Photodynamics and Luminescence of Mono- and Tri-Nuclear Lanthanide Complexes in the Gas Phase and in Solution.

Lanthanide ions (DyIII , EuIII ) are stabilized by coordination with two Schiff base ligands in compounds [Dy{H3 L}2 ](NO3 )(EtOH)(H2 O)8 (1) and [Eu{H3 L}2 ](NO3 )(H2 O)8 (3) (H4 L, 2,2'-{[(2-aminoethyl)imino]bis[2,1-ethanediyl-nitriloethylidyne]}bis-2-hydroxy-benzoic acid). The latter is reported here for the first time. Both luminescence and ultrafast photodynamics after photoexcitation via a ligand absorption band (∼400 nm) have been studied. In solution, only the [Eu{H3 L}2 ]+ ([3]+ ) complex displays the typical lanthanide emission lines, whereas in gas phase both, [Dy{H3 L}2 ]+ ([1]+ ) and [3]+ , show their corresponding transitions depending on excitation energy. The ultrafast excited state dynamics, obtained in gas phase and in solution, are assigned to excited state intramolecular proton transfer processes in the ligands. The antenna ligand moiety of these complexes provides pockets for stabilization of two MnII ions so that we additionally investigated the photophysical behavior of the corresponding tri-nuclear (NHEt3 )2 [Ln{MnL}2 ](ClO4 )(H2 O)2 (Ln=DyIII , EuIII ) compounds (2, 4). Interestingly, the related complexes do not show lanthanide emission, neither in solution nor in gas phase. Transient data in solution and gas phase suggests an efficient quenching of the ligand's electronically excited state by strong interaction with the MnII ions. This effect could possibly be developed further into a design principle for luminescence-based sensing devices for metal cations.

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Self-pairing of 1-methylthymine mediated by two and three Ag(I) ions: a gas phase study using infrared dissociation spectroscopy and density functional theory.

Metal base pairs of Ag(I) cations and 1-methylthymine (1MT) or deprotonated 1-methylthymine (1MT-H) are produced and analyzed by electrospray ionization mass spectrometry (ESI-MS). Mass-selected ions of type [Ag2(1MT)(1MT-H)](+) and [Ag3(1MT-H)2](+) are interrogated by infrared multiple-photon dissociation (IRMPD) in an ion trap in the range of 1200-3700 cm(-1). Supporting spectroscopic data were obtained from the investigation of the analogous 2'-deoxy-thymidine complexes which exhibit advantageously high fragment yields. By comparison with calculated linear IR spectra (obtained by density functional theory, DFT) we assign the structures and the possible isomeric forms of these metal base pairs and their dependence on the number of mediating Ag(I) ions. Based on the observed Ag(+)/1MT complexes and related polarizable continuum model DFT calculations we describe the probable formation pathways in aqueous solution. The present findings pave the way for subsequent UV investigations of the multi-metal mediated base pairs.

Investigation by two-color IR dissociation spectroscopy of Hoogsteen-type binding in a metalated nucleobase pair mimic.

Two Ag(I) ions, deprotonated 1-methyl-thymine (1MT-H)(-) and 1,3-dideaza-adenine (DDA), self-assemble in methanolic solution to a cationic [(Ag)2(1MT-H)(DDA)](+) complex as identified by electrospray ionization mass spectrometry. Assignment of vibrational bands and identification of the silver coordination pattern arise from comparison of one- and two-color Infrared Multiple Photon Dissociation (IRMPD) spectra (1000-4000 cm(-1)) of isolated and trapped complexes to calculated spectra from density functional theory. This approach reveals two structurally and energetically close isomers that resemble a metalated Hoogsteen-like binding motif. The two color IR/IR double resonance scheme proved in particular useful to observe weakly absorbing or weakly fragmenting vibrational modes.

Vibrational signatures of Watson-Crick base pairing in adenine-thymine mimics.

The vibrational fingerprints of hydrogen-bonding associated with the adenine-thymine (A-T) Watson-Crick (WC) base pair have been identified in an infrared study of the A-T mimics 4-aminopyrimidine-1-methylthymine (4APM-1MT) and 4-aminopyrimidine-6-methyl-4-pyrimidinone (4APM-M4PMN) in the gas-phase. The IR vibrational spectra were measured via a double resonance scheme utilizing femtosecond multiphoton ionization. The changes in the molecular structure, anharmonic vibrational parameters, and the assignment of the observed vibrational spectra in the NH/CH stretch region were investigated by carrying out high-level theoretical calculations of the anharmonic spectra. The experimental observations and theoretical calculations indicate that the hydrogen bonds associated with WC base-pairing are relatively stronger than those associated with reverse WC (rWC) base pairing. This is manifested in a more pronounced red-shift of the H-bonded vibrational modes associated with the WC as compared with the rWC base-pairing. An analysis of the factors contributing to the anharmonicity of the vibrational modes associated with H-bonding reveals that the magnitude of the off-diagonal anharmonic coupling of the H-bonded -NH2 stretch and the -NH2 bend is much smaller in WC base-pairing than in the corresponding rWC base-pairing. The chemical and biological implications of these results, especially in the context of using vibrational spectroscopy as a tool for identifying the signatures of nucleotide base vibrations is addressed.

On the nature of the long-lived "dark" state of isolated 1-methylthymine.

The photoinduced excited-state relaxation dynamics of gaseous thymine and 1-methylthymine are studied by both femtosecond and nanosecond pump-probe ionization spectroscopy on the sub-picosecond to microsecond timescale. A threefold exponential decay is observed with time constants of 80±40 fs, 4.8±2 ps, and 280±30 ns for thymine and 70±40 fs, 3.4±1.1 ps, and 310±30 ns for 1-methylthymine using a 267 nm excitation and subsequent 800 nm multiphoton ionization. In addition, a vibrational spectrum in the NH stretch region of the long-lived "dark" electronic state of isolated 1-methylthymine is reported for the first time. This spectrum, in combination with the dependence of the dark-state ionization rate on the laser intensity, allows assignment of the dark state of 1-methylthymine to the lowest triplet state of the keto tautomer, thus excluding enol tautomers as well as the nπ* excited state and a hot electronic ground state from the consideration. Very similar excited-state relaxation dynamics of thymine and 1-methylthymine justify the conclusion that the long-lived dark state of isolated thymine is also of triplet nature.

Specific photodynamics in thymine clusters: the role of hydrogen bonding.

A photoionization detected IR study of thymine and 1-methylthymine monohydrates and of their homodimers was carried out to shed some light on the structure of the thymine clusters whose complex photodynamics has recently been the subject of great interest. Under supersonic jet conditions, thymine forms doubly H-bonded cyclic clusters with water or another base preferentially via its N1-H group and the adjacent carbonyl group. This hydrate is of no biological relevance since the N1-H group is the sugar binding site in thymidine. On the other hand, 1-methylthymine forms the donor H-bonds only via the N3-H group. Hence, properties of the N1-H and the N3-H bound clusters of thymine can be studied using thymine and 1-methylthymine molecules, respectively. No biologically relevant conformations of the dimers and hydrates of thymine, contrary to those of 1-methylthymine, are observed under supersonic jet conditions. Thymine homodimer, which extensively fragments upon UV ionization by formation of a protonated monomer, exhibits two N1-H···O═C2 hydrogen bonds. The photodynamics of hydrated thymines is found to be extremely sensitive to the hydration site: ranging from an ultrafast relaxation in less than 100 fs up to formation of a dark state with the lifetime on the microsecond time scale.

4-Aminobenzimidazole-1-methylthymine: a model for investigating Hoogsteen base-pairing between adenine and thymine.

We report the infrared spectrum of the 4-aminobenzimidazole-1-methylthymine (4ABI:1MT) heterodimer, detected by femtosecond multiphoton ionization. Based on calculations of both the harmonic and the anharmonic frequencies, the observed vibrational spectrum is assigned to a structure that mimics the Hoogsteen base pairing of adenine and thymine. A notable observation made in the course of this study is that there is a significant imbalance in the observed strengths of the H-bonds. While the N···H-N bond reveals a large red shift of >700 cm(-1) for the NH stretch frequency, the N-H···O bond is characterized by only a 50 cm(-1) shift. The importance of this observation in the formation of Hoogsteen duplexes by thymine-based oligonucleotides is discussed.

The structure of adenine monohydrates studied by femtosecond multiphoton ionization detected IR spectroscopy and quantum chemical calculations.

We present femtosecond multiphoton ionization detected infrared spectra of jet-cooled monohydrates of adenine and 9-methyladenine. By quantum chemical vibrational analysis and comparison with available literature data we identified two isomers of adenine hydrate with one water molecule hydrogen-bonded to either the amino or the N9-H group. These two monohydrates revealed different fragmentation patterns in the ion depletion spectra, indicating isomer specific intermolecular dynamics. This different behaviour is discussed in terms of competing electronically excited state relaxation and dissociation processes.

Structure and hydrogen-bond vibrations of water complexes of azaaromatic compounds: 7-(3'-pyridyl)indole.

Complexes with water have been studied in the regime of supersonic jet isolation for 7-(3'-pyridyl)indole, a bifunctional molecule possessing hydrogen bond donor and acceptor groups. Two rotameric forms, syn and anti, are possible, of which only the former is able to form cyclic hydrogen bonds with protic solvents. Infrared-induced ion depletion spectroscopy was used to obtain vibrational patterns for 1:1 and 1:2 complexes in the hydrogen bond stretching region. The analysis of the spectra, supported by DFT calculations, revealed that for both stoichiometries the dominant forms correspond to cyclic, doubly or triply hydrogen-bonded species. The frequencies of NH...O, OH...N, and OH...O stretching vibrations were compared with the literature data to assess the strength of single vs multiple hydrogen bonds. Several new assignments and reassignments were proposed.

Specific microsolvation triggers dissociation-mediated anomalous red-shifted fluorescence in the gas phase.

Ion-depletion IR spectroscopy has revealed that at least two water molecules are required in complexes with 4-(dimethylamino)benzoic acid methyl ester (DMABME) for anomalous red-shifted fluorescence to occur in the gas phase. Through the use of high-level quantum-chemical calculations, two experimentally observed isoenergetic isomers are assigned to complexes in which a water dimer is hydrogen-bonded either to the carbonyl oxygen of the ester function or to the amino nitrogen. Surprisingly, computed IR spectra reveal that the N-bonded isomer is responsible for the observed red-shifted fluorescence. For an explanation, the mechanism of twisted intramolecular charge-transfer (TICT) formation and energy dissipation is investigated in detail. In general, for red-shifted fluorescence to occur, the N-bonded complexes must be able to dissipate energy, which in the gas phase can only happen nonradiatively via fragmentation. Arguments are given that only the N-bonded isomer photodissociates rapidly enough into free DMABME and a water dimer as a result of the immediate repulsion between the amino nitrogen and the water dimer in the TICT state. The O-bonded isomer, on the other hand, stays intact because the hydrogen bond is strengthened by additional electrostatic attraction in the ICT state. Furthermore, an experiment to further corroborate that mechanism is suggested.

Separation of different hydrogen-bonded clusters by femtosecond UV-ionization-detected infrared spectroscopy: 1H-pyrrolo[3,2-h]quinoline.(H2O)(n=1,2) complexes.

Experimental and theoretical studies are presented for complexes of water with 1H-pyrrolo[3,2-h]quinoline (PQ), a bifunctional compound acting simultaneously as a hydrogen-bond donor and acceptor. A 1:1 complex, which is not fluorescent and only very short-lived in the electronically excited state, was analyzed by isolating the complex under supersonic jet conditions and characterizing its structure by infrared-induced ion depletion spectroscopy utilizing multiphoton ionization by femtosecond UV pulses (IR/fsMPI spectroscopy). On the other hand, a long-lived 1:2 complex was identified as the smallest microhydrate of PQ contributing to the laser-induced fluorescence excitation spectrum. Its structure was assigned by fluorescence-detected IR spectra and analyzed using density functional theory. The structures of the 1:1 and 1:2 clusters are assigned to species in which the water molecule(s) form a hydrogen-bonded solvent bridge between the two functional groups. In accord with calculations, both 1:1 and 1:2 PQ/water complexes reveal weaker hydrogen bonding than the analogous clusters of PQ with methanol.

Supersonic jet studies of solvation effects on the spectroscopy and photophysics of 4-diethylaminopyridine.

We present the results of spectroscopic and photophysical investigations of 4-diethylaminopyridine (DEAP) and its 1 : 1 complexes with a number of protic solvents such as water and various alcohols of different acidity isolated under supersonic jet conditions. While a double resonance vibrational spectroscopic method was employed to investigate the size and geometrical structure of jet-cooled clusters, laser-induced fluorescence spectroscopy was used to examine the changes of photophysics induced by complexation of DEAP with solvent molecule(s). The results obtained from ab initio calculations enable the assignment of geometries and of the vibrational spectra of the clusters in the OH-stretch region. The comparison of the experimental and calculated vibrational spectra indicates that the solvent molecule is hydrogen-bonded to the pyridine nitrogen atom. Dual luminescence is observed only for the complexes with alcohols of relatively strong acidity.

Fluorescence quenching in cyclic hydrogen-bonded complexes of 1H-pyrrolo[3,2-h]quinoline with methanol: cluster size effect.

Laser induced fluorescence (LIF) excitation spectra of molecular complexes of 1H-pyrrolo[3,2-h]quinoline (PQ) with methanol (n= 1, 2, 3) as well as vibrational spectra of their electronic ground state are reported. The latter have been recorded in the mid-infrared region by fluorescence depletion (FDIR). The only PQ.methanol(n) complex clearly identified in the LIF spectrum is the triply hydrogen-bonded cyclic 1 ratio 2 aggregate. Its stoichiometry has been proven by the femtosecond multiphoton ionization detected infrared measurements [J. Am. Chem. Soc., 2006, 128, 10 000]. The structure of the 1 ratio 2 cluster is determined by means of FDIR spectroscopy in combination with ab initio and DFT calculations. No fluorescence was detected that could be attributed to the 1 ratio 1 cluster. This behaviour of the 1 ratio 1 complex is explained in terms of rapid excited state double proton transfer followed by a non-radiative relaxation. The n= 3 and heavier clusters are fluorescent. Their electronic spectra overlap, preventing the selective measurement of the FDIR spectra of individual complexes.

Detection and structural characterization of clusters with ultrashort-lived electronically excited states: IR absorption detected by femtosecond multiphoton ionization.

The vibrational fingerprint of the electronically excited short-lived complex of 1-H-pyrrolo[3,2-h]quinoline:methanol was measured using femtosecond multiphoton ionization detected infrared (IR/fsMPI) spectroscopy under supersonic jet conditions. A cyclic doubly hydrogen-bonded structure of the cluster has been proven from the comparison of the measured vibrational spectrum with that calculated with density functional theory. The employed nsIR-fsUV double resonance scheme is shown to be an effective tool for structural analysis of precursors that undergo fast deactivation and/or photoreactions.