High-resolution ultraviolet spectroscopy of neutral and ionic clusters: hydrogen bonding and the external heavy atom effect

Mechanistic Understanding and Rational Design of Quantum Dot/Mediator Interfaces for Efficient Photon Upconversion

The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet-triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet-triplet energy losses compared to metal-ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step's efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems.This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 and TET2 efficiencies. The agreement between calculated and measured quantum efficiencies suggests a firm understanding of QD-sensitized photon upconversion. We believe the above conclusions are general and should be widely applicable to similar systems, including singlet fission in hybrid organic-nanocrystal materials.

Tuning the Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-Visible Photon Upconversion Systems

Lead sulfide (PbS) quantum dots (QDs) have shown promising performance as a sensitizer in infrared-to-visible photon upconversion systems. To investigate the key design rules, we compare three PbS-sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Using transient absorption spectroscopy, we directly measure the triplet energy-transfer rates and efficiencies from the QD to the mediator and from the mediator to the emitter. With increasing distance between the mediator and PbS QD, the efficiency of the first triplet energy transfer from the QD to the mediator decreases because of a decrease in the rate of this triplet energy-transfer step, while the efficiency of the second triplet energy transfer from the mediator to the emitter increases because of a reduction in the QD-induced mediator triplet state decay. The latter effect is a result of the slow rate constant of the second triplet energy-transfer process, which is 3 orders of magnitude slower than the diffusion-limited value. The combined results lead to a net decrease of the steady-state upconversion quantum yield with distance, which could be predicted by our kinetic model. Our result shows that the QD/mediator interface affects both the first and second triplet energy transfer processes in the photon upconversion system, and the QD/mediator distance has an opposite effect on the efficiencies of the first and second triplet energy transfer. These findings provide important insight for the further rational improvement of the overall efficiency of QD-based upconversion systems.

Estimating Strengths of Individual Hydrogen Bonds in RNA Base Pairs: Toward a Consensus between Different Computational Approaches

Noncoding RNA molecules are composed of a large variety of noncanonical base pairs that shape up their functionally competent folded structures. Each base pair is composed of at least two interbase hydrogen bonds (H-bonds). It is expected that the characteristic geometry and stability of different noncanonical base pairs are determined collectively by the properties of these interbase H-bonds. We have studied the ground-state electronic properties [using density functional theory (DFT) and DFT-D3-based methods] of all the 118 normal base pairs and 36 modified base pairs, belonging to 12 different geometric families (cis and trans of WW, WH, HH, WS, HS, and SS) that occur in a nonredundant set of high-resolution RNA crystal structures. Having addressed some of the limitations of the earlier approaches, we provide here a comprehensive compilation of the average energies of different types of interbase H-bonds (E _HB). We have also characterized each interbase H-bond using 13 different parameters that describe its geometry, charge distribution at its bond critical point (BCP), and n → σ*-type charge transfer from filled π orbitals of the H-bond acceptor to the empty antibonding orbital of the H-bond donor. On the basis of the extent of their linear correlation with the H-bonding energy, we have shortlisted five parameters to model linear equations for predicting E _HB values. They are (i) electron density at the BCP: ρ, (ii) its Laplacian: ∇²ρ, (iii) stabilization energy due to n → σ*-type charge transfer: E(2), (iv) donor-hydrogen distance, and (v) hydrogen-acceptor distance. We have performed single variable and multivariable linear regression analysis over the normal base pairs and have modeled sets of linear relationships between these five parameters and E _HB. Performance testing of our model over the set of modified base pairs shows promising results, at least for the moderately strong H-bonds.

"Through-Space" Relativistic Effects on NMR Chemical Shifts of Pyridinium Halide Ionic Liquids

We have investigated, using two-component relativistic density functional theory (DFT) at ZORA-SO-BP86 and ZORA-SO-PBE0 level, the occurrence of relativistic effects on the ¹ H, ¹³ C, and ¹⁵ N NMR chemical shifts of 1-methylpyridinium halides [MP][X] and 1-butyl-3-methylpyridinium trihalides [BMP][X₃ ] ionic liquids (ILs) (X=Cl, Br, I) as a result of a non-covalent interaction with the heavy anions. Our results indicate a sizeable deshielding effect in ion pairs when the anion is I^- and I₃ ^- . A smaller, though nonzero, effect is observed also with bromine while chlorine based anions do not produce an appreciable relativistic shift. The chemical shift of the carbon atoms of the aromatic ring shows an inverse halogen dependence that has been rationalized based on the little C-2s orbital contribution to the σ-type interaction between the cation and anion. This is the first detailed account and systematic theoretical investigation of a relativistic heavy atom effect on the NMR chemical shifts of light atoms in the absence of covalent bonds. Our work paves the way and suggests the direction for an experimental investigation of such elusive signatures of ion pairing in ILs.

Non-covalent interactions in anisole–(CO2)n (n = 1, 2) complexes

Non-covalent interactions are a ubiquitous binding motif and a challenge for theory and experiments.

Exploring the mechanism of isomerisation and water-migration in the water-complexes of amino-acid l-proline: electrostatic potential and vibrational analysis

This work reveals interesting pathways for water-migration and neutral ↔ zwitterionic isomerisation in the water complexes of l-proline.

Probing Ultrafast Purely Electronic Charge Migration in Small Peptides

A pump–probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that short peptide terminated by an aromatic amino acid are particularly suitable test compounds. The pump pulse needs to ionize the molecule on a time scale short compared to the period of the electronic motion, typically sub-fs. However, ionization occurs preferentially when the electrical field of the light is maximal so that the duration of the pulse envelope can be somewhat longer. Detection by photoelectron spectrometry of the peptide cation, to produce a dication, is shown to be able to probe the electronic rearrangement.

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule–Solvent Recognition

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

ZEKE-PFI spectroscopy of benzocaine

The adiabatic ionization threshold (AIT) of trans- and gauche-benzocaine has been measured by zero electron kinetic energy-pulsed field ionization (ZEKE-PFI) spectroscopy to be 7.8412+/-0.0008 eV (lasers at 34,134.4 and 29,109.3 cm(-1)) and 7.8421+/-0.0004 eV (34,144.8+29105.7 cm(-1)), respectively. AITs computed at the B3LYP/AUG-cc-p-VDZ level for the two conformers are some approximately 2,500 cm(-1) lower than the experimental; in contrast their energy difference is very close. The trans-benzocaine cation ZEKE spectra has been recorded taking a number of S(1) intermediate vibronic states. The spectra provide an energy threshold for the appearance of intramolecular vibrational redistribution (IVR) around approximately 540 cm(-1) in the S(1) state.

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.

Theoretical and experimental studies of water complexes of p- and o-aminobenzoic acid

We report studies of supersonically cooled water complexes of p- and o-aminobenzoic acid with one or two water molecules using two-color resonantly enhanced multiphoton ionization (REMPI) spectroscopy. Density functional theory calculations are carried out to identify structural minima of water complexes in the ground state. According to the calculation, water molecules are bound to both the C=O and -OH groups to form a cyclic hydrogen-bond network in the most stable isomer. Vibrational frequency calculations for the first electronically excited state of the most stable isomer agree well with the experimental observation. On the basis of this agreement, we believe that only one isomer exists in our molecular beam. The frequency shifts of a few normal modes caused by the water molecules further confirm the site of water addition. A surprising observation is that, for OABA(H2O)n complexes, abundant intermolecular vibrational modes are clearly observable in the REMPI spectra, while for PABA(H2O)n complexes, these modes are conspicuously missing. A red shift in the transition energy is observed for OABA(H2O)1, while blue shifts are observed for the rest of the complexes. This difference alludes to the relative stabilities of the water complexes of the two aminobenzoic acids in both the ground and excited electronic states. These observations will be discussed in comparison with those from the meta isomer.

Comparison of Photoelectron-Spectroscopy Results to Ab-Initio and Density Functional Calculations: The Ethylbenzene Cation

In this work resonant S 0–S 1 two-photon ionization (R2PI) and high-resolution R(1+1’)PI photoelectron spectroscopy (PES) as well as ab initio and density functional (DFT) calculations of ethylbenzene (EB) are combined. Conformer energies and equilibrium geometries have been calculated for neutral and cationic EB with the HF, UHF, B3LYP and the MP2 methods and different basis sets. In agreement with previous results the tail-to-chromophore orientation of neutral EB is orthogonal. This conformer is also the most stable structure in the cation, but a second local minimum in which all carbons lie in a plane (termed “planar” conformer) lays 325cm-1 higher in energy. R(1+1’)PI PE spectra were recorded by time-of-flight spectrometer with an energy resolution (Δ E) below 8 cm-1 and an absolute accuracy of ± 10 cm-1 for electron energies below 200 meV. Because the experiment starts in the orthogonal conformer and ionization is vertical, the recorded PE spectra show the cation ground state vibrations of this conformer. Beside benzene modes also low-energetic tail-to-chromophore modes are observed and assigned by DFT vibrational mode analysis. The differences of the calculated vibrational frequencies between the two conformers are comparable to the deviation between experiment and theory and a conformer assignment by comparison of theory and experiment would be difficult. R(1+1’)PI PE spectra recorded via selected S 1 vibrations provide vibrational assignments for S 1, qualitative S 1–D 0 geometry changes, vibrational symmetries as well as internal vibrational redistribution dynamics in S 1. Charge and spin densities of the neutral and cation were calculated to elucidate the problem of charge delocalization and electronic tail-to-chromophore coupling.

Van der Waals Bonding to a Molecule with π-Electron Conjugation: Styrene-Argon Complex Studied by Mass Selective High Resolution R2PI UV Spectroscopy and Ab-initio Calculations

Combining high-resolution mass-selective resonance-enhanced two-photon ionization spectroscopy analysed by genetic-algorithm-based computer-assisted rotational fit and high-level ab initio calculations we were able to determine the structure and the transition moment orientation of the styrene-Ar complex. The results demonstrate that Ar binds to the benzene ring of styrene. The binding site of Ar is 3.42 Å above the benzene-ring plane and is close to the C 6 symmetry axis of the aromatic ring, but slightly shifted to the vinyl group, which implies that the binding pattern is perturbed by the presence of the bonded to the benzene ring vinyl group. The transition moment ratio of the styrene-Ar complex is considerably different from the one of bare styrene this being a purely mass effect stemming from the reorientation of the principal axes of inertia upon the cluster formation. The red shift of the 00 0 origin band of the styrene-Ar2 complex is almost twice that of the single complex, indicating that the red-shift additivity rule holds in this case. On this basis we conclude that the second Ar atom is bound to the opposite side of the benzene ring.

Accurate ab Initio Study on the Hydrogen-Bond Pairs in Protein Secondary Structures

Ab initio calculations up to the MP2/aug-cc-pVQZ//MP2/6-311+G** level have been carried out to characterize the four patterns of hydrogen-bond (H-bond) pairs in protein secondary structures. The unblocked and methyl-blocked glycine dipeptide dimers were arranged to model the H-bond pairs in α-helix (αHH) and antiparallel (Aββ-C5 and Aββ-C7) and parallel β-sheet (Pββ) secondary structures. The study uncovers that, in addition to the primary CO⋯NH H-bonds and the crossing secondary interactions, the CH⋯OC H-bonds and the tertiary effect (as we call it) also contribute substantially. The tertiary effect is due to the interpolarization between the donor and acceptor of a H-bond. This effect, which enhances the dipole-dipole interactions between two nearby H-bonds, stabilizes the β-sheet-like but destabilizes the helix-like H-bond pairs. The MP2 binding energies of the complexes were further refined by extrapolating to the complete basis set limit (CBS) according to Truhlar and co-workers and by a three-basis-set-based method. The best extrapolated CBS(aD-aT-aQ) binding energies of the unblocked dimers are -13.1 (αHH), -11.3 (Aββ-C5), -19.2 (Aββ-C7), and -14.8 kcal/mol (Pββ). For the methyl-blocked counterparts, the best extrapolated CBS(D-T-Q) binding energies are -14.8, -13.4, -20.8, and -16.7 kcal/mol, respectively. The interactions in the parallel β conformations are very close to the averages of the C5 and C7 antiparallel β conformations, and both are stronger than the helical dimers. Because the additive force fields are unable to account for the tertiary effect owing to the lack of polarization, all examined additive force fields significantly overestimate the interaction energies of the helix conformations relative to the β-sheet conformations. Notably, the agreement between molecular mechanical and quantum mechanical binding energies is improved after turning on the polarization. The study provides reference ab initio structures and binding energies for characterizing the backbone H-bonds of the protein secondary structures, which can be used for the parametrization of empirical molecular mechanics force fields.

Evidence for a C–H⋯π type weak interaction: 1 : 1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

The 1 : 1 complex of styrene with acetylene has been studied by mass selective low- and high-resolution UV resonance-enhanced two-photon ionisation (R2PI) spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution, and high level ab initio quantum chemistry calculations. Two stable conformeric geometries of the 1 : 1 complex of styrene and acetylene have been theoretically found: one with acetylene binding to styrene as a proton donor, and one with acetylene acting as a proton acceptor. From the analysis of the vibronic structure of the S1<-- S0 spectrum and the fit of the highly resolved spectrum of the 0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene. The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4 degrees relative to the C6 symmetry axis of the benzene ring, most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

Using high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with genetic-algorithm-based computer-aided rotational fit analysis and ab initio quantum chemistry calculations we determined the conformational structure and transition moment orientation in 2-phenylethanol and its 1:1 clusters with argon. The results clearly demonstrate that the gauche structure of 2-phenylethanol, which is stabilized by the intramolecular pi-hydrogen bond between the folded side chain and the benzene ring, is the most abundant in the cold molecular beam. In this conformer the transition moment is rotated by 18 degrees from the short axis of the aromatic ring. Two distinct 1:1 complexes of 2-phenylethanol with argon in a cis- and trans-configuration with respect to the side chain have been found. Employing the Kraitchman [Am. J. Phys. 21, 17 (1953)] analysis we have found that the structure of the 2-phenylethanol moiety and the orientation of the transition moment do not change after the complexation with argon within the experimental accuracy. From the measured band intensities we conclude that in addition to the dispersion interaction of the argon atom with the aromatic ring a hydrogen-bond-type interaction with the terminal -OH group of the side chain stabilizes the cis-structure of the 1:1 complex of 2-phenylethanol with argon.

The World of Non-Covalent Interactions: 2006

The review focusses on the fundamental importance of non-covalent interactions in nature by illustrating specific examples from chemistry, physics and the biosciences. Laser spectroscopic methods and both ab initio and molecular modelling procedures used for the study of non-covalent interactions in molecular clusters are briefly outlined. The role of structure and geometry, stabilization energy, potential and free energy surfaces for molecular clusters is extensively discussed in the light of the most advanced ab initio computational results for the CCSD(T) method, extrapolated to the CBS limit. The most important types of non-covalent complexes are classified and several small and medium size non-covalent systems, including H-bonded and improper H-bonded complexes, nucleic acid base pairs, and peptides and proteins are discussed with some detail. Finally, we evaluate the interpretation of experimental results in comparison with state of the art theoretical models: this is illustrated for phenol...Ar, the benzene dimer and nucleic acid base pairs. A review with 270 references.

The Fate of Dicationic States in Molecular Clusters of Benzene and Related Compounds

Calculations employing density functional theory indicate that, rather than undergoing fragmentation, dicationic clusters of benzene, hexafluorobenzene, and naphthalene produced by sequential one-electron or sudden double-ionization experiments on the neutrals can relax via the formation of inter-ring covalent C-C bonds, along with a series of proton transfers that enable a substantial reduction of inter- and intramolecular Coulomb repulsions. The theoretically predicted chemically bound structures correspond to deep local energy minima on the potential energy surface pertaining to the lowest electronic state of the dications and can therefore be regarded as metastable (kinetically long-lived) species. This discovery invalidates on theoretical grounds the liquid-droplet model of multiply charged clusters and sheds very unexpected light on possible consequences in chemistry of the intermolecular Coulombic decay (ICD) mechanism [Cederbaum, L. S.; et al. Phys. Rev. Lett. 1997, 79, 4778; Jahnke, T.; et al. Phys. Rev. Lett. 2004, 93, 163401] for deep inner-valence ionized states. Propagation of charge rearrangement reactions and proton transfers to several monomers may eventually lead to the formation of rather extended dicationic assemblies.

High-resolution ultraviolet spectroscopy of p-fluorostyrene-water: Evidence for a σ-type hydrogen-bonded dimer

Ab initio calculations predict four stable conformational structures of the singly hydrated cluster of p-fluorostyrene: two out of plane with pi- and two in plane with sigma-type intermolecular hydrogen bonding between p-fluorostyrene and water. We employed mass-selective resonance-enhanced two-photon ionization high-resolution (70-MHz FWHM laser bandwidth) spectroscopy to partially resolve the rotational structure of the 0(0) (0) origin band of the S(1) <--S(0) electronic transition. A computer-aided fit based on genetic algorithms was used to analyze the experimental high-resolution spectrum and to determine the observed conformational structure. The good agreement between the experimental and the simulated spectra of the 0(0) (0) band and the assignment of the other prominent bands as inter- and intramolecular vibrational progressions clearly demonstrates that the anti in-plane conformer is the most abundant one in the molecular beam. The existence of the sigma-type hydrogen bond between p-fluorostyrene and water manifests that the electron attracting effect of fluorine dominates over the releasing mesomeric effect of the vinyl group and thus a pi-type hydrogen bonding with the aromatic ring is not favored in this case.

Interaction of Ionic Biomolecular Building Blocks with Nonpolar Solvents: Acidity of the Imidazole Cation (Im+) Probed by IR Spectra of Im+−Ln Complexes (L = Ar, N2; n ≤ 3)

The intermolecular interaction between the imidazole cation (Im+ = C3N2H4+) and nonpolar ligands is characterized in the ground electronic state by infrared photodissociation (IRPD) spectroscopy of size-selected Im+-Ln complexes (L = Ar, N2) and quantum chemical calculations performed at the UMP2/6-311G(2df,2pd) and UB3LYP/6-311G(2df,2pd) levels of theory. The complexes are created in an electron impact cluster ion source, which predominantly produces the most stable isomers of a given cluster ion. The analysis of the size-dependent frequency shifts of both the N-H and the C-H stretch vibrations and the photofragmentation branching ratios provides valuable information about the stepwise microsolvation of Im+ in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between various intermolecular binding motifs (H-bonding and pi-bonding) and their interaction energies, and the acidity of both the CH and NH protons. In line with the calculations, the IRPD spectra show that the most stable Im+-L dimers feature planar H-bound equilibrium structures with nearly linear H-bonds of L to the acidic NH group of Im+. Further solvation occurs at the aromatic ring of Im+ via the formation of intermolecular pi-bonds. Comparison with neutral Im-Ar demonstrates the drastic effect of ionization on the topology of the intermolecular potential, in particular in the preferred aromatic substrate-nonpolar recognition motif, which changes from pi-bonding to H-bonding. .

Fragmentation and conformation study of ephedrine by low- and high-resolution mass selective UV spectroscopy

The neurotransmitter molecule, ephedrine, has been studied by mass-selective low- and high-resolution UV resonance enhanced two-photon ionization spectroscopy. Under all experimental conditions we observed an efficient fragmentation upon ionization. The detected vibronic peaks in the spectrum are classified according to the efficiency of the fragmentation, which leads to the conclusion that there exist three different species in the molecular beam: ephedrine-water cluster and two distinct conformers. The two-color two-photon ionization experiment with a decreased energy of the second photon leads to an upper limit of 8.3 eV for the ionization energy of ephedrine. The high-resolution (70 MHz) spectrum of the strongest vibronic peak in the spectrum measured at the fragment (m/z=58) mass channel displays a pronounced and rich rotational structure. Its analysis by the use of a specially designed computer-aided rotational fit process yields accurate rotational constants for the S(0) and S(1) states and the transition moment ratio, providing information on the respective conformational structure.

Enhancement of the phosphorescence of organic luminophores upon interaction with a mercury trifunctional Lewis acid

The interaction of the trinuclear mercury(II) complex [(o-C(6)F(4)Hg)(3)] (1) and pyrene leads to the formation of the 1:1 adduct 1.pyrene. The crystal structure of this adduct reveals the existence of supramolecular stacks in which molecules of 1 and molecules of pyrene alternate along the infinite chains. Steady-state and time-resolved photoluminescence measurements indicate the occurrence of a heavy atom effect which results in red, green, and blue (RGB) phosphorescent emissions for 1.pyrene, 1.naphthalene, and 1.biphenyl, respectively.