic ion trap. Upon exposure to UV irradiation, efficient fragmentation of (Ubi+zH)+z(N2)m occurs, primarily yielding bare (Ubi+zH)+z ions as fragments. The significant mass difference between the parent and fragment ions facilitates the acquisition of UV-UV HB spectra, which reveal the presence of at least two distinct conformers. Molecular dynamics simulations suggest that these conformers correspond to A-state structures, differing only in the interactions of a tyrosine residue with neighboring residues. Our findings underscore UV-UV HB spectroscopy of protein ions as a powerful tool for exploring diverse protein isomers.

MODE-specific deactivation of adenine at the singlet excited states.

The deactivation process of adenine excited near the band origin of the lowest ππ* state ((1)L(b)) is investigated using picosecond (ps) time-resolved photoionization spectroscopy. The transients obtained with a ps pump pulse at the sharp vibronic bands, 36,105 cm(-1) (D) and 36,248 cm(-1) (E), in the resonant two-photon ionization spectrum exhibit a bi-exponential decay with two distinct time constants of τ1 ~ 2 ps and τ2 > 100 ps, whereas the transients with the pump at other wavenumbers in this energy region show a single exponential decay with τ = 1-2 ps. We suggest that the τ1 represents the lifetimes of the (1)nπ∗ energy levels near the D and E peaks, which are excited together by the ps pump pulse having a broad spectral bandwidth, and the τ2 shows the lifetimes of D and E peaks. The long lifetime of D level is attributed to a small barrier for internal conversion from the minimum of the (1)L(b) state to the (1)nπ* state. On the other hand, the long lifetime of E level is ascribed to the nuclear configuration of adenine at this level, which is unfavorable to reach the seam of the conical intersection leading to nearly barrierless deactivation to the electronic ground state. This study shows that the ps time-resolved spectroscopy provides a powerful tool to study mode- and energy-specific deactivation processes occurring in a multi-dimensional potential energy surface.

Hydrated alizarin complexes: hydrogen bonding and proton transfer.

We investigated the hydrogen bonding structures and proton transfer for the hydration complexes of alizarin (Az) produced in a supersonic jet using fluorescence excitation (FE), dispersed laser induced fluorescence (LIF), visible-visible hole burning (HB), and fluorescence detected infrared (FDIR) spectroscopy. The FDIR spectrum of bare Az with two O-H groups exhibits two vibrational bands at 3092 and 3579 cm(-1), which, respectively, correspond to the stretching vibration of O1-H1 that forms a strong intramolecular hydrogen bond with the C9=O9 carbonyl group and the stretching vibration of O2-H2 that is weakly hydrogen-bonded to O1-H1. For the 1:1 hydration complex Az(H(2)O)(1), we identified three conformers. In the most stable conformer, the water molecule forms hydrogen bonds with the O1-H1 and O2-H2 groups of Az as a proton donor and proton acceptor, respectively. In the other conformers, the water binds to the C10=O10 group in two nearly isoenergetic configurations. In contrast to the sharp vibronic peaks in the FE spectra of Az and Az(H(2)O)(1), only broad, structureless absorption was observed for Az(H(2)O)(n) (n≥ 2), indicating a facile decay process, possibly due to proton transfer in the electronic excited state. The FDIR spectrum with the wavelength of the probe laser fixed at the broad band exhibited a broad vibrational band near the O2-H2 stretching vibration frequency of the most stable conformer of Az(H(2)O)(1). With the help of theoretical calculations, we suggest that the broad vibrational band may represent the occurrence of proton transfer by tunnelling in the electronic ground state of Az(H(2)O)(n) (n≥ 2) upon excitation of the O2-H2 vibration.

Ultraviolet-ultraviolet hole burning spectroscopy in a quadrupole ion trap: dibenzo[18]crown-6 complexes with alkali metal cations.

Conformation selective: A new technique of ultraviolet-ultraviolet hole burning spectroscopy that can be applied to ions stored in a quadrupole ion trap (QIT) is developed and used to obtain the conformation-selective electronic spectra of dibenzo[18]crown-6 complexes with alkali metal cations (M(+), see picture; F(+) = fragment).

Femtosecond decay dynamics of intact adenine and thymine base pairs in a supersonic jet.

We investigated the decay dynamics of the DNA base pairs adenine-adenine (A(2)), adenine-thymine (AT), and thymine-thymine (T(2)) produced in a supersonic jet by femtosecond (fs) time-resolved photoionization spectroscopy. The base pair was excited by a fs pump pulse at 267 nm and the population change of its excited state was monitored by non-resonant three-photon ionization using a fs probe pulse at 800 nm after a certain time delay. All of the transients recorded in the mass channel of the parent ion exhibited a tri-exponential decay, with time constants ranging from 100 fs to longer than 100 ps. Most of these time constants coincide well with the previous values deduced indirectly from the transients of protonated adenine (AH(+)) and thymine (TH(+)), which were assumed to be produced by fragmentation of the base-pair ions. Notably, for the transient of T(2), we observed a new decay component with a time constant of 2.3 ps, which was absent in the transient of TH(+). We suggest that the new decay component arises from the decay of stacked T(2) dimers that are mostly ionized to T(2)(+), whereas the decay signal recorded in the mass channel of TH(+) is merely from the relaxation of hydrogen-bonded T(2) dimers. From the amplitude of the new decay component, the population of the stacked T(2) dimers relative to the hydrogen-bonded dimers was estimated to be ∼2 % in the supersonic jet, which is about fifteen times higher than the theoretical value.

Theoretical calculations for mass resolution of a quadrupole ion trap reflectron time-of-flight mass spectrometer.

We have developed a theoretical method of predicting the mass resolution for a quadrupole ion trap reflectron time-of-flight (QIT-reTOF) mass spectrometer as a function of the spatial and velocity distributions of ions, voltages applied to the electrodes, and dimensions of the instrument. The flight times of ions were calculated using theoretical equations derived with an assumption of uniform electric fields inside the QIT and with the analytical description of the potential including the monopole, dipole, and quadrupole components. The mass resolution was then estimated from the flight-time spread of the ions with finite spatial and velocity distributions inside the QIT. The feasibility of the theoretical method was confirmed by the reasonable agreement of the theoretical resolution with the experimental one measured by varying the extraction voltage of the QIT or the deceleration voltage of the reflectron. We found that the theoretical resolution estimated with the assumption of the uniform electric fields inside the QIT reproduced the experimental one better than that with the analytical description of the potential. The possible applications of this theoretical method include the optimization of the experimental parameters of a given QIT-reTOF mass spectrometer and the design of new instruments with higher mass resolution.

Ultraviolet photodepletion spectroscopy of dibenzo-18-crown-6-ether complexes with alkaline Earth metal divalent cations.

Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkaline earth metal divalent cations (A(2+)-DB18C6, A = Ba, Sr, Ca, and Mg) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. Each spectrum exhibits the lowest energy absorption band in the wavenumber region of 35 400-37 800 cm(-1), which is tentatively assigned as the origin of the S(0)-S(1) transition of A(2+)-DB18C6. This origin band shows a red shift as the size of the metal dication increases from Mg(2+) to Ba(2+). The binding energies of the metal dications to DB18C6 at the S(0) state were calculated at the lowest energy structures optimized by the density functional theory and employed with the experimental energies of the origin bands to estimate the binding energies at the S(1) state. We suggest that the red shifts of the origin bands arise from the decrease in the binding energies of the metal dications at the S(1) state by nearly constant ratios with respect to the binding energies at the S(0) state, which decrease with increasing size of the metal dication. This unique relationship of the binding energies between the S(0) and S(1) states gives rise to a linear correlation between the relative shift of the origin band of A(2+)-DB18C6 and the binding energy of the metal dication at the S(0) state. The size effects of the metal cations on the properties of metal-DB18C6 complex ions are also manifested in the linear plot of the relative shift of the origin band as a function of the size to charge ratio of the metal cations, where the shifts of the origin bands for all DB18C6 complexes with alkali and alkaline earth metal cations are fit to the same line.

Excited-state lifetime of adenine near the first electronic band origin.

The excited-state lifetime of supersonically cooled adenine was measured in the gas phase by femtosecond pump-probe transient ionization as a function of excitation energy between 36 100 and 37 500cm(-1). The excited-state lifetime of adenine is ∼2ps around the 0-0 band of the (1)L(b) ππ(∗) state (36 105cm(-1)). The lifetime drops to ∼1ps when adenine is excited to the (1)L(a) ππ(∗) state with the pump energy at 36 800cm(-1) and above. The excited-state lifetimes of (1)L(a) and (1)L(b) ππ(∗) states are differentiated in accordance with previous frequency-resolved and computational studies.

Chiral and Isomeric Discrimination of Chiral Molecular Ions by Cold Ion Circular Dichroism Spectroscopy.

Circular dichroism (CD) spectra contain information about absolute configurations and conformations of chiral compounds. However, extracting this information from CD spectra in solution is challenging, because the spectra exhibit only the averaged CD values of all different conformers. CD spectroscopy of jet-cooled molecules can provide conformation-specific CD spectra, but its application to biomolecules has been limited due to the difficulty of their production in the gas phase. Here, we obtained the first CD spectra of chiral molecular ions produced by electrospray ionization (ESI) using cold ion CD spectroscopy. Protonated l- or d-phenylalanine ions produced by ESI were stored in a cold quadrupole ion trap and irradiated by multiple laser pulses with left or right circular polarization. The CD spectra exhibited well-resolved CD bands of two conformers, whose signs were opposite to each other. This study will broaden the scope of conformation-resolved CD spectroscopy to large molecular ions without size limitations.